Quantifying the source and formation of nitrate in PM2.5 using dual isotopes combined with Bayesian mixing model: A case study in an inland city of southeast China

Abstract

Atmospheric nitrate has been attracting increasing attention because it is one of the important components of PM2.5. Understanding the sources and formation mechanism of nitrate in PM2.5 is essential to take effective measures to prevent and control the emission of nitrogen oxides in the atmosphere and reduce the formation of haze. In this study, PM2.5 samples were collected from Ganzhou, an inland city in southeast China, during summer and winter. The concentrations of PM2.5 and NO3− were determined, and the isotopic compositions of NO3− (δ15N–NO3− and δ18O–NO3−) were analyzed in order to quantify the relative contributions of different emission sources and formation pathways of nitrate in PM2.5. The results showed that PM2.5 and NO3− concentrations were lower in summer (39.80 ± 11.10 μg·m−3 and 1.79 ± 0.70 μg·m−3) while higher in winter (69.85 ± 29.58 μg·m−3 and 10.83 ± 9.89 μg·m−3). The values of δ18O–NO3- and δ15N–NO3- ranged from 42.84‰ to 56.80‰ and from −11.17‰ to −2.08‰ in summer, while from 55.86‰ to 78.66‰ and from −10.63‰ to −1.88‰ in winter, respectively. The results of δ15N–NO3− combined with Bayesian isotope mixing model showed that the relative contributions of vehicle exhaust, soil microbial activity, biomass combustion and coal fired power plants were 59.3%, 28.5%, 8.7% and 3.4% in summer, while 65.1%, 20.1%, 10.6% and 4.1% in winter, respectively. The results of δ18O–NO3- combined with Bayesian isotope mixing model showed that the possible relative contributions of pathway 1 (P1) (NO2 + ·OH), pathway 2 (P2) (NO3 + HC) and pathway 3 (P3) (N2O5 + H2O) were 73.5%, 12.4% and 14.1% in summer, while 41.6%, 28.9% and 29.5% in winter, respectively. Moreover, P2 and P3 contributed more when NO3− concentrations were higher, suggesting that P2 and P3 were of significance to the formation of nitrate in PM2.5, especially during winter.