What is the purpose of this site?

Over 30 years ago scientists first realized that man-made CFCs being released into the atmosphere could destroy large amounts of ozone in the stratosphere through a previously unrecognized catalytic reaction.

In 1975 Congress directed NASA "to provide for an understanding of and to maintain the chemical and physical integrity of the Earth's upper atmosphere."

Here we provide the results and data related to these ongoing studies.

Is there a list of acronyms?

Yes!

ATBD

       Algorithm Theoretical Basis Document

BUV

       Backscattered UltraViolet

DOAS

       Differential Optical Absorbtion Spectroscopy

DU

       Dobson Unit

EP

       Earth Probe instrument

ESDT

       Earth Science Data Type

GOME

       Global Ozone Monitoring Experiment

GOME-2

       Global Ozone Monitoring Experiment-2

HDF

       Hierarchical Data Format

HDF-EOS

       Hierarchical Data Format - Earth Observing System

IDL

       Interactive Data Language

KNMI

       Koninklijk Nederlands Meteorologisch Instituut
(Royal Netherlands Meteorological Institute)

M3

       Meteor 3 instrument

MODIS

       MODerate resolution Imaging Spectroradiometer

N7

       Nimbus 7 instrument

NASA

       National Aeronautics and Space Administration

NOAA

       National Oceanic and Atmospheric Administration

OMI

       Ozone Monitoring Instrument

OMPS

       Ozone Mapping and Profiler Suite

OMSAO

       OMI/Smithsonian Astrophysical Observatory

OSST

       OMI Science Support Team

PGE

       Product Generation Executive/Executable

SBUV

       Solar Backscatter UltraViolet instrument

TOMS

       Total Ozone Mapping Spectrometer

UV

       UltraViolet

What is meant by Level 1, Level 2, Level 3, etc. products?

Here is a NASA page that describes this well.

What is OMI?

OMI is a nadir-viewing near-UV/Visible CCD spectrometer aboard NASA's Earth Observing System's (EOS) Aura satellite. Aura flies in formation about 15 minutes behind Aqua, both of which orbit the earth in a polar Sun-synchronous pattern. Aura was launched on July 15, 2004, and OMI has collected data since August 9, 2004.

OMI measurements cover a spectral region of 264-504 nm (nanometers) with a spectral resolution between 0.42 nm and 0.63 nm and a nominal ground footprint of 13 x 24 km² at nadir. Essentially complete global coverage is achieved in one day. The significantly improved spatial resolution of OMI measurements as well as the vastly increased number of wavelengths observed, as compared to TOMS, GOME and SCIAMACHY, sets a new standard for trace gas and air quality monitoring from space. The OMI observations provide the following capabilities and features:

• A mapping of ozone columns at 13 km x 24 km and profiles at 13 km x 48 km (a continuation of TOMS and GOME ozone column data records and the ozone profile records of SBUV and GOME)
• A measurement of key air quality components: NO2, SO2, BrO, HCHO, and aerosol (a continuation of GOME measurements)
• The ability to distinguish between aerosol types, such as smoke, dust and sulfates
• The ability to measure aerosol absorption capacity in terms of aerosol absorption optical depth or single scattering albedo
• A measurement of cloud pressure and coverage
• A mapping of the global distribution and trends in UV-B radiation
• A combination of processing algorithms including TOMS Version 8, DOAS (Differential Optical Absorption Spectroscopy), Hyperspectral BUV retrievals and forward modeling to extract the various OMI data products
• Near real-time measurements of ozone and other trace gases

The OMI is a contribution of NIVR (Netherlands Institute for Air and Space Development) of Delft, in collaboration with FMI (Finnish Meteorological Institute), Helsinki, Finland, to the EOS Aura mission. The Dutch industrial efforts focused on the optical bench design and assembly, thermal design and project management. The detector modules and the readout and control electronics were provided by Finnish industrial partners.

For more details on the OMI instrument and project, see the OMI web site, the OMI Project Guide, and the OMI instrument overview. For details on OMI results, see the OMI publications web site.

OMI multimedia information

• Movies on illustrations of OMI viewing the earth
• Tropospheric Ozone over South America
• The 2005 Antarctic Ozone Hole

What is the OMI swath?

The instantaneous swath of any imaging instrument, including OMI, is the width of the region that is actually observed across the track of the instrument at any time during any particular overflight. The global measurement mode is the default mode, sampling the complete swath of 2600 km for the complete wavelength range. The ground pixel size at nadir position in the global mode is 13 x 24 km2 (along-track x cross-track) for the UV-2 and VIS channels, and 13 x 48 km2 for the UV-1 channel.

