The TOMS instrument and data products

Information compiled by the BADC team


  1. The instrument
  2. TOMS data
  3. Data quality overview
  4. Transferring TOMS data from the BADC

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1. The instrument

1.1 Instrument description and measurement technique

1.1.1 Nimbus-7 TOMS

Sources: "Nimbus-7 TOMS Data Products Users Guide" (NASA reference publication 1384), Goddard Space Flight Centre TOMS home page, Goddard DAAC WWW pages

TOMS was one of eight instruments designed to provide continuous, long-term monitoring of atmospheric, oceanic and surface parameters on a global basis throughout most of the 1980s. The Nimbus-7 TOMS instrument operated from 1st November 1978 to 5th May 1993.

The Nimbus-7 TOMS instrument measures backscattered ultraviolet radiance from Earth at wavelength bands centred at 312.5, 317.5, 331.3, 339.9, 360.0 and 380.0 nanometres. The first four wavelengths are sensitive to ozone; the two longer wavelengths are used for estimating the scene reflectivity necessary for deriving ozone amounts. The instrument is a single stage fixed-grating Ebert-Fastie monochromater with a rotating chopper wheel to resolve the incoming light into these 6 wavelength bands with a one nanometer bandpass.

Total column ozone is inferred from the differential absorption of scattered sunlight in the ultraviolet using the ratio of two wavelengths, 312nm and 331nm for instance, where one wavelength is strongly absorbed by ozone while the other is weakly absorbed. TOMS scans in the cross-track direction in 3 degree steps from 51 degrees on one side of nadir to 51 degrees on the other, for a total of 35 samples. The instantaneous field-of-view (IFOV) of 3 degrees x 3 degrees results in a footprint varying from a 50 km x 50 km square at nadir to a 130 km by 300 km diamond at the scan extremes. The total swath width is 3000 km, implying that consecutive orbits overlap to create a contiguous mapping of ozone data. Approximately 200,000 measurements are made on a daily basis during the sunlit portions of the orbits.

Nimbus-7 is in a south-north sun synchronous polar orbit such that it is always close to a local noon/midnight beneath the satellite. Thus, ozone over the entire world is measured every 24 hours.

Launch date                     10/24/78
Orbit                           Sun-synchronous, near polar
Nominal Altitude (km)           955
Inclination (deg)               104.9
Nodal Period (min.)             104
Equator Crossing Time           1200 noon (ascending)
Nodal Increment (deg)           26.1

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1.1.2 Meteor-3 TOMS

Sources: Goddard Space Flight Center online documentation and WWW pages.

Meteor-3 was launched on August 15, 1991 from Plesetsk, Russia, and failed on 27th December 1994. The orbit drifts with respect to the sun angle with a period of 212 days. Complete global measurements are made except when the orbit is near the day-night terminator. Under these conditions, the TOMS instrument may not be able to view one of the hemispheres for a period of up to 3 weeks. This differs from the Nimbus-7/TOMS ozone data obtained in a near-noon sun synchronous orbit.

Because of the precessing nature of the Meteor-3 orbit (precession period = 212 days), the equator crossing time goes from noon (overhead sun, solar zenith angle = 0 deg) to the terminator (solar zenith angle = 90 deg). The best data quality is when the equator crossing time of the ascending node of the orbit is between 10am and 2pm or between 10pm and 2am (which indicates that the equator crossing time of the descending node of the orbit is between 10am and 2pm). Good data quality is obtained over a wider range of equator crossing times.

Some other potentially useful data concerning the Meteor orbit and Meteor-3/TOMS characteristics:

	altitude		1202 km
	orbit inclination	82.5 deg
	orbit period		109 min
	orbit eccentricity	< 2 x 10^-3
	orbit precession	212 days
	launch date		08/15/91
	launch time		12:15 Moscow standard time
	1st ozone data		08/22/91
	# orbits/day		13.21
	field of view		3 x 3 degrees
	wavelengths		312.35, 317.4, 331.13
				339.73, 359.0, 380.16 nm
	bandwidth		1.1 nm
	cross-track scan angle	+/- 51 deg
	number of views/scan	35
	nadir ground size	64 x 64 km
	extreme off nadir view  260 x 70 km  (approx) rotated 45 deg.

