Home

ADEOS Total Ozone Mapping Spectrometer (TOMS) Data Products

Contents

1. 331 nm residue Figure 6 1 TOMS Derived Ozone Error as a Function of Aerosol Index in the Presence of Tropospheric Absorbing Aerosol D Desert dust C1 and C2 Smoke 27 6 2 Additional Scan Angle Dependence For the near local noon equator crossing time Sun glint can occur over water for clear sky and near overhead Sun Under these conditions the derived surface reflectivity is enhanced a result of the extra radiation reflected from the surface The consequence is that derived ozone is low under these conditions slightly west of nadir in the vicinity of scan position 13 This effect is illustrated in Figure 6 2 which shows weekly averages of equatorial ozone as a function of scan position away from and near equinox The bottom panel shows a modest impact in the weekly mean but individual scans can show a 2 3 percent effect The upper panel shows ozone as a function of sample number for an unaffected scan It shows a smaller scan angle dependence of about 1 percent probably arising from scattering due to background aerosols and by cirrus cloud neither of which is fully treated in the radiative transfer calculation described in Section 5 1 260 Equatorial Februory 4 1998 259 250 245 Ozone D U 240 235 O 10 20 30 Scan Position 275 Equatorial March 25 1998 270 265 260 Ozone D U 255 250 0 10 20 30 Scan Position Figure 6 2 Derived Total Ozone as a Function of Scan Position Top Ty
2. Applied Optics 19 236 242 Komhyr W D R D Grass and R K Leonard 1989 Total Ozone Ozone Vertical Distributions and Stratospheric Temperatures at South Pole Antarctica in 1986 and 1987 J Geophys Res 94 11 429 11 436 Krotkov N A I V Geogdzhaev N Ye Chubarova S V Bushnev V U Khattatov T V Kondranin December 1996 A new database program for spectral surface UV measurements J of Oceanic and Atmos Tech 13 1291 1299 Krueger A J 1983 Sighting of El Chichon Sulfur Dioxide Clouds With the Nimbus 7 Total Ozone Mapping Spectrometer Science 220 1377 1378 Krueger A J M R Schoeberl and R S Stolarski 1987 TOMS Observations of Total Ozone in the 1986 Antarctic Spring Geophys Res Lett 14 527 530 Labow G J L E Flynn M A Rawlins R A Beach C A Simmons and C M Schubert 1996 Estimating Ozone with Total Ozone Portable Instruments II Practical Operation and Comparisons Appl Opt 35 6084 6089 Larko D E L W Uccellini and A J Krueger 1986 Atlas of TOMS Ozone Data Collected During the Genesis of Atlantic Lows Experiment GALE NASA TM 87809 99 pp Lienesch J H and P K K Pandey 1985 The Use of TOMS Data in Evaluating And Improving The Total Ozone from TOVS Measurements Rep NOAA TR NESDIS 23 Issue 22 3814 3828 Logan J A 1985 Tropospheric Ozone Seasonal Behavior Trends and Anthropogenic Influen
3. AQ AEGORIT M 100 da ARIES ee ge 11 4 1 Theoretical Foundation 0 ccc cece eee teen neta teen cere nsec eee eees 11 42 CalculationofRadiances c creeranno 13 AS Surface Reflection i lt Lirica ne A AAA AS 15 44 Initial B Pair Estimate 0c ccc ccc cee ee reece eee eee terete eee ar ro 16 45 BestOZone cosa vc fase cS Rw hse ES as FE ia SR Cert AAA 16 4 6 Validity Checks 0 0 cece cere 19 4 7 Level 3 Gridding Algorithm 0 0 ccc cece eee eee eee eet teen nee neers 21 5 0 GENERAL UNCERTAINTIES 2 2 cc cece ee eee eee eee eee eee eee een e ener eeeees 22 5 1 Accuracy and Precision of TOMS Measurements 2 6 66 eee eee rr 22 5 2 Calculated Radiances and Their Use in the Algorithm 2 0 cee cee eee ee cee eee ees 23 5 3 Comparison with Fairbanks Ozone Sondes 0 2 2 eee eee teen ene eee e ence teens 24 5 4 Comparison with Ground Based Measurements 5 e eee reece rr 24 6 0 PROBLEMS LOCALIZED IN SPACE AND TIME eee een eee eee eens 27 6 1 Aerosol Contamination ce ee eee eee ee eee eae n nnn 27 6 2 Additional Scan Angle Dependence eee teen een enn enn ees 28 6 3 Solar Eclipses 0 0 0 ccc cece ee cee ieri 29 6 4 Polar Stratospheric Clouds 0 29 6 5 High Teran sico sacs hawker A Hee Hag bo ee OE eee pria 29 7 0 DATA FORMATS visit AAA AA Ea TA AA AR ROSE Ge ARAS See Se
4. SYNC Flag for chopper non synchronization occurrence LATITUDE LONGITUDE SOLAR_ZENITH_ANGLE PHI NVALUE SENSITIVITY dN dR RESIDUE TOTAL _OZONE REFLECTIVITY ERROR_FLAG OZONE_BELOW_CLOUD TERRAIN_PRESSURE CLOUD_PRESSURE SOI ALGORITHM_FLAG CLOUD_FRACTION MIXING_FRACTION CATEGORY 0 Does not occur in current or next scan 1 Occurs in current scan not in next 2 Occurs in next scan not current 3 Occurs in both current and next scan IFOV latitude from 90 N 90 S degrees x 100 IFOV longitude from 180 W 180 E degrees x 100 IFOV solar zenith angle degrees x 100 Angle q between Sun and satellite measured at IFOV degrees x 100 N values as defined in Section 4 5 at 6 wavelengths shortest first x 50 Sensitivity dN dQ at 5 shortest wavelengths shortest first obtained by table interpolation matm cm x 10 000 N value sensitivity to reflectivity dN dR at 6 wavelengths shortest first x 50 Adjusted residues see Sections 4 5 at 5 shortest wavelengths shortest first x 10 127 Total Ozone matm cm x 10 Effective reflectivity assuming Lambertian surface x 100 Error Flag O good data 1 good data 84 lt solar zenith angle lt 88 2 residue at 331 nm greater than 4 in N value units 3 triplet residue too large 4 SOI gt 13 SO contamination 5 Atleast one residue has absolute value larger than 12 5 A value of 10 is added to the error flag for all scans on descending mi
5. Solar measurements made using the working diffuser have been used to estimate rs equation 2 Weekly measurements of the Working surface are presented in Figure 3 1 where the initial values have been normalized to and signals have been corrected for Sun Earth distance as well as the 1 5 percent initial working degradation In the figure the 360 nm signal is shown along with the 331 360 nm signal ratio and the 313 331 360 triplet ratio Plots for the other wavelength ratios are qualitatively similar to Figure 3 1 The nearly 15 percent decrease at 360 nm is substantial but still less than EP TOMS The decrease in throughput is believed to be optical degradation of the fore optics probably the scan mirror The rapidity of the decrease is not understood Since C and C of equation 3 cannot be measured simultaneously C must be characterized for intervening times at which C are measured A regression of the data shown in Figure 3 1 was chosen to smooth through variations in solar measurements These variations a few tenths of a percent in the wavelength ratios are not large enough to be seen in total ozone retrieval 3 3 Wavelength Monitoring Following the laboratory calibration an on board wavelength monitor has tracked changes in the wavelength scale both before launch and in orbit Change might be produced by excessive temperature differentials or mechanical displacement of the wavelength determining components resulting from shock o
6. geog_flag G indicating global data 7 Northern latitude north_lat 90 00 8 Southern latitude south_lat 90 00 9 East longitude east_lon 180 00 10 West longitude west_lon 180 00 11 Day night flag day_night_flag D indicating daytime data 12 Granule version granule_version 01 indicating first archive version 13 Producer granule ID producer_granule_id algY YDDD hdf where YY is 2 digit year and DDD is day of year both with leading zeroes as necessary 14 Fill value for ozone miss_val_ozone 0 15 Fill value for reflectivity miss_val_ref 999 16 Local time of ascending node equator crossing lect YYYY MM DD HH MM SS where Y Y Y Y is year MM is month of year 1 12 DD is day of month HH is hour of day MM is minute of hour and SS is second of minute The data stored in the SDSs are on a fixed 1 degree latitude by 1 25 degree longitude grid The gridded ozone values are stored as 3 digit integers in units of matm cm Reflectivity in percent is also stored as 2 byte integers Grid cells that are missing data due to lack of sunlight or other problems will be filled with 0 for ozone 999 for reflectivity The two Coordinate SDSs stored in the Level 3 product are listed in Table 7 5 Table 7 5 TOMS Level 3 HDF Coordinate SDSs Name Type Scaletype Scalemin Scalemax Latitude 4 byte real regular 89 5 89 5 Longitude 4 byte real regular 179 375 179 375 7 2 Native Format
7. 15 16 day of month 17 eS 18 ASCII blank 19 22 year 23 24 ASCII blanks 25 35 ADEOS TOMS 40 Table 7 11 Format of Header Line of CDTOMS Daily Grid Continued III ROcq qktr _E__ 2 ml18_____ ______rrcrr tm Character Contents 36 37 ASG i banks 38 40 STD 41 ASCII blank 42 46 OZONE 47 50 ASCII blanks 51 60 GEN yy ddd 61 ASCII blank 62 70 Asc LECT 71 ASCII blank 72 73 hour local of ascending node equator crossing 74 ASCII 75 76 minute local of ascending node equator crossing 77 ASCII blank 78 79 AM or PM indicating morning or aftemoon evening ascending node equator crossing 80 ASCII blank 81 lt lf gt line feed character i e HEX OA Day 180 Jun 29 1997 ADEOS TOMS STD OZONE GEN 97 182 Asc LECT 10 40 AM Longitudes 288 bins centered on 179 375 W to 179 375 E 1 25 degree steps Latitudes 180 bins centered on 89 5 S to 89 5 N 1 00 degreesteps 211211211211207207207207205205205205205205205205205205205205205205205205205205205205200200200 200199199199199197197197197195195195195194194194194194194194194194194194194194194194194194194 194194194194194194187187187187189189189189192192192192192192192192192192192192192192192192188 188188188188188188188188188188188188188188188189189189189189189189189189189189189195195195195 195195195195195195195195195195195195197197197197200200200200198198198198199199199199201201201 2012012012
8. 7 2 1 TOMS Ozone File Level 2 Data Product The TOMS Ozone File also called the Level 2 Data Product is a binary file written as FORTRAN unformatted records It is generated under UNIX These files are used primarily as part of the TOMS processing They are not normally distributed but may be obtained by special arrangement Each file contains all of the data processed for a single day The first record of the file is a header written in character format containing information on the production hardware and software for both the Level 2 product and the Level 1 product used to generate it the date and time the Level 2 file was generated and the time period that the data on the file cover The data records follow ordered chronologically by time GMT of observation and grouped by TOMS orbit Each data record contains the information processed from one scan of the TOMS instrument Only daylight scans where the solar zenith angle at the nadir view for the scan is less than or equal to 92 degrees have been processed by the ozone algorithm and written to the ozone file The end of an orbit is indicated by a record called the orbital summary record which contains the date time and location of the start and end of the orbit and of the equator crossing counts of the number of scans processed and those flagged for various reasons and other summary and ancillary information for the orbit The last record of the file called the trailer record contai
9. C J N C Hsu J R Herman P K Bhartia O Torres W I Rose D J Schneider and N Krotkov 1997 Detection of Volcanic Ash Clouds From Nimbus 7 TOMS Reflectivity Data J Geophys Res 102 16 749 16 759 Torres O Z Ahmad and J R Herman 1992 Optical Effects of Polar Stratospheric Clouds on the Retrieval of TOMS Total Ozone J Geophys Res 97 13 015 13 024 Torres O J R Herman P K Bhartia and Z Ahmad 1995 Properties of Mount Pinatubo Aerosols as Derived From Nimbus 7 Total Ozone Mapping Spectrometer Measurements J Geophys Res 100 14 043 14 055 Torres O P K Bhartia J R Herman Z Ahmad and J Gleason 1998a Derivation of Aerosol Properties from Satellite measure ments of Backscattered Ultraviolet Radiation Theoretical Basis J Geophys Res In press Torres O L Moy P K Bhartia 1998b Impact of Aerosol Absorption on Total Ozone Retrieval from Satellite Measurements of Backscattered Ultraviolet Radiation submitted to J Geophys Res Wellemeyer C G S L Taylor G Jaross M T DeLand C J Seftor G Labow T J Swissler and R P Cebula 1996 Final Report on Nimbus 7 TOMS Version 7 Calibration NASA Contractor Report 4717 National Aeronautics and Space Admin istration Washington DC Wellemeyer C G S L Taylor C J Seftor R D McPeters P K Bhartia 1997 A correction for TOMS profile shape errors at high latitude J Geophy
10. T and Sy from the tables for the geometry of the measurement and a single ozone profile the low latitude profile for measurements at latitudes 15 degrees and lower the mid latitude profile for 15 degrees lt latitude lt 60 degrees and the high latitude profile at latitudes higher than 60 degrees These radiances are then corrected for rotational Raman scattering the Ring effect The correction factors based on the results of Joiner et al 1995 are shown in Table 4 3 They were computed using a solar zenith angle of 45 degrees and a nadir scan The dependences on solar and scan angles which are small under most conditions are neglected Two sets were calculated one at 1 atm and the assumed 8 percent ground reflectivity for use with the 1 atm radiance tables and the other at 0 4 atm and the assumed 80 percent cloud reflectivity for use with the 0 4 atm tables This correction greatly reduces the biases that had been seen between ozone values Table 4 3 Rotational Raman Scattering Corrections Radiance Correction Pressure 0 4 atm Reflectivity 80 Pressure 1 0 atm Actual Wavelength nm Reflectivity 8 308 68 0 295 0 167 312 59 0 17 0 006 317 61 0 598 0 311 322 40 0 126 0 056 331 31 0 310 0 139 360 11 0 430 0 175 The ground radiance is then derived by interpolating between values for the two pressures to derive the radiance for the pressure at the terrain height from the grid A similar proces
11. and cloud The average ground terrain heights are from the National Oceanic and Atmospheric Administration NOAA National Meteorological Center NMC provided in km for a 0 5 degree x 0 5 degree latitude and longitude grid These heights are converted to units of pressure using a U S Standard Atmosphere ESSA 1966 and interpolated to the TOMS IFOVs to establish the pressure at the Earth s surface Probabilities of snow ice cover from around the globe are collected by the Air Force Global Weather Center and mapped on a polar stereographic projection These data have been averaged to provide a monthly snow ice climatology mapped onto a 1 degree x 1 degree latitude and longitude grid and used to determine the presence or absence of snow in the TOMS IFOV If the probability is 50 percent or greater snow ice is assumed to be present For cloud heights a climatology based upon the International Satellite Cloud Climatology Project ISCCP data set is used It consists of the climatological monthly averages over a 0 5 x 0 5 degree latitude longitude grid The impact of the use of this climatology on the TOMS derived ozone is discussed in Hsu et al 1997 Refiectivity is determined from the measurements at 360 nm For a given TOMS measurement the first step is to determine calculated radiances at 360 nm for reflection off the ground and reflection from cloud based on the tables of calculated 360 nm radiances For reflection from the ground the terrain he
12. and reference are arranged as the sides of an equilateral triangle and mounted on a carousel so that a given diffuser can be rotated into view on demand The working diffuser was exposed once per week and the cover diffuser was exposed for the remainder of the time whether or not the solar flux was being measured The reference surface was exposed only twice early in the data record The measured degradation rate of the cover diffuser was used to infer that the degradation of the working diffuser was negligible A new feature on ADEOS TOMS is the ability to monitor solar diffuser reflectance A device referred to as the Reflectance Calibration Assembly RCA was added to the new series of TOMS This assembly employs a phosphor light source with peak emission over the TOMS wavelength range When powered on the lamp illuminates the exposed diffuser surface which is then viewed using the TOMS scan mirror The scan mirror also rotates to view the phosphor surface directly The ratio of signals at the two scan mirror positions is a measure of relative diffuser reflectance The ADEOS TOMS has 11 operating modes during normal operations The most important of these are 1 Standby mode 2 Scan mode 3 Solar calibration mode 4 Wavelength monitoring mode 5 Electronic calibration mode 6 Reflectance calibration mode 7 Direct control mode The primary operating mode of the TOMS is scan mode It is in this mode that the scanning mirror sampl
13. appropriate absorption coefficient and temperature dependence Paur and Bass 1985 for each wavelength The I F for the entire band A A is then given by the following expression AQ ADF FAIS 8 where A A at at wavelength F A solar flux at wavelength A I A Earth radiance at wavelength A and S A Instrument response function at wavelength A The wavelength dependence of the solar flux is based on SOLSTICE measurements Woods et al 1996 This detailed calculation replaces the effective absorption coefficients used in Version 6 Table 4 2 shows effective absorption coefficients for the ADEOS TOMS wavelengths As discussed above effective absorption coefficients are not used in the Version 7 algorithm The same method of calculation was used as in Version 6 integrating the monochromatic laboratory values over the TOMS bandpass for the following conditions a mid latitude profile for Q 350 a path length of 2 5 and a wavelength independent solar flux These effective absorption coefficients are given in Table 4 2 Because the effective absorption coefficient depends on the ozone profile optical path length and solar flux spectrum the Version 7 technique of calculating IF at individual 13 wavelengths and then integrating over the TOMS bandpass eliminates the imprecision arising from using one set of effective absorption coefficients derived for a particular set of conditions for all calrulations Table 4 2 a
