Nitrogen isotope (delta N-15) has been employed to differentiate major sources of atmospheric N. However, it remains a challenge to quantify contributions of multiple sources based on delta N-15 values of the N mixture in atmospheric samples. This study measured delta N-15 of bulk N in PM 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, delta N-15 values of PM2.5, (-4.1 parts per thousand to +13.5 parts per thousand, +2.8 +/- 6.4 parts per thousand) 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, delta N-15 values of PM2.5 (+8.0 parts per thousand to +27.9 parts per thousand, +18.5 +/- 5.8 parts per thousand) 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 SO42- in PM2.5 at the background site suggested that the equilibrium of NH3 <-> NH4+ caused apparent N-15 enrichments in ammonium. Results of the SIAR model showed that 39 and 32% of bulk N in PM, 5 of Beijing site were contributed from N emissions of coal combustion and vehicle exhausts, respectively, whereas N in PM,, 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 delta N-15 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.