The spatial zoom-in mode has a nadir ground pixel size of 13 x 12 km2, but the swath width has a minimum of 725 km. The spatial zoom-in mode is used one day each 32 days, always above the same geo-locations. In the spectral range of 264-311 nm, the pixel size in the cross-track direction is twice as large (that is, a nadir ground pixel size of 13 x 24 km2). The swath is symmetric with respect to the sub-satellite track. The spatial zoomin mode results in two products:

• A Zoom Radiance product consisting of all the zoom data.
• A Global Radiance product in which the zoom data are effectively degraded to match the resolution of images produced in the global mode but only cover half the normal mode radiance.

The spectral zoom-in mode has a nadir ground pixel size of 13 x 12 km2 and a full swath of 2600 km. It has a limited spectral coverage of 307-432 nm to cover the most important scientific products. This mode was tested during the pre-launch period and run a few times between early August and early October 2004, during Launch and Early Operations (LEO). Because this mode has not been used since that time, it is not addressed in this document.

What is OMI's orbit?

The Aura satellite orbits at an altitude of 705 km in a sun-synchronous polar orbit with an exact 16-day repeat cycle and with a local equator crossing time of 13.45 (1:45 P.M.) on the ascending node. The orbital inclination is 98.1 degrees, providing latitudinal coverage from 82° N to 82° S.

What is OMI's ground path?

Every 233 orbits, the EOS-Aura orbital repeat cycle, the spacecraft covers the exact same ground track. By describing the ground coverage for each orbit in the orbital repeat cycle, the ground coverage of a science data product can be described in terms of a reference to one of these pre-defined paths, rather than using bounding boxes or polygons. This method for describing the ground coverage of a science data product in metadata is called NOSE, Nominal Orbit Spatial Extent. The NOSE metadata can be used for geo-spatial searches. To refine these searches, each ground track is split up in blocks. Each science data product then only needs to refer to the paths and blocks covered by the product in order to allow for geo-spatial searches. The definition of the NOSE paths and blocks needs to be included in the search system.

For OMI, 466 NOSE paths are defined. Paths 1 .. 233 specify the ground coverage of nominal global measurements for each orbit in the orbital repeat cycle, paths 234 .. 466 specify the ground coverage of spatial zoom-in measurements. The paths are split up in blocks: each block covers about two minutes of measurement time.

What is the data coverage for OMI?

The OMI instrument has daily global coverage of data. More information on data coverage is available in the summary page.

Where can I find detailed information about the contents of OMI data products?

See the OMI Data User Guide and OMI Data Products and Data Access Information. The theory behind the algorithms used to process OMI data is in the Algorithm Theoretical Basis Documents (ATBD).

What is meant by "Collection"?

As OMI continues to reprocess the data products, Collections of scientifically consistent sets of data product versions become available. Collections may contain multiple format, production, and ESDT Versions. The current collection version is collection 3.

What is OMI's file naming convention?

The file names used for OMI products all contain two date/time stamps: the data date and the production date. The data date is the start time for the OMI granule or orbit measurement data in the file. The processing date/time indicates when the file for the given product was created. Here is an example of an aerosol product Level 2 file name.

OMI-Aura_L2-OMAERO_2008m0830t1828-o21953_v003-2008m0831t021052.he5

OMI = Instrument Name
Aura = Satellite Name
L2 = Level
OMBRO = Product Name
2008m0830t1828 = Data Date
o21953 = Orbit Number
v003 = Collection Number
2008m0831t021052 = Production Date
he5 = Extension

OMI data set file naming convention description can be found in OMI Level 2 Aerosol Data Product Specification.

What is the quality of the OMI data products?

Intensive assessment of OMI product data quality is an ongoing activity. Users can refer to the Quality Assessment Document for more information.

Where can I obtain OMI imagery?

OMI SO2 imagery can be obtained from SO2 group's web site, which includes daily images. OMI O3, NO2, UV Index and other images are available from Tropospheric Emission Monitoring Internet Service (TEMIS).

How do I obtain OMI information and data?

OMI data is availabe from Goddard Earth Sciences Data and Information Center (DISC) site. Follow the steps below to order the data you need.

• Go to Goddard Earth Sciences Data and Information Center (DISC) site
• You will see all the data products listed (Level 1, Level 2 and Level 3). Click on the product you need. For example you need OMAERUV Level 2 data. Then click on OMAERUV
• You will arrive in Data Access Page. Click on "WHOM - search & download OMAERUV data"
• Then select the "Year" you want the data product for. You will also have other selection options. For example: you want data for 2007. Then click on 2007.
• Then you can choose the months and days by selecting them
• You will then get instructions on what to do to get the data files. For example:
  1. Download the "FTP script for full size granules" provided after the table of granules below.
  2. On SGI or Linux machines, run: ftp -p -n aurapar2u.ecs.nasa.gov < script
  3. Or on SunOS, Windows/DOS, or Mac platforms, run: ftp -n aurapar2u.ecs.nasa.gov < script
• You will also have options to select granules, select parameters and do spatial subsetting. Choose your selections and the click on "Submit SUBSET Request".
• You will then get instructions for single and multiple file download. For example for multiple download you will get instructions such as:
  1. Download the FTP_script: order_xxx....txt
  2. On SGI or Linux machine, run: ftp -p -n aurapar2u.ecs.nasa.gov < order_16498.txt
  3. On DOS, SunOS or Windows/Mac platforms, run: ftp -n aurapar2u.ecs.nasa.gov < order_16498.txt
• Follow the instructions and you get your data.

