[1] Ground-based UV-visible instruments for NO 2 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) NO 2 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 NO 2 operational data (standard product) and correlative ground-based twilight measurements in midlatitudes, at Issyk-Kul and OHP, in 2004-2006. The stratospheric NO 2 columns, observed by OMI and our ground-based instruments, have been corrected for NO 2 diurnal change and normalized to local noon values using a photochemical box model. According to our comparison, OMI stratospheric NO 2 columns underestimate ground-based measurements by (0.3 ± 0.3) Â 10 15 molecules/cm 2 and (0.7 ± 0.6) Â 10 15 molecules/cm 2 at Issyk-Kul and OHP, respectively. The effect of tropospheric pollution on the NO 2 measurements in both regions of observations has been identified and discussed.

[1] Aerosol optical depth measurements over Issyk-Kul Lake acquired with the handheld sun photometer Microtops II are analyzed. Aerosol found over the mountainous region at the elevation of 1650 m above sea level resembles mostly clean background conditions. The yearly aerosol optical depth at a wavelength 500 nm $0.10 ± 0.03 is in agreement with the multi-year means from the background sites of the Aerosol Robotic Network (AERONET). Over a period of 4 years optical depth showed a seasonal pattern, with a maximum observed during summer. A link has been made between new and previously acquired data in order to provide a reliable trend for the region.

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