Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 6 2 2006 10.5194/acpd-6-2189-2006 http://www.atmos-chem-phys-discuss.net/6/2189/2006/ http://www.atmos-chem-phys-discuss.net/6/2189/2006/acpd-6-2189-2006.html http://www.atmos-chem-phys-discuss.net/6/2189/2006/acpd-6-2189-2006.pdf 2189 2239 2006-03-31 Comparison of GOME tropospheric NO₂ columns with NO₂ profiles deduced from ground-based in situ measurements D. Schaub K. F. Boersma J. W. Kaiser A. K. Weiss D. Folini H. J. Eskes B. Buchmann Swiss Federal Institute for Materials Science and Technology (Empa), Ueberlandstrasse 129, CH-8600 Duebendorf, Switzerland Royal Netherlands Meteorological Institute (KNMI), P.O. Box 201, 3730 AE, De Bilt, The Netherlands European Centre for Medium-Range Weather Forecasts (ECMWF), Shinfield Park, Reading, RG2 9AX, UK Nitrogen dioxide (NO₂) vertical tropospheric column densities (VTCs) retrieved from the Global Ozone Monitoring Experiment (GOME) are compared to coincident ground-based tropospheric NO₂ columns. The ground-based columns are deduced from in situ measurements at different altitudes in the Alps for 1997 to June 2003, yielding a unique long-term comparison of GOME NO₂ VTC data retrieved by KNMI/BIRA with independently derived tropospheric NO₂ profiles. A first comparison relates the GOME columns to the ground-based NO₂ profiles that are directly integrated to tropospheric columns. A second comparison includes averaging kernel (AK) information, which makes the comparison independent from the a priori NO₂ profile used in the GOME retrieval. This allows splitting the total difference between the column data sets into two contributions: one that is due to differences between the a priori and the ground-based NO₂ profile shapes, and another that can be attributed to uncertainties in both the remaining retrieval parameters and the ground-based in situ NO₂ profiles. For anticyclonic clear sky conditions the comparison indicates a good agreement between the columns (n=157, R=0.70/0.74 without/with AK included). Both data sets show a similar seasonal behaviour with a distinct maximum of spring NO₂ VTCs. The mean relative difference (with respect to the ground-based columns) is −7% with a standard deviation of 40% and GOME on average slightly underestimating the ground-based columns. Further analysis indicates small GOME columns being systematically smaller than the ground-based ones. The influence of different shapes in the a priori and the ground-based NO₂ profile is analysed by considering AK information. It is moderate and indicates similar shapes of the profiles for clear sky conditions. Only for large GOME columns, differences between the profile shapes explain the larger part of the relative difference. In contrast, the other error sources give rise to the larger relative differences found towards smaller columns. Further, for the clear sky cases, errors from different sources are found to compensate each other partially. The comparison for cloudy cases indicates a poorer agreement between the columns (n=60, R=0.61). The mean relative difference between the columns is 60% with a standard deviation of 118% and GOME on average overestimating the ground-based columns. The clear improvement after inclusion of AK information (n=60, R=0.87) suggests larger errors in the a priori NO₂ profiles under cloudy conditions and demonstrates the importance of using the kernel information for (partially) clouded scenes.