14. forof 3 5 Values of fprof Outside this limit require such a degree of extrapolation that the profile is not considered highly reliable If the data fail the relevant test the error flag is set to 3 The next check uses the 331 nm residue If this residue exceeds 4 in N value units the error flag is set to 2 Flag values of 3 or 2 resulting from large residues imply that the values of I F may be inconsistent with the assumption that the linear correction can be used For solar zenith angles greater than 84 degrees the algorithm loses accuracy Most retrievals must make use of the C triplet which is not highly sensitive to ozone In addition the conditions depart from those for which the radiative transfer code was designed in particular the extreme geometry Caudill et al 1997 For this case the error flag is set to 1 Finally the value 10 is added to the flag value for the data that are taken in polar summer on the descending north to south part of the orbit While all flagged ozone values appear on the Level 2 data sets only ozone values 20 with the flag set to 0 for a good retrieval from the ascending part of the orbit are used to derive the gridded means of Level 3 Table 4 4 summarizes the error flags when they are set and their significance Table 4 4 Error Flags Flag Criterion Significance 0 No other flag set Good value l Solar zenith angle gt 84 Algorithm less accurate 2 r 331 gt 4 N value Linear correct
15. the emphasis in TOMS calibration is not in determining k or k separately but rather their ratio K The primary quantity measured by TOMS and used to derive ozone is the normalized radiance F_ The advantage of this approach is that the spectrometer sensitivity changes affecting both the Earth and solar measurements f _ cancel in the ratio The ratio becomes la C G Fo a KG set 3 m I I where K is a combined calibration constant for TOMS normalized radiances referred to as the albedo calibration constant Table 3 1 Radiance and irradiance measurements are sometimes made in different gain ranges at high solar zenith angles Evidence indicates that G has been very accurately characterized see Section 3 4 Therefore the initial absolute TOMS calibration involves knowledge of the quantity k g k The angular dependence g is dominated by the diffuser Bi directional Reflectivity Distribution Function BRDF and is measured prior to launch Since the instrument changes affecting both the Earth and solar measurements cancel in the I F ratio the quantity critical to the time dependent calibration of the normalized radiance is the diffuser plate reflectivity p t Table 3 1 ADEOS TOMS Albedo Calibration Constants and Gain Range Ratios Wavelength Albedo Cal Constant Adjustment Factor nm steradian ratio 308 68 0 093 1 000 312 59 0 094 1 000 317 61 0 094 1 000 322 40 0 095 1 005 331 31 0 096 1 000 360
16. 1 Modes of Equatorial Distributions of Residues for each of the ADEOS TOMS Channels as a Function of Wavelength Residues are reported on the Level 2 product in units of N value A difference of 1 N value is equal to 2 31 The A tnplet residue can be defined as A 360 A 331 360 33 The modal A triplet residues for the 309 318 and 322 nm channels are equal to their vertical displacement from the A triplet line in Figure 4 1 These non zero triplet residues indicate some wavelength dependent inconsistency in the measurement system This may be due to calibration error some systematic error in the atmospheric radiation transfer model used in the retrieval or systematic wavelength dependence in the effective surface reflectivity at the bottom of the atmosphere As discussed in Section 3 2 2 a calibration adjustment has been made at 322 nm to remove the modal 0 5 A triplet residue shown in Figure 4 1 This removes the systematic offset that would occur between A triplet ozone and C triplet ozone It also serves to normalize the triplet residues for use in the profile mixing scheme described below Note that similar definitions of B triplet residue and C triplet residue can be constructed relative to total ozone derived using these triplets as well r 18 For retrievals at latitudes where two profiles are used an ozone value appropriate to the latitude of the measurement is then derived from the ozone values for the two profiles using an
17. F values at 1 AU watts cm for current day at the six instrument wavelengths shortest first Calibration constants The counts to radiance conversion factors in units of watts cm steradian count given for each of the four gain ranges for each of the six wavelengths in order words 67 80 309 nm words 87 90 360 nm Nominal spacecraft zenith angle 0 80 degrees at each scan position A A A I A Notes The logical sequence number is a 16 bit integer that occupies the left half two most significant bytes of word 3 Words 53 127 are stored in IEEE 754 32 bit floating point format REAL 4 all others are 4 byte 32 bit integer format with the most significant byte first 5 k _ _ __ _ OK Table 7 10 Format of Trailer Record NR rrddazaTZT 0 et _v_7 Word Description e nr rrr P eleltleeia aumeauSddIIITdWET reTtCYTYTeET SOJA A a dn Du WN Orbit number of last scan GMT seconds of first scan of last orbit of day Logical sequence number 1 2 most significant bytes Day of year of first scan of last orbit of day Year of first scan of last orbit of day Latitude 90 S 90 N for first scan nadir view degrees x 100 Longitude 180 W 180 E for first scan nadir view degrees x 100 GMT seconds of last scan of last orbit of day Day of year of last scan of last orbit of day Year of last scan of last orbit of day Latitude 90
18. equation of the following form Q I S prof S tower id prof higher 19 18 where Q best ozone Qiower Ozone retrieved using lower latitude profile Qhigher Ozone retrieved using higher latitude profile fprof weight given to higher latitude profile Thus forof Will be O if only the lower latitude profile is selected 1 if only the higher latitude profile is selected and in between for a combination of the two profiles The choice of pairs and fpro depends upon the optical path length Qp sec Oy sec 8 in atm cm For path lengths less than 1 5 a value of fprof obtained by simple linear interpolation in latitude latitude latitude ower J 20 prof latitude y o y gy llatitudel TARDO is used for latitudes between 15 and 75 degrees using the two profiles appropriate to the latitude The low latitude profile alone is used from the equator to 15 degrees and the high latitude profile alone is used from 75 degrees to the pole For a path length less than or equal to 1 0 the A triplet wavelengths are used in Equation 16 for a path length greater than and no greater than 1 5 the B triplet is used with the same latitude interpolation For longer path lengths the profile mixing scheme mentioned above in Section 4 2 is used to determine the profile mixing factor forof The basic principle is to improve the triplet ozone using profile shape information in the triplet residue of a shorter wavelength to determi
19. layer in matm cm 3 SOI in matm cm 50 4 Pressure derived from ISCCP cloud climatology in atm x 100 19 l Algorithm flag flag flag 10 for snow assumed present 1 11 A triplet used 2 12 B triplet used 3 13 B triplet used with profile selection B mix 4 14 C triplet used with profile selection C mix 2 Effective cloud fraction x 100 3 Profile mixing fraction x 10 1 lt f lt 2 profile between low and mid latitude 2 lt f lt 3 profile between mid and high latitude 4 Surface category code O water 1 land 2 low inland below sea level 3 land and water 4 land and low inland 5 water land and low inland 20 523 Same as 6 19 for samples 2 37 524 525 Spares Table 7 9 Format of Orbital Summary Record Word Description 1 Orbit number 2 GMT seconds of first scan of orbit 3 Negative logical sequence number 2 most significant bytes 4 Day of year of first scan of orbit 5 Year of first scan of orbit 4 digits 6 Latitude 90 S 90 N for first scan nadir view degrees x 100 7 Longitude 180 W 180 E for first scan nadir view degrees x 100 8 GMT seconds of last scan of orbit 9 Day of year of last scan of orbit 10 Year of last scan of orbit 4 digits 11 Latitude 90 S 90 N for last scan nadir view degrees x 100 12 Longitude 180 W 180 E for last scan nadir view degrees x 100 13 Local time seconds at equator crossing or 77 if unavailable 14 Day of year local time at equator crossing 15 Ye
20. maximum at higher latitude This correlation was used as the basis for lowering the heights of the ozone maxima at high latitudes and raising them in the tropics when extending the original climatology to represent the more extreme profile shapes Wellemeyer et al 1997 Given the wavelength total ozone and ozone profile surface pressure satellite zenith angle at the field of view and solar zenith angle the quantities Im Ia T and Sy of Equations 4 and 5 can then be calculated at the six TOMS wavelengths For the tables used in the algorithm these terms are computed at the TOMS wavelengths for all 26 standard profiles and two reflecting surface pressure levels 1 0 atm and 0 4 atm For each of these cases Iw Ia T are calculated for 10 choices of solar zenith angle from 0 88 degrees spaced with a coarser grid at lower zenith angles and a finer grid for higher zenith angles and for six choices of satellite zenith angle five equally spaced from 0 60 14 degrees and one at 70 degrees In Version 6 the tables extended only to a satellite zenith angle of 63 3 degrees The fraction of reflected radiation scattered back to the surface Sp does not depend on solar or satellite zenith angle 4 3 Surface Reflection To calculate the radiances for deriving ozone from a given measurement requires that the height and reflectivity of the reflecting surface be known The TOMS algorithm assumes that reflected radiation can come from two levels ground
21. ozone and effective reflectivity adjusted residues can be computed for the remaining wavelengths 309 nm 318 nm and 322 nm These residues specifically characterize the inconsistency of the measured radiances with the total ozone and reflectivity derived using the A triplet Modal residues from the global population of A triplet retrievals were used to estimate the necessary adjustment see Table 3 1 The largest residue occurs in the 309 nm channel However since the 309 nm channel is not used to derive the best ozone the derived adjustment factor of 0 986 was not applied The 318 nm channel was consistent to within about 1 dobson unit and no adjustment was needed The 322 nm channel required the adjustment indicated in Table 3 1 These modal residues are illustrated in Figure 4 1 No adjustment has been made to the absolute scale 360 nm albedo calibration value 3 2 3 Time Dependent Calibration Archival Product As discussed in the introduction to this section the time dependent calibration requires a correction for changes in the reflectivity of the solar diffuser plate The ADEOS TOMS was equipped with a carousel with three diffusers that were exposed to the degrading effects of the Sun at different rates The cover diffuser was exposed almost constantly the working diffuser was exposed weekly and the reference diffuser was exposed only twice While the cover diffuser degraded quite rapidly there is no indication that the working or the r
22. wavelengths is far larger than the separation between the pairs thus the 360 nm measurement provides a long baseline for deriving wavelength dependence This process may be iterated using the results of the first triplet calculation as the new initial estimate Table 4 1 lists the wavelengths of the pairs and triplets and the range of optical path length over which they are used Here we define optical path length s2Q sec seco Table 4 1 Pair Triplet Wavelengths Pair Triplet Ozone Sensitive Ozone Insensitive Reflectivity Range of Application Designation Wavelength nm Wavelength nm Wavelength nm Optical Path s atm cm A 312 6 331 3 360 1 s lt l B 317 6 331 3 360 1 1 lt s lt 3 C 322 4 331 3 360 1 s gt 3 4 2 Calculation of Radiances To carry out the calculation described in Section 4 1 requires the following information e Ozone absorption coefficients as a function of temperature for the wavelengths in the TOMS bandpasses e Atmospheric Rayleigh scattering coefficients Climatological temperature profiles e Climatological ozone profiles e Solar zenith angle Satellite zenith angle at the IFOV e Angle between the solar vector and the TOMS scan plane at the IFOV e Pressure at the reflecting surface Because of its finite bandwidth TOMS does not measure a monochromatic radiance For comparison with the TOMS measurements radiances are calculated at approximately 0 05 nm intervals across each of the TOMS slits using the
23. 0 120 Dobson Units Mean V7 0 5 Sigma V7 3 9 5 ud v E O N O 9 O HL 2 3 4 5 6 Optical Path Length atm cm Figure 5 1 Summary of ADEOS TOMS Sonde Comparisons A Comparison of profile shapes used in TOMS retrievals with simple latitude mixing dots and profile selection dashes with a coincident ozone sonde from Fairbanks solid Each shape plotted contains the same total ozone but the impact of the selected shape on TOMS derived ozone at 83 4 degrees solar zenith angle relative to the sonde amount is illustrated A coincident NOAA 9 SBUV 2 measurement is used to provide ozone amounts in the uppermost six layers B Residual error in ADEOS TOMS retrievals relative to all sixteen coincident composite profiles from ozone sonde and SBUV 2 at high solar zenith angles 25 Percent Difference Mean Difference 1 5 Standard Deviation 0 8 96 6 96 8 97 0 97 2 97 4 Year Figure 5 2 Percentage Difference of ADEOS TOMS Ground Ozone Measurements as a Function of Time 6 0 PROBLEMS LOCALIZED IN SPACE AND TIME 6 1 Aerosol Contamination Increased Mie scattering resulting from the presence of tropospheric aerosols modifies the radiative properties of the atmosphere and may significantly affect the radiances measured by TOMS The triplet formulation described in Section 4 is designed to correct for such departures if they result in algorithmic residues that are linear with wavelength This appears t
24. 01201201201201201202202202202202202202202192192192192192192192192192192192192192192 192192192192192192192192192192208208208208208208208208208208208208208208208208208208208208197 197197197197197197197200200200200200200200200202202202202203203203203201201201201201201201201 201201201201201201201201201201201201201201201201197197197197199199199199198198198198196196196 196196196196196192192192192 lat 89 5 217217217217217217217217209209209209203203203203202202202202202202202202202202202202199199199 199198198198198191191191191191191191191191191191191191191191191191191191191189189189189189189 189189189189189189186186186186186186186186187187187187187187187187187187187187187187187187187 187187187188188188188190190190190190190190190190190190190190190190190190190190190191191191191 192192192192192192192192192192192192193193193193193193193193200200200200200200200200200200200 200200200200200201201201201202202202202211211211211211211211211211211211211211211211211211211 211211213213213213217217217217217217217217220220220220216216216216219219219219219219219219211 211211211211211211211209209209209221221221221220220220220218218218218218218218218217217217217 216216216216214214214214208208208208206206206206206206206206206206206206203203203203198198198 198196196196196196196196196 lat 88 5 Figure 7 1 Sample CDTOMS Daily Grid File Excerpt 41 REFERENCES Ahmad Z and P K Bhartia 1995 Effect of Molecular Anisotropy on the Backscattered UV Radiance Applied Opti
25. 11 0 010 1 000 Gain Range Ratios Range 2 1 Range 3 2 10 032 10 007 3 2 1 Prelaunch Calibration ADEOS TOMS prelaunch characterization includes determination of the albedo calibrations K and band center wavelengths Both are reported in Table 3 1 Several different methods were employed to measure the values of K for the six TOMS channels These included separate characterization of radiance and irradiance sensitivity and direct measurement of the flight diffuser reflectance Only one method was chosen to represent the instrument calibration The technique selected to calibrate the instrument radiance and irradiance sensitivity ratio albedo calibration involves calibration transfer from a set of laboratory diffuser plates These Spectralon diffusers were independently characterized by GSFC and by NIST National Institute of Standards and Technology In the calibration a NIST calibrated tungsten halogen lamp is used to illuminate a Spectralon plate which in turn is viewed by the instrument This yields an estimate of the radiance calibration constants k The same lamp illuminating the instrument directly yields the irradiance calibration constants k In the ratio of calibration constants many systematic error sources such as absolute lamp irradiance cancel The value of k is also measured at various illumination angles to determine the angular correction g A film strip technique was used to determine instrument wavelength selection Photo sen
