Different microphysical, optical and radiative properties of aerosol were analyzed during the severe fires in summer 2010 over Central Russia using ground measurements at two AERONET sites in Moscow and Zvenigorod (Moscow suburb) and radiative measurements in Moscow. Volume aerosol size distribution in smoke conditions was shown to have a bimodal character with the significant prevalence of fine mode aerosol particles which effective radius shifted to higher values (<i>r</i>_eff-fine = 0.24 μm against approximately 0.15 μm in typical conditions). Imaginary part of refractive index in visible region was characterized by lower values compared with typical conditions (REFI = 0.006 against REFI = 0.01) and single scattering albedo (SSA) was significantly higher (SSA_{&lambda;=675nm} = 0.95 against SSA_{&lambda;=675nm} ~ 0.9). Extremely high daily average AOT's were observed on 6–8 August reaching the absolute maximum on 7 August up to AOT500 = 6.4 in Moscow and AOT500 = 5.9 at Zvenigorod. A dramatic attenuation of solar irradiance at ground in cloudless but smoky conditions was also observed. Maximum irradiance loss has reached 64% for global shortwave irradiance, 91% for UV radiation 300–380 nm and 97% for erythemally-weighted UV irradiance even at relatively high solar elevation due to extremely high AOT and smaller SSA values in UV (0.8–0.9) compared with SSA in visible region of spectrum. The assessments of radiative forcing effect (RFE) at the TOA indicated a significant cooling of the smoky atmosphere. Instant RFE reached −167 Wm⁻² at AOT500 = 6.4 while climatological RFE calculated for monthly mean AOT in August 2010 was about −65 Wm⁻² compared with −20 Wm⁻² for typical aerosol conditions according to the 10 year period of measurements in Moscow

Abstract.Simultaneous measurements by the collocated AERONET CIMEL sun/sky photometers at the Moscow State University Meteorological Observatory (MSU MO) and at the Zvenigorod Scientific Station (ZSS) of the A. M. Obukhov Institute of Atmospheric Physics during September 2006-April 2009 provide the estimates of the effects of urban pollution on various aerosol properties in different seasons. The average difference in aerosol optical thickness between MO MSU and ZSS, which can characterize the effect of aerosol pollution, has been estimated to be about dAOT = 0.02 in visible spectral region. The most pronounced difference is observed in winter conditions when relative AOT difference can reach 26%. The high correlation of the AOT's, the Angstrom exponent values and the effective radii between the sites confirms that natural processes are the dominating factor in the changes of the aerosol properties even over the Moscow megacity area. The existence of positive correlation between dAOT and difference in water vapor content explains many cases with large dAOT between the sites by the time lag in the airmass advection. However, after excluding the difference due to this factor, AOT in Moscow remains higher even in a larger number of cases (more than 75%) with the same mean dAOT = 0.02. Due to the negative average difference in aerosol radiative forcing at the TOA of about dARF TOA = −0.9 W m −2 , the aerosol urban pollution provides a distinct cooling effect of the atmosphere. The PAR and UV irradiance reaching the ground is only 2-3% lower in Moscow due to the pollution effects, though in some situations the attenuation can reach 13% in visible and more than 20% in UV spectral region.

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