Ozone is calculated from the measured radiances by first forming the ratio with measured irradiances (solar view) in each wavelength channel. These ratios, known as directional albedos, are further combined into ratios of directional albedos from 2 wavelength channels.

	N = -100 Log (A1/A2)

Tables of ozone amount vs N-value are precomputed from a variety of atmospheric models using a radiative transfer computer program. For details of this computation and the corrections involved, see Herman et al, "A new self-calibration method applied to TOMS and SBUV backscattered ultraviolet data to determine long-term global ozone change", J. Geophys. Res., 96, 7531-7545, 1991.

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1.1.3 Earth-Probe TOMS

Sources: Goddard Space Flight Center WWW pages.

Earth-Probe TOMS was launched on 2nd July 1996. The satellite is being flown in a 500km polar orbit, rather than the 950 km orbit that was originally planned. While ADEOS TOMS gives full daily global coverage, Earth-Probe will be unable to do so for ozone measurements within 60 degrees of the equator. However it will give full coverage at the poles. In addition, the lower orbit will give increased resolution and increase the probability of making measurements over cloudless scenes, due to the smaller "footprint". The lower orbit will also improve the ability of the TOMS instrument to make measurements of UV-absorbing aerosols in the troposphere, a capability that was recently developed using earlier TOMS data. The increased probability of making measurements over cloud-free areas will enhance the capability of converting the TOMS aerosol measurements into geophysical quantities such as optical depth. The increased resolution available with the lower orbit may even result in the ability of the TOMS instrument to detect urban-scale aerosols such as pollution. The wavelengths used by Earth-Probe TOMS are:

Band 1 - 360.0 +/- 0.2 nm
Band 2 - 331.2 +/- 0.1 nm
Band 3 - 322.3 +/- 0.1 nm
Band 4 - 317.5 +/- 0.1 nm
Band 5 - 312.5 +/- 0.1 nm
Band 6 - +/- 0.1 nm


Sources: Goddard Space Flight Center WWW pages.

The ADEOS TOMS instrument was launched on 17th Aug 1996 from Tanegashima Space Center in Japan. ADEOS is the Advanced Earth Observing Satellite, developed by NASDA (the National Space Development Agency of Japan). The instrument is designed to operate for 3 years. The satellite is flying in a 800km orbit, giving full daily global coverage, and uses the following wavebands: 308.6, 312.5, 317.5, 332.2, 331.2, 360.0 nm.

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1.2 Data Processing

Source: "Nimbus-7 TOMS Data Products Users Guide" (NASA reference publication 1384)

The retrieval of total ozone is based on a table look-up and interpolation process. A table is constructed that gives backscattered radiance as a function of total ozone, optical slant path length, surface pressure, surface reflectivity and latitude. Given the computed radiances for the latitude, surface pressure, reflectivity and slant path for a particular radiance measurement, the total ozone value for the scan can be derived by interpolation in the table.

The two shortest TOMS wavelengths are strongly absorbed by ozone and only very weakly absorbed by other atmospheric components. At wavelengths above 310 nm, the backscattered radiance is primarily solar radiation reflected from the Earth's surface and the troposphere. Since 90% of the ozone is in the stratosphere, the backscattered radiance at 310nm and higher is only weakly dependent on the vertical distribution of ozone.

Derivation of ozone values from backscattered radiance requires an understanding of reflections from the Earth's surface and scattering from clouds and other aerosols. The scattered or reflected light depends upon both the incident angle of the sunlight and the satellite viewing angle. In practice, the cloud and aerosol contributions can be treated as if the effective lower boundary of the atmosphere is located at an average pressure in the troposphere (the "scene pressure"), with an effective reflectivity (the "scene reflectivity") that accounts for scattering from clouds, tropospheric aerosols and the Earth's surface. Values for these quantities are derived for every instantaneous field-of-view (IFOV) of the instrument, although this is not possible where stratospheric aerosols are present (e.g. from volcanic eruptions).