26. 4296 Fraser R S and Z Ahmad 1978 The Effect Of Surface Reflection and Clouds on the Estimation of Total Ozone From Satellite Measurements Fourth NASA Weather and Climate Program Science Review NASA Conf Publ 2076 247 252 National Aeronautics and Space Administration Washington DC NTIS N7920633 Fleig A J P K Bhartia and David S Silberstein 1986 An Assessment of the Long Term Drift in SBUV Total Ozone Data Based on Comparison With the Dobson Network Geophys Res Lett 13 1359 1362 Fleig A J D S Silberstein R P Cebula C G Wellemeyer P K Bhartia and J J DeLuisi 1989 An Assessment of the SBUV TOMS Ozone Data Quality Based on Comparison With External Data Ozone in the Atmosphere Proceedings of the Qua drennial Ozone Symposium 1988 and Tropospheric Ozone Workshops edited by R D Bojkov and P Fabian 232 237 A Deepak Hampton VA Gleason J F N C Hsu and O Torres 1998 Biomass Burning Smoke Measured Using Backscattered Ultraviolet Radiation SCAR B and Brazilian Smoke Interannual Variability accepted to J Geophys Res Gleason J F P K Bhartia J R Herman R McPeters P Newman R S Stolarski L Flynn G Labow D Larko C Seftor C Wellemeyer W D Komhyr A J Miller and W Planet 1993 Record Low Global Ozone in 1992 Science 260 523 526 Heath D F A J Krueger H R Roeder and B D Henderson 1975 The Solar Backscatter
27. Aerosol Index 0 6 cece eee eee eee ene 27 6 2 Derived Total Ozone as a Function of Scan Position 2 0 eee erence eens 28 7 1 Sample CDTOMS Daily Grid File Excerpt 02 e cc bio 41 LIST OF TABLES Table Page 3 1 ADEOS Albedo Calibration Constants and Gain Range Ratios oooooooorocrrrcnrormor 6 4 1 Pair Triplet Wavelengths 2 ccc ccc ce iii 13 4 2 Effective Absorption and Scattering Coefficients 20 eee ee cer ee eee teen eens 14 4 3 Rotational Raman Scattering Corrections 0 6 cece cc eee ere eee tent ee enna 16 AA Enor Flags kino ei ad Ses See hae ters Bask antenati ano AAA 21 5 1 Errors in Retrieved ADEOS TOMS Ozone 2 0 2 ii 22 7 TOMS Level HDF SDFS uri ai paia Sa sae GPRS ole See A eae 32 7 2 Detailed Description of TOMS Level 2 SDSs 0 32 7 3 Fill Values for Missing Scans 0 ccc cece ec ee ee cee eee e cnet e ene e eee ee eee te erences 34 7 4 TOMS Level 2 HDF Coordinate SDSs cece cee eee eee teen cere ne neees 34 7 5 TOMS Level 3 HDF Coordinate SDSS PRIORE IAT TTT ORION 35 7 6 Format of TOMS Ozone File Header Record iii 36 1T FormatorData RECO Sr A ARI A e 36 78 Detaled Desenpions rici A A AA e A ade la ARAS 37 vii 7 9 7 10 7 11 A l A 2 A 3 LIST OF TABLES Continued Page Format of Orbital Summary Record 38 Formator Traner Record ais iaa idad 39 Format of Header Lin
28. C Hsu C Seftor E Celarier 1997 Nimbus TOMS Absorbing Aerosols J Geophys Res 102 16911 16922 Herman J R R Hudson R McPeters R Stolarski Z Ahmad X Y Gu S Taylor and C Wellemeyer 1991 A New Self Cal ibration Method Applied to TOMS SBUV Backscattered Ultraviolet Data to Determine Long Term Global Ozone Change J Geophys Res 96 7531 7545 Herman J R P K Bhartia A J Krueger R D McPeters C G Wellemeyer C J Seftor G Jaross B M Schlesinger O Torres G Labow W Byerly S L Taylor T Swissler R P Cebula and X Gu October 1996 Meteor 3 Total Ozone Mapping Spec trometer TOMS Data Products User s Guide NASA Reference Publication 1393 Hsu N C J R Herman P K Bhartia C J Seftor O Torres A M Thompson J F Gleason T F Eck and B N Holben 1996 Detection of biomass burning smoke from TOMS measurements Geophys Res Lett 23 745 748 Hsu N Christina R D McPeters C J Seftor and A M Thompson 1997 The Effect of An Improved Cloud Climatology on the TOMS Total Ozone Retrieval J Geophys Res 102 4247 4255 lig D F Baker and M Folk 1993 HDF Specification and Developer s Guide Version 3 2 National Center for Supercomputing Applications Champaign IL Jaross G A J Krueger R P Cebula C Seftor U Hartman R Haring and D Burchfield 1995 Calibration and Postlaunch Performance of the Meteor 3 TOMS Instru
29. C triplet for ozone determination The ADEOS and Earth Probe TOMS algorithms are identical except for small differences in the band center wavelengths 4 1 Theoretical Foundation To interpret the radiance measurements made by the TOMS instrument requires an understanding of how the Earth s atmosphere scatters ultraviolet radiation as a function of solar zenith angle Incoming solar radiation undergoes absorption and scattering in the atmosphere by atmospheric constituents such as ozone and aerosols and by Rayleigh scattering Radiation that penetrates to the troposphere is scattered by clouds and aerosols and radiation that reaches the ground is scattered by surfaces of widely varying reflectivity The backscattered radiance at a given wavelength depends in principle upon the entire ozone profile from the top of the atmosphere to the surface The three shortest wavelengths used in the TOMS ozone measurements were selected because they are strongly absorbed by ozone At these wavelengths absorption by other atmospheric components except volcanic SO is negligible compared to that by ozone At wavelengths longer than approximately 310 nm the intensity is determined primarily by the total optical depth above the scattering layer in the troposphere The amount of ozone below the scattering layer is small and can be estimated with sufficient accuracy to permit derivation of total column ozone Because most of the ozone is in the stratosphere the pri
30. ITUDE 500 x 37 2 byte integer SOLAR_ZENITH_ANGLE 500 x 37 2 byte integer PHI 500 x 37 2 byte integer NVALUE 500 x 37 x 6 2 byte integer SENSITIVITY 500 x 37 x 5 2 byte integer dN dR 500 x 37 x 6 l byte unsigned integer RESIDUE 500 x 37x 5 byte unsigned integer TOTAL_OZONE 500 x 37 2 byte integer REFLECTIVITY 500 x 37 2 byte integer ERROR_FLAG 500 x 37 2 byte integer OZONE_BELOW_CLOUD 500 x 37 1 byte unsigned integer TERRAIN_PRESSURE 500 x 37 1 byte unsigned integer CLOUD_PRESSURE 500 x 37 1 byte unsigned integer SOI 500 x 37 1 byte unsigned integer ALGORITHM_FLAG 500 x 37 1 byte unsigned integer CLOUD_FRACTION 500 x 37 byte unsigned integer MIXING_FRACTION 500 x 37 1 byte unsigned integer CATEGORY 500 x 37 1 byte unsigned integer The last index varies most rapidly in all arrays 0 TE PS A A NA AAA ARE aaa Table 7 2 Detailed Description of TOMS Level 2 SDSs SDS Name Description LSEQNO Sequence number of scan within orbit YEAR Year four digits at start of scan GMT DAY Day of year 1 366 at start of scan GMT GMT Greenwich Mean Time in seconds of day at start of scan 1 86 400 ALTITUDE Spacecraft altitude at start of scan km NADIR Nadir scan angle used to express the spacecraft s attitude error the angle between the vectors from the S C to the local normal and from the S C to the FOV 0 lt nadir angle lt 180 x 100 32 Table 7 2 Detailed Description of TOMS Level 2 SDSs Continued SDS Name Description
31. Level 3 product contains global total ozone on a fixed 1 degree latitude by 1 25 degree longitude grid It is available at URL ftp jwocky gsfc nasa gov pub adeos Except for some changes in the header line the Version 7 Level 3 product is identical to the Nimbus 7 TOMS Version 6 CD ROM product and the CDTOMS ozone product that was available by ftp One global grid is stored in each CDTOMS file Table 7 11 provides a detailed description of the first line of a daily grid file Figure 7 1 shows an example of the header and the first two latitude zones in a CDTOMS daily file from the ADEOS TOMS The gridded ozone values are stored as 3 digit integers in units of matm cm Each of the 180 latitude zones requires 12 lines They are ordered from south to north with the first zone centered at 89 5 degrees Within each latitude zone values are given for each of 288 longitude zones from 180 W through 0 Greenwich to 180 E The first longitude zone is centered at 179 375 degrees As shown in Figure 7 1 annotation is present after all values are given for a latitude zone Zeroes denote flagged data that is data that could not be collected due to lack of sunlight or other problems Table 7 11 Format of Header Line of CDTOMS Daily Grid Character Contents ASCII blank HEX 20 2 5 Day quotes indicate fixed content 6 ASCII blank 7 9 day of year 10 ASCII blank 11 13 month Jan Feb Mar 14 ASCII blank
32. NASA TP 95 206857 ADEOS Total Ozone Mapping Spectrometer TOMS Data Products User s Guide A Krueger PK Bhartia R McPeters and J Herman Goddard Space Flight Center Greenbelt MD C Wellemeyer G Jaross C Seftor O Torres G Labow W Byerly L Moy S Taylor T Swissler and R Cebula Raytheon STX Corporation Lanham Maryland National Aeronautics and Space Administration Goddard Space Flight Center Greenbelt Maryland 20771 eE AAA NN ee __P1_________ __ _ yymT tsmoek o July 1998 Available from NASA Center for AeroSpace Information Parkway Center 7121 Standard Drive Hanover MD 21076 1320 Price Code A17 National Technical Information Service 5285 Port Royal Road Springfield VA 22161 Price Code A10 ACKNOWLEDGMENTS The Level 2 and Level 3 data products described in this User s Guide were prepared by the Ozone Processing Team OPT of NASA Goddard Space Flight Center Please acknowledge the Ozone Processing Team as the source of these data whenever reporting on results obtained using the TOMS data The TOMS algorithm development evaluation of instrument performance ground truth validation and data production were carried out by the OPT at NASA GSFC The OPT is managed by the Nimbus Project Scientist R D McPeters The current OPT members include Z Ahmad G Batluck E Beach P Bhartia W Byerly R Cebula E Celarier S Chandra M DeLand D F
33. Ozone Mapping Spectrometer Instruments J Geophys Res 100 14057 14076 42 REFERENCES Continued McPeters R D A J Krueger P K Bhartia J R Herman A Oakes Z Ahmad R P Cebula B M Schlesinger T Swissler S L Taylor O Torres and C G Wellemeyer 1993 Nimbus 7 Total Ozone Mapping Spectrometer TOMS Data Products User s Guide NASA Reference Publication 1323 National Aeronautics and Space Administration Washington DC McPeters R D et al 1996 Nimbus 7 Total Ozone Mapping Spectrometer TOMS Data Product s User s Guide NASA Ref erence Publication 1384 National Aeronautics and Space Administration Washington DC McPeters R D and G J Labow 1996 An Assessment of the Accuracy of 14 5 Years of Nimbus 7 TOMS Version 7 Ozone Data by Comparison with the Dobson Network Geophys Res Lett 23 3695 3698 National Center for Supercomputing Applications 1994 Hierarchical Data Format http www ncsa uiuc edu SDG software HDF HDFIntro html hypertext file Paur R J and A M Bass 1985 The Ultraviolet Cross Sections of Ozone II Results and Temperature Dependence Atmospher ic Ozone edited by C S Zerefos and A Ghazi 611 616 D Reidel Dordrecht Schaefer S J J B Kerr M M Millan V J Realmuto A J Krueger N A Krotkov C Seftor 1 E Sprod 1997 Geophysicists Unite to Validate Volcanic SO2 Measurements EOS Trans 78 217 223 Seftor
34. S 90 N for last scan nadir view degrees x 100 Longitude 180 W 180 E for last scan nadir view degrees x 100 Total number of input output errors Total number of scans read Total number of scans written Total number of good samples written Total number of samples out of range Total number of samples out of range for 39 Table 7 10 Format of Trailer Record Continued Word Description 18 Zenith angle gt 88 degrees 19 Latitude out of range absolute value gt 90 degrees normally zero 20 Counts out of range negative 21 Number of samples written that were bad algorithm flag not 0 1 10 or 11 total 22 27 Totals of error flag counts for algorithm flag 1 22 Total number of samples that had error flag 0 or 10 23 Total number of samples that had error flag 1 or 11 24 Total number of samples that had error flag 2 or 12 25 Total number of samples that had error flag 3 or 13 26 Total number of samples that had error flag 4 or 14 27 Total number of samples that had error flag 5 or 15 28 33 Same as 22 27 for Algorithm Flag 2 34 39 Same as 22 27 for Algorithm Flag 3 40 45 Same as 22 27 for Algorithm Flag 4 46 525 Spare The trailer record identifier 1 is a 16 bit integer that occupies the left half two most significant bytes of word 3 All other values are stored as 4 byte integers MSB first A TA AE AAA ESA RAN AAA AAN 7 2 2 CDTOMS Level 3 Data Product The CDTOMS
35. S on board the ADEOS I satellite measured incident solar radiation and backscattered ultraviolet sunlight Total ozone was derived from these measurements To map total ozone TOMS instruments scan through the sub satellite point in a direction perpendicular to the orbital plane The ADEOS TOMS instrument is identical to two other instruments one of which was flown aboard an Earth Probe satellite in 1996 and the other of which is scheduled to be launched on a Russian Meteor 3M satellite in August of 2000 These three are essentially the same as the first two TOMS fiown aboard Nimbus 7 and Meteor 3 a single fixed monochromator with exit slits at six near UV wavelengths The slit functions are triangular with a nominal nm bandwidth The order of individual measurements is determined by a chopper wheel As it rotates openings at different distances from the center of the wheel pass over the exit slits allowing measurements at the different wavelengths The sampling wavelength is interleaved to minimize the effect of scene changes on the ozone retrieval The IFOV of the instrument is 3 degrees x 3 degrees A mirror scans perpendicular to the orbital plane in 3 degree steps from 54 degrees on the left side of spacecraft nadir to 54 degrees on the right relative to direction of flight for a total of 37 samples At the end of the scan the mirror quickly returns to the first position making no measurements on the retrace Six seconds after the start of t
36. Section 2 2 and described in detail in Section 4 This algorithm is identical to the one used for the Version 7 Nimbus 7 and Meteor 3 TOMS data archive Because of this the initial archive of the ADEOS TOMS data set is also referred to as Version 7 A radiative transfer model is used to calculate backscattered radiances as a function of total ozone latitude viewing geometry and reflecting surface conditions Ozone can then be derived by comparing measured radiances with theoretical radiances calculated for the conditions of the measurement and finding the value of ozone that gives a computed radiance equal to the measured radiance Section 2 provides a general overview of the ADEOS TOMS instrument the algorithm the uncertainties in the results and of other basic information required for best use of the data files It is designed for the user who wants a basic understanding of the products but does not wish to go into all the details Such a user may prefer to read only those parts of Sections 3 through 6 addressing questions of particular interest In Section 3 the instrument its calibration and the characterization of its changes with time are discussed The algorithm for retrieval of total ozone and its theoretical basis are described in Section 4 Section 5 describes the overall uncertainties in the ozone data and how they are estimated while Section 6 discusses particular problems that may produce errors in specific time intervals and geographic
37. Ultraviolet and Total Ozone Mapping Spectrometer SBUV TOMS for Nimbus G Opt Eng 14 323 331 Heath D F A J Krueger and H Park 1978 The Solar Backscatter Ultraviolet SBUV and Total Ozone Mapping Spectrometer TOMS Experiment in The Nimbus 7 Users Guide edited by C R Madrid 175 211 NASA Goddard Space Flight Center Greenbelt MD RELATED LITERATURE Continued Heath D F 1988 Non Seasonal Changes in Total Column Ozone From Satellite Observations 1970 1986 Nature 332 219 227 Heath D F 1990 Changes in the Vertical Distribution of Stratospheric Ozone and the Associated Global Scale Changes in Total Ozone From Observations With the Nimbus 7 SBUV Instrument 1978 1986 Proceedings of the International Ozone Sym posium 1988 and Tropospheric Ozone Workshops edited by R Bojkov and P Fabian 810 A Deepak Hampton VA Herman J R R D Hudson and G Serafino 1990 Analysis of the Eight Year Trend in Ozone Depletion From Empirical Models of Solar Backscattered Ultraviolet Instrument Degradation J Geophys Res 95 7403 7416 Herman J R R McPeters R Stolarski D Larko and R Hudson 1991 Global Average Ozone Change From November 1978 to May 1990 J Geophys Res 96 17 279 17 305 Herman J R R McPeters and D Larko 1993 Ozone Depletion at Northern and Southern Latitudes Derived From January 1979 to December 1991 Total Ozone Mapping Spectrometer D
38. ace reflectivity of approximately 0 5 percent This drift which is consistent with our estimate of working diffuser degradation would have no significant effect on derived ozone Absolute changes in spectrometer sensitivity have been observed by studying signals measured at the nadir over Antarctica and Greenland and corrected for solar zenith angle dependence The ice signal time series is plotted with the solar Working measurements in Figure 3 2 Greenland and Antarctica results have been combined in a single data set by normalizing results during their overlap at each equinox Solar and ice data are further normalized to 1 during the first week of data Ice results represent weekly average sensitivity values determined for all available zenith angles up to 83 degrees The solar and ice results at 360 nm exhibit good agreement with deviations of less than 1 O O e _ 1 ACF 360 E X ice Radiances O O 0 95 N Na Par gt 0 90 gt E D UN 0 85 ANTARCTICA GREENLAND 96 6 96 8 970 97 2 97 4 97 6 YEAR Figure 3 2 Comparisons of Estimates of Instrument Change in the ADEOS TOMS Based on Solar Output and the Reflectivity of Antarctica and Greenland 10 4 0 ALGORITHM The ADEOS TOMS algorithm is based on the one used for Nimbus 7 and Meteor 3 TOMS The major differences concern the use of the 360 nm wavelength for reflectivity instead of 380 nm and the use of 322 nm and 331 nm instead of 331 nm and 340 nm in the
