Columnar and surface urban aerosol in Moscow megacity according to measurements and simulations with COSMO-ART model.

Urban aerosol pollution was analyzed over the Moscow megacity region using COSMO-ART chemical transport model and intensive measurement campaigns at the Moscow State University Meteorological Observatory (MSU MO, 55.707° N, 37.522° E) during April-May period in 2018 and 2019. We analyzed mass concentrations of Particulate Matter with diameter smaller 10 micron (PM₁₀), Black Carbon (BC), and aerosol gas precursors (NOx, SO₂, CHx) as well as columnar aerosol parameters for fine and coarse modes together with different meteorological parameters including an index characterizing the Intensity of Particle Dispersion (IPD). Both model and experimental datasets have shown a statistically significant linear correlation of BC with NO₂ and PM₁₀ mass concentrations, which indicates mostly common sources of emissions of these substances. There was a pronounced increase in the BC / PM₁₀ ratio from 0.7 % to 5.9 % with the decrease in IPD index related to the amplification of the atmospheric stratification. We also found an inverse dependence between the BC / PM₁₀ ratio and columnar single scattering albedo (SSA) for the intense air mixing conditions. This dependence together with the obtained negative correlation between wind speed and BC / PM₁₀ may serve an indicator of changes in the absorbing properties of the atmosphere due to meteorological factors. On average, relatively low for urban regions BC / PM₁₀ ratio of 4.7 % is the cause of the observed relatively high SSA = 0.94 in Moscow. Using long-term parallel aerosol optical depth (AOD) measurements over the 2006-2020 period at the MSU MO and in upwind clean background conditions at Zvenigorod Scientific Station (ZSS) of the IAP RAS (55.7° N, 36.8° E), we estimated the urban component of AOD (AOD_urb) and some other parameters as the differences at these sites. The average AOD_urb at 550 nm was about 0.021 with more than 85 % of fine aerosol mode. The comparisons between AOD_urb obtained from model and measurements during the experiment have revealed a similar level of aerosol pollution of about AOD_urb = 0.015-0.019, which comprised 15-19 % of the total AOD at 550 nm. The urban component of PM₁₀ (PM_10urb) was about 0.016 mg m^-3 according to the measurements and 0.006 mg m^-3 according to the COSMO-ART simulations. We obtained a pronounced diurnal cycle of PM_10urb and urban BC (BC_urb), as well as their strong correlation with the IPDs. With the IPD index change from 3 to 1 at night, there was about 4 times increase in PM_10urb (up to 0.030-0.040 mg m^-3) and 3 times increase in BC_urb (up to 0.003-0.0035 mg m^-3). At the same time, no pronounced daily cycle was found for the columnar urban aerosol component (AOD_urb), although there is a slight tendency to the increase in model AOD_urb at night. We also obtained a close relationship between the calculated and measured PM_10urb values, their dependence on IPD index, and the pronounced growth of PM_10urb with the PM₁₀ increase. [ABSTRACT FROM AUTHOR]