Seasonal Trends and Site Differences of Nitroaromatic Compounds in PM2.5 in Sichuan Basin and Their Effects on Light Absorption of Brown Carbon

Nitroaromatic compounds (NACs) have adverse effects on human health and climate. Daily PM_2.5 samples were collected in winter and summer of 2022 in two cities, Chengdu (CD) and Mianyang (MY), located in Sichuan Basin of southwestern China. Four types of NACs in PM_2.5, containing nitrophenols, nitrocatechols, nitrosalicylic acids, and nitronaphthol, were analyzed. The mean concentration of a total of 10 NACs (ΣNACs) in winter at the suburban MY site (71.7 ± 35.6 ng m^-3) was higher than that in urban CD (29.5 ± 16.2 ng m^-3), while in summer, the mean concentrations of ΣNACs in the two cities were similar, around 2.2 ng m^-3. The much higher concentrations of ΣNACs in winter were attributed to the impact of biomass burning. 4-Nitrocatechol (4NC) was the most abundant species during the sampling period, accounting for 35-56% of ΣNACs mass. In winter, the mean light absorption coefficient of methanol-soluble brown carbon (Abs_365,M) was 10.5 ± 3.4 and 13.6 ± 4.3 Mm^-1 in CD and MY, respectively, which was about 4-7 times that of summer. The contributions of light absorption of ΣNACs at 365 nm to Abs_365,M were 1.6-3.6% in winter and 0.5-0.7% in summer, with 4NC contributing the most to brown carbon among all NACs. The geographical origins of potential sources of NACs at both sites were mainly distributed within the basin.