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 GOME and ENVISAT 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 mid-latitudes – 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)×1015 molec/cm2 and (0.7±0.6)×1015 molec/cm2 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.