The backscattered radiance is dependent on atmospheric and surface components, where only the surface component depends on the scene reflectivity. The scene reflectivity can be calculated from the longest two TOMS wavelengths, which are not sensitive to ozone. The measured backscattered radiances are corrected for wavelength drift and changes in instrument optics and sensitivity, and used in the formulation of "N-values", which depend on the ratio of backscattered intensity to the incident intensity at the level of the instrument. A set of tables is created to relate total ozone to this ratio, taking into account climatological ozone profiles, pressure of the reflecting surface (2 values are used, 1000 and 400mb), solar zenith angle and satellite zenith angle at the IFOV.

In calculating total ozone, radiance ratios called Pair values are computed. These are ratios of the backscattered to incident radiation ratio at two wavelengths, where the longer wavelength is insensitive to ozone while the shorter is sensitive. The values are separated by 20nm or less in order to keep the scattering effects approximately the same and ensure that their relative attenuation is sensitive mostly to ozone absorption. The choice of pair depends on the conditions: high or low sun angle, ozone values etc.

For each of the three Pair values, four total ozone estimates are made by table interpolation. The four values correspond to the two pressure values, 1000 and 400 mb, and two standard latitudes situated on either side of the measured latitude. Linear interpolation in pressure and then in latitude yields the Pair ozone value at the scene pressure and measured latitude.

A "best" value comes from a weighted average of the total ozone from the A, B and C Pairs, taking into account sensitivity of each pair to ozone profile shape, solar zenith angle and changes in total ozone. The daily gridded product is then calculated by averaging the data over 1 degree (latitude) by 1.25 degree (longitude) cells, after rejecting values with stratospheric aerosol contamination, inconsistent pair ozone or anomalous ozone or reflectivity values.

The first TOMS data released by NASA was processed using version 4 of the software; versions 1 to 3 were for development only. In 1985, Version 5 introduced newly approved ozone absorption coefficients (Bass and Paur). Version 6 introduced a stabilised long-term calibration to compensate for the observed drift in derived total ozone values resulting from an error in the correction for the diffuser plate degradation. The most recent processing version, Version 7, uses a revised instrument calibration based on analysis of the entire 14.5 year data record, as well as an improved algorithm. Improvements include:

For information on the algorithms used for retrieving ozone from the TOMS instrument, contact the Ozone Processing Team at Goddard Space Flight Centre.

See the "Nimbus-7 TOMS Data Products Users Guide" (NASA reference publication 1384) for more information

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2. TOMS data

2.1 'Column' or 'total' ozone measurements and units

TOMS measures `column' ozone (sometimes referred to as `total' ozone), ie the total amount of ozone in a column of unit area above the Earth's surface. The data is stored as gridded averages.

Column ozone is usually expressed in terms of the equivalent thickness of the ozone layer at standard temperature and pressure (0 degrees C, 1013.25 mb). If an entire column of ozone (i.e. the mass per unit area in a column of air extending from the surface to the top of the atmosphere) is brought to standard temperature and pressure, the thickness of the column would be, on average, about 3 millimetres.

The units of the TOMS column ozone measurements are Dobson Units (DU), where 1 DU = 10 E-5 metres, also expressed as a milli-atmosphere-centimetre of ozone. Hence, a typical value of column ozone is of the order of 300 DU.

2.2 Spatial coverage and resolution

TOMS measurements cover the whole globe from -90 to 90 degrees latitude, -180 to 180 degrees longitude. However, an individual day's coverage can never be truly global since the instrument relies on backscattered UV and there are always some areas of the Earth in shade.

The data is gridded such that the cell area over which each measurement is made is roughly constant. The Earth is divided into 1 degree latitude zones, each of which is subdivided into a number of longitude cells. The number of cells is allowed to vary with latitude, from 288 at the equator to 72 at the poles, as shown in the table below. Because the constant separation in degrees between orbits corresponds to a smaller distance in kilometres closer to the poles, a particular cell may be viewed from several adjacent orbits in the higher latitudes (at approx. 100 minutes time interval).

In the ASCII data files, higher latitude measurements are repeated such that there is a constant 288 measurements per latitude zone.

