Contribution of urban and advective fine aerosol to PM₁₀ variability in Moscow

Seasonal and daily concentrations of fine suspended particles (РМ₁₀) in surface air are determined by the influence of their sources, including the state of the underlying surface, but the main share of variability is due to the impact of meteorological processes. The average annual and average daily variations in РМ₁₀ in the middle seasonal months in Moscow are shown based on observations from the Mosecomonitoring network; the heterogeneity of the PM₁₀ field in the metropolis and the differences in PM₁₀ levels between highway areas and residential areas are discussed. It is shown that an increase in PM10 concentration and the achievement of critical levels of surface air pollution (aerosol episodes) are observed in two typical situations. Most often during periods of adverse meteorological conditions (AMC) for the dispersion of impurities, when PM₁₀ increases due to local urban sources.

Aerosol episodes are also formed under the influence of an external factor – long-range transfer. It manifests itself when dusty air masses arrive from areas of soil drought, from desert areas of wind-blown fine particles, in particular from the Caspian region and western Kazakhstan, as well as when pyrogenic aerosol is transferred from areas of natural fires. The transfer of fine particles over long distances is provided by specific atmospheric processes, as a rule, these are the western peripheries of vast anticyclones with accompanying mesojet currents in the lower 2-km layer. Examples are given of an abnormal increase in temperature in the upper part of the boundary layer as a result of direct heating of the transferred aerosol. It is noted that aerosol episodes due to long-range transfer are recorded in Moscow in spring and autumn, and episodes of local pollution after the melting of the snow cover – in dry weather from April to October.

 To identify AMC, developed meteorological index of pollution dispersion (MIPP) is used, based on quantitative assessment of the effect of the transfer velocity in the lower part of the atmospheric boundary layer, thermal mixing and precipitation. Using the above parameters calculated according to the data of the numerical model COSMO–RuENA6.6, the MIPP is forecasted, including the type of meteorological conditions that contribute to the accumulation of impurities in the surface air. Forecasting of PM₁₀ concentrations in the Moscow region in the Hydrometeorological Center of Russia is carried out within the framework of the current technology of numerical calculations using the chemical transport models (CTM) CHIMERE and COSMO-RuART with grid step of 2 km. Quality indicators of numerical forecasts of PM₁₀ based on CTM are given. It is noted that aerosol episodes of urban pollution in Moscow are predicted satisfactorily by the CTM technology, in contrast to episodes of long-range transport, as a consequence of the absence of accounting for irregular emissions of suspended particles of natural origin in remote regions in the CTM technology; examples of aerosol pollution by PM₁₀ in Moscow under AMC and during long-range transport are given, confirmed by trajectory analysis.