OMI Level 3 Data File Format
Ozone, Aerosol, and Radiative Cloud Fraction
The data files for OMI standard L3 products (ozone, radiative cloud fraction, aerosol, 1.00 degree longitude x 1.00 degree latitude gridding) are in ASCII file format. The format is the same as that used for all previous TOMS CD-ROMs and the products offered on the TOMS website. Software written to read those files may be used to read all OMI standard L3 data files by changing filename and date entries. The file rdgrid_omi.for contains a sample FORTRAN routine to read the data files. This same format is used for the OMI High resolution L3 products whose gridding is .25 degree longitude x .25 degree latitude.
Data for each day are gridded into 1 degree latitude zones by 1.00 degree longitude zones. Latitudes go from -90 degrees (the south pole) to 0 degrees (the equator) to +90 degrees (the north pole) in 1 degree steps, so there are 180 latitude zones. The first zone extends from -90 to -89, so the grid cell is considered to be centered on -89.5. Similarly, longitudes go from -180 (west longitude) to 0 (Greenwich, England) to +180 (east longitude) in 1.00 degree steps, so there are 360 longitude zones (360/1.00). The first longitude zone extends from -180 to -179.00, so the grid cell is considered to be centered on -179.50.
As an example (using an OMI ozone data file), the first few lines of a data file should look like the following:
Day: 290 Oct 17, 2007 OMI TO3 STD OZONE GEN:08:011 Asc LECT: 01:51 pm Longitudes: 360 bins centered on 179.5 W to 179.5 E (1.00 degree steps) Latitudes : 180 bins centered on 89.5 S to 89.5 N (1.00 degree steps) 0158 0 0159154 0 0162156158153168156158156 0157 0159158 0155163170 0158 0160157158 0 0 0158156 0162 0157154159 0 0 0157155163159155 0 0158159155 0156 0157153154158156 0154159156 0 0 0152152160154 0 161158153155 0 0155161153154158154154 0156154153 0 0 0154157 0152160 0162152154 0 0156156155157 0152 0154152 0152152 0154157 0159162153 0163153151156158151154 0157 0153158152151153 0150157154 0162 0158 0 0156156 0150158151156 0152156151159152151154 0158150154 0158 0 0150 159159152 0150154154 0 0153 0159153151152158 0 0155159153 0 0 0154 163 0 0153156153154153 0156153162154153154160166 0155 0155 0 0 0163 154 0 0160156151155 0160 0160158 0163161156 0165151 0 0162 0166 0 0157167 0158154158 0166169159165 0165163160164159 0156 0163 0167 0 0 0 0155164156161 0169161164170 0169165166 0153160169159165169 0 0 0 0 0162166171160168160171171168166 0169166160 0 0163 0168 0 0 0 0 0161167170175 0163 0168164 0172 0167164 0 0 0167166172164 0 0 0156166170171 0163165162167 lat = -89.5 171 0165165170168166161178170165 0168169169162166169165164163164162165169 165161163163160159164159 0166164160164164163161164158161161161162160163163 156160161158157164158157160160159157161158158157158155158159157156159157156 162157155154161158152154159156154156157152153156155154156154154153156151155 158154151155159156152157152158151154157151152154152153154151153152153151153 156150151153154 0150155153149151154149148153148148154150151151151150151153 0152148154 0 0150152 0148150153146151147147152152150149149150149151152 157148149152 0146151152 0147152146146152147147152151149148150148151152151 158149151153 0151152150148152153147152152148152152151151150152153152153157 0152153153 0157154 0153157149152158152156155155157153157158154160155160 155163155156166165154 0161158154161160157161162159165162163165163171162160 167169159160172170161173171160166169163166170167170172173169174171168168170 171 0168169174 0171172175170171173176162172174175175177173176171174175170 169 0173171 0 0175172183173174 0174172175176177176173176173175176177168 167180172170 0181170 0175 0 lat = -88.5
The first three lines are header information which includes the date the data were taken, the instrument, the type of processing (eg., production, near real time, etc), the feature of study (ozone or aerosols), the generation date of the file.
Data values at 360 longitudes for the latitude zone centered at -89.5 degrees are given. (Note: negative latitudes are South, negative longitudes are West). The next 360 values are given for the latitude centered at -88.5 degrees and so on. The zeros denote missing or flagged data, i.e. data that could not be collected due to lack of sunlight or other problems for ozone. In the case of aerosols or radiative cloud fraction, "999" is used for missing or flagged data. All ozone measurements are given in Dobson units and are integers with 3 significant figures. A Dobson Unit represents the physical thickness of the ozone layer if it were brought to the Earth's surface. A value of 300 Dobson units equals three millimeters or 1/10th of an inch.
The aerosol monthly average data sets were computed using only positive values (i.e. absorbing aerosol indices) of the aerosol index for each month. Values of zero are used in the averaging anytime the aerosol index is negative. The final monthly average data sets contain aerosol index values greater than or equal to 0.7.