39. al areas Both sections identify some anomalies remaining in the data and discuss what is known about them The structure of the data products is identical to those of previous TOMSs This information is presented in Section 7 Appendix A tabulates the standard atmospheric ozone and temperature profiles used in the algorithm for ozone retrieval Appendix B describes software available for reading the data files and Appendix C provides information on data availability 2 0 OVERVIEW 2 1 Instrument The ADEOS TOMS one of nine instruments aboard the ADEOS I satellite was launched by a H II rocket on August 17 1996 The nominal satellite orbit was established on September 8 1996 at which time the mean local time of the descending node was 10 41 AM It remained in the range 10 45 AM to 11 45 AM throughout the ozone data record from September 11 1996 to June 29 1997 The orbital inclination was 98 6 degrees and the nominal orbit altitude was 800 km with an orbital period of 100 8 minutes The ADEOS TOMS achieved daily global coverage of the sunlit portions of the Earth by scanning perpendicular to either side of the sub orbital track in 3 degree steps to an angle of 54 degrees Two additional scan positions were added relative to previous TOMS instruments to provide global coverage at the lower ADEOS orbit altitude see Figure 2 1 In normal Earth scanning mode ADEOS TOMS measured the Earth backscatter ultraviolet at the six wavelength channels given
40. ar local time at equator crossing 16 GMT seconds at equator crossing or 77 if unavailable 17 Day of year GMT at equator crossing 18 Year GMT at equator crossing 19 Longitude at equator crossing or 77777 if unavailable nadir view degrees x 100 20 Altitude km at last scan 21 Number of input output errors for this orbit 22 Number of scans read for orbit ee e eee ee 38 Table 7 9 Format of Orbital Summary Record Continued Word Description 23 Number of scans written for orbit 24 25 26 24 28 29 34 29 30 31 32 33 34 35 40 41 46 47 52 53 54 55 60 61 66 67 90 91 127 Number of samples out of range total Number of samples out of range for Zenith angle gt 88 degrees Latitude out of range gt 90 degrees Counts negative Number of bad samples written algorithm flag not 0 1 10 or 11 total Counts of error flags for Algorithm Flag 1 see data record for description of error flags number of samples that had error flag 0 or 10 number of samples that had error flag 1 or 11 number of samples that had error flag 2 or 12 number of samples that had error flag 3 or 13 number of samples that had error flag 4 or 14 number of samples that had error flag 5 or 15 Same as 29 34 for Algorithm Flag 2 Same as 29 34 for Algorithm Flag 3 Same as 29 34 for Algorithm Flag 4 Minimum ozone for orbit Maximum ozone for orbit The six instrument wavelengths Solar irradiance
41. ata J Geophys Res 98 12 783 12 793 Herman J R D Larko 1994 Nimbus 7 TOMS November 1978 to May 6 1993 Low Ozone Amounts During 1992 1993 From Nimbus 7 and Meteor 3 Total Ozone Mapping Spectrometer J Geophys Res 99 3483 3496 Herman J R P K Bhartia J Ziemke Z Ahmad and D Larko 1996 UV B Increases 1979 1992 from Decreases in Total Ozone Geophys Res Lett 23 2117 2120 Hudson R D J R Herman and G Serafino 1989 On the Determination of Long Term Trends From SBUV Ozone Data Ozone in the Atmosphere Proceedings of the Quadrennial Ozone Symposium 1988 and Tropospheric Ozone Workshops ed ited by R Bojkov and P Fabian 189 192 A Deepak Hampton VA Janz S E Hilsenrath J Butler D F Heath and R P Cebula 1996 Uncertainties in Radiance Calibrations of Backscatter Ul traviolet BUV Instruments as Determined from Comparisons of BRDF Measurements and Integrating Sphere Calibrations Metrologia 32 637 641 Jaross G A J Krueger 1993 Ice Radiance Method for Backscatter UV Instrument Monitoring in Atmospheric Ozone edited by T Henriksen 94 101 D Reidel Norwell Mass Jaross G A J Krueger H Park and R Haring 1996 Improved Ozone Trend Measuring Capabilities of TOMS Instruments SPIE Proceedings 7 8 August 1996 Denver Colorado 2831 48 56 Klenk K F 1980 Absorption Coefficients of Ozone for the Backscatter UV Experiment
42. ce J Geophys Res 90 10 463 10 482 McPeters R D S M Hollandsworth L E Flynn J R Herman and C J Seftor 1996 Long Term Ozone Trends Derived From the 16 Year Combined Nimbus7 Meteor 3 TOMS Version 7 Record Geophys Res Lett 23 3699 3702 Pommereau J P F Goutail H LeTexier and T S Jorgensen 1989 Stratospheric Ozone and Nitrogen Dioxide Monitoring at Southem and Northem Polar Latitudes Our Changing Atmosphere Proceedings of the 28th Liege International Astrophysi cal Colloquium edited by P Crutzen J C Gerard and R Zander University de Liege Liege Belgium me Se Se E _______E lt T____
43. cs Caudill T R D E Flittner B M Herman O Torres and R D McPeters 1997 Evaluation of the Pseudo spherical Approxi mation for Backscattered Ultraviolet Radiances and Ozone Retrieval J Geophys Res 102 3881 3890 Cebula R P H Park and D F Heath 1988 Characterization of the Nimbus 7 SBUV Radiometer for the Long Term Monitoring of Stratospheric Ozone J Atm Ocean Tech 5 215 227 Chu W P M P McCormick J Lenoble C Brogniez and P Pruvost 1989 SAGE II Inversion Algorithm J Geophys Res 94 8339 8351 Dave J V 1964 Meaning of Successive Iteration of the Auxiliary Equation of Radiative Transfer Astrophys J 140 1292 1303 Dave J V 1978 Effect of Aerosols on the Estimation of Total Ozone in an Atmospheric Column From the Measurement of its Ultraviolet Radiance J Atmos Sci 35 899 911 Environmental Science Services Administration National Aeronautics and Space Administration and United States Air Force 1966 U S Standard Atmosphere Supplements U S Government Printing Office Washington DC Fleig Albert J R D McPeters P K Bhartia Barry M Schlesinger Richard P Cebula K F Klenk Steven L Taylor and D F Heath 1990 Nimbus 7 Solar Backscatter Ultraviolet SBUV Ozone Products User s Guide NASA Reference Publication 1234 National Aeronautics and Space Administration Washington DC Herman J R P K Bhartia O Torres
44. culated for pure ground and pure cloud origin The calculation of radiances at each pressure level follows the formulation of Dave 1964 A spherical correction for the incident beam has been incorporated and Version 7 treats molecular anisotropy Ahmad and Bhartia 1995 Consider an atmosphere bounded below by a Lambertian reflecting surface of reflectivity R The backscattered radiance emerging from the top of the atmosphere as seen by a TOMS instrument I is the sum of purely atmospheric backscatter I and reflection of the incident radiation from the reflecting surface I 1 A 8 8p amp Poy R L A 8 8p 0 22 Po A 8 Op gt Q Po R 4 where wavelength 9 satellite zenith angle as seen from the ground 8 solar zenith angle azimuth angle 11 Q column ozone amount Po pressure at the reflecting surface R effective reflectivity at the reflecting surface The surface reflection term can be expressed as follows where RT A 9 80 Q Po ud e Ri 5 Pol A B 8 Q Po R IRSA Pa 5 b 0 T 0 90 Q Po a ges Bo Q2 Po f A 6 Q Po 6 where Sp fraction of radiation reflected from surface that atmosphere reflects back to surface Iy total amount of direct and diffuse radiation reaching surface at Po f fraction of radiation reflected toward satellite in direction 0 that reaches satellite and the other symbols have the same meaning as before The denominator of Equation 5 accounts for m
45. d Fraction Mixing Fraction Surface Category 20 523 Same as 6 through 19 for samples 2 to 37 524 525 Spares Notes All values are stored in INTEGER format MSB first Values stored in one byte are always positive with a value of 255 indicating missing data Values stored in two bytes can be either positive or negative with values of 32767 indicating missing data Some values have had constants added or multiplied to accommodate integer storage 36 Table 7 8 Detailed Descriptions Word Bytes Description l Orbit number starting at ascending node 2 Greenwich Mean Time at start of scan in seconds 1 86 400 3 1 2 Sequence number of record in orbit 3 4 Flag for chopper non synchronization O Does not occur in current or next scan 1 Occurs in current scan not in next 2 Occurs in next scan not current 3 Occurs in current and next scan 4 1 2 Day of Year 1 366 at start of scan 3 4 Year at start of scan 4 digits 5 1 2 Spacecraft altitude in kilometers at start of scan 3 4 Sample 1 view angle is the same for all scenes since nominal attitude is assumed 6 1 2 IFOV latitude from 90 S 90 N in degrees x 100 3 4 IFOV longitude from 180 W 180 E in degrees x 100 7 1 2 IFOV solar zenith angle in degrees x 100 3 4 Angle q between Sun and satellite measured at IFOV in degrees x 100 8 1 2 309 nm N value x 50 N value is defined in Section 4 5 3 4 313 nm N value x 50 9 l 2 318 nmN value x 50 3 4 322nmN value x 50 10 1 2 331 n
46. dnight part of orbit Estimated ozone below cloud layer matm cm Ground pressure derived from NOAA NMC grid atm x 100 Cloud pressure from ISCCP climatology atm x 100 Sulphur dioxide index SOI matm cm 50 Algorithm flag identifies triplet s used 1 A triplet alone used 2 B triplet alone used 3 B triplet used with profile selection B mix 4 C triplet used with profile selection C mix Effective cloud fraction as defined in Section 4 3 percent Mixing fraction frog Which parameterizes contributions of lower and higher latitude profiles in ozone determination as described in Section 4 5 values range from 0 5 to 3 5 x 10 1 0 pure low latitude 2 0 pure mid latitude 3 0 pure high latitude Surface Category code 0 ocean land low inland below sea level mixed land and ocean mixed land and low inland mixed ocean land and low inland ee ae te on 33 Table 7 3 Fill Values for Missing Scans Data Type Decimal Hexadecimal byte unsigned integer 255 xFF 2 byte integers 32767 x7FFF 4 byte integers 2147483647 x7 FFFFFFF Table 7 4 TOMS Level 2 HDF Coordinate SDSs Name Type Scaletype Scalemin Scalemax scan number 2 byte int regular 0 scans 1 scene number 2 byte int regular 0 scenes wavelength_6 4 byte real irregular n a 6 TOMS wavelengths wavelength_5 4 byte real irregular n a 5 shortest wavelengths 7 1 2 Level 3 Hierarchical Data Format Product The standard archival Level 3 product contains
47. e and DD DD is latitude in degrees 9 East longitude east_lon SDDD DD where S is for east of the Prime Meridian and for west of the Prime Meridian and DDD DD is longitude in degrees Pe ee 10 11 12 13 14 15 16 17 18 19 20 21 West longitude west_lon SDDD DD where S is for east of the Prime Meridian and for west of the Prime Meridian and DDD DD is longitude in degrees Day night flag day_night_flag D indicating daytime data Granule version granule_version 01 indicating first archive version Producer granule ID producer_granule_id alsNNNNN hdf where NNNNN is orbit number with leading zeroes as necessary Number of scans including fill last_seq_index X XX where XXX is number of scans Date and time of ascending node equator crossing date_eqx YYY Y MM DD HH MM SS where YYYY is year MM is month of year 1 12 DD is day of month HH is hour of day MM is minute of hour and SS is second of minute in UT Longitude of ascending node equator crossing long_eqx SDDD DD where S is for east of the Prime Meridian and for west of the Prime Meridian and DDD DD is longitude in degrees Spacecraft altitude at last scan altitude XXX where XXX is altitude in km Orbit number orbit NNNNN where NNNNN is orbit number Fill value for 4 byte signed integer miss_val_signed_4_byte Ox7 ffffftt Fill value for 2 byte signed integer miss_val_signed_2_byte 0x7ff
48. e data within the Level 2 HDF It has the following form TOMS_ADEOS_yyddd_nnnnn where yy is the two digit year ddd is the three digit day of year and nnnmn is the lifetime orbit number i e revolution since launch where orbit 1 is defined to start with the first ascending node equator crossing Leading zeroes are used for yy ddd and nnnnn when applicable The File Description is a field of up to 40 000 ASCII characters which describes in free form text the Level 2 product and its generation algorithm Metadata include the following Data set name data_set TOMS Data product name data_product Level 2 orbital data Granule size granule_size XXXXXXX where XXXXXXX is in bytes Time of first scan begin_date Y Y Y Y MM DD HH MM SS where Y Y Y Y is year MM is month of year 1 12 DD is day of month HH is hour of day MM is minute of hour and SS is second of minute in UT 5 Time of last scan end_date Y Y Y Y MM DD HH MM SS where Y Y Y Y is year MM is month of year 1 12 DD is day of month HH is hour of day MM is minute of hour and SS is second of minute in UT 6 Geographical flag geog_flag 0 indicating orbital data 7 Northern latitude north_lat SDD DD where S is for northern hemisphere and for southern hemisphere and DD DD is latitude in degrees 8 Southern latitude south_lat SDD DD where S is for northern hemisphere and for southern hemispher
49. e of CDTOMS Daily Grid ooooooonoornrro e ne teeeenees 40 TOMS Version 7 Standard Ozone Profiles o oooooooooormromrrrrrrrrrnrr amooo 49 TOMS Version 7 Standard Temperature Profiles Lee 50 UMKehrLayefS 56 sica Aaa 50 viii 1 0 INTRODUCTION This document is a guide to the data products derived from the measurements made by the Total Ozone Mapping Spectrometer TOMS flown on the Japanese meteorological satellite ADEOS and processed by the National Aeronautics and Space Administration NASA It discusses the calibration of the instrument the algorithm used to derive ozone values from the measurements uncertainties in the data and the organization of the data products The data began September 11 1996 and ended on June 29 1997 when contact was lost with the ADEOS spacecraft These data are archived at the Goddard Space Flight Center GSFC Distributed Active Archive Center DAAC The ADEOS TOMS was one of nine instruments launched on board a Japanese meteorological satellite on August 17 1996 The instrument provided daily global coverage of the sunlit portions of the Earth by scanning perpendicular to the sub orbital track and measuring the Earth backscatter ultraviolet at six discrete wavelength channels Nominally the data are continuous though no ozone data were taken during the first month of flight and some days are missing during the second month while testing of the spacecraft systems wa
50. e of up to 2 percent at solar zenith angles greater than 80 degrees Larger errors up to 6 percent may be introduced by Type II PSCs of optical depth 0 05 water ice particle mean radius 5 50 pm Underestimates as large as 50 percent may occur when Type II PSCs of optical depth 0 4 associated with lee waves are present No corrections have been made for the presence of PSCs but they tend to be very localized in time and space lasting 3 5 days with typical sizes of 1000 3000 km Torres et al 1992 6 5 High Terrain Users may note an apparent anticorrelation of ozone with terrain height particularly in the form of ozone dips above high mountain ranges These dips occur because the algorithm retrieves the actual column ozone above the surface not above sea level The atmospheric ozone that would normally be present between sea level and the actual terrain height is missing Column ozone actually is lower above the mountains in the same way as other atmospheric constituents The relation between column ozone and altitude is thus not an artifact of the measurement but simply reflects the fact that when the surface is higher there is less atmosphere above it Some TOMS data users have made use of this effect to infer tropospheric ozone amounts in regions adjacent to high mountains It should be noted the measurement efficiency of the BUV technique used by TOMS is reduced for tropospheric ozone Klenk et al 1982 29 7 0 DATA FORMATS 7 1 H
51. e off nadir At higher latitudes where orbital overlap occurs the orbit that provides the best view of a given cell is used In practice cell averages are computed separately for each TOMS orbit and the one with the shortest average path index is selected The path index is calculated as sec 8p 2sec 0 where Qo and are the solar zenith and spacecraft zenith angles respectively defined at the IFOV This index is designed to place more importance on the spacecraft zenith angle than on solar zenith angle relative to the proper calculation of geometric path sec 00 sec 9 The TOMS level 3 product is non synoptic The Western Pacific is measured near the beginning of the GMT day and the Eastern Pacific is measured near the end of the GMT day There is a 24 hour discontinuity in the data at 180 meridian Individual TOMS IFOVs are sorted into different days across the 180 meridian to ensure that this is the only place where such a time discontinuity occurs TOMS level 3 products are archived at the Goddard DAAC in Hierarchical Data Format as described in Section 7 1 2 The derived total ozone and effective surface reflectivity are available in this form The TOMS near real time level 3 products are available via anonymous ftp in their native format which is described in Section 7 2 2 21 5 0 GENERAL UNCERTAINTIES There are three areas in which uncertainties can be introduced into the ozone derived from TOMS the accuracy and precision of
