Isotopic partitioning of nitrogen in PM_2.5 at Beijing and a background site of China

Abstract. Using isotope mixing model (IsoSource) and natural δ¹⁵N method, this study evaluated contributions of major sources to N of PM_2.5 at Beijing (collected during a severe haze episode of January 22nd–30th, 2013) and a background site (Menyuan, Qinghai province; collected from September to October of 2013) of China. At Beijing, δ¹⁵N values of PM_2.5 (−4.1 – +13.5 ‰; mean = +2.8 ± 6.4 ‰) distributed within the range reported for major anthropogenic sources (including NH₃ and NO₂ from coal combustion, vehicle exhausts and domestic wastes/sewage). However, δ¹⁵N values of PM_2.5 at the background site (+8.0 – +27.9 ‰; mean = +18.5 ± 5.8 ‰) were significantly higher than that of potential sources (including NH₃ and NO₂ from biomass burning, animal wastes, soil N cycle, fertilizer application, and organic N of soil dust). Evidences from molecular ratios of NH₄₊ to NO₃₋ and/or SO₄²⁻ in PM_2.5, NH₃ to NO2 and/or SO₂ in ambient atmosphere suggested that the equilibrium of NH₃ ↔ NH₄ + caused apparent ¹⁵N enrichment only in NH₄ + of PM_2.5 at the background site due to more abundant NH₃ than SO₂ and NO₂. Therefore, a net ¹⁵N enrichment (33 ‰) was assumed for NH₃ sources of background PM_2.5 when fractional contributions were estimated by IsoSource model. Results showed that 41 %, 30 % and 14 % of N in PM_2.5 of Beijing originated from coal combustion, vehicle exhausts and domestic wastes/sewage, respectively. Background PM_2.5 derived N mainly from biomass burning (58 %), animal wastes (15 %) and fertilizer application (9 %). These results revealed the regulation of the stoichiometry between ammonia and acidic gases on δ¹⁵N signals in PM_2.5. Emissions of NO₂ from coal combustion and NH₃ from urban transportation should be strictly controlled to advert the risk of haze episodes in Beijing.