Formation mechanisms and source apportionments of nitrate aerosols in a megacity of eastern China based on multiple isotope observations.

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 ¹⁸O-NO ₃ ⁻, δ ¹⁷O-NO ₃ ⁻, and δ ¹⁵N-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 ¹⁷O-NO ₃ ⁻: 20.0‰–37.9‰) and nitrogen isotope of nitrate (δ ¹⁵N-NO ₃ ⁻: −2.9‰ to 18.1‰) values were higher in winter and lower in summer. Based on Δ ¹⁷O-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.