52. e temperature retrieval and tropospheric ozone errors as random is based upon an approach in which the atmospheric variations are not known and are treated as random variability about the climatology However if independent measurements of any of these quantities are available for a scan then such measurements can be used to correct the ozone values derived from TOMS and the error would no longer be random 5 3 Comparison with Fairbanks Ozone Sondes A number of ozone sondes were flown from Fairbanks Alaska during fall of 1996 in support of the TOMS validation effort These measurements have been used to validate the profile selection scheme described in Section 4 5 Sixteen coincidences have been identified between these Fairbanks ozone sondes and ADEOS TOMS retrievals in which the profile selection method was applied Coincidences were also identified between the ozone sondes and measurements of the ozone profile by the SBUV 2 instrument on board the NOAA 9 Spacecraft so that a composite profile could be constructed of the lower atmosphere measured by the ozone sonde and the upper levels measured by SBUV 2 Figure 5 1a shows a sample composite profile compared to TOMS standard profiles for the same total ozone amount with profile shape selected purely by latitude TOMS Version 6 and with profile shape determined using the mixing fraction frog in Equation 21 Figure 5 1b summarizes estimated errors in ADEOS TOMS total ozone relative to the composite
53. eS 30 7 1 Hierarchical Data Format 2 0 Le 30 7 1 1 Level 2 Hierarchical Data Format Product 2 0 0 cc eee eee eee eee eee 30 7 1 2 Level 3 Hierarchical Data Format Product 0 0 cece eee eee ee eee teen nes 34 TABLE OF CONTENTS TABLE OF CONTENTS Continued Section Page Wee Nalve Pormmaliosa ici etilico 35 7 2 1 TOMS Ozone File Level 2 Data Product 00 c ccc cc ccc cece ccecuecceuee 35 7 2 2 CDTOMS Level 3 Data Product LL iii 40 REFERENCES abitare tana 42 RELATED LITERATURE 0 0 e did e 44 LIST OF ACRONYMS INITIALS AND ABBREVIATIONS o 47 Appendixes APPENDIX A STANDARD OZONE AND TEMPERATURE PROFILES 0ccccccccccceee 49 APPENDIX B SOFTWARE TO READ HDF OZONE DATA ccccccccccccvccccccevececcce 51 APPENDIX C DATA AVAILABILITY 00 ccc ccc irene eee eee 52 ABSIRACL cibi nananana 53 vi LIST OF FIGURES Figure Page 2 1 ADEOS TOMS Instantaneous Fields of View Li 2 3 1 Estimated Change in ADEOS TOMS Instrument Sensitivity 00 8 3 2 Comparisons of Estimates of Instrument Change i 10 4 1 Modes of Equatorial Distributions of Residues rro rr 18 5 1 Summary of ADEOS TOMS Sonde Comparisons 2 2 2 eee ec ee ee eet eee eens 25 5 2 Percentage Difference of ADEOS TOMS Ground Ozone Measurements as a Function of Time 26 6 1 TOMS Derived Ozone Error as a Function of
54. eference diffuser have degraded significantly The term p t drops out of Equation 3 if no diffuser change occurred and the angular response g is the sole external characterization needed Unlike other TOMS working diffuser changes on ADEOS TOMS were not measured The Reflective Calibration Assembly RCA suffered from long term drifts and the two solar measurements made using the reference diffuser are too few for working reference comparisons The ratios of solar flux derived from the cover diffuser to that derived from the working diffuser change with time indicating a degradation rate per exposure hour of the cover diffuser When the same rate is applied to the working diffuser the degradation estimate is approximately 0 5 percent at all wavelengths The maximum spectral dependence is 0 3 percent These estimates for diffuser degradation have not been applied to the calibration Measurements of nadir radiances over ice described in Section 3 6 are consistent with the estimates of working diffuser degradation Studies using the spectral discrimination technique as described in Wellemeyer et al 1997 are also consistent with the estimated ADEOS TOMS long term diffuser degradation The RCA results indicate that a 1 5 percent wavelength independent decrease occurred in the reflectance of all three diffuser surfaces on the first day of science operations Solar and ice measurements confirm this result so the change is accounted for in the calibration
55. elength A Equation 17 becomes ry 5 Q Q a bh 13 The radiation at 360 nm is insensitive to ozone and therefore s3g0 O Further since the reflectivity was derived at 360 nm the residue is zero at that wavelength Substituting into Equation 13 and solving yields a 360b 14 and therefore for the ozone sensitive wavelengths Pa 5 Q Qg b A 360 15 There are two unknowns Q and b Let AA A 360 Using measurements at two wavelengths labeled A and A it is possible to solve for 2 r n ra A rg 2 i 16 Q OQ 0 Sj AA SAA Equation 16 is the form in which the algorithm applies the correction Ozone values are derived for each of the two profiles selected Another form of this equation is risi Q Qo Ak 77 57 2 90 17 This form illustrates how the correction is equivalent to assuming that the size of that part of the residual not arising from ozone error is linear with wavelength 17 This situation is illustrated in Figure 4 1 which shows the modes of the equatorial distributions of residues at each channel as a function of wavelength These modal residues represent a huge population but they serve to illustrate concepts applicable to individual retrievals as well Because the A triplet is used exclusively at path lengths encountered in the tropics the modal residues at 313 331 and 360 nm are co linear N value 310 320 330 340 350 360 Wavelength nm Figure 4
56. engths used to derive the SOI are all zero when the C triplet is used to derive ozone with the B triplet to select the profile SOI is not evaluated for path lengths greater than 3 the output data set will contain a fill value SO gt contaminated data will still be likely to be flagged by the remaining residue tests but the presence of SO will not be identified In principle Equation 22 could be used to simultaneously solve for ozone and SOI However the wavelengths best for ozone determination at a given path length are not necessarily the best for SOI determination The more complicated expression for ozone that would result would significantly increase the computer time required and the accuracy of the corrected ozone would likely be poor For further information about SO derived from TOMS measurements see Krueger et al 1995 and 1998 Schaefer et al 1997 and Krotkov et al 1997 The next check assesses triplet consistency If a single triplet is used the triplet residue defined in Equation 18 is checked for the ozone sensitive wavelength not used in the ozone determination 317 nm in the case of the A triplet and 312 nm for the B triplet The maximum residues allowed in N value units are 1 1 at 317 nm when an A triplet determination is checked and 0 9 at 312 nm when a B triplet determination is checked If a second triplet is used to determine the profile then the requirement is that a value of forof Can be found such that 0 5 lt
57. es the 37 scenes corresponding to the scanner view angles measuring the backscattered Earth radiances used for deriving col umn ozone During the nighttime portion of the orbit the instrument is placed in standby mode at which time the scan mirror points into the instrument at a black surface During solar calibration mode the scanner moves to view the ex posed diffuser surface The remaining modes are specialized for calibration purposes as the names indicate The direct control mode is used when overriding the standard instrument modes On ADEOS TOMS the reflectance calibrations were re defined using direct control 3 2 Radiometric Calibration Conceptually the calibration of the TOMS measured Earth radiance and solar irradiance may be considered separately The Earth radiance can be written as a function of the instrument counts in the following way Im CHG pb insi 1 where L t derived Earth radiance C counts detected in Earth radiance mode k radiance calibration constant G gain range correction factor fai correction for instrument changes The measured solar irradiance F_ can be written as 2 FD CikiGifinsi D 80M 2 where C irradiance mode counts k irradiance calibration constant G gain range correction factor fins Correction for instrument changes p t solar diffuser plate reflectivity normalized at t 0 g relative angular correction for diffuser reflectivity In practice however
58. f Fill value for 1 byte unsigned integer miss_val_unsigned_1_byte 0x7f Note C code assignment shown for values in Table 7 3 The following netCDF style attributes are included l 4 5 Quality flag counters 32 Number of input output errors for this orbit 2 Number of scans read for orbit 3 Number of scans written for orbit 4 Number of samples out of range total of 5 7 Number of samples out of range for 5 Zenith angle gt 88 degrees 6 Latitude 7 Instrument counts negative 8 Number of samples written that were bad total of 9 32 Numbers of individual error flags for each Algorithm Flag see Table 7 2 for description of error and algorithm flags 9 Number of samples that had error flag 0 or 10 10 Number of samples that had error flag 1 or 11 1 Number of samples that had error flag 2 or 12 12 Number of samples that had error flag 3 or 13 13 Number of samples that had error flag 4 or 14 14 Number of samples that had error flag 5 or 15 15 20 Same as 9 14 for Algorithm Flag 2 21 26 Same as 9 14 for Algorithm Flag 3 27 32 Same as 9 14 for Algorithm Flag 4 TOMS band center wavelengths nm shortest first Solar irradiance F values at 1 A U watts cm for the current day at the six TOMS wavelengths shortest first Count to radiance conversion factors watts cm steradian count for each of the four gain ranges for each of the six wavelengths shortest first Nominal spacec
59. global arrays of total ozone and effective surface reflectivity stored as daily HDF files A Level 3 file is generated from each complete daily set of Level 2 files The Level 3 HDF file is comprised of the following elements a File Label a File Description Metadata stored as a second file description 2 Data Scientific Data Sets SDS 2 Coordinate SDSs The File Label is TOMS_ADEOS_DAILY_GRIDDED_DATA_mm_dd _yy where mm is month of year 1 12 dd is day of month and yy is 2 digit year Leading zeroes are used in these substitutions o AN The Level 3 file names have the following form alg Y YDDD hdf where Y Y is a 2 digit year and DDD is day of year The File Description provides background on the TOMS instrument processing algorithms and data products in free format The following metadata are included 1 Data set name data_set TOMS 2 Data product name data_product Level 3 daily gridded data 3 Granule size granule_size XXXXXXX where XXXXXXX is in bytes 4 Begin date and time begin_date YYYY MM DD HH MM SS where Y Y Y Y is year MM is month of year 1 12 DD is day of month HH is hour of day MM is minute of hour and SS is second of minute in UT 5 End date and time end_date YYYY MM DD HH MM SS where YY YY is year MM is month of year 1 12 DD is day of month HH is hour of day MM is minute of hour and SS is second of minute in UT 6 Geographical flag
60. grees N values are calculated using middle and high latitude profiles and for latitude gt 75 degrees only N values for high latitude profiles are calculated Values of dN dQ are calculated as well In general these calculated N values will not equal the measured N values In the derivation of the initial ozone estimate reflectivity is assumed to be independent of wavelength but for some surface conditions such as sea glint desert dust or ice the reflectivity will be wavelength dependent In addition residual errors in the instrument calibration can produce a wavelength dependent artifact in the measured N value Because of these effects on the spectrum of backscattered radiation and because of the simplifications used in its derivation the initial ozone estimate will not be equal to the true ozone value This error in ozone will also contribute to the discrepancy between the measured N value N and the value No calculated from the initial ozone estimate The initial ozone estimate should however be sufficiently close to the true value to derive a correction using a first order Taylor expansion in the difference The wavelength dependent contribution from factors other than ozone such as reflectivity and residual errors in the instrument characterization is assumed to be a linear function of wavelength a bA Then dN Na Not 2 0 4 bi 12 Let ri Nm No be the residue at wavelength A and Ss ea be the sensitivity at wav
61. gth calibration may lead to a time invariant systematic zero point error or bias in the retrieved ozone The third is possible changes with time in the instrument sensitivity An error here may cause a drift with time of the derived total ozone values 22 Instrument noise has been reduced in the new TOMS instruments and does not contribute significantly to errors in derived ozone The total random instrumental error is 0 1 percent This error is the first entry under random errors in Table 5 1 The uncertainty of the initial radiometric calibration of ADEOS TOMS is about 1 percent in derived total ozone Uncertainties in the radiometric calibration at individual wavelengths may be somewhat larger than this but since the ozone is derived using wavelength triplets the impact on derived ozone remains small Errors in the instrument wavelength scale also can generate uncertainties in the retrieved ozone The radiances that are calculated for comparison with measurements must be derived for the wavelengths and slit sensitivity of the TOMS instrument If there is an error in the wavelengths assumed then the calculated radiances will not be the same as those actually measured by the TOMS instrument leading to an error in the retrieved ozone As discussed in Section 3 3 it is estimated that the initial TOMS wavelength calibration was known to 0 04 nm accuracy This uncertainty corresponds to a possible systematic error of less than 1 percent in derived o
62. he previous scan another begins One significant difference from the first two TOMS is a change in the wavelength selection for the 6 channels of the three new instruments Four of the band center wavelengths Table 3 1 remain the same on all TOMS Channels measuring at 340 nm and 380 nm have been eliminated in favor of 309 nm and 322 nm on the new TOMS Ozone retrieval at 309 nm is advantageous because of the relative insensitivity to wavelength dependent calibration errors though retrievals are limited to equatorial regions Ozone retrievals at high latitudes are improved because 322 nm is a better choice for the optical paths encountered there The TOMS instrument response to solar irradiance is measured by deploying a ground aluminum diffuser plate to reflect sunlight into the instrument Severe degradation of the Nimbus 7 diffuser plate was observed over its 14 5 year lifetime and determining the resultant change of the instrument sensitivity with time proved to be one of the most difficult aspects of the instrument calibration Cebula et al 1983 Fleig et al 1990 Herman et al 1991 McPeters et al 1993 Wellemeyer et al 1996 The three diffuser system aboard Meteor 3 and subsequent TOMS reduces the exposure and degradation of the diffuser used for the solar measurements and allows calibration through comparison of signals reflected off diffusers with different rates of exposure Jaross et al 1995 The diffusers designated cover working
63. heric ozone 0 05 percent change May be 5 percent or higher at very high solar zenith angles Value for comparisons with non UV instruments or UV measurements evaluated using different ozone absorption cross sections e It is important to recognize that the use of a single number to describe the uncertainty from any source is an oversimplification In all cases the uncertainty in total ozone depends upon the wavelengths used in determining ozone the uncertainty in the measurement at those wavelengths and the sensitivity of the retrieved ozone to a change in the value of I F at that wavelength In addition the error from a particular source will depend on the conditions of measurement with values higher than the usual values under certain conditions The entries in Table 5 1 represent values for the most common conditions Some cases where the uncertainty may differ significantly from the values in the table are noted 5 1 Accuracy and Precision of TOMS Measurements There are three separate components to determining the accuracy and precision of the normalized radiances that are used in the total ozone retrieval from TOMS First is the precision of the radiances which is governed by instrument noise and by the digitization of the TOMS output These factors produce random errors in the value that is given for measured radiance The second is the initial laboratory calibration An error in the absolute radiometric calibration or in the wavelen
64. ierarchical Data Format TOMS data products will be available electronically from the Distributed Active Archive Center DAAC in the form of Hierarchical Data Format HDF files Ilg et al 1993 Kalman 1994 Along with the files the DAAC will distribute HDF software tools for reading the files A detailed HDF description is provided below for completeness but the HDF tools available at the DAAC and elsewhere make it unnecessary to understand this detail except under special circumstances 7 1 1 Level 2 Hierarchical Data Format Product The standard archival Level 2 products are stored in HDF files one for each orbit at the GSFC DAAC They are generated using version 3 3 release 4 of HDF available from the University of Illinois National Center for Supercomputing Applications NCSA and endorsed by the Earth Observing System Data Information System EOSDIS Project The Level 2 file contains all output from the Version 7 ozone processing including ozone and reflectivity products as well as diagnostic parameters and a SOI on a scan by scan basis for each TOMS daylit FOV The Level 2 HDF file consists of the following components A File Label A File Description Metadata stored as a second file description Network Common Data Form netCDF style attributes Multiple Data Scientific Data Sets SDSs Multiple Coordinate SDSs AMP A The File Label is a string that identifies the instrument the spacecraft date and orbit number of th
