World Ozone and Ultraviolet Radiation Data Centre (WOUDC)

Total Ozone Mapping Spectrometer

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Time series plots of total column ozone, derived from the spaceborne, Total Ozone Mapping Spectrometer (TOMS) instrument, are available from the WOUDC ftp site for selected WOUDC stations. These plots are available as Post Script (*.ps) or PDF files (*.pdf). If you wish to view the "PDF" plots you will require the Adobe® Acrobat® viewer available available at Adobe® Acrobat®.

TOMS data measurements began in 1978. The ground-based data used in the analysis were obtained from the WOUDC total ozone data archive (unless otherwise indicated). The TOMS plots are provided courtesy of Dr. Richard McPeters and Mr. Gordon Labow of the NASA Goddard Space Flight Centre located in Greenbelt, Maryland, USA. Further information about these plots or about the TOMS instrument may be found in the following article or by visiting the NASA-GSFC TOMS web site.

Total Ozone Mapping Spectrometer (TOMS) instruments have now been flown on four spacecraft: Nimbus 7 (11/78 - 5/93), Meteor 3 (8/91 - 12/94), ADEOS (9/96 - 6/97), and Earth Probe (7/96 - present), giving a satellite ozone data record covering 20 years, except for a year and a half gap in 1995/96. The data have all been processed using the version 7 algorithm. McPeters and Labow [1996] compared the Nimbus 7 TOMS data with an average of 30 northern hemisphere ground-stations and with the World Standard Dobson Instrument I83. They found that the TOMS ozone values were within 1% to 1.5% of those from I83 and that the long term calibration showed almost no time dependent drift (less than 1% per decade). Since there are now data from multiple instruments there are questions of instrument-to-instrument calibration differences. For example, ground comparisons suggest that data from Earth Probe TOMS are 1% to 2% high relative to previous TOMS instruments.

The following plots show comparisons of total column ozone measured by TOMS on Nimbus 7, Meteor 3, and Earth Probe with total column ozone measured by individual ground-stations. (ADEOS comparisons are not shown because they overlap Earth Probe completely.) The purpose of these plots is to allow the comparison of the performance of individual ozone measuring instruments with other instruments by using TOMS as a transfer standard. If a sudden change in the TOMS versus ground measurement comparison is seen at only one location, the source of the change is most likely to be the instrument at the ground-station. Otherwise, if many stations observe the same change(s) versus TOMS, the cause would most probably be a change in the TOMS instrument characteristics. It is hoped that these comparisons can be used to improve the performance of all instruments.

While Nimbus-7 and Earth Probe were in near-noon sun synchronous polar orbits, the orbit of Meteor 3 precessed with a period of 212 days, leading to near-terminator observing conditions every 106 days. Measurements taken when the orbit parallels the terminator have increased uncertainty and are not used, leading to data gaps in the 1993/94 time period. The Meteor data are plotted as a dashed line. Only the period after the failure of the Nimbus 7 TOMS is plotted.

TOMS makes 35 individual scans across the orbital track. The TOMS FOV (field of view) on the surface of the earth is a 26 km to 50 km (depending on orbit altitude) square at nadir, somewhat larger for the off-nadir scans. For each day the ozone measured within a single FOV most nearly co-located with a ground-station is taken as the TOMS value for that day. The TOMS measurement is almost always located within 1 degree of the station and within two hours of local noon. When multiple matches are possible, at high latitudes in summer for instance, the TOMS measurement with the shorter optical path is chosen.

The ground based data consist of measurements made by Dobson, M83/124, and Brewer instruments. At some stations there have been changes in instrument type (e.g. Dobson to Brewer) and TOMS data are useful to check continuity. All ground-station data were taken from the World Ozone and Ultraviolet radiation Data Center (WOUDC) as of January 1999. Two graphs are used to show each matched set of satellite/ground-station measurements. The top graph shows the TOMS ozone amounts for each week in which there exist both TOMS and ground-station data. The lower plot shows the percent difference between TOMS and the ground-station for all observational codes for each week. The value (TOMS - ground)/TOMS x 100 is plotted.

For the latest information on data from the different TOMS instruments and the most recent overpass updates, check the TOMS homepage at: http://toms.gsfc.nasa.gov

If there are any questions about these comparisons, please contact:

Reference: "An assessment of the accuracy of 14.5 years of Nimbus 7 TOMS version 7 ozone data by comparison with the Dobson network," R.D. McPeters and G.J. Labow, Geophys. Res. Lett., 23, 3695-3698, 1996.