Seasonal differences in sources and formation processes of PM 2.5 nitrate in an urban environment of North China

Nitrate (NO 3 ???) has been the dominant ion of secondary inorganic aerosols (SIAs) in PM 2.5 in North China. Tracking the formation mechanisms and sources of particulate nitrate are vital to mitigate air pollution. In this study, PM 2.5 samples in winter (January 2020) and in summer (June 2020) were collected in Jiaozuo, China, and water-soluble ions and ( 815 N, 818 O)NO 3 ??? were analyzed. The results showed that the increase of NO 3 ??? concentrations was the most remarkable with increasing PM 2.5 pollution level. 818 O-NO 3 ??? values for winter samples (82.7 %o to 103.9 %o ) were close to calculated 818 O-HNO 3 (103 %o ?? 0.8 %o ) values by N 2 O 5 pathway, while 818 O-NO 3 ??? values (67.8 %o to 85.7 %o ) for summer samples were close to calculated 818 O-HNO 3 values (61 %o ?? 0.8 %o ) by OH oxidation pathway, suggesting that PM 2.5 nitrate is largely from N 2 O 5 pathway in winter, while is largely from OH pathway in summer. Averaged fractional contributions of P N2O5 + H2O were 70% and 39% in winter and summer sampling periods, respectively, those of P OH were 30% and 61%, respectively. Higher 815 N-NO 3 ??? values for winter samples (3.0 %o to 14.4 %o ) than those for summer samples (-3.7 %o to 8.6 %o ) might be due to more contributions from coal combustion in winter. Coal combustion (31% ?? 9%, 25% ?? 9% in winter and summer, respectively) and biomass burning (30% ?? 12%, 36% ?? 12% in winter and summer, respectively) were the main sources using Bayesian mixing model. These results provided clear evidence of particulate nitrate formation and sources under different PM 2.5 levels, and aided in reducing atmospheric nitrate in urban environments. ?? 2022 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.