65. ight pressure is used and the reflectivity is assumed to be 0 08 For cloud radiances a pressure corresponding to the cloud height from the ISCCP based climatology is used and the reflectivity is assumed to be 0 80 The ground and cloud radiances are then compared with the measured radiance If Igrouna Imeasured cioud and snow ice is assumed not to be present an effective cloud fraction f is derived using fx I measured I round 9 I cloud I round If snow ice is assumed to be present then the value of fis divided by 2 based on the assumption that there is a 50 50 chance that the high reflectivity arises from cloud The decrease in f means that there is a smaller contribution from cloud and a higher contribution from ground with a high reflectivity off snow and ice Equation 9 is solved for a revised value of Isround and the ground reflectivity is calculated from Equation 5 For the ozone retrieval the calculated radiances are determined assuming that a fraction f of the reflected radiance comes from cloud with reflectivity 0 80 and a fraction f from the ground with reflectivity 0 08 when snow ice is absent and with the recalculated reflectivity when snow ice is present An effective reflectivity is derived from the cloud fraction using the following expression R RI f RS 10 where R is 0 08 when snow ice cover is assumed absent and has the recalculated value when it is assumed present This reflectivity is included i
66. in Table 3 1 0 500 1000 1500 2000 Figure 2 1 ADEOS TOMS Instantaneous Fields of View Projected onto Earth s Surface The right portion samples 1 19 of two consecutive scans are shown and a portion of a scan from the previous orbit is also shown to illustrate the inter orbit coverage at the equator In descending mode North to South ADEOS TOMS scans West to East The ozone retrieval uses the atmospheric reflectivity the ratio of the backscattered Earth radiance to the incident solar irradiance This requires periodic measurements of the solar irradiance To measure the incident solar irradiance the TOMS scanner is positioned to view one of three ground aluminum diffuser plates housed in a carousel The selected diffuser reflects sunlight into the instrument The diffuser plate is the only component of the optical system not common to both the Earth radiance and the solar irradiance measurement Only a change in the reflectivity of the diffuser plate can cause a change of the radiance irradiance ratio with time In principle an accurate characterization of these changes will yield the correct variation of this ratio and hence an accurate long term calibration of the instrument The three diffuser plates are exposed at different rates allowing calibration by examining the differences in degradation of diffuser reflectivity resulting from the different rates of exposure This approach was first used with Meteor 3 TOMS Jaross et al 1995 and
67. ion inadequate 3 Fyrip 317 gt 1 1 N value Linear correction inadequate if A triplet alone used Ftrip 31 2 gt 0 9 N value if B triplet alone used forof lt 0 5 Or fprof gt 3 5 Anomalous Profile profile selection 4 SOI gt 13 Sulfur dioxide contamination 5 any residue gt 12 5 Unusual atmospheric conditions or data stream problems 10 Descending orbit Data taken during descending north to south portion of orbit OO 4 7 Level 3 Gridding Algorithm The level 3 gridding algorithm is used to combine the orbital TOMS measurements into a daily map product with a fixed global grid The grid used is 1 degree in latitude by 1 25 degrees longitude over the entire globe Only high quality level 2 data with a quality flag of zero as defined in Table 4 4 are included in the cell averages The cell averages are computed as weighted averages of TOMS parameters derived for IFOVs that overlay the given cell For this purpose a simple rectangular model is used for the actual TOMS IFOV which is illustrated in Figure 2 1 The area of overlap between the rectangular IFOV and a given cell is used to weight its contribution to the given grid cell average A single TOMS IFOV can contribute weight to more than one cell average within a single degree latitude band Contributions outside the latitude band are ignored as a simplification of the calculation The dimensions of the model IFOV vary from 42 x 42 km at nadir to 80 x 210 km at the extrem
68. is based upon the signal level Thus knowledge of the gain ratios between ranges represents part of the determination of instrument linearity and the stability of the gain ratios can affect the time dependent calibration of the normalized radiance Equation 3 The two ratios were determined electronically prior to launch The value of the ratios directly affects the ozone retrieval because the solar calibration takes place in only the least sensitive range and Earth measurements occur in all three ranges Gain ratios are monitored using signals which are simultaneously amplified in all three ranges These simultaneous readings are reported in the instrument telemetry for one scene each scan Thus Earth radiances can be used to verify the interrange ratios when the signals fall within the operating range for both amplifiers This tends to occur near the day night terminator in the orbit Interrange ratios were found to be constant in time with average values close to the prelaunch characterization The postlaunch averages used in ozone processing are reported in Table 3 1 3 5 Attitude Determination Spacecraft attitude as reported by ADEOS operations was within 0 01 degree of nominal on all three axes However between December 1996 and May 1997 larger attitude excursions were observed Variations up to 0 1 degrees in roll and yaw were observed during the day side of the orbit No adjustment has been made in the processing to account for these exc
69. latitudes in steps of 50 D Us The profiles are given in Appendix A Differences between these assumed climatological ozone profiles and the actual ozone profile can lead to errors in derived total ozone at very high solar zenith angles The longer wavelength triplets are used at high path lengths because they are much less sensitive to profile shape effects The differential impact of the profile shape error at the different wavelengths indicates however that profile shape information is present in the TOMS measurements at high solar zenith angles An interpolation procedure has been developed to extract this information Wellemeyer et al 1997 and implement it in the Version 7 algorithm To use the new Version 7 ozone profile weighting scheme for high path lengths it was necessary to extend the standard profiles beyond the available climatology To minimize the use of extrapolation in this process profile shapes were derived by applying a Principal Component Analysis to a separate ozone profile climatology derived from SAGE II Chu et al 1989 and balloon measurements to derive Empirical Orthogonal Functions EOFs The EOFs corresponding to the two largest eigenvalues represented more than 90 percent of the variance The EOF with the greatest contribution to the variance was associated with variation in total ozone The second most important EOF was associated with the height of the ozone maximum and correlated well with latitude showing a lower
70. littner L Flynn J Gleason X Gu J Herman E Hilsenrath S Hollandsworth C Hsu R Hudson G Jaross N Krotkov A Krueger G Labow D Larko J Miller L Moy R Nagatani P Newman H Park W Planet D Richardson C Seftor T Swissler R Stolarski S Taylor O Torres C Wellemeyer R Wooldridge and J Ziemke The TOMS instrument was built by Orbital Sciences Corporation of Pomona California and launched aboard the Japanese meteorological satellite ADEOS seo ni Section Page tO INTRODUCTION cosas leer Ha ese eee A O AA l O OVERVIEW sli AE DAS A Di ii 2 O O lie belin iii edit 2 2 2 AlgorithM ooo ccc cece i iii 2 23 Data UACOMaintes os ccc ocd bak whe st A he eet lose OE ee ea 3 JA Archived Products silicio Whe een AA AA ear e 3 30 INSTRUMENT 00 040 IS Se we eee ao 4 3 1 Description 2 ccc teen eee e ene e nee n eee n nner betes rece eens 4 3 2 Radiometric Calibration iii ee eae naa eee eS eee ee ee hae chee ene eee 5 31 Prelaunch Calibration li eee ae eas oh 99 85 Rew Reis Se dla 6 3 2 2 Radiance Based Calibration Adjustments 0 6 0 0 Li 6 3 2 3 Time Dependent Calibration Archival Product 0 6 eee eee eee eee eee eee eens 7 3 3 Wavelength Monitoring rr 7 3 4 Gain Monitoring 000 00 9 35 Attitude Determination o ocooocnsono oo eee eee cere ene rana rar oror coo 9 36 VAMO ri a A AI RS AS AA AAA 9
71. lso contains the Rayleigh scattering coefficients and the regression equations used for the temperature dependence of the ozone coefficients The values shown in the table are purely to illustrate the magnitude of the change they have not been used in the algorithm Table 4 2 Effective Absorption and Scattering Coefficients Effective Ozone Vacuum Wavelength Absorption Coefficient Temperature Dependence Rayleigh Scattering nm atm cm at 0 C Coefficients Coefficient atm Co C C2 308 68 3 25 7 64 x 10 3 78 x 10 gt 1 076 312 59 1 77 6 10 x 10 3 21 x 103 1 020 317 61 1 0753 3 58 x 10 2 14x 10 gt 0 952 322 40 0 542 2 08 x 1073 1 22 x 10 0 893 331 31 0 197 9 10 x 10 4 90 x 10 0 795 360 11 lt 10 zi 0 559 Correction to ozone absorption for temperature Ozone absorption Cy CT CT where T is in degrees C Ozone and temperature profiles were constructed using a climatology based on SBUV measurements above 15 km and on balloon ozone sonde measurements Klenk et al 1983 for lower altitudes Each standard profile represents a yearly average for a given total ozone and latitude Profiles have been constructed for three latitude bands low latitude 15 degrees mid latitude 45 degrees and high latitude 75 degrees There are 6 profiles at low latitudes and 10 profiles each at middle and high latitudes for a total of 26 These profiles cover a range of 225 475 D Us for low latitudes and 125 575 for middle and high
72. m N value x 50 3 4 360 nm N value x 50 11 1 2 309 nm sensitivity dN dQ in matm cm x 10 000 3 4 313 nm sensitivity dN dQ in matm cm x 10 000 12 1 2 318 nm sensitivity dN dQ in matm cm x 10 000 3 4 322 nm sensitivity dN dQ in matm cm x 10 000 13 1 2 331 nm sensitivity dN dQ in matm cm x 10 000 3 4 Effective Reflectivity in percent x 100 14 1 2 Total Ozone in matm cm x 10 14 3 4 Error Flag flag flag 10 for data taken during descending N S orbit 0 10 good data 1 11 good data 84 lt SZA lt 88 2 12 pair residue too large 3 13 triplet residue too large A triplet r3 7 gt 1 1 N value units B triplet r3 gt 0 9 N value units B mix fprof lt 0 5 or gt 3 5 C mix fprof lt 0 5 or gt 3 5 4 14 SOI flag set SO is present 5 15 fatal set when the absolute value of any residue is larger than 12 5 ozone and SOI set to fill values 15 l 309 nm dN dR reflectivity sensitivity in percent x 50 2 313 nm dN dR in percent x 50 3 318 nm dN dR in percent x 50 4 322 nm dN dR in percent x 50 16 l 331 nm dN dR in percent x 50 2 360 nmdN dR in percent x 50 3 spare byte 4 Terrain pressure in atm x 100 17 l 309 nm residue x 10 127 2 313 nm residue x 10 127 37 Table 7 8 Detailed Descriptions Continued Word Bytes Description 3 318 nm residue x 10 127 4 322 nm residue x 10 127 18 l 331 nm residue x 10 127 2 Amount of ozone added below cloud
73. me invariant error group of Table 5 1 show the effect of the uncertainties in these quantities on derived ozone In addition the absorptivity of ozone is a function of the temperature The calculated radiances are based upon climatological temperature profiles Appendix A however if the temperature structure departs from the climatology the absorption coefficient may change from that assumed in the algorithm producing an error in retrieved ozone The size of this error is shown in the second line of the random error group The third random error component listed in Table 5 1 called retrieval error arises from variations of the properties of the real atmosphere about those assumed for the calculation of radiances The most important of these is the difference between the actual vertical distribution of ozone and the standard profile used to compute the look up tables At low to moderate solar zenith angles the TOMS derived total ozone is not significantly dependent on the ozone profile used At high solar zenith angles however profile sensitivity is a significant source of error The profile interpolation procedure described in Section 4 5 reduces this error but does not eliminate it Wellemeyer et al 1997 The fourth random error in Table 5 1 arises from possible variations in tropospheric ozone in particular from cases where changes in tropospheric ozone do not affect the measured radiance TOMS cannot measure ozone that is hidden from the ins
74. ment J Geophys Res 100 2985 2995 Joiner J P K Bhartia R P Cebula E Hilsenrath and R D McPeters 1995 Rotational Raman Scattering Ring Effect in Sat ellite Backscatter Ultraviolet Measurements Applied Optics 34 4513 4525 Kalman L 1994 HDF Reference Manual Version 3 3 National Center for Supercomputing Applications Champaign IL Klenk K F P K Bhartia A J Fleig V G Kaveeshwar R D McPeters and P M Smith 1982 Total Ozone Determination From the Backscattered Ultraviolet BUV Experiment J Appl Meteorol 21 1672 1684 Klenk K F P K Bhartia E Hilsenrath and A J Fleig 1983 Standard Ozone Profiles From Balloon and Satellite Data Sets J Climate Appl Meteorol 22 2012 2022 Krotkov N A A J Krueger and P K Bhartia 1997 Ultraviolet Optical Model of Volcanic Clouds for Remote Sensing of Ash and Sulfur Dioxide J Geophys Res 102 21891 21904 Krotkov N A P K Bhartia J R Herman V Fioletov and J Kerr 1998 Satellite Estimation of Spectral Surface UV Irradiance in the Presence of Tropospheric Aerosols 1 Cloud Free Case J Geophys Res In press Krueger A J L S Walter I E Sprod N A Krotkov C C Schnetzler 1998 Low Resolution SO Alert DPD3281 Krueger A J L S Walter P K Bhartia C C Schnetzler N A Krotkov I E Sprod and G J S Bluth 1995 Volcanic Sulfur Di oxide Measurements from the Total
75. n the TOMS data products but plays no role in the retrieval If the measured radiance is less than the ground radiance then the radiation is considered to be entirely from surface terrain with a reflectivity less than 0 08 Equations 4 and 5 can be combined to yield I I a R 4 _ 11 T S O 15 The ground reflectivity can be derived using an I obtained assuming ground conditions Similarly if the measured radiance is greater than the cloud radiance when snow ice are absent the reflected radiance is assumed to be entirely from cloud with reflectivity greater than 0 80 and an I derived using the cloud conditions is used in Equation 11 to derive the effective reflectivity If snow ice are present the cloud and ground are assumed to contribute equally to Im at 360 nm Equation 11 can then be used to calculate new values of both ground and cloud reflectivities from these radiances Radiances at the shorter wavelengths are calculated using these reflectivities and a value of 0 5 for f 4 4 Initial B Pair Estimate The initial ozone is calculated using the B pair which provides good ozone values over the largest range of conditions of any of the pairs The first step is to calculate radiances for the conditions of the measurement geometry latitude cloud and terrain height and cloud fraction For each ozone value in the table radiances are calculated for the 1 0 atm and 0 4 atm levels using ground reflectivity and the values of I
76. ncipal effect of atmospheric ozone at these wavelengths is to attenuate both the solar flux going to the troposphere and the component reflected back to the satellite Derivation of atmospheric ozone content from measurements of the backscattered radiances requires a treatment of the reflection from the Earth s surface and of the scattering by clouds and other aerosols These processes are not isotropic the amount of light scattered or reflected from a given scene to the satellite depends on both the solar zenith angle and view angle the angle between the scene and the nadir as seen at the satellite Earlier TOMS algorithms previous to the current version 7 algorithm were based on the treatment of Dave 1978 who represented the contribution of clouds and aerosols to the backscattered intensity by assuming that radiation is reflected from a particular pressure level called the scene pressure with a Lambert equivalent scene reflectivity R When this method was applied at the non ozone absorbing wavelengths the resulting reflectivity exhibited a wavelength dependence correlated with partially clouded scenes To remove this wavelength dependence a new treatment has been developed based on a simple physical model that assumes two separate reflecting surfaces one representing the ground and the other representing clouds The fractional contribution of each to the reflectivity is obtained by comparing the measured radiances with the values cal
77. ne amount is multiplied by the cloud fraction f to derive the ozone in a particular field of view that is under cloud The sensitivities are calculated from the sensitivities for the two profiles using the same weighting as for ozone 4 6 Validity Checks The algorithm contains several validity checks for maintaining data quality Before measured radiances are accepted for use in ozone determination the solar zenith angle satellite attitude and instrument status are checked to ensure the suitability of the radiances and other geophysical input to the algorithm This section describes the quality checks performed to identify invalid and lower quality ozone values caused either by bad input data that passed preprocessing checks or by limitations of the ozone algorithm It also explains the significance of the error flags that are set 19 The principal tool used to investigate the validity and quality of a total ozone value is the set of residues The residues measure how well radiances calculated based on the ozone derived using one set of wavelengths match the radiances measured at the other wavelengths The usual significance of a large residue is that the atmospheric or surface conditions deviate significantly from those assumed in the algorithm for example if reflectivity has a non linear dependence on wavelength The final triplet residues for wavelengths used in the retrieval will be zero The first check is of all the non zero residues if a
