Measurement report: Molecular composition, sources, and evolution of atmospheric organic aerosols in a basin city in China

Although organic aerosols (OAs) have important impacts on the climate, environment, and human health, research on OAs in the Sichuan Basin (SCB), one of the heavily polluted areas in China, is still scarce. In this study, samples of particulate matter with an aerodynamic diameter of ≤ 2.5 µm (PM_2.5) were collected during winter 2023 in Chengdu, the capital city of Sichuan Province, and analyzed for organic compounds using gas chromatography–mass spectrometry. The total average concentration of 125 organic compounds was 2013.4 ± 902.4 ng m⁻³ (mean ± standard deviation), and these compounds were dominated by fatty acids (28.9 %), phthalate esters (28.4 %), and anhydrosugars (18.0 %). Anthropogenic sources, such as fossil fuel and biomass burning, were the main sources of aliphatic lipids. Softwood burning was the main source of anhydrosugars. Although both are related to the aging of polycyclic aromatic hydrocarbons (PAHs), oxygenated PAHs and phthalic acids demonstrated different generation mechanisms. The isoprene secondary OA (SOA) tracers were strongly affected by NO_x, relative humidity, and aerosol acidity. Biomass burning was an important source of biogenic SOA tracers. Tracer-based methods revealed that anthropogenic sources (11.6 %), β-caryophyllene (11.0 %), and biomass burning (10.0 %) were important sources of organic carbon (OC). Positive matrix factorization (PMF) analysis demonstrated that secondary formation (22.2 %) was the greatest source of OC, followed by dust (20.4 %), vehicular emissions (17.6 %), plastic-related sources (17.4 %), biomass burning (11.3 %), coal combustion (6.2 %), and primary biogenic emissions (5.0 %). As pollution worsened, the proportions of secondary inorganic species and secondary OC in PM_2.5 increased substantially; PMF analyses indicated that the OC increase was caused mainly by secondary formation and biomass burning. These results are of great value with respect to understanding the characteristics and formation mechanisms of OA as well as the contribution of OA to air pollution in the SCB.