How are OMI science data products generated?

OMI data products are grouped into three processing levels. Level 1 processing provides corrected (or calibrated) instrument data. These data are processed and calibrated to remove many of the instrument effects. The resulting products thus contain minimal instrument or spacecraft artifacts and are most suitable for subsequent scientific derivations. Level 2 processing provides retrieval of derived scientific quantities, such as atmospheric aerosol and cloud measurements. Level 3 processing produces global grids of various parameter elements from the Level 2 products. These global grids are produced daily.

What OMI products are currently available?

Public release information for OMI Level 1, and 2 products is available at KNMI website. Information about Level 3 globally gridded products is available at Goddard Giovanni site.

How do I decide which OMI product files to order?

Chapter 3, OMI Data Products, of the OMI Data User's Guide may be helpful in determining which data products to order.

What OMI Level 1 products are available?

See the DISC web site OMI Level 1 Products for a complete list of data products The main products are OMI geolocated earth radiances and OMI solar irradiances.

What OMI Level 2 products are available?

Level 2 products can be divided into products in 3 different categories. They are: Ozone Clouds, Aerosols, and Surface UV Irradiance * Trace Gases

A complete list of Level 2 products is available at the DISC web site.

What types of OMI Level 3 products are available?

OMI Level 3 products are produced for some Level-2 standard products. Each Level-2 product file contains data from a single orbit. For each Level-2 product there will be 14 files per day. OMI Level-3 daily global products are produced by averaging data over small equal angle grids (0.25 deg x 0.25 deg), (0.5 deg x 0.5 deg) or (1 deg x 1 deg) covering the whole globe. Each grid also contains the corresponding statistical parameters (number of pixels, minimum, maximum, and standard deviation).

What tools are available to work with OMI data products?

The OMI data products are in HDF-EOS format. The Aura Tools site lists available tools for reading and working with OMI data. Tools are also available from KNMI's site.

What is OMPS?

OMPS is the Ozone Mapping and Profiler Suite.

What OMPS Level 1 products are available?

Take a look at the OMPS data portal on this site: http://ozoneaq.gsfc.nasa.gov/data/omps

Is there a description of the SBUV Version 8 Algorithm?

V8 Algorithm Description

The Solar Backscatter Ultraviolet instruments, SBUV on Nimbus 7 and SBUV/2s on NOAA-9, -11, -14, -16 and -17, are nadir-viewing instruments that infer total column ozone and the ozone vertical profile by measuring sunlight scattered from the atmosphere in the ultraviolet spectrum. Heath et al. (1975) describes the SBUV flown on Nimbus-7. Frederick et al. (1986), and Hilsenrath et al. (1995) describe the follow-on SBUV/2 instruments flown on the NOAA series of spacecraft.

The instruments are all of similar design: nadir-viewing double-grating monochromators of the Ebert-Fastie type. The instruments step through 12 wavelengths in sequence over 24 seconds, while viewing the Earth in the fixed nadir direction with an instantaneous field of view (IFOV) on the ground of approximately 180 km by 180 km. To account for the change in the scene-reflectivity due to the motion of the satellite during the course of a scan, a separate co-aligned filter photometer (centered at 343 nm on SBUV; 380 nm on SBUV/2) makes 12 measurements concurrent with the 12 monochromator measurements. Each sequence of measurements is separated by 8 seconds from the next, producing a complete set every 32 seconds on the daylight portions of an orbit.

The instruments are flown in polar orbits to obtain global coverage. Since the SBUV ozone measurements rely on backscattered solar radiation, data are only taken on the dayside of each orbit. There are about 14 orbits per day with 26 degrees of separation at the equator. Unfortunately, the early NOAA polar orbiting satellites are not sun-synchronous. For example the NOAA-11 equator crossing times drifted from 1:30 pm (measurements at 30 degrees solar zenith angle at the equator) at the beginning of 1989 to 5:00 pm by the end of 1995 (measurements at 70 degrees solar zenith angle). As the orbit drifts, the terminator crossing location moves to lower latitudes and coverage decreases.

Ozone profiles and total column amounts are derived from the ratio of the observed backscattered spectral radiance to the incoming solar spectral irradiance. This ratio is referred to as the backscattered albedo. The only difference in the optical components between the radiance and irradiance observations is the instrument diffuser used to make the solar irradiance measurement; the remaining optical components are identical. Therefore, a change in the diffuser reflectivity will result in an apparent trend in ozone. This is the key calibration component for the SBUV(/2) series. See Hilsenrath et al. (1995) for a longer discussion.

