ACPD Atmospheric Chemistry and Physics Discussions ACPD 1680-7375 Copernicus GmbH Göttingen, Germany 10.5194/acpd-9-22271-2009 Comparison of OMI NO<sub>2</sub> tropospheric columns with an ensemble of global and European regional air quality models Huijnen V. 1 Eskes H. J. 1 Amstrup B. 2 Bergstrom R. 3 Boersma K. F. 1 Elbern H. 4 Flemming J. 5 Foret G. 6 Friese E. 4 Gross A. 2 D'Isidoro M. 7 Kioutsioukis I. 8 Maurizi A. 7 Melas D. 8 Peuch V.-H. 9 Poupkou A. 8 Robertson L. 3 Sofiev M. 10 Stein O. 11 12 Strunk A. 4 Valdebenito A. 13 Zerefos C. 8 Zyryanov D. 6 14 Royal Netherlands Meteorological Institute, De Bilt, The Netherlands Danish Meteorological Institute, Copenhagen, Denmark Swedish Meteorological and Hydrological Institute, Norrkoping, Sweden Rhenish Institute for Environmental Research at the University of Cologne, Köln, Germany European Centre for Medium-Range Weather Forecasts (ECMWF), Reading, UK Laboratoire Interuniv. des Systèmes Atmosphériques, CNRS/Univ. Paris 12 et 7, Créteil, France Institute of Atmospheric Sciences and Climate, Consiglio Nazionale delle Ricerche, Bologna, Italy Laboratory of Climatology, Faculty of Geology, University of Athens, Athens, Greece CNRM-GAME, Météo-France and CNRS URA 1357, Toulouse, France Air Quality Research, Finnish Meteorological Institute, Finland Institute for Chemistry and Dynamics of the Geosphere (ICG), FZ Jülich, Germany Max Planck Institute for Meteorology, Hamburg, Germany Norwegian Meteorological Institute, Norway Laboratoire de Météorologie Dynamique, CNRS/IPSL, Ecole polytechnique, Palaiseau, France 22 10 2009 9 5 22271 22330 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys-discuss.net/9/22271/2009/acpd-9-22271-2009.html The full text article is available as a PDF file from http://www.atmos-chem-phys-discuss.net/9/22271/2009/acpd-9-22271-2009.pdf

We present model results for tropospheric NO<sub>2</sub> from 9 regional models and 2 global models that are part of the GEMS-RAQ forecast system, for July 2008 to June 2009 over Europe. These modeled NO<sub>2</sub> columns are compared with OMI NO<sub>2</sub> satellite retrievals and surface observations from the Dutch Air Quality Network. The participating models apply principally the same emission inventory, but vary in model resolution (0.15 to 0.5°), chemical mechanism, meteorology and transport scheme. For area-averaged columns only a small bias is found when the averaging kernel is neglected in the comparison to OMI NO<sub>2</sub> columns. The reason for this is that TM4 a priori profiles have higher NO<sub>x</sub> concentrations in the free troposphere (where sensitivity to NO<sub>2</sub> is high) and higher NO<sub>x</sub> concentrations in the surface layers (where sensitivity to NO<sub>2</sub> is low) than RAQ models, effectively cancelling the effect of applying the averaging kernel. We attribute these low NO<sub>2</sub> concentrations in the RAQ models to missing emissions from aircraft and lightning. It is also shown that the NO<sub>2</sub> concentrations from the upper part of the troposphere (higher than 500 hPa) contribute up to 20% of the total tropospheric NO<sub>2</sub> signal observed by OMI. Compared to the global models the RAQ models show a better correlation to the OMI NO<sub>2</sub> observations, which are characterized by high spatial variation due to the short lifetime for NO<sub>2</sub>. The spread in the modeled tropospheric NO<sub>2</sub> column is on average 20–40%. In summer the mean of all models is on average 46% below the OMI observations, whereas in winter the models are more in line with OMI. On the other hand the models on average under-predict surface concentrations in winter by 24% and are more in line with observations in summer. These findings suggest that OMI tropospheric columns in summer over polluted regions are biased high by about 40%. The diurnal cycle and profiles in the regional models are well in line, and the profile shapes correspond well to results from the global models. The analyses against OMI observations have proven to be very useful to initiate model improvements, and to quantify uncertainties in the retrieval product.

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