78. ne the profile mixing factor defining a linear combination of the standard profiles that best explains the radiances at all four wavelengths This profile mixing factor is defined as r lower m 21 prof r lower r higher a f where lower and higher refer to latitudes of the two profiles used and r refers to the B triplet residue for the 313 nm channel for 1 5 lt s lt 3 0 and to the C triplet residue at the 318 nm channel for s 2 3 In most cases the appropriate profile will be between the higher and lower latitude profiles and the residues will be of opposite sign thus the denominator represents a distance between the residues or sensitivity to profile shape and the numerator a fraction of this distance When the low and mid latitude profiles are used if the derived value of fprof is greater than 1 the process is repeated using the mid and high latitude profiles similarly if fpro lt O when using mid and high latitude profiles the process is repeated using the low and mid latitude profiles The final step is to estimate the amount of the derived ozone that is beneath clouds Estimates of the ozone amount under the cloud level pressure level are obtained for each of the two latitude profiles used to derive Best Ozone and the two tabulated ozone values on either side of the derived Best Ozone The column ozone beneath cloud is then derived by interpolating in ozone and using fprof to weight the latitudes Finally this ozo
79. ns the time and date of the first and last scan of the last orbit of the day and the total number of the scans processed and flagged for various reasons for all orbits Each type of record other than the header can be identified by the logical sequence number which is stored as an integer in the two most significant bytes of the third word of the record All data records have a positive logical sequence number that counts the order of that record within the orbit to which it belongs starting with a value of 1 for the first data record of the orbit The orbital summary record for each orbit has a negative logical sequence number whose absolute value is one greater than that of the last data record of the orbit The trailer record contains the unique logical sequence number of 1 which may be used to identify the end of the file Tables 7 6 7 10 contain in order the format of the header record the format of the data records a detailed description of selected words in the data record the format of the orbital summary record and the format of the trailer record 35 Table 7 6 Format of TOMS Ozone File Header Record Bytes Character Representation Description 1 9 ADEOS Spacecraft identification 10 14 FM 4 Flight model identifier l Nimbus 2 Meteor 3 Earth Probe 4 ADEOS 15 22 LEVEL 2 Data product identification 23 38 BY XXXXXXXXXXXX Program name in 12 characters e g ozt f 39 51 VERSION XXXX Program version in 4 characte
80. ny is greater than 12 5 in units of N value the error flag is set to 5 This condition usually arises when problems in the data stream lead to incorrect values for the measured radiance or when the atmospheric conditions are so unusual that the assumptions used in the calculation of radiances do not hold Data that pass flag 5 are checked for sulfur dioxide contamination The SO index SOI is defined by the following equation dN dN This equation is formulated in the same way as Equation 13 the basic equation for the ozone correction with an additional term for sulfur dioxide contamination The physical interpretation is that the mismatch between calculated and measured radiance has a component due to SO in addition to the components due to ozone error wavelength dependent reflectivity and residual calibration error accounted for in Equation 15 Using three wavelengths provides three equations which can be solved for SOI as a function of the residues the sensitivities and the wavelengths The algorithm uses the residues at 317 nm 322 nm and 331 nm The 312 nm wavelength is not used because it is more affected by aerosols If the SOI is greater than 13 the error flag is set to 4 The limit corresponds to a 40 2 D U departure from zero as determined from examination of a day of data that is known not to be contaminated The 2 D U is added to account for additional variability due to aerosol effects Since the triplet residues at the wavel
81. o work quite well except in the situation where absorbing aerosols are present A careful study of this effect using a variety of absorbing aerosol models has indicated that absorbing aerosols are generally associated with a positive residue at 331 nm the aerosol index and that the resulting error in TOMS derived ozone is roughly linear with the 331 nm residue Torres et al 1998b This finding is illustrated in Figure 6 1 Retrievals with 331 nm residues greater than four are flagged with an error code of 1 and are excluded from the level 3 product However Figure 6 1 indicates that significant errors in derived ozone may still be present These situations occur in northern Africa and the equatorial Atlantic during late summer and fall when large quantities of desert dust are present in the atmosphere They also occur when large quantities of smoke due to bio mass burning or forest fires are present Interested users may correct these data based on the results summarized in Figure 6 1 using the 331 nm residue aerosol index reported on the level 2 product A level 3 product containing the aerosol index is planned for release later in 1998 We hesitate to provide a corrected data set because the modeling is quite specific and other sources of uncertainty contribute to the 331 nm residue However we think that corrections based on Figure 6 1 during episodes of absorbing aerosols in the troposphere will give 2 percent accuracy Estimated Ozone Error
82. of time a time dependent drift or a systematic error that will appear only under particular circumstances For ADEOS TOMS total ozone the absolute error is 3 percent the random error is 2 percent though somewhat higher at high latitudes and the drift over the 9 month data record is less than 0 5 percent More detailed descriptions of the different sources of uncertainty and the extent to which each contributes to the overall uncertainty appear in Sections 3 5 and 6 Section 3 discusses uncertainties due to errors in the characterization of the instrument sensitivity Section 5 discusses other sources of random errors absolute error and drift combining them with the instrument error to yield the overall estimates given above Section 6 discusses errors that are limited in their scope to specific times places and physical conditions Sections 5 and 6 also describe the remaining anomalies that have been identified in the ADEOS TOMS data set with a discussion of what is known of their origin Comparisons with ground based measurements of total ozone indicate that the ADEOS TOMS data are consistent with these uncertainties The ADEOS TOMS ozone is approximately 1 5 percent higher than a 45 station network of ground measurements whereas Nimbus 7 TOMS is about 0 5 percent higher than a similar ground based network McPeters and Labow 1996 None of the TOMS ozone data sets show any significant drift relative to the ground based networks Data
83. oud fraction and the contribution from each level can be derived Using this effective cloud fraction and the radiances measured at one pair of wavelengths an initial ozone estimate is derived using the tables This ozone estimate is then used to calculate the residues at all TOMS wavelengths except the longest A correction to the initial ozone estimate is then derived from the residues at selected wavelengths Applying this correction produces the Best Ozone value The choice of wavelengths is based upon the optical path length of the measurement The OPT has developed algorithms for the derivation of other parameters from the TOMS measurements in addition to total ozone These include estimates of sulfur dioxide UVB flux at the surface Krotkov et al 1998 and aerosol loading Hsu et al 1996 Seftor et al 1997 Herman et al 1997 and Torres et al 1995 and 1998a 2 3 Data Uncertainties Uncertainties in the ozone values derived from the TOMS measurements have several sources errors in the measurement of the radiances errors in the values of input physical quantities obtained from laboratory measurements errors in the parameterization of atmospheric properties used as input to the radiative transfer computations and limitations in the way the computations represent the physical processes in the atmosphere Each of these sources of uncertainty can be manifested in one or more of four ways random error an absolute error that is independent
84. pical weekly average scan dependence Bottom Weekly average scan dependence affected by sea glint 28 6 3 Solar Eclipses When the Sun is eclipsed the decrease in incoming solar irradiance leads to a decrease in the backscattered Earth radiance However because the solar irradiance used for the ozone retrieval is derived from measurements of the uneclipsed Sun the derived I F is not correct during times of eclipse Consequently ozone values are not retrieved for periods of time and ranges of latitude where the radiances are affected by a solar eclipse In actual production tabulated eclipse information is part of the input stream for the job run and is used by the software to exclude the eclipse periods and regions In the ADEOS data record only one eclipse event was excluded It occurred in the northern hemisphere for the last 20 minutes of March 8 1997 and the first 3 5 hours of the next day GMT 6 4 Polar Stratospheric Clouds The effect of anomalously high clouds can be a significant error source for localized regions in the Arctic and Antarctic Polar Stratospheric Clouds PSCs above the ozone peak may cause the TOMS retrieved total ozone to be underestimated for solar zenith angles larger than 70 degrees Models indicate that the impact of these clouds on TOMS retrieved total ozone is a strong function of optical depth Type I PSCs of optical depth 0 01 composed of HNO 3H 0 particle mean radius 0 5 pm may produce an underestimat
85. profiles that are due to differences between the profile shape estimated using the TOMS profile selection scheme and that measured by ozone sonde and SBUV 2 These are quite small considering that most of these retrievals are at solar zenith angles higher than 84 degrees See the first footnote in Table 5 1 5 4 Comparison With Ground Based Measurements The ADEOS TOMS data have been compared with ground based measurements made by a network composed of 45 mid northern latitude stations with Dobson and Brewer ozone measuring instruments Each ground measurement was paired with the TOMS sample whose center was closest to the station if two measurements were equally near the one measured closest to nadir was used A daily network mean was then calculated using the daily TOMS ground differ ences at all available stations Figure 5 2 shows the percentage difference of ADEOS TOMS ground ozone measurements as a function of time The ADEOS TOMS total ozone is about 1 5 percent higher than the ground measurements Similar comparisons of the Nimbus 7 McPeters and Labow 1996 and Meteor 3 TOMS Herman et al 1996 with ground measurements indicate smaller biases of about 0 5 percent and 0 0 percent respectively There is no significant trend in the bias so only the mean bias and its standard deviation are noted in the figure 0 A FB Sonde 330 DU TOMS V6 358 DU A TOMS V7 336 D U E Dd 10 0 49 UN a 100 0 1000 0 O 20 40 60 80 10
86. proved to be very successful In addition the ADEOS TOMS is equipped with UV lamps for monitoring the reflectivity of the solar diffusers A more detailed description of the instrument and its calibration appears in Section 3 2 2 Algorithm Retrieval of total ozone is based on a comparison between the measured normalized radiances and radiances derived by radiative transfer calculations for different ozone amounts and the conditions of the measurement It is implemented by using radiative transfer calculations to generate a table of backscattered radiance as a function of total ozone viewing geometry surface pressure surface reflectivity and latitude Given the computed radiances for the particular observing conditions the total ozone value can be derived by interpolation in radiance as a function of ozone It is also possible to reverse this process and use the tables to obtain the radiances that would be expected for a given column ozone and conditions of the measurement The logarithm of the ratio of this calculated radiance to the measured radiance is the residue a parameter that has proved useful for the detection of tropospheric aerosols The reflecting surface is assumed to consist of two components a surface component of lower reflectivity and a cloud component of higher reflectivity By comparing the measured radiance at the ozone insensitive 360 nm wavelength with that calculated for cloud and for ground reflection alone the effective cl
87. quality flags are provided with the derived ozone in the TOMS Ozone File Level 2 data product Only the data quality flag values of 0 are used to compute the averages provided in the Level 3 product Other flag values indicate retrieved ozone values that are of lower quality allowing the users of Level 2 to decide whether or not they wish to accept such data for their applications 2 4 Archived Products The ADEOS TOMS total ozone products are archived at the GSFC DAAC in Hierarchical Data Format HDF There are two kinds of total ozone products the TOMS Level 2 orbital data and the Level 3 gridded data The orbital files contain detailed results of the TOMS ozone retrieval for each IFOV in time sequence One file contains all the data processed for a single orbit The gridded files contain daily averages of the retrieved ozone and effective surface reflectivity in a 1 degree latitude by 1 25 degree longitude grid In areas of the globe where orbital overlap occurs the view of a given grid cell closest to nadir is used and only good quality retrievals are included in the average TOMS Level 3 data have also been made available over the internet in the native format at the site given in Appendix C Each native Level 3 file contains one daily TOMS map 0 4 megabyte day Detailed descriptions of these products are provided in Section 7 These data will be made available as Level 3 products sometime in 1993 3 0 INSTRUMENT 3 1 Description The TOM
88. r vibration Scans of an internal mercury argon lamp for in flight monitoring of the wavelength selection were executed once per week during nighttime The wavelength calibration was monitored by observing two wavelength bands on either side of the 296 7 nm Hg line Relative changes in the signal level indicate wavelength shifts which are nearly equivalent at all six wavelengths The prelaunch data indicated two major shifts totalling 0 15 nm occurred prior to launch Wavelength monitor results indicate a drift in band centers since launch of less than 0 02 nm The wavelengths presented in Table 3 1 take into account all indicated shifts No additional shifts were detected after the first month of operation 360 nm 1 00 0 98 0 96 0 94 0 92 0 90 0 88 0 86 96 6 96 8 97 0 97 2 97 4 97 6 YEAR 331 360 nm 1 ACF 360 1 000 0 995 0 990 1 ACF 331 360 0 985 96 6 96 8 97 0 97 2 97 4 97 6 YEAR A Triplet 1 008 1 006 1 004 1 ACF A trp 1 002 1 000 96 6 96 8 97 0 97 2 97 4 97 6 YEAR Figure 3 1 Estimated Change in ADEOS TOMS Instrument Sensitivity Based on Solar Measurements Using the Working and Reference Diffusers Fit characterizations used in the archive processing are shown 3 4 Gain Monitoring The current from the Photo Multiplier Tube PMT is fed to three electronic amplifiers in parallel each of which operates in a separate gain range The choice of amplifier recorded for output
89. raft zenith angle degrees at each scan position There are 26 Data SDSs stored in the Level 2 product Their names dimensions and data types are listed in Table 7 1 More detailed descriptions units offsets and scale factors are listed in Table 7 2 The data are stored as integers to convert to the physical units they must be added to the offset and then multiplied by the scale factor Table 7 3 lists the fill values used for different data types for missing scans An exception to these fill values has been identified in the Level 2 HDF for ADEOS and left uncorrected An ozone value of 1 is given on rare occasions when an error flag 31 of 3 is returned for algorithm flag 3 or 4 due to lack of convergence in the ozone algorithm One dimensional SDSs are stored in a TOMS scan number domain Two dimengional SDSs are stored in a TOMS scan number by TOMS scene number domain Three dimensional SDSs are stored in a TOMS scan number by TOMS scene number by TOMS wavelength domain The dimension of 500 column 2 is nominal The actual dimension is scan number Table 7 4 The four Coordinate SDSs stored in the Level 2 product are listed in Table 7 4 Table 7 1 TOMS Level 2 HDF SDSs Name of SDS Dimensions Data Type LSEQNO 500 2 byte integer YEAR 500 2 byte integer DAY 500 2 byte integer SECOND OF DAY 500 4 byte integer ALTITUDE 500 2 byte integer NADIR 500 2 byte integer SYNC 500 2 byte integer LATITUDE 500 x 37 2 byte integer LONG
