[Analysis of Formation Processes and Sources of PM_2.5 Ammonium During Winter and Summer in Suburban Area of the Yangtze River Delta]

As the main pollutants of secondary inorganic aerosols(SIAs) in fine particulate matter(PM_2.5), aerosol ammonium(p-NH₄⁺) plays a significant role in the formation of haze. However, the contribution ratio of each potential source of atmospheric NH_x(p-NH₄⁺ and NH₃) still remains controversial. In this study, 3 h high-frequency PM_2.5 samples were collected in Dongshan, Suzhou during winter and summer in 2015, respectively. Meanwhile, we determined concentrations and δ¹⁵N isotope ratios of total nitrogen(TN) and p-NH₄⁺ and quantitatively analyzed formation processes and sources of p-NH₄⁺ based on the Bayesian mixing model(SIAR). SO₄^2-, NO₃^-, and NH₄⁺ were the main water-soluble ions(WSIs) both in winter and summer, accounting for more than 70% in general. The concentration change trends of PM_2.5, TN, and p-NH₄⁺ were consistent, and the concentrations in winter were 2-3 times those in summer. The δ¹⁵N-NH₄⁺ value was in direct proportion to the p-NH₄⁺ concentration both in winter and summer. However, δ¹⁵N-NH₄⁺ values in winter(-1.7‰±4.3‰) were lower than those in summer(7.8‰±5.9‰). This indicated that the differences in δ¹⁵N-NH₄⁺ were caused by different contribution rates of each potential source within each season, whereas it was mainly led by nitrogen isotope fractionation during ammonium-ammonia gas particle distribution in different seasons. The SIAR model calculated that non-agricultural sources were the dominant source of p-NH₄⁺ in Dongshan, with the contribution rate of 59% in winter and 69% in summer, which indicated that NH₃ emitted by fossil fuel combustion more easily formed particle aerosol ammonium than that volatilized from agricultural sources.