Wildfire-induced suppression of anthropogenic sulfate formation in downwind regions of Northeast Asia

Wildfires are widely known to deteriorate air quality by increasing particulate matter concentrations both locally and in downwind regions. However, wildfire-induced decreases in particulate matter remain poorly understood. In this study, we investigated a reduction in sulfate (SO₄^2-) concentrations observed over Northeast Asia during the Siberian wildfire event in July 2014. The Community Multiscale Air Quality (CMAQ) model was applied in conjunction with the high-resolution Fire INventory from the National Center for Atmospheric Research (FINN) to simulate wildfire emissions and their impacts. Large amounts of pollutants emitted from the Siberian wildfires were transported over long distances, generally leading to increases in particulate matter concentrations across Northeast Asia. However, during 26-29 July 2014, an unusual decrease in SO₄^2- concentrations of up to -1.56 μg m^-3 was identified over Northeast China, the Shandong Peninsula, and the Yellow Sea. This decrease was attributed to the scavenging of hydroxyl radicals (OH) by wildfire-emitted gaseous species such as CO and volatile organic compounds (VOCs), which suppressed the oxidation of anthropogenic SO₂ and consequently reduced sulfate formation. Source apportionment analysis revealed that the reduced sulfate originated predominantly from inland anthropogenic SO₂ emissions (92.30%), with a smaller contribution from oceanic sources (7.70%). These findings demonstrate that wildfire emissions can reduce secondary inorganic aerosol formation in downwind regions through complex chemical interactions with anthropogenic pollutants. This mechanism has important implications for PM_2.5 composition and air quality management in Northeast Asia, where inorganic aerosols constitute a substantial fraction of fine particulate matter.