Formation and driving factors of sulfate in PM2.5 at a high-level atmospheric SO2 city of Yangquan in China

Abstract

Sulfate constitutes a significant portion of fine particulate matter (PM2.5) in China, and its formation has been of great interest in the environmental and research community in recent years. In this study, PM2.5 sampling was conducted along with gaseous pollutant concentration measurements and meteorological parameters in the city of Yangquan, China, which has high levels of atmospheric sulfur dioxide (SO2), over the course of four seasons in 2018. PM2.5 levels in daily samples ranged from 20.4 to 250.2 μg m−3, with a mean of 75.1 μg m−3. PM2.5 levels in Yangquan exhibited clear seasonal variations, with the highest seasonal average level of 108.2 μg m−3 found in the winter, likely due to coal combustion for heating purposes, and the lowest level of 45.5 μg m−3 in the summer. The results showed that oxidation by ozone (O3) played a major role in the formation of sulfate in the spring, summer, and autumn, while nitrogen dioxide (NO2) oxidation played a major role in the formation of sulfate in the winter. As a precursor of O3, NO2 also played a key role in its formation. Therefore, controlling NO2 emissions could effectively reduce the formation of sulfate in all four seasons via reductions in O3. Meanwhile, reducing SO2 alone did not effectively decrease sulfate levels when the variations in the concentration of NO2 were small, which indicated the importance of NO2 to the formation of sulfate (SO42−) between 2014 and 2018 in Yangquan. Thus, the control and reduction of NO2 emissions should be strengthened, leveraging existing control efforts of SO2 emissions, in order to most efficiently reduce PM2.5.