Isotopic source analysis of nitrogen-containing aerosol: A study of PM_2.5 in Guiyang (SW, China)

The source of fine particulate matter (PM_2.5) has been a longstanding subject of debate, the nitrogen-15 isotope (δ¹⁵N) has been used to identify the major sources of atmospheric nitrogen. In this study, PM_2.5 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_2.5. The results showed an average PM_2.5 of 33.0 μg m^-3 ± 20.0 μg m^-3. The concentration of PM_2.5 was higher in Winter, lower in Summer. The major water resolved inorganic ions (WSIIs) were Ca²⁺, NH₄⁺, Na⁺, SO₄^2-, NO₃^-, Cl^-. Nitrogen-containing aerosols (i.e., NO₃^- and NH₄⁺) suddenly strengthened during the winter, when NO₃^- became the dominant contributor. Over the sampling period, the molar ratio of NH₄⁺/(NO₃^- + 2 × SO₄^2-) ranged from 0.1 to 0.9, thus indicating the full fixation of NH₄⁺ by existing NO₃^- and SO₄^2- in PM_2.5. 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 NO₃^- occurred every season and was most influential during the winter. The total particulate nitrogen (TN) δ¹⁵N value of PM_2.5 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_2.5 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.