Isotopic partitioning of nitrogen in PM 2.5 at Beijing and a background site of China

Abstract. Using isotope mixing model (IsoSource) and natural δ 15 N 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, δ 15 N values of PM 2.5 (−4.1 – +13.5 ‰; mean = +2.8 ± 6.4 ‰) distributed within the range reported for major anthropogenic sources (including NH 3 and NO 2 from coal combustion, vehicle exhausts and domestic wastes/sewage). However, δ 15 N 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 NH 3 and NO 2 from biomass burning, animal wastes, soil N cycle, fertilizer application, and organic N of soil dust). Evidences from molecular ratios of NH 4+ to NO 3− and/or SO 4 2− in PM 2.5 , NH 3 to NO 2 and/or SO 2 in ambient atmosphere suggested that the equilibrium of NH 3 ↔ NH 4 + caused apparent 15 N enrichment only in NH 4 + of PM 2.5 at the background site due to more abundant NH 3 than SO 2 and NO 2 . Therefore, a net 15 N enrichment (33 ‰) was assumed for NH 3 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 δ 15 N signals in PM 2.5 . Emissions of NO 2 from coal combustion and NH 3 from urban transportation should be strictly controlled to advert the risk of haze episodes in Beijing.