The spectral resolutions for SBUV(/2) monochromators are all approximately 1.1 nm, full-width at half-maximum (FWHM) with triangular bandpasses. The bandwidths of the photometers are approximately 3 nm FWHM. The wavelength channels used for Nimbus 7 SBUV were: 256, 273, 283, 288, 292, 298, 302, 306, 312, 318, 331, and 340 nm. The wavelengths for NOAA-9 and the other NOAA SBUV/2 instruments were very similar except that the shortest channel was moved from 256 nm to 252 nm in order to avoid emission in the nitric oxide gamma band that contaminated the SBUV Channel 1 measurement. Data from the 256 and 252 nm channels are not used in this Version 8 processing for any of the instruments.

Version 8 SBUV(/2) Ozone Profile Retrieval Algorithm (V8A)

The Version 8 SBUV(/2) ozone profile retrieval algorithm combines backscattered ultraviolet measurements and a priori _ profile information in a maximum likelihood retrieval. See _Rodgers (1990) for an analysis of this class of retrievals. It improves on the Version 6 SBUV(/2) algorithm described in Bhartia et al. (1996). Among the improvements are the following:

1. The V8A has a new set of a priori _profiles varying by month and latitude, leading to better estimates in the troposphere (where SBUV/2 lacks retrieval information) and allowing simplified comparisons of SBUV/2 results to other measurement systems (in particular, to Umkehr ground-based ozone profile retrievals which use the same _a priori data set).
2. The V8A has a true separation of the a priori and first guess. This simplifies averaging kernel analysis. Examples and further information are provided below.
3. The V8A has improved multiple scattering and cloud and reflectivity modeling. These corrections are updated as the algorithm iterates toward a solution.
4. Some errors present in the V6A will be reduced. These include the elimination of errors on the order of 0.5% by improved fidelity in the bandpass modeling.
5. The V8A incorporates several ad hoc Version 6 algorithm improvements directly. These include better modeling of the effects of the gravity gradient, better representation of atmospheric temperature influences on ozone absorption, and better corrections for wavelength grating position errors.
6. The algorithm uses improved terrain height information and gives profiles relative to a climatological surface pressure.
7. The V8A is also designed to allow the use of more accurate external and climatological data and allow simpler adjustments for changes in wavelength selection.
8. Finally, the V8A is designed for expansion to perform retrievals for hyperspectral instruments, such as the Ozone Monitoring Instrument (OMI), the Global Ozone Monitoring Experiment (GOME-2) and the Nadir Profiler in the Ozone Mapping and Profiler Suite (OMPS).

The atmospheric ozone absorption decreases by several orders of magnitude over the 252 to 340 nm wavelength range. The V8A uses a variable number of backscattered ultraviolet measurements depending on the solar zenith angle (SZA) of the observations to maintain its sensitivity to ozone changes in the lower atmosphere. For small SZAs (the sun high in the sky), only six wavelengths are used in the retrievals. They are at 273 nm, 283 nm, 288 nm, 292 nm, 298 nm, and 302 nm. As the SZA increases the 306 nm, then the 313 nm and finally the 318 nm channels are added to the retrieval.

Version 8 Algorithm A Priori Profiles

The a priori profile database is provided in a data file (climat.txt). The profile data set gives the climatological averages for 18 10-degree latitude bands and 12 months. These profiles can be used to determine the information used in a specific retrieval by interpolating in latitude and day with the apriori FORTRAN code. The lowest layer from the _a priori _program will differ from that used in the retrieval if the surface pressure is not 1 atmosphere. Another data file (terrain.txt) and the terrpres FORTRAN code are provided to generate the surface pressure for a given latitude and longitude.

The a priori covariance is constructed as follows: the diagonal elements correspond to 50% variance and the non-diagonal covariance elements fall off with a correlation length of twelve fine layers (approximately two Umkehr layers). The measurement covariance is diagonal and corresponds to radiance errors of 1% in each channel.

The profile database was created from 15 years (1988 to 2002) of ozonesonde measurements and SAGE (Version 6.1) and/or UARS-MLS (Version 5) data. Over 23,400 sondes from 1988 to 2002 were used in producing this climatology. Data was "filtered", i.e., obvious bad data points were removed. Data from balloons that burst below 250 hPa were discarded. Data from bouncing balloons were sorted by pressure. Note: No total ozone correction factors (TOMS or Dobson) filtering were used. The stations were weighted equally for each band so that we do not introduce any additional longitudinal biases (e.g., Resolute and Nyalesund have equal weights in December even though Nyalesund has three times as many sondes as Resolute for that month). The SAGE data was also "screened" to remove anomalous retrievals. Average profiles from ozonesondes and SAGE are merged over a 4-km range with the sonde weight decreasing from 80 to 60 to 40 to 20% and the SAGE weight increasing correspondingly.