                                 No of     
                      No. of     observations  Expected no.
           Longitude  cells      permitted     of orbits    Resolution
 Latitude  size       in zone    per cell      per cell     (km x km)
 ========  =========  ========   ============  ===========  ==========

 0 to      1.25 deg.  288        1             1.0          110 x 138
 50 deg.                                       1.6          110 x 88.4
 50 to     2.5 deg.   144        2             1.6          110 x 76
 70 deg.                                       3.5          110 x 94.1
 70 to     5 deg.      72        4             3.5          110 x 188
 80 deg.                                       7.3          110 x 95.5
 80 to     5 deg.      72        4             7.3          110 x 95.5
 90 deg.                                       14.0         110 x 0.0

2.3 Temporal coverage

The TOMS instrument on Nimbus-7 operated from 1st Nov. 1978 to 6th May 1993. From 22nd June 1979 TOMS operated full-time. Prior to this date, TOMS followed a regular on/off schedule for spacecraft power management, operating on 10 of each 12 days. At times the instrument was also operated on scheduled off days, resulting in an actual duty cycle of greater than 83%.

Meteor-3 TOMS operated from 22nd August 1991 until 27th Dec. 1994. Data coverage during the latter part of 1994 was sporadic. Due to the precessing nature of the Meteor-3 orbit, there were periods (when the orbit was near the day-night terminator) when the TOMS instrument was unable to view one of the hemispheres for up to 3 weeks.

Meteor-3 TOMS data is only available (at level 3) for high data quality periods, namely:

  22nd Aug. 91 -  2nd Sep. 91
  26th Oct. 91 - 17th Dec. 91
   9th Feb. 92 -  2nd Apr. 92
  26th May  92 - 17th July 92
  31st Aug. 92 -  9th Nov. 92
  16th Dec. 92 - 24th Feb. 93
   1st Apr. 93 - 10th June 93
  17th July 93 - 25th Sep. 93
  31st Oct. 93 -  1st Sep. 94
  14th Feb. 94 - 25th Apr. 94
   1st June 94 - 10th Aug. 94
  15th Sep. 94 - 24th Nov. 94

After a gap of one and a half years, two new instruments were launched on the Earth-Probe and ADEOS satellites in July and August 1996.

Nov 1978 -/~/- Aug 1991 -/~/- May 1993 -/~/- Dec 1994 -/~/- July 1996 - Aug 1996

Nimbus-7 launch ------------> N-7 TOMS fails

               Meteor-3 launch ------------> M-3 TOMS fails
                                                            Earth-Probe launch---->
                                                                        ADEOS launch

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3. Data quality overview

Source: "Nimbus-7 TOMS Data Products User's Guide", McPeters et al, NASA Reference Publication 1384. See also "Meteor-3 Total Ozone Mapping Spectrometer (TOMS) Data Products User's Guide", NASA reference publication 1393

TOMS data is subject to uncertainties from several sources, a few of which are outlined here. For a full discussion of these uncertainties you should refer to the source document.

3.1 Accuracy and precision of the TOMS albedo

Uncertainty in the albedo used in total ozone retrieval depends on three factors:

The laboratory calibration depends on the accuracy of the standard calibration lamp and the accuracy with which the reflecting properties of the instrument diffuser plate are known. This is known to be better than +/-3%. The accuracy of the long-term calibration depends on the accuracy with which changes with time of the diffuser plate optical properties can be modelled. Studies by Herman et al (1991) estimate that this accuracy produces a 1.3% uncertainty in long-term ozone trends over the first 10 years of data. The study by Gleason et al (1993) indicates that this accuracy was maintained through the first 14 yeas of data. The precision of the TOMS albedo measurement is better than 0.8% at all wavelengths.

Uncertainties in the instrument wavelength can also lead to uncertainties in the retrieved ozone, since radiance is measured at a "known" wavelength. If the measurement wavelengths are not those specified, the backscattered radiation will not match the algorithm, leading to an error in the ozone value. It is estimated that the initial Nimbus-7 TOMS wavelength calibration was known to +/-0.5nm accuracy and that the calibration drifted by less than 0.005nm over the life of the instrument.

3.2 Random, time-invariant and time-dependent errors

TOMS ozone retrievals are subject to random, time-invariant and time-dependent errors. Random errors of approx 2% net arise from instrument noise, digitization, atmospheric temperature and retrieval errors. Time-invariant errors occur due to Rayleigh scattering, wavelength and radiometric calibrations, retrieval error (all of order <1%) and the ozone absorption cross-section (<3%). Time-dependent errors are due to radiometric and wavelength calibrations (<0.1 and <0.02 %/yr respectively), atmospheric temperature (0.16%/K) and tropospheric ozone (0.05%/% change).

