Influence of anthropogenic pollution on the molecular composition of organic aerosols over a forest site in the Qinling Mountains region of central China

Abstract. Biogenic organic aerosols interacting with anthropogenic pollutants lead to large uncertainties in aerosol properties and impacts, yet the underlying mechanisms remain to be fully elucidated. To explore the ­anthropogenic–biogenic interactions in the Qinling Mountains region of central China, we investigated the molecular composition of organic aerosols in atmospheric PM_2.5 at a forest site in summer and winter of 2021/2022, using ultrahigh performance liquid chromatography coupled with Orbitrap mass spectrometry. Organic species were more abundant and chemically diverse in winter compared with those in summer, as revealed by their higher numbers and peak area intensities. The molecular characteristics of organic species exhibited distinct seasonal variabilities, with higher peak-area-weighted mean values of molecular weight and oxidation state but lower unsaturation degree in summer, possibly associated with more biogenic emissions and intense photochemical processes. A variety of organic tracer species were identified in the two seasons, among which the biogenic ones were relatively more abundant in summer, contrasting with the substantial increase of anthropogenic ones in winter. A higher ambient relative humidity, except for heavy precipitation, usually promoted the production of nitrogen- and sulfur-containing organic species by involving more anthropogenic pollutants. The synergistic effects of meteorology and anthropogenic pollution greatly affected the organic aerosol production in this forest atmosphere, thereby altering their molecular composition and related properties under different environmental conditions. The combined set of results herein provides direct evidence for the anthropogenic perturbations on air quality, atmospheric chemistry, and associated climate impacts in the Qinling Mountains region.