Averaging Kernel Plots for Nimbus 7

This section gives some sample averaging kernel plots to help describe the V8A retrieval capabilities. The averaging kernels give the theoretical responses in the retrieval layer amounts to changes in the true atmospheric profiles.

Fig. 1 shows Averaging Kernels (AKs) (for fractional changes in ozone) at the 15 pressure levels where the ozone mixing ratios are provided on this DVD. The short horizontal lines on the right side of the graph show the pressure levels and point to the corresponding AK. The horizontal and AK lines – styles correspond. In general, the (fractional) variation in the mixing ratio reported by SBUV at a given pressure level is a weighted average of the (fractional) variation of the mixing ratio at surrounding altitudes, relative to the a priori profile. Since the SBUV V8 a priori profiles have no inter-annual variation, the AKs also show how the algorithm would smooth a long-term trend in ozone mixing ratio. Note, however, that individual SBUV profiles usually have structures that are finer than those implied by the AKs; these structures come from the assumed a priori profile, rather than from the measurements themselves. This figure shows typical AKs at the equator. The AKs show best resolution of ~6 km near 3 hPa, degrading to ~10 km at 1 and 20 hPa. Outside this range the retrieved profiles have little information. For example, the (fractional) variation in ozone mixing ratio seen at 0.5 hPa actually represents the (fractional) variation from the region around 1 hPa, and the variation around 50 hPa represents the variation from around 30 hPa.

Fig. 2 shows typical AKs for March at 40N latitude. At this latitude the 50 hPa AK does capture some of the atmospheric variation, albeit with a resolution of ~11 km. In general, the upper AKs get progressively better as the solar zenith angle increases, and the lower AKs become better as the ozone density peak drops in altitude.

Fig. 3 shows typical AKs for March at 80N latitude. One can see the improvement in AKs for the upper portions of the profile, especially the 0.5, 0.7, 1, and 1.5 hPa AKs, in capturing the atmospheric variation more accurately than in Figs. 2 and 1, with better resolution of ~6 km at 0.7 hPa.

References

Bhartia, P.K., S. Taylor, R.D. McPeters, and C. Wellemeyer, Application of the Langley plot method to the calibration of the solar backscatter ultraviolet instrument on the Nimbus 7 satellite, J. Geophys. Res., 100, 2997-3004, 1995.

Bhartia, P.K., R.D. McPeters, C.L. Mateer, L.E. Flynn, and C.G. Wellemeyer, Algorithm for the estimation of vertical profiles from the backscattered ultraviolet technique, J. Geophys. Res., 101, 18,793-18,806, 1996.

Frederick, J.E, R. P. Cebula, and D. F. Heath, Instrument characterization for the detection of long-term changes in stratospheric ozone: An analysis of the SBUV/2 radiometer, J. Atmos. Oceanic Technol., 3, 472-480, 1986.

Gleason, J.F, R.D. McPeters, Correction to the Nimbus 7 solar backscatter ultraviolet data in the "nonsync" period (February 1987 to June 1990), J. Geophys. Res., 100, 16,873-16,877, 1995.

Heath, D. F., A. J. Krueger, H. R. Roeder, B. D. Henderson, The solar backscatter ultraviolet and total ozone mapping spectrometer (SBUV/TOMS) for Nimbus G, Optical Engineering, 14, 323-331, 1975.

Heath, D.F., Z. Wei, W.K. Fowler, and V.W. Nelson, Comparison of Spectral Radiance Calibrations of SSBUV-2 Satellite Ozone Monitoring Instruments using Integrating Sphere and Flat-Plate Diffuser Technique, Metrologia, 30, 259-264, 1993.

Hilsenrath, E., R.P. Cebula, M.T. Deland, K. Laamann, S. Taylor, C. Wellemeyer, and P.K. Bhartia, Calibration of the NOAA-11 Solar Backscatter Ultraviolet (SBUV/2) Ozone Data Set from 1989 to 1993 using In-Flight Calibration Data and SSBUV, J. Geophys. Res., 100, 1351-1366, 1995.

Rodgers, C. D., The Characterization and Error Analysis of Profiles Retrieved from Remote Sounding Measurements, J. Geophys. Res., 95, 5587-5595, 1990.

Is there a description of the SBUV Version 8 profile data?

