Unraveling the main chemical mechanism of PM2.5 pollution caused by open burning of sugarcane straw during winter in southern China: The Pivotal Role of NOx and NH3 Oxidation-Neutralization.

Guangxi is one of the main sugarcane-producing areas in China, with its cultivated sugarcane area accounting for more than 60 % of the national total. Open burning of sugarcane stalks is thus a significant contributor to fine particulate matter (PM2.5) pollution in the region during winter. In this study, a PM2.5 pollution event is analyzed in Nanning, the capital of Guangxi, from February 24-28, 2022. Results showed a significant increase in PM2.5 concentrations and its components during the pollution period. Among them, K+, Cl-, Na+, and EC, which are heavily influenced by open straw burning, increased by 4.50, 4.25, 3.50, and 3.02 times, respectively, compared to the non-polluted period. PM2.5 concentrations rose sharply at night (19:00 on February 26), and the subsequent accumulation of pollutants increased atmospheric oxidizing capacity the following day. This facilitated the conversion of SO2 and NOx, leading to a sudden increase in secondary inorganic aerosols (SIA) and causing PM2.5 concentrations to peak again during the daytime. The concentration characteristics of SO2, NOx, and NH3 indicated that nitric acid was the limiting species in the SIA generation process. This revealed that oxidation and neutralization reactions involving NOx and NH3 were critical mechanisms for the increase in PM2.5 concentrations during the open burning of sugarcane straw in winter in Guangxi. This study reveals the importance of strengthening the control of anthropogenic emissions of NOx and NH3 during wintertime straw burning in Southern China.