Comparisons of Total Column Ozone Measured by TOMS with Ozone
                   Measured by Ground Stations

   Data from the Total Ozone Mapping Spectrometer (TOMS)
onboard the Nimbus-7 (1978-1993) and Earth Probe (1996-present)
satellites have been reprocessed with an improved algorithm. The
new algorithm (Version 8) has implemented more accurate
instrument calibrations and the following improvements:

   - New climatological values for temperature and ozone profiles
     (latitude, altitude and time)
   - Temperature climatology derived from NCEP
   - Ozone climatology derived from SAGE & ozonesondes
   - Uses one wavelength pair (317.5 & 360.0 nm)
   - Uses a more accurate cloud height climatology derived from
     NIMBUS-7 Temperature-Humidity Infrared Radiometer
   - Improved instrument calibration
   - Improved surface reflectivity model to discriminate between
     snow and cloud
   - New aerosol corrections
   - Implemented sea glint corrections
   - Improved Radiative Transfer Model
   - Corrections made for O2-O2 dimer absorption at 360nm
   - Ring (rotational Raman scattering) correction

   The following plots show comparisons of total column ozone
measured by TOMS and by individual ground stations. Each plot
shows a series of three graphs which are the following:

   - Graph 1: Weekly averaged ozone values (blue=TOMS)
   - Graph 2: Difference in percent between the TOMS and the
              ground instrument as calculated by: 100* (TOMS-Ground)/TOMS
   - Graph 3: Difference in percent between the two measurements
              as a function of the average of the TOMS & ground station
              total ozone.

   The purpose of these plots is to allow the comparison of the
performance of individual ozone monitoring instruments with other
instruments by using TOMS as a transfer standard.  If a sudden
change in the difference between TOMS and a single ground station
is seen in the time series, the source of the change is most
likely the instrument at the ground-station.  If many stations
observe the same change(s) versus TOMS, the cause would most
probably be a change in the TOMS instrument characteristics. The
total ozone differences are particularly interesting because they
indicate instrument performance issues such as stray light
effects and/or instrument nonlinearities. It is hoped that these
comparisons can be used to improve the performance of all
instruments

   The ground based data consist of measurements made by Dobson,
M83/124 filter ozonemeters and/or Brewer spectrophotmeters. At
some stations there have been changes in instrument type (e.g.
Dobson to Brewer) and the TOMS data are useful to check
continuity.  Other stations have more than one instrument
operating simultaneously and each ground instrument will have a
comparison file in this dataset.

   The file naming convention is as follows:

   S### INST .TOMS.pdf
   
   Where ### is the 3 digit WMO station number, INST is B for brewer, 
   D for Dobson and F for M83/124
   TOMS is either N7 for Nimbus 7 (1978-1993) or EP for Earth Probe
   (1996-present)
   
   Example: s065d.n7.pdf is a pdf file containing the Nimbus-7 TOMS
            comparisons with the Toronto Dobson.

NOTICE:

   1) There are no comparisons for the Meteor 3 TOMS (1991-1993)
      instrument because the Version 8 algorithm has not yet been
      applied to the M3 dataset.  We expect M3 data to be available by
      summer 2005.
   2) The Earth Probe TOMS data from year 2000 onward were
      processed using a calibration that was stabilized against the
      NOAA 16 SBUV/2.  Because of continuing optical degradation of the
      EP scan mirror, data after 2002 should not be used in trend
      studies.  There are known latitude dependent errors of -2% to -4%
      in this period.

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

Gordon Labow
NASA Goddard Space Flight Center
Mail Case 916
Greenbelt, Md 20771
labow@qhearts.gsfc.nasa.gov

Richard McPeters
NASA Goddard Space Flight Center
Mail Case 916
Greenbelt, Md 20771
mcpeters@wrabbit.gsfc.nasa.gov