Ground-based validation of EOS-Aura OMI NO2 vertical column data in the midlatitude mountain ranges of Tien Shan (Kyrgyzstan) and Alps (France)

Ground-based UV-visible instruments for NO2 vertical column measurements have been operating at Issyk-Kul station, in Kyrgyzstan, and Observatoire de Haute-Provence (OHP), in France, since 1983 and 1992, respectively. These measurements have already been used for validation of ERS-2 Global Ozone Monitoring Experiment (GOME) and Envisat Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY) NO2 column data. Building upon the successful missions of GOME and SCIAMACHY, the Ozone Monitoring Experiment (OMI) was launched by NASA onboard the EOS Aura satellite in July 2004. Here we present the results of recent comparisons between OMI NO2 operational data (standard product) and correlative ground-based twilight measurements in midlatitudes, at Issyk-Kul and OHP, in 2004-2006. The stratospheric NO2 columns, observed by OMI and our ground-based instruments, have been corrected for NO2 diurnal change and normalized to local noon values using a photochemical box model. According to our comparison, OMI stratospheric NO2 columns underestimate ground-based measurements by (0.3 +/- 0.3) x 10(15) molecules/cm(2) and (0.7 +/- 0.6) x 10(15) molecules/cm(2) at Issyk-Kul and OHP, respectively. The effect of tropospheric pollution on the NO2 measurements in both regions of observations has been identified and discussed.