Isotopic source analysis of nitrogen-containing aerosol: A study of PM in Guiyang (SW, China).

The source of fine particulate matter (PM) has been a longstanding subject of debate, the nitrogen-15 isotope (δN) has been used to identify the major sources of atmospheric nitrogen. In this study, PM samples (n = 361) were collected from September 2017 to August 2018 in the urban area of Guiyang (SW, China), to investigate the chemical composition and potential sources of PM. The results showed an average PM of 33.0 μg m ± 20.0 μg m. The concentration of PM was higher in Winter, lower in Summer. The major water resolved inorganic ions (WSIIs) were Ca, NH, Na, SO, NO, Cl. Nitrogen-containing aerosols (i.e., NO and NH) suddenly strengthened during the winter, when NO became the dominant contributor. Over the sampling period, the molar ratio of NH/(NO + 2 × SO) ranged from 0.1 to 0.9, thus indicating the full fixation of NH by existing NO and SO in PM. The annual value of NOR was 0.1 while rised to 0.5 in Winter. The variations of NOR (Nitrogen oxidation ratio) (0.1-0.5) values suggest that the secondary formation of NO occurred every season and was most influential during the winter. The total particulate nitrogen (TN) δN value of PM ranged from -5.9‰ to 25.3‰ over the year with annual mean of +11.8‰ ± 4.7‰, whereas it was between -5.9‰ and 14.3‰ during the winter with mean of 7.0‰ ± 3.8‰. A Bayesian isotope mixing model (Stable Isotope Analysis in R; SIAR) was applied to analyze the nitrogen sources. The modeling results showed that 29%, 21%, and 40% of TN in PM during the winter in Guiyang was due to nitrogen-emissions from coal combustion, vehicle exhausts, and biomass burning, respectively. Our results demonstrate that biomass burning was the main contributor to PM during the winter, 80% of the air mass comes from rural areas of Guizhou border, this transport process can increase the risk of particulate pollution in Guiyang.