Measurement report: Molecular composition, size distribution, sources and evolution of organic aerosols in the Sichuan Basin, China–Part I: Non-polar organic compounds

Abstract. This study measured non-polar organic compounds (NPOCs) in atmospheric particles in two medium-sized cities (Nanchong (NC) and Yibin (YB)) of China’s Sichuan Basin across summer and winter. Winter PM2.5 concentrations were 3.4 and 3.5 times summer values for NC and YB, respectively, with higher concentrations at YB. Forty-eight NPOCs were identified; their summer concentrations were 1.3 (NC) and 1.6 (YB) times winter levels. The NPOCs were grouped into n-alkanes, PAHs, hopanes, and phthalate esters (PAEs). PAEs dominated NPOCs in summer (71.2 %–74.9 %), while n-alkanes prevailed in winter (69.6 %–76.5 %). High summer temperatures favored high PAEs, whereas elevated anthropogenic emissions coupled with adverse meteorology led to increased wintertime n‑alkane and PAHs concentrations. Vehicular emissions dominated summer PM2.5, accompanied by considerable plastic-related contributions. Secondary inorganic and biomass burning increased in winter, alongside a substantial rise in coal combustion at YB. PAHs, hopanes, and n-alkanes mainly accumulated in fine particles, whereas PAEs were richer in coarse particles. PM2.5-bound PAHs posed low health risks in summer and moderate risks in winter. Vehicular emissions contributed nearly half of summer life-time lung cancer risk (LLCR), while coal combustion and biomass burning determined winter LLCR. With worsening pollution, PAEs dominated with rising n-alkanes during YB summer, and n-alkanes became more dominant in winter. Vehicular emissions showed the largest increase and governed PM2.5 for YB summer pollution, while winter pollution in both cities was driven by combustion and secondary inorganic sources. Except for hopanes at NC, all NPOCs exhibited particle-size coarsening as pollution worsened in winter.