Greg Michalski

Inorganic nitrate (NO 3 − ) is a crucial component of fine particulate matter (PM 2.5 ) in haze events in China. Understanding the formation mechanisms of nitrate and the sources of NO x was critical to control the air pollution. In this study, measurements of multiple isotope compositions of nitrate (δ 18 O‐NO 3 − , δ 17 O‐NO 3 − , and δ 15 N‐NO 3 − ) in PM 2.5 were conducted in Hangzhou from 9 October 2015 to 24 August 2016. Our results showed that oxygen anomaly of nitrate (Δ 17 O‐NO 3 − : 20.0‰–37.9‰) and nitrogen isotope of nitrate (δ 15 N‐NO 3 − : −2.9‰ to 18.1‰) values were higher in winter and lower in summer. Based on Δ 17 O‐NO 3 − observation and a Bayesian model, NO 3 radical chemistry was found to dominate the nitrate formation in winter, while photochemical reaction (NO 2 + OH) was the main pathway in summer. After considering the nitrogen isotopic fractionation in the NO x (g) ‐NO 3 − (p) conversion, the average contributions of coal combustion, vehicle exhausts, biomass burning, and soil emission were 50% ± 9%, 19% ± 12%, 26% ± 15%, and 5% ± 4%, respectively, to nitrate aerosols during the whole sampling period. Coal combustion was the most important nitrate source in Hangzhou, especially in winter (∼56%). The contribution of soil emission increased significantly in summer due to active soil microbial processes under high temperature environment.