Formation and driving factors of sulfate in PM 2.5 at a high-level atmospheric SO 2 city of Yangquan in China

Sulfate constitutes a significant portion of fine particulate matter (PM 2.5 ) in China, and its formation has been of great interest in the environmental and research community in recent years. In this study, PM 2.5 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 (SO 2 ), over the course of four seasons in 2018. PM 2.5 levels in daily samples ranged from 20.4 to 250.2 μg m −3 , with a mean of 75.1 μg m −3 . PM 2.5 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 (O 3 ) played a major role in the formation of sulfate in the spring, summer, and autumn, while nitrogen dioxide (NO 2 ) oxidation played a major role in the formation of sulfate in the winter. As a precursor of O 3 , NO 2 also played a key role in its formation. Therefore, controlling NO 2 emissions could effectively reduce the formation of sulfate in all four seasons via reductions in O 3 . Meanwhile, reducing SO 2 alone did not effectively decrease sulfate levels when the variations in the concentration of NO 2 were small, which indicated the importance of NO 2 to the formation of sulfate (SO 4 2− ) between 2014 and 2018 in Yangquan. Thus, the control and reduction of NO 2 emissions should be strengthened, leveraging existing control efforts of SO 2 emissions, in order to most efficiently reduce PM 2.5 .