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 NOx 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 NOx 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 NOx sources (lightning). The results highlight the importance of RO2 (or HO2) in NOx oxidation pathways in typhoons and provide valuable insight into the NOx sources of typhoons.