Version 8.0 SBUV Profile Data Readme

Introduction

This web site contains a complete time series record of atmospheric ozone profiles from November 1978 to December 2003 derived from satellite Solar Backscattered Ultraviolet (SBUV) measurements. Data from the SBUV instrument on the Nimbus-7 satellite and the SBUV/2 instruments on NOAA-11, NOAA-16 and NOAA-9 satellites are selected to give a complete time series using the highest quality data available. As discussed further in V8 Data Documentation and indicated by the Error Codes and the Residue Quality Control (Residue QC) parameters in the data files, the overall data quality varies with instrument and time. Parts of Nimbus-7, NOAA-11 and NOAA-16 have the best quality. The data from the NOAA-9 SBUV/2 instrument are considered to be of poorer quality, and are included only to complete gaps in time not covered by the other instruments. The Noaa09_post92 directory contains data covering the period when there are no NOAA-11 data. The Noaa09_pre89 directory contains data covering the Nimbus-7 out-of-synchronization period. Some overlap of instrument measurements is provided. See V8 Data Documentation for guidelines to select the best quality data and some discussion of instrument problems that affect data quality. Links to the latest information on the data provided on this DVD can be found at: NOAA SBUV/2 V8 DVD Link

Description of Data

In the V8DATA directory, there are subdirectories for each instrument. These have subdirectories for each year containing the daily data files in ASCII format. A sample ASCII file shows the beginning of a daily file with eight lines of header and the subsequent first three 3-line data records. As briefly described in the header, each data record starts with 11 parameters relative to the profile, followed by ozone amounts in Dobson Units for 13 layers and finally the ozone concentrations in PPMV units at 15 pressure levels. A brief description of the first eleven parameters is as follows:

• Year : The four digit Common Era year Day_of_Year : The day of the year with January 1st equal to 1 GMT_seconds : The Greenwich Mean Time at the start of the measurement Latitude : The Latitude (Degrees North) of the measurement Longitude : The Longitude (Degrees East) of the measurement Solar_Zenith : The Solar Zenith Angle (Degrees) of the measurement Total_Ozone : The total column ozone in Dobson Units from the ground up Reflectivity : Effective reflectivity from the ground and/or clouds. Aerosol_Index : Significant quantities indicate the presence of aerosols. Quality_Residue : Average of the absolute final residues (See V8 Data Documentation). Error_Flag : Three digit error code (See V8 Data Documentation).

Note: a daily file may contain part of an orbit of data from the following day, and some of the data for the start of a day may only appear in the preceding day's file.

These files can be accessed through the SBUV Ozone Profile page or by modifying one of the sample IDL or FORTRAN 90 programs to read in a day's data.

Documentation Files

1. V8 Algorithm Description
2. V8 Data Quality
3. V8 Validation Results

Acknowledgments

The reprocessed SBUV/2 data were made available with support from the NOAA Climate and Global Change Program, Atmospheric Chemistry Element and the NOAA National Environmental Satellite Data and Information Services, Product Systems Development and Implementation Program. Support for SSAI contributors was provided under NASA contracts NAS5-00220 and NAS5-01008.

What is the Earth Probe TOMS Data Coverage?

Summary of Level 2 Data Coverage

EP/TOMS was launched into a 500 km sun synchronous orbit on
July 2, 1996.  The first EP/TOMS Earth scan data were taken
during orbit 216 on July 16, 1996.  Normal science operations
began during orbit 339 on July 24, 1996.  Orbits prior to 7903
(December 4, 1997) were at the initial 500 km altitude.  Orbits
after 8037 (December 13, 1997) were at 740 km altitude after an
orbit boosting maneuver.

No Earth scan science data (and therefore no Level 2 data) were
acquired for the orbits listed in Table 1. Seventy-two (72) of
these orbits were before the start of normal operations.

Table 2 lists orbits that have incomplete data coverage.
Nine (9) of these orbits were prior to normal operations.

Table 3 lists orbits that have some fixed scene view ("stare mode")
data that interrupts the usual continuous daytime Earth scan data.
Each affected orbit has a minimum 3 minute Level 2 data gap.

----------------------------------------------------------------
                         Table 1
           EP/TOMS Orbits with No Level 2 Data
----------------------------------------------------------------
Jul 17-19, 1996: 220, 228, 233-263
Jul 22-24, 1996: 300-338     (Prior to Normal Operations)
Nov    28, 1996: 2258
Nov 16-19, 1997: 7640-7675   (Attitude Anomaly)
Dec  4-13, 1997: 7903-8037   (Orbit Boost)
Nov 17-18, 1998: 12935-12951 (Leonid Meteors)
Dec    13, 1998
 -Jan   2, 1999: 13311-13610 (Spacecraft Anomaly)
Nov 17-18, 1999: 18209-18225 (Leonid Meteors)
May    14, 2000: 20798       (Operations Error)
Nov 17-19, 2001: 28783-28802 (Leonid Meteors)
Aug  2-12, 2002: 32516-32663 (Spacecraft Anomaly)
Nov 18-19, 2002: 34085-34100 (Leonid Meteors)
May 15-22, 2003: 36657-36767 (Spacecraft Anomaly)

