Molecular characteristics and formation mechanisms of biogenic secondary organic aerosols in the mountainous background atmosphere of southern China

Biogenic secondary organic aerosols (BSOA) play a crucial role in atmospheric chemistry, with significant implications for air quality and human health. However, the mechanisms by which anthropogenic emissions influence the formation of BSOA in the real atmosphere have not been fully understood. In this study, fine aerosol (PM2.5) samples were collected during both the wet and dry seasons at a high-altitude (1690 m) background site in the Nanling Mountains. Eleven typical secondary organic aerosols (SOA) tracers derived from isoprene (SOAI), α/β-pinene (SOAM), β-caryophyllene (SOAS), and toluene (SOAA), as well as water-soluble inorganic ions and the ambient concentrations of isoprene and its two primary oxidation products, were measured. The results showed that the concentrations and composition of SOA tracers exhibited significant seasonal variations. The total concentration of SOA tracers in the wet season (159 ± 100 ng m-3) was almost three times higher than that in the dry season (61 ± 68 ng m-3). SOAI dominated in both seasons, followed by SOAM, SOAS, and SOAA. Compared with other studies, the concentrations of SOAI and SOAA in the wet season were relatively high, which was mainly attributed to the rapid oxidation of their precursors due to the higher atmospheric oxidative capacity. C5-alkene triols had a significantly higher proportion in SOAI species than in other mountaintop sites. Temperature was the main factor causing the seasonal difference in the concentration of SOAI. Anthropogenic pollutants significantly promoted the formation of SOAI in the wet season. The influence of anthropogenic pollutants on the reaction pathway of IEPOX was consistent in both seasons, favoring the formation of C5-alkene triols. Our study provides vital insights into the molecular characteristics and formation mechanisms of BSOA in a high-altitude background region. It also highlights that the complex air pollution not only affects BSOA concentrations but also the reaction pathways of biogenic volatile organic compounds (BVOCs) transforming into BSOA.