Nitrate sources and its formation in precipitation during typhoons (In-fa and Chanthu) in multiple cities, East China

A clear understanding of the factors governing dual isotopes (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-) in typhoons is essential for understanding their NO₃^- sources and its formation mechanisms. In this study, sequential precipitation samples during typhoons, including In-fa and Chanthu, were collected from Ningbo, Hangzhou and Huzhou. The chemical compositions, nitrogen and oxygen isotopes of NO₃^- and oxygen isotopes of H₂O (δ¹⁸O-H₂O) were measured. The results showed that the δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^- values ranged from -6.3‰ to 6.0‰, and 38.0‰ to 66.5‰, respectively. The lower δ¹⁸O-NO₃^- values (less than 52‰) indicated the importance of peroxy radicals (RO₂ or HO₂) in NO_x oxidation to NO₃^- formation pathways. By the Monte Carlo simulation of δ¹⁸O-NO₃^- values of typhoons, the calculated oxidation proportions of NO by RO₂ (or HO₂) during the OH· pathway ranged from 0% to 27% of In-fa and from 0% to 32% of Chanthu, respectively, in the three cities. More NO_x emissions from marine microbial processes caused the lower δ¹⁵N-NO₃^- values of typhoons in Ningbo than those in Hangzhou and Huzhou. The variation in δ¹⁵N-NO₃^- values in sequential samples in In-fa reflected the decreased marine sources (lightning) and the increased anthropogenic sources in land (coal combustion and microbial N cycle) from Phrase I to Phrase II and III. Based on the improved Bayesian model with nitrogen isotopic fractionation, the contributions of lightning + biomass burning, coal combustion, mobile sources and the microbial N cycle were 35.7%, 22.5%, 27.1% and 14.7% in In-fa, and 28.3%, 32.3%, 28.0% and 11.4% in Chanthu, respectively, in the three cities, emphasizing the influence of marine NO_x sources (lightning). The results highlight the importance of RO₂ (or HO₂) in NO_x oxidation pathways in typhoons and provide valuable insight into the NO_x sources of typhoons.