Nitrate (NO3−), one of the most important inorganic aerosols in the atmosphere, is mainly formed by oxidation of NOx by the hydroxyl radical (OH) and ozone (O3) 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 (PM2.5) samples in a second‐tier city in southeast China from 1 September to 31 December 2017 and measured the NO3− and nitrate isotopic compositions (δ15N and δ18O). The average concentration of NO3−, δ15N, and δ18O values were 14.7 ± 11.6 μg/m3, (+4.3 ± 4.3)‰, and (+71.8 ± 14.7)‰ with the ranges from 0.8 to 57.7 μg/m3, −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 NO3−, contributions of (37.1 ± 33.4)%, (60.3 ± 32.2)%, and (2.6 ± 2.7)% to NO3− could be attributed to OH oxidation, N2O5 hydrolysis, and NO3 + hydrocarbon (HC) pathways, respectively. Higher OH radical concentrations with higher ratios of OH to O3 led to lower NO3− concentrations, while lower OH radical concentrations with higher ratios of O3 to OH led to higher contributions of N2O5 hydrolysis, forming higher NO3− concentrations in winter. Under low OH, an increased O3 to NOx ratio increased the contribution of the NO3 + HC pathway. The comprehensive analysis of the isotopic compositions of nitrate helped identify the importance of major oxidation pathways of NOx in this city.

Atmospheric reactive nitrogen deposition is an important process in the nitrogen cycle of natural ecosystems, especially in oligotrophic oceans. Increased land‐based atmospheric nitrogen deposition over the ocean changes the stoichiometric balance of marine ecosystems; this causes changes in ecosystem functions and biogeochemical cycles. Current studies have shown that atmospheric reactive nitrogen is mainly derived from land‐based human activities, such as the use of fertilizers, combustion of fossil fuels, and forest fires. Forest fires can provide a vast amount of reactive nitrogen and other nutrients over short time scales and may greatly influence marine ecosystems. Here, we document a large change in nitrogen concentration and primary productivity in upper levels of the South China Sea (SCS), coincident with Indonesian forest fires between August and September of 2012. Using back trajectories, fire spot maps, geographical distributions of the smoke, vertical distributions of the depolarization ratio, and nitrogen isotope values of nitrate in rainwater, we found that the change in nitrogen could be attributed to the forest fires. Our results show that the SCS received about 180 Gg of N as wet deposition during the sampling period. This atmospheric nitrogen deposition caused high primary productivity (40.679 ± 15.852 mg C·m−2·hr−1) in the upper levels of the SCS, which tripled values recorded in other years. This suggests that high nitrogen levels as well as other nutrients derived from tropical Asian forest fires are of great importance to the marine ecosystem of the SCS and also likely affect global marine biogeochemical cycles.