90. reflecting surface The calculated N value for a given scene is then obtained by interpolation in this grid of theoretical N values The ozone derivation is a two step process In the first step an initial estimate is derived using the difference between N values at a pair of wavelengths one wavelength is significantly absorbed by ozone and the other is insensitive to ozone Use of a difference provides a retrieval insensitive to wavelength independent errors in particular any in the zero point calibration of the instrument In deriving the initial estimate the same pair is always used In the second step N values at all wavelengths are calculated using this ozone estimate In general these calculated values will not equal the measured N values The differences in the sense Nyneas Neaic are called the residues Using the residues at a properly chosen triplet of wavelengths it is possible to simultaneously solve for a correction to the original ozone estimate and for an additional contribution to the radiances that is linear with wavelength arising primarily from wavelength dependence in the surface reflectivity but also possibly originating in the instrument calibration The triplet consists of two pair wavelengths as described above plus 360 nm which is insensitive to ozone The pair wavelengths used are those most sensitive to ozone at the optical path length of the measurement The 12 separation of the 360 nm wavelength from the pair
91. rs e g 1 0 52 63 MMM DD YYYY Program date in month day year e g JUL 01 1994 64 83 6ONpXXXXXXXXXXXXXXXX Processing environment char e g ALPHA UNIX V 84 106 GEN MMM DD YYYY HHMMSS Time in month day year hours minutes and seconds corresponding to generation time of file 107 135 DATA SPAN MMM DD YYYY HHMMS Time in month day year hours minutes and seconds Sb corresponding to start of data span on file 136 159 TO MMM DD YYYY HHMMSS Time in month day year hours minutes and seconds corresponding to end of data span on file 160 170 LEVEL 1 BY Indicates that Level 1 program name follows 172 220 rufgen c Program name and version information 221 2100 Blanks Blank space Character is used to indicate a blank character Table 7 7 Format of Data Records Word Byte 1 Byte 2 Byte 3 Byte 4 l Orbit number 2 GMT seconds of day at start of scan 3 Logical sequence number Chopper synchronization flag 4 Day of year at start of scan Year at start of scan 5 Altitude Sample 1 view angle 6 Latitude Longitude 7 Solar Zenith Angle Angle 8 N309 N313 9 N318 N322 10 N331 N360 11 AN d02 309 dN dQ2 3 3 12 dN d 3 318 dN d amp Q2 3 gt 5 13 dN dQ2 33 Reflectivity 14 Total Ozone Error Flag 15 dN dR 309 AN dR 3 3 AN dR 3 18 dN dR 32 gt 16 AN dR 33 AN dR 360 Fill Terrain pressure 17 RES N309 RES N313 RES N3 8 RES N32 18 RES N331 Ozone Below Cloud SOI Cloud pressure 19 Algorithm Flag Eff Clou
92. rueger and L S Walter 1993 New Constraints on Sulfur Dioxide Emissions From Global Volcanism Nature 366 327 329 Bowman K P 1986 Interannual Variability of Total Ozone During the Breakdown of the Antarctic Circumpolar Vortex Geo phys Res Lett 13 1193 1196 Bowman K P and A J Krueger 1985 A Global Climatology of Total Ozone From the Nimbus 7 Total Ozone Mapping Spec trometer J Geophys Res 90 7967 7976 Bowman K P 1988 Global Trends in Total Ozone Science 239 48 50 Chandra S 1986 The Solar and Dynamically Induced Oscillations in the Stratosphere J Geophys Res 91 2719 2734 Chandra S 1993 Changes in Stratospheric Ozone and Temperature Due to the Eruption of Mt Pinatubo Geophys Res Lett 20 33 36 Chandra S and R S Stolarski 1991 Recent Trends In Stratospheric Total Ozone Implications of Dynamical and El Chich n Perturbation Geophys Res Lett 18 2277 2280 Dave J V 1965 Multiple Scattering in a Non Homogeneous Rayleigh Atmosphere J Atmos Sci 22 273 279 Dave J V and Carlton L Mateer 1967 A Preliminary Study on the Possibility of Estimating Total Atmospheric Ozone From Satellite Measurements J Atmos Sci 24 414 427 Eck T F P K Bhartia P H Hwang and L L Stowe 1987 Reflectivity of Earth s surface and Clouds in Ultraviolet From Sat ellite Observations J Geophys Res 92 4287
93. s Res 102 9029 9038 Woods T N ef al 1996 Validation of the UARS Solar Ultraviolet Irradiance Comparison With the Atlas 1 2 Measurements J Geophys Res in press 43 RELATED LITERATURE Bates D R 1984 Rayleigh Scattering by Air Planet Sp Sci 32 785 790 Bhartia P K J R Herman R D McPeters and O Torres 1993 Effect of Mount Pinatubo Aerosols on Total Ozone Measure ments From Backscatter Ultraviolet BUV Experiments J Geophys Res 98 18547 18554 Bhartia P K K F Klenk D Gordon and A J Fleig Nimbus 7 total Ozone Algorithm 1983 Proceedings 5th Conference on Atmospheric Radiation American Meteorological Society Baltimore MD Bhartia P K K F Klenk C K Wong D Gordon and A J Fleig 1984 Intercomparison of the Nimbus 7 SBUV TOMS Total Ozone Data Sets With Dobson and M83 Results J Geophys Res 89 5239 5247 Bhartia P K D Silberstein B Monosmith and Albert J Fleig 1985 Standard Profiles of Ozone From Ground to 60 km Ob tained by Combining Satellite and Ground Based Measurements Atmospheric Ozone edited by C S Zerefos and A Ghazi 243 247 D Reidel Dordrecht Bluth G J S S D Doiron C C Schnetzler A J Krueger and L S Walter 1992 Global Tracking of the SO Clouds From the June 1991 Mount Pinatubo Eruptions Geophys Res Lett 19 151 154 Bluth G J S S D Doiron C C Schnetzler A J K
94. s being done Since the ADEOS TOMS data record is only 9 months in duration it may not prove useful for monitoring of long term changes in ozone but it provides daily global coverage of total ozone during this period for monitoring of short term variations in the total ozone field Other monitoring capabilities include detection of smoke from bio mass burning identification of desert dust and aerosols as well as sulfur dioxide and ash emitted by large volcanic eruptions e g Nyamuragira The ADEOS TOMS is the second of three instruments built by Orbital Sciences Corporation to continue the TOMS Mission The first was launched aboard the dedicated Earth Probe Satellite on July 2 1996 The third is scheduled for launch aboard the Russian Meteor 3M in August of 2000 These instruments are similar in design to the previous Nimbus 7 and Meteor 3 TOMS instruments They provide enhanced systems to monitor long term calibration stability and a redefinition of two wavelength channels to aid in calibration monitoring and increased ozone sensitivity at very high solar zenith angles Further discussion of the ADEOS TOMS instrument is provided in Sections 2 1 and 3 The TOMS instruments also measure solar irradiance at the six discrete wavelength channels in order to provide for normalization of the measured Earth radiances The algorithm used to retrieve total column ozone also referred to as total ozone from these radiances and irradiances is outlined in
95. s is carried out for both pressures using cloud reflectivity and the cloud radiance is derived by linear interpolation for the pressure level at the height given by the ISCCP cloud height climatology Finally the appropriate fractions of ground and cloud radiances determined as described in Section 4 3 are added to yield I F for all ozone values These results are then converted to N values The next step is to compare the measured radiance with the calculated radiance The two tabulated ozone values whose calculated B pair N value differences bracket the measured N value difference are identified in the table A climatological ozone amount below the terrain pressure level is subtracted from these two bracketing table ozone values and the initial ozone estimate is derived by linearly interpolating between the two resultant values using the measured N value and the two calculated N values 4 5 Best Ozone Once an initial estimate of ozone has been obtained it is used to calculate N values at all TOMS wavelengths in the way described in Section 4 2 applying the rotational Raman scattering correction described in Section 4 4 N values are calculated for each measurement using one profile or two depending upon the latitude For latitude lt 15 degrees only the low latitude profiles are used for 15 degrees lt latitudes lt 45 degrees one set each is calculated using low and 16 middle latitude profiles for 45 degrees lt latitudes lt 75 de
96. sitive film is placed to cover the six exit slits prior to final instrument assembly The film is then exposed through the monochromator using several emission line sources placed at the entrance slit of the instrument An image of the exit slits is also obtained by exposing the film with the slit plate acting as a mask The film images of the exit slits overlap the emission lines thus providing for relative measurement of the two Several films are used to provide optimum exposure and to give the best estimate of the band centers The TOMS wavelength monitor on board TOMS is used subsequent to the film measurements to detect any shift in the band center wavelengths The wavelength monitor indicated a 0 15 nm shortward shift of all wavelengths prior to launch 3 2 2 Radiance Based Calibration Adjustments The initial albedo calibration of one of the wavelength channels has been adjusted prior to processing The main motivation for this adjustment is algorithmic Since different wavelengths are used to determine total ozone in different solar zenith angle regimes it is imperative that the wavelength dependence of the initial calibration be consistent with the forward model calculation of the theoretical radiances used in the retrieval Any inconsistencies can be identified through analysis of the residues see Section 4 5 for further discussion of the residues In cases where the A triplet 313 nm 331 nm and 360 nm wavelengths are used to determine total
97. the measurements the value of the radiances calculated from the radiative transfer model and the process of comparing the measured and calculated radiances to derive ozone In each of these areas errors of three kinds are possible random errors time invariant systematic errors and time dependent systematic errors Table 5 1 summarizes the estimated uncertainties in the retrieved ADEOS TOMS ozone They are organized by kind of error rather than by where they originate in the ozone retrieval process This organization makes it clearer how the errors are to be combined to derive a total error for the retrieval However the following discussion will be organized by where the error arises in the retrieval process to clarify the relationship between the individual uncertainties and how they arise Table 5 1 Errors in Retrieved TOMS Ozone one sigma Source Error Random not applicable to long term change typical values may be larger in winter months or under disturbed atmospheric conditions Instrument noise 0 1 Atmospheric temperature l Retrieval error l Tropospheric ozone 1 5 Net Root sum of squares 2 0 Time Invariant Rayleigh scattering lt 0 5 Ozone absorption cross section lt 2 Wavelength calibration lt 1 Radiometric calibration Retrieval error lt l Net Root sum of squares 3 Time Dependent over first year Radiometric calibration lt 0 5 Wavelength calibration lt 0 5 Atmospheric temperature 0 16 K Troposp
98. trument by thick cloud In the TOMS algorithm a climatological tropospheric ozone amount IS assumed to be present beneath the cloud fraction identified by the reflectivity channel of TOMS Thus the error due to hiding by clouds in a given measurement is equal to the error in tropospheric ozone times the cloud fraction and the algorithm will in general be less sensitive to errors in tropospheric ozone if the cloud fraction is low About 6 percent of total ozone is in the lowest 5 km with a 50 percent variability The radiation from the troposphere has both surface and atmospheric components the surface component traverses the troposphere and provides a measure of tropospheric ozone while the atmospheric component arising from Rayleigh scattering is not as sensitive to the 23 ozone amount Over surfaces with low reflectivity the Rayleigh scattering component dominates and the measured radiance will not be sensitive to departures from the standard tropospheric ozone profile When the surface is highly reflective the ozone sensitive surface component is more important and the TOMS estimate of tropospheric ozone improves thus the problem of tropospheric ozone is less significant over ice covered regions such as the Antarctic The retrieval also improves at low solar zenith angles when incident UV penetrates further into the troposphere Klenk et al 1982 Overall TOMS measures roughly half of the tropospheric ozone variation Assignment of th
99. ultiple reflections between the ground and the atmosphere The intensity of radiation as it passes through a region where it is absorbed and scattered can be described in general terms as having a dependence I exp t For a simplified case where all processes can be treated as absorption the optical depth t depends on the number of absorbers n in a column and the absorption efficiency a of the absorbers that is I o lt exp na The column number should thus scale approximately as log I The ozone algorithm therefore uses ratio of radiance to irradiance in the form of the N value defined as follows I N 10010g 0 7 7 The N value provides a unit for backscattered radiance that has a scaling comparable to the column ozone the factor of 100 is to produce a convenient numerical range This same unit is used in the derivation of ozone from the ground based Dobson and Brewer networks The basic approach of the algorithm is to use a radiative transfer model to calculate the N values that should be measured for different ozone amounts given the location of the measurement viewing conditions and surface properties and then to find the column ozone that yields the measured N values In practical application rather than calculate N values separately for each scene detailed calculations are performed for a grid of total column ozone amounts vertical distributions of ozone solar and satellite zenith angles and two choices of pressure at the
100. ursions Calibration errors associated with incorrect solar angles due to attitude errors do not exceed 0 5 percent in any one channel Only the spectral dependence of this error affects derived ozone however and this is quite small The main ozone retrieval error is a cross track bias due to roll error These errors are less than 1 D U at the extreme off nadir where they are most severe 3 6 Validation Several techniques are employed to validate characterizations of instrument performance Among these is the residue method described in Section 4 5 This method is used to verify wavelength dependent changes in the spectrometer sensitivity but cannot directly detect absolute changes at a single wavelength The spectral discrimination technique was first applied as the primary calibration technique for the Nimbus 7 TOMS which had no on board diffuser calibration apparatus Wellemeyer et al 1996 This method has been applied to the ADEOS TOMS data record The trend in the 331 nm residue over highly reflective equatorial clouds indicates that the wavelength dependent calibration of ADEOS TOMS is stable to within a few tenths of a percent Using the spectral discrimination technique the difference in trend between the 331 nm residue over low reflecting surfaces and the 331 nm residue over highly reflective clouds can be used to derive the drift in calibration at the 360 nm reference channel This analysis indicates a small upward trend in derived surf
101. zone constant with time A wavelength calibration drift could produce a time dependent error in ozone As noted in Section 3 3 the wavelength calibration drifted by less than 0 02 nm over the 9 month data record corresponding to a possible drift of less than 0 5 percent in ozone The upper limit to the possible change appears on the second line under the time dependent changes of Table 5 1 The uncertainty in the time dependence of the radiometric calibration is estimated to be less than 0 5 percent in ozone This uncertainty is relatively small because of the diffuser carousel used for ADEOS This situation is illustrated in Figure 3 1 by the fit of the A triplet wavelengths It is the uncertainty in the determination of the wavelength dependent calibration that is critical to the TOMS total ozone determination 5 2 Calculated Radiances and Their Use in the Algorithm Errors in the calculation of radiances have two principal origins in the physical quantities whose values are obtained from laboratory physics and in the atmospheric properties assumed for the radiative transfer calculations Calculation of radiative transfer through the atmosphere requires values for the ozone absorption and Rayleigh scattering coefficients The values used in the algorithm are obtained from laboratory measurements Any error in the laboratory values will propagate through the algorithm to produce a systematic error in the derived ozone The first two lines in the ti

Download Pdf Manuals

Related Search

Related Contents

GA 24 Technical Manual Manual Técnico  Instruction Manual v1_0  LED CONTROLLER TOUCH RGB  こちらのPDF  Bendix Air Disc Brake Manual  Laboratoire et taxe : la SMSR appelle à la grève administrative  Elektronischer Tresor 20EA  

Copyright © All rights reserved. Failed to retrieve file