Impact of Wildfires on Chemical Compositions of Atmospheric Particulate in European Megacity Moscow

<p indent="0mm"> Wildfires are an important source of particulate matter in the atmosphere and have a significant impact on the regional atmospheric environment through atmospheric transportation. In this study, PM₁₀ samples were collected at Moscow urban background from June to September 2022, covered the periods of wildfires occurred in the Ryazan forest, <sc>200 km</sc> from the city. Mass concentrations of carbonaceous components (organic carbon, OC, and elemental carbon, EC) and water soluble inorganic ions (WSll_s) were analyzed to assess the impact of wildfires on the PM₁₀ composition. Based on backward air mass trajectories and fire spot maps analysis, the two pollution events with significantly increased PM₁₀ concentrations (Plume 1 and Plume 2) that occurred in August were identified. During Plume 1 and Plume 2, averaged PM₁₀ were <sc>44.1±7.2</sc> and <sc> 71.0±19.2 μg/m³, </sc> respectively, 1.9 and 3.0 times higher than during urban background period, nighttime PM₁₀ concentrations were higher than daytime. SOC concentrations increased by 2.0 and 4.8 times, respectively, compared to urban background <sc> (3.9±1.9 μg/m³). </sc> OC and EC concentrations (Plume 1: <sc>18.8±7.0</sc> and <sc> 3.9±1.2 μg/m³; </sc> Plume 2: <sc>32.5±15.1</sc> and <sc> 5.4±2.4 μg/m³) </sc> were significantly higher than those during urban background period <sc>(7.7±2.8</sc> and <sc> 2.1±1.0 μg/m³). </sc> Proportion of organic matter (OM) in PM₁₀ and SOC in OC increased to 68.3% and 73.2% and up to more than 60%. The influence of the transportation of wildfire plumes increased the proportion of OM, especially SOC. The total concentrations of WSll_s during Plume 1 and Plume 2 increased by 1.6 and 4.2 times compared to urban background <sc> (1.5±0.9 μg/m³), </sc> and the proportion of secondary inorganic ions (SNA) in WSll_s also increased. Under plumes impact, significant correlations between SNA and K⁺ were found, and NO₃^–/SO₄^2– were less than 1, different from urban background (NO₃^–/SO₄^2–​&gt;1), indicating that SNA contributed by wildfire smoke. By comparison of nitrogen oxidation (NOR) and sulfur oxidation (SOR) rates, it indicated the promotion of the conversions of SO₂ and NO₂ to SO₄^2– and NO₃^– as a result of wildfire smoke plumes transportation to the megacity, especially the conversion of SO₂ at night.