Long-term variability of aerosol optical thickness in Eastern Europe over 2001-2014 according to the measurements at the Moscow MSU MO AERONET site with additional cloud and NO2 correction

The atmospheric aerosol properties were obtained within the framework of the AERONET program at the Moscow State University Meteorological Observatory (Moscow MSU MO) over the 2001-2014 period. The quality data control has revealed the necessity of additional cloud screening and NO2 correction. The application of additional cloud screening according to hourly visual cloud observations provides a decrease in monthly average aerosol optical thickness (AOT) at 500 nm of up to 0.03 compared with the standard data set. We also show that the additional NO2 correction of the AERONET version 2 data is needed in large megalopolis, like Moscow, with 12 million residents and NOx emission rates of about 100 kt yr(-1). According to the developed method, we estimated monthly mean NO2 content, which provides an additional decrease of 0.01 for AOT at 340 nm, and of about 0.015 - for AOT at 380 and 440 nm. The ratios of NO2 optical thickness to AOT at 380 and 440 nm are about 5-6% in summer and reach 15-20% in winter when both factors have similar effects on UV irradiance. Seasonal cycle of AOT at 500 nm is characterized by a noticeable summer and spring maxima, and a minimum in winter conditions, changing from 0.08 in December and January up to 0.3 in August. The application of the additional cloud screening removes a local AOT maximum in February. Statistically significant negative trends in annual AOT for UV and mid-visible spectral range have been obtained both for average and 50% quantile values. The pronounced negative changes were observed in most months with the rate of about 1-5% yr(-1) and could be attributed to the negative trends in emissions (E) of different aerosol precursors of about 135 Gg yr(-2) in E SO x, 54 Gg yr(-2) in E-NMVOC, and slight negative changes in NO x over the European part of Russia. No significant influence of natural factors on temporal AOT variations has been revealed.