Nitrate dynamics and source identification of rainwater in Beijing during rainy season: Insight from dual isotopes...

Anthropogenic reactive nitrogen emissions have a significant impact on atmospheric chemical composition and earth surface ecosystem. As one of the most important sinks of atmospheric nitrogen, the wet deposition of nitrate (rainwater NO₃^-) has been widely concerned. Yet, the sources and transformation processes of wet deposited NO₃^- were not well revealed in megacity during rainy season in the context of global climate change. Here, we investigated the concentrations of nitrogen components and dual isotopes of rainwater nitrate collected in Beijing during July to August 2021 (rainy season). The main findings showed that the concentrations of NH₄⁺-N, NO₃^--N, and NO₂^--N ranged 0.5- 6.7 mg L^-1, 0.3- 4.5 mg L^-1, and 0.05- 0.18 mg L^-1, respectively, with the average relative percentages of 69 %, 29 %, and 2 %. The stoichiometry analysis of characteristic ion ratios indicated that the contribution of municipal wastes and agricultural sources to rainwater NH₄⁺-N is relatively significant, while traffics were the major contributor of NO₃^--N instead of the fixed emission. Rainwater δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^- presented slightly ¹⁵N-depleted characteristic compared to previous studies with the average values of -3.9 ± 3.1 ‰ and 58.7 ± 12.6 ‰. These isotope compositions suggesting an origin of rainwater NO₃^- from the mixing of multi-sources and was mainly generated via the pathway of OH radical oxidization. Further source apportionment of rainwater NO₃^- by Bayesian mixing model evaluated that traffic (30.3 %) and soil (30.3 %) emissions contributed mostly to NO₃^-, while the contribution of biomass burning (18.8 %) and coal combustion (20.6 %) were relatively lower. This study highlighted the important role of dual isotopes in rainwater nitrate source identification and formation processes in megacity.