---------------------------------------------------------------
                         Table 2
          EP/TOMS Orbits with Partial Level 2 Data
          (percent available shown in parenthesis)
---------------------------------------------------------------
Jul 16, 1996: 216 (84%), 219 (61%)
Jul 17, 1996: 221 (40%), 227 (61%), 229 (40%), 232 (29%)
Jul 19, 1996: 264 (39%)
Jul 22, 1996: 299 (12%)
Jul 24, 1996: 339 (40%)  (all before normal operations)
Oct  9, 1996: 1505 (64%), 1506 (80%)
Nov 28, 1996: 2257 (38%), 2259 (62%)
Dec 31, 1996: 2773 (77%)
Mar  9, 1997: 3794 (65%)
Nov 16, 1997: 7639 (41%)
Nov 19, 1997: 7676 (60%)
Dec  4, 1997: 7902 (38%)
Dec 13, 1997: 8038 (67%)
Apr 14, 1998: 9807 (82%)
May  1, 1998: 10045 (75%)
May 12, 1998: 10212 (96%), 10213-10216 (38% each)
May 13, 1998: 10217-10223 (38% each), 10224 (40%)
May 24, 1998: 10376 (61%), 10377 (39%)
Nov 17, 1998: 12934 (40%)
Nov 18, 1998: 12952 (61%)
Dec 13, 1998: 13310 (37%)
Jan  3, 1999: 13610 (63%)
Nov 17, 1999: 18208 (39%)
Nov 18, 1999: 18226 (62%)
Feb  5, 2000: 19365 (68%)
May 14, 2000: 20797 (59%), 20799 (52%)
Apr  9, 2001: 25572 (57%)
Nov 17, 2001: 28782 (13%)
Nov 19, 2001: 28803 (49%)
Dec 31, 2001: 29423 (38%)
Jan  1, 2002: 29424 (65%)
Aug  2, 2002: 32515 (59%)
Aug 12, 2002: 32664 (77%)
Nov 18, 2002: 34084 (65%)
Nov 19, 2002: 34101 (63%)
Dec 31, 2002: 34712 (69%)
May 15, 2003: 36656 ( 8%)
May 22, 2003: 36768 (41%)

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                         Table 3
    EP/TOMS Orbits with Fixed Scene ("stare mode") Data
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Apr  1, 1997: 4152
Apr  2, 1997: 4167
Apr  6, 1997: 4228, 4230
Apr 11, 1997: 4306
Apr 15, 1997: 4367
Apr 16, 1997: 4382
Apr 20, 1997: 4443
Apr 21, 1997: 4458
Apr 25, 1997: 4519
Apr 30, 1997: 4595
May  4, 1997: 4656
May  5, 1997: 4671
May  8, 1997: 4717
May  9, 1997: 4732
May 10, 1997: 4747
May 13, 1997: 4793
May 14, 1997: 4808
May 18, 1997: 4869
May 19, 1997: 4884
May 23, 1997: 4945
May 27, 1997: 5006
May 28, 1997: 5021
Jun  1, 1997: 5082
Jun  2, 1997: 5097
Jun  5, 1997: 5143
Jun  6, 1997: 5158
Jun  7, 1997: 5173
Jun 10, 1997: 5219
Jun 11, 1997: 5233, 5234
Jun 12, 1997: 5240, 5250
Jun 14, 1997: 5277, 5280
Jun 15, 1997: 5295
Jun 16, 1997: 5309, 5310
Jun 17, 1997: 5316, 5326
Jun 19, 1997: 5353, 5356
Jun 20, 1997: 5371
Jun 21, 1997: 5386
Jun 24, 1997: 5432
Jun 25, 1997: 5446, 5447
Jun 28, 1997: 5493
Jun 29, 1997: 5508

Last updated July 16, 2003

Is there a TOMS data quality assessment?

Until late in its life, EP/TOMS data were of high quality, consistent with nominal uncertainties.

Frequently asked questions regarding the Earth Probe TOMS near real-time data:

• How did the most recent EP/TOMS data compare with historical values?
• What geophysical events affected EP/TOMS data quality?
• Does EP/TOMS have missing data or data taken in an alternate instrument mode?
• Briefly discuss EP/TOMS instrument calibration
• What effect did spacecraft attitude anomalies have on EP/TOMS data?
• What is the data coverage for EP/TOMS?

What are the TOMS uncertainties?

The nominal uncertainties in the Earth Probe TOMS near real-time data products are summarized in the tables below:

The effective Lambertian equivalent surface reflectivity and the aerosol index are not physical quantities. Because of this, no algorithmic error sources have been included. Nominal instrument calibration errors of 1.5% at 360 nm and 0.75% at 331 nm relative to 360 nm have been assumed. Evidence of uncorrected diffuser degradation drives the errors in long-term mean for the reflectivity and aerosol index, but there is no significant effect on derived ozone.

More information on error sources is found in the EP TOMS Data Products User's Guide.

What is a Dobson Unit?

