Rainwater records of atmospheric nitrogen-bearing pollutants in a plateau city, China: Compositions, evolution, and sources

Rainfall-related nitrogen (N) deposition has important influences on the earth surface due to its various eco-environmental effects (e.g., water eutrophication, biodiversity decreases). However, the evaluation of wet N deposition in plateau urban areas has received little attention to date, particularly regarding the composition, historical variations, and sources. Here, rainwater in the typical plateau city (Kunming) was sampled daily from June 2022 to May 2023 for N chemical measurement, and the historical data was applied for comparison. The concentrations of measured rainwater N species components, including NH₄⁺ -N, NO₃^--N, and dissolved organic nitrogen (DON), were 0.46, 0.45, and 0.88 mg N L⁻¹, respectively. Accordingly, the wet deposition fluxes in dissolved total nitrogen (DTN) were 14.20 kg N ha⁻¹ yr⁻¹, close to the average level in southwest China, the NH₄⁺-N, NO₃^--N, and DON were 26.0 %, 24.8 %, and 49.2 % of DTN, respectively. The significant variation in N concentration (highest in November with the lowest rainfall amount) and wet N deposition fluxes (highest in August with the highest rainfall amount) were observed in this study, which highlighted the leading role of rainfall amount and air pollutant content on rainwater N relative to other environmental factors. Moreover, the stoichiometric method shows that agricultural and non-agricultural factors controlled the rainwater N. The correlation between historical atmospheric nitrogen-bearing pollutant emissions and wet N deposition fluxes reveals that local human activities and transboundary pollutants are the primary factors influencing the wet N deposition in such a plateau city (Kunming). This study provides beneficial observed data on wet N deposition in plateau urban regions, which is also helpful in assessing urban nitrogen-bearing pollution and managing anthropogenic N emissions in plateau cities.