1. OMI ST meeting KNMI 22-6-2006 Validation of OMI total ozone using ground-based Brewer and Dobson observations D. Balis 1, E. Brinksma 2, M. Kroon 2,V. Amiridis 1 and C. Zerefos 3 1 Laboratory of Atmospheric Physics, Aristotle University of Thessaloniki, Greece 2 KNMI 3 Geology Department, University of Athens, Greece

2. OMI ST meeting KNMI 22-6-2006 Data processed  Preliminary near-to-real time Brewer data collected daily by the WMO Northern Hemisphere Ozone Mapping Centre.  Archived Dobson, Brewer and M-124 total ozone measurements have been extracted from the World Ozone Data Centre of WMO at Toronto, Canada  OMI-TOMS overpass data publicly available from NASA/GSFC for the period 2004-2005.  Provisional OMI-DOAS overpass data available form AVDC/NASA for the period 2004-2005.

3. OMI ST meeting KNMI 22-6-2006 Quality control of ground based stations  Ground-based data have been updated from WOUDC and checked for possible revised datasets.  Stations showing systematically inconsistencies between nearby stations or unreasonable small correlation between satellite and GB have been excluded from the summary validation statistics. (Same set of stations used in GOMEv4 and TOMSv8 comparisons)  Finally 22 Brewer and 47 Dobson records have been used for the comparisons with OMI-TOMS and OMI-DOAS

4. OMI ST meeting KNMI 22-6-2006 Examples of rejected ground-based data (based on previous validation studies)

5. OMI ST meeting KNMI 22-6-2006 Error sources in ground-based comparisons In standard WOUDC data: For Dobsons: The temperature dependence of the ozone absorption coefficients used in the retrievals might account for a seasonal variation in the error of up to ±1.7% and for a systematic errors of up to 4% (can increase in extreme cold conditions) For Brewers: The effect is smaller because of the different wavelengths used. Both might suffer from long-term drifts associated with calibration changes. Additional stray-light problems for SZA>75°, mainly for single Brewers and Dobsons, less for double Brewers. Dobson: A pair 305.5 and 325.4 D pair 317.6 and 338.8 Brewer: MS4=306.3/316.8, MS5=310.1/316.8, MS6=313.5/316.8, MS7320.1/316.8

6. OMI ST meeting KNMI 22-6-2006

16. Conclusions (1/2)  The average difference between OMI-DOAS and Brewer observations is 1.03% while the corresponding difference between OMI-TOMS and Brewer observations is -0.12% (valid mainly for 30-60 o N).  The average difference between OMI-DOAS and Dobson observations is about 2.7%, showing however better agreement over 30-40 o S. The average difference between OMI-TOMS and Dobson observations is about 1%, with higher values for the tropics.  OMI-DOAS comparisons show a seasonal dependence with an amplitude of 1.5% for the Brewer comparisons and slightly larger but in phase (2%) for the Dobson comparisons. This seasonality is similar and in phase with the one found in GDP4.0-ground comparisons.  OMI-TOMS-Brewer comparisons presented do not show any seasonality and are remarkably stable around 0%. OMI-TOMS-Dobson comparisons show seasonality similar to the OMI-DOAS-Dobson comparison with reduced amplitude  The seasonal variability in the southern hemisphere both for OMI-TOMS and OMI-DOAS is less than the one found in the northern hemisphere comparisons

17. OMI ST meeting KNMI 22-6-2006 Conclusions (2/2)  There is no SZA dependence in the OMI-TOMS comparisons. OMI-DOAS comparisons show a SZA dependence, which however might also be attributed to the seasonal dependence (not conclusive)  Comparison results from high latitude stations cannot be considered at present significant since they are based on few observations. Over Antartica OMI- TOMS underestimates ozone, while OMI-DOAS overestimates by 2%  The different temperature dependence of the ozone absorption coefficients between the various algorithms may account for part of the observed seasonalities.