Source appointment of nitrogen in PM_2.5 based on bulk δ¹⁵N signatures and a Bayesian isotope mixing model

Nitrogen isotope (δ¹⁵N) has been employed to differentiate major sources of atmospheric N. However, it remains a challenge to quantify contributions of multiple sources based on δ¹⁵N values of the N mixture in atmospheric samples. This study measured δ¹⁵N of bulk N in PM_2.5 at an urban site of Beijing during a severe haze episode of 22–30 January 2013 and a background site of Qinghai, north-western China from 6 September to 15 October 2013, then applied a Bayesian isotope mixing model (SIAR, Stable Isotope Analysis in R) to analyse the N sources. At Beijing site, δ¹⁵N values of PM_2.5 (−4.1‰ to +13.5‰, +2.8 ± 6.4‰) were distributed within the range of major anthropogenic sources (including NH₃ and NO₂ from coal combustion, vehicle exhausts and domestic wastes/sewage). At Menyuan site, δ¹⁵N values of PM_2.5 (+8.0‰ to +27.9‰, +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 dust N). High molar ratios of NH₄⁺ to NO₃^- and/or SO₄^2- in PM_2.5 at the background site suggested that the equilibrium of NH₃ ↔ NH₄⁺ caused apparent ¹⁵N enrichments in ammonium. Results of the SIAR model showed that 39 and 32% of bulk N in PM_2.5 of Beijing site were contributed from N emissions of coal combustion and vehicle exhausts, respectively, whereas N in PM_2.5 at Menyuan site was derived mainly from N emissions of biomass burning (46%) and NH₃ volatilization (34%). These results revealed that the stoichiometry between NH₃ and acidic gases plays an important role in controlling the bulk δ¹⁵N signatures of PM_2.5 and emissions of reactive N from coal combustion and urban transportation should be strictly controlled to advert the risk of haze episodes in Beijing.