Source appointment of nitrogen in PM2.5 based on bulk δ15N signatures and a Bayesian isotope mixing model

Abstract

Nitrogen isotope (δ15NN) has been employed to differentiate major sources of atmospheric N. However, it remains a challenge to quantify contributions of multiple sources based on δ15NN values of the N mixture in atmospheric samples. This study measured δ15NN of bulk N in PM2.5 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, δ15NN values of PM2.5 (-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, δ15NN values of PM2.5 (+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 NH+4 to NO-3 and/or SO2-4 in PM2.5 at the background site suggested that the equilibrium of NH3 ↔ NH+4 caused apparent 15N enrichments in ammonium. Results of the SIAR model showed that 39 and 32% of bulk N in PM2.5 of Beijing site were contributed from N emissions of coal combustion and vehicle exhausts, respectively, whereas N in PM2.5 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 δ15NN signatures of PM2.5 and emissions of reactive N from coal combustion and urban transportation should be strictly controlled to advert the risk of haze episodes in Beijing.