A dobson unit is the most basic measure used in ozone research. The unit is named after G.M.B. Dobson, one of the first scientists to investigate atmospheric ozone (~1920 - 1960). He designed the 'Dobson Spectrometer' - the standard instrument used to measure ozone from the ground. The Dobson spectrometer measures the intensity of solar UV radiation at four wavelengths, two of which are absorbed by ozone and two of which are not.

The illustration above shows a column of air, 10 deg x 5 deg, over Labrador, Canada. The amount of ozone in this column (i.e. covering the 10 x 5 deg area) is conveniently measured in Dobson Units.

If all the ozone in this column were to be compressed to stp (0 deg C and 1 atmosphere pressure) and spread out evenly over the area, it would form a slab approximately 3mm thick.

1 Dobson Unit (DU) is defined to be 0.01 mm thickness at stp; the ozone layer over Labrador then is ~300 DU.

NOTE: This page, including the copyrighted graphic, is based on a page developed by Owen Garrett for the Centre for Atmospheric Science at Cambridge University, UK. The center has kindly given us permission to reproduce it. (Take their excellent Multimedia Ozone Hole Tour!)

How did the most recent EP/TOMS data compare with historical values?

How do the 2005 ozone data (plus) compare with the 2004 data (solid) and with the entire Nimbus 7 TOMS climatology (shaded)? (10K/graph). The white curve shown on each of the plots is the climatological mean.

What geophysical events affected EP/TOMS data quality?

Solar eclipses occurred on February 16 and August 11 of 1999. When the Sun is obscured or partly obscured, the calibration of the TOMS measurement is compromised. For this reason, data covering the region of the eclipses are excluded from the Level-2 (orbital) and Level-3 (map) data products.

Elevated values of Aerosol Index (AI) starting July 23, 1999 in North America are associated with Canadian Forest Fires. This may cause small underestimation of total column ozone amount in the vicinity of these fires. This map also shows dust emanating from Northern Africa and smoke from biomass burning in Southern Africa. The impact of dust and smoke on ozone derived from EP/TOMS measurements is described in Section 6.1 of the TOMS Data User's Guide.

The eruption of Soufriere Hills Volcano on Monserat (lat, Lon) on July 21, 1999 caused a slight elevation in the Aerosol Index (AI) reported on the EP/TOMS Level-2 data product (not readily apparent in Level-3). The effect is small and does not lead to any data rejection from the Level-3 product, nor to any significant error in the total column ozone amounts.

During spring, some data were being rejected by the EP/TOMS algorithm in the vicinity of the North Pole. This was the result of air masses of very high total ozone amounts but with unusually low ozone concentration in the upper stratosphere. This situation was more prevalent in spring of 1999 than in previous years. This might possibly be associated with the two stratospheric warmings that occurred this Spring. Missing data in the TOMS Level-3 map products near the North Pole on April 25, for example, are the result of this type of data rejection. The white hole in the map is where the missing data is.

For more details on the TOMS profile mixing scheme, please refer to the algorithm section of the TOMS data User's Guide.

The eruption of Shishaldin (54.76 N, 163.97 W) on April 19. 1999 produced an ash cloud visible in the TOMS aerosol index (AI) over the Alaskan peninsula. Some of the TOMS ozone retrievals in this region are rejected for contamination due to the sulfur dioxide cloud produced by the eruption.

Does EP/TOMS have missing data or data taken in an alternate instrument mode?

The tables below indicate data that are permanently missing from the Level-2 EP/TOMS data record. Some filling can occur in the Level-3 product due to overlapping data from adjacent orbits. Also, additional gaps may occur in the Level-3 data resulting from bad retrievals due to desert dust or smoke from forest fires as described in the EP/TOMS Data Products User's Guide.

EP/TOMS Orbits with no Derived Products

EP/TOMS Orbits with Partial Coverage

Briefly discuss EP/TOMS instrument calibration

As discussed in the User's Guide, the EP/TOMS working diffuser is deployed every week to make solar measurements. The EP/TOMS reference diffuser is deployed only every 10 weeks, So the working to reference ratio indicates degradation of the working diffuser, which is critical to the long-term calibration of EP/TOMS. Every time a working diffuser measurement is made, the near real-time calibration of EP/TOMS is extrapolated for another week to permit continuous processing of the data. The following graphics are used to monitor the quality of this calibration system. The upper plot in both graphics pertains to the single 360 nm channel and affects the calibration of the derived reflectivity. The middle plot pertains to the 331 nm / 360 nm ratio, which affects the aerosol index. The bottom plot pertains to the A-triplet of channels, which corresponds to ozone errors.

The working diffuser degradation relative to reference (5K)	The degree to which extrapolated calibration tracks actual solar measurements (4K)

What effect did spacecraft attitude anomalies have on EP/TOMS data?

The EP/TOMS spacecraft has experienced a number of attitude anomalies listed in the table below. These anomalies are short lived and tend to affect only the extreme off-nadir scans. The ozone errors are typically 1 D.U. or less though they may become larger during extreme events.