Differentiation Between Nitrate Aerosol Formation Pathways in a Southeast Chinese City by Dual Isotope and Modeling Studies
Hong‐Wei Xiao, Ren‐Guo Zhu, Yuanyuan Pan, Wei Guo, Nengjian Zheng, Yong‐Hui Liu, Cheng Liu, Zhongyi Zhang, Jingfeng Wu, Chang‐An Kang, Li Luo, Huayun Xiao
Abstract
Abstract Nitrate (NO 3 − ), one of the most important inorganic aerosols in the atmosphere, is mainly formed by oxidation of NO x by the hydroxyl radical (OH) and ozone (O 3 ) in urban atmospheres. However, the fractional contributions of its various oxidation pathways remain unclear. Here, we collected particulate matter with aerodynamic diameter less than 2.5 μm (PM 2.5 ) samples in a second‐tier city in southeast China from 1 September to 31 December 2017 and measured the NO 3 − and nitrate isotopic compositions (δ 15 N and δ 18 O). The average concentration of NO 3 − , δ 15 N, and δ 18 O values were 14.7 ± 11.6 μg/m 3 , (+4.3 ± 4.3)‰, and (+71.8 ± 14.7)‰ with the ranges from 0.8 to 57.7 μg/m 3 , −10.5‰ to +12.5‰ and +34.5‰ to +91.9‰, respectively. All three species were significantly higher in winter than in summer. Based on a Bayesian mixing model with a dual isotope array for NO 3 − , contributions of (37.1 ± 33.4)%, (60.3 ± 32.2)%, and (2.6 ± 2.7)% to NO 3 − could be attributed to OH oxidation, N 2 O 5 hydrolysis, and NO 3 + hydrocarbon (HC) pathways, respectively. Higher OH radical concentrations with higher ratios of OH to O 3 led to lower NO 3 − concentrations, while lower OH radical concentrations with higher ratios of O 3 to OH led to higher contributions of N 2 O 5 hydrolysis, forming higher NO 3 − concentrations in winter. Under low OH, an increased O 3 to NO x ratio increased the contribution of the NO 3 + HC pathway. The comprehensive analysis of the isotopic compositions of nitrate helped identify the importance of major oxidation pathways of NO x in this city.