All values expressed here are typical and may not be representative under extreme conditions. In all cases, the uncertainty in total ozone depends on the wavelengths used, the uncertainty in the measurement at those wavelengths and the sensitivity of the retrieved ozone to a change in albedo at that wavelength.

The time-invariant errors due to Rayleigh scattering and absorption cross-section coefficients arise through errors in the laboratory measurements used to determine these values. These are values required for the calculation of the scattering of atmospheric radiation by ozone. Errors propagate through the retrieval algorithm to produce a systematic offset.

The Nimbus-7 TOMS instrument was in orbit for 15 years, during which time the instrument sensitivity changed significantly. NASA's Ozone Processing Team has devoted considerable effort to understanding these changes, which account for about 1% per decade change in the instrument calibration.

Differences between the actual vertical distribution of ozone and the standard profiles can contribute to significant random errors at high solar zenith angles. Thus, the combination of long-term decreases in ozone at high altitudes and a drift in the Nimbus-7 orbit that resulted in a small increase in SZA, can lead to systematic errors up to -5%/decade in Version 6 TOMS data at latitudes greater than 70 deg. during Northern hemisphere winter. In lower latitudes, the seasonal trend is estimated to be less than 2%/decade.

No detectable change with time has been found in the Nimbus-7 TOMS wavelength scale. The upper limit for the possible change is 0.02%/year.


3.3 Problems localised in space and time

Volcanic SO2 and aerosol contamination prevented TOMS from accurately measuring total ozone when volcanic clouds were present, prior to the development of an algorithm to detect the presence of sulphur dioxide. Since this algorithm was developed over 100 eruptions have been detected and measured, the most notable being El Chichon in 1982 and Mt. Pinatubo in 1991. Values that are flagged for SO2 contamination are not used to calculate grid means, since the error in the ozone values can be as high as 50DU.

Other localised problems include scan angle dependence (of the order 1%, rising to 2% if there is sun glint), solar eclipses (when no ozone values are retrieved due to the decrease in solar irradiance), the presence of polar stratospheric clouds which can lead to total ozone being underestimated at solar zenith angles greater than 70 degrees, and high terrain.

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4. Transferring TOMS data from the BADC

You can obtain copies of TOMS ASCII data files via this WWW server, or via the BADC ftp service on For the inexperienced user, help is available on how to use FTP (File Transfer Protocol). The daily data files are relatively small (approx. 170 kilobytes) and easy to transfer.

4.1 Finding the data directory for FTP

After logging in, cd /badc/toms to reach the top of the TOMS directory tree. You will then see the data and software directories. The ASCII data files are in subdirectories for all four TOMS satellites below the data directory. (These subdirectories contain further subdirectories for the processing version and year).

GIF images can be found in the images directory, where there are some sample files and subdirectories for northern and southern hemisphere plots. A full set of plots for the two hemispheres is currently held online, although these may be removed if there is pressure on disk space in the future. The IDL code used to plot the sample files is available in the 3rdparty software directory.

4.2 File names

The file name convention for daily ASCII data files is:


ga = gridded ASCII
sat = satellite ID:
sat = m3t = Meteor-3 TOMS
sat = n7t = Nimbus-7 TOMS
sat = ept = Earth-Probe TOMS
sat = a1t = ADEOS TOMS
yy, mm and dd = year, month and day (all two digits)

The GIF image files also use the yymmdd date convention, as follows:


where sat = satellite, but the 3-letter code above is replaced by a 1-letter code
h = hemisphere ('n' or 's') for north or south, e.g. a1t961001s.gif for ADEOS TOMS data for the southern hemisphere, 1st Oct. 1996.

4.3 Read software

There is an example FORTRAN program called READGRID.FOR, which is supplied with the Nimbus-7 ASCII files obtainable on CD-ROM. The Meteor-3 format is also readable with this program.

Some additional software is available on the Nimbus-7 CD-ROM. We also hold this software online.