Source apportionment and evolution of reactive nitrogen in an East Asian mountain forest: A dual-isotope and modeling approach

Anthropogenic activities have led to a rapid increase of reactive nitrogen (Nr) in the Earth system, contributing to climate change, biodiversity loss, acid deposition, and air pollution. Among Nr species, particulate ammonium (pNH₄⁺) and nitrate (pNO₃⁻) derived from ammonia (NH₃) and nitrogen oxides (NO_x) are key pollutants affecting air quality. However, their sources and formation pathways vary by location and remain poorly understood. This study investigates the sources and atmospheric processing of Nr in an East Asian mountain forest, using nitrogen (δ¹⁵N) and oxygen (δ¹⁸O) isotope compositions of pNH₄⁺ and pNO₃⁻. A field campaign was conducted in Xitou, Taiwan (23.40°N, 120.47°E, 1179 m above sea level) from April 17 to 24, 2021. Size-segregated aerosol particles ranging from 0.056 to 18 µm were collected using a micro-orifice uniform deposit impactor (MOUDI) and analyzed for mass concentrations and isotopic compositions using Fourier-transform infrared spectroscopy with attenuated total reflection (FTIR-ATR) and gas chromatography-isotope ratio mass spectrometer (GC-IRMS), respectively. Additionally, a stable isotope mixing model (MixSIAR) was applied to quantify source contributions of Nr based on δ¹⁵N signatures. Xitou, located downstream of metropolitan coastal areas during the daytime, receives air pollutants transported inland by sea breezes and valley winds, combined with local emissions. During the campaign, the average mass concentrations of pNH₄⁺ and pNO₃⁻ were 3.7 and 2.4 µg m⁻³, respectively. The mean δ¹⁵N values of pNH₄⁺ (10.8 ± 2.7‰) and pNO₃⁻ (−3.0 ± 2.0‰) reflect their emission sources and isotopic fractionation during gas-particle partitioning. δ¹⁸O values of pNO₃⁻ ranged from 32.0‰ to 73.3‰, indicating distinct chemical formation pathways: pNO₃⁻ formed via O₃ reactions exhibited higher δ¹⁸O values, while those formed via peroxy radicals (RO₂) had lower values. Two distinct groups of pNO₃⁻ were identified based on δ¹⁵N-pNO₃⁻and δ¹⁸O-pNO₃⁻ signatures. The first group, characterized by higher δ¹⁵N (−5.6 to 0.8‰) and δ¹⁸O (55 to 83‰), likely formed in metropolitan areas via O₃ oxidation before being transported to the mountain observation site. The second group, consisting of smaller particles with lower δ¹⁵N (−10.1 to −2.1‰) and δ¹⁸O (8.6 to 38‰), was likely produced locally with RO₂ as the dominant oxidant. Source apportionment analysis of δ¹⁵N revealed that combustion-related sources, including fossil fuel combustion and NH₃ slip, accounted for 63% of NH₃ emissions, while anthropogenic NO_x sources such as biomass burning, coal combustion, and mobile sources contributed approximately 68% of total NO_x emissions. These findings highlight the importance of targeted emission control policies to reduce Nr pollution and mitigate its adverse environmental impacts, including air quality degradation and ecosystem harm.