Nitrate Isotopic Composition in Precipitation at a Chinese Megacity: Seasonal Variations, Atmospheric Processes, and Implications for Sources

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Nitrogen stable isotope composition (δ15N) of nitrate (NO3−) deposition can aid in source apportionment of its precursor emissions, nitrogen oxides (NOx), with implications in mitigation policy to address serious air pollution. However, potential δ15N fractionation during atmospheric NO3− formation may hinder accurate quantification of NOx contributions. Previously, NOx photochemical reactions have been suggested to be the dominant δ15N fractionation process in NO3− formation. Here, we have quantified the potential fractionation effects associated with NOx photochemical reactions based upon δ15N‐NO3− measured in 1 year of daily‐based bulk deposition samples in Shenyang, a megacity with distinct seasonal fossil fuel combustion in northeastern China. The mean δ15N was 0.9 ± 4.0‰ and ranged from −4.9 to 8.3‰, with the lowest and highest values observed during summer and winter, respectively. Calculated NOx photochemical equilibrium fractionation may account for up to 42% of the observed seasonal δ15N change in NO3− deposition. However, the relative NOx source contribution trends, estimated based upon a δ15N Bayesian mixing model, were insensitive to NOx photochemical fractionation considerations. Overall, NOx source partitioning based upon averaged year‐long δ15N of bulk NO3− deposition estimated the following relative trend in NOx emission sources: coal combustion > biomass burning = vehicle emissions ≫ soil emissions. Seasonally, the increase of coal combustion emissions from summer to winter drives the seasonality of δ15N in NO3− deposition, indicating a necessity to control NOx emissions from coal combustion to improve wintertime air quality.