Altitudinal and seasonal differences in PM2.5 sources, secondary formation, and particle mixing states in the basin-plateau transition zone of Southwest China.

To explore the evolutionary characteristics of atmospheric pollution in the transition zone from basins to plateaus in southwest China, this study analyzed the chemical composition and sources of PM2.5 during summer and winter, as well as the wintertime evolution of single particles, in two cities at different altitudes in this area, i.e., Chengdu (500 m a.s.l.) and Kangding (2500 m a.s.l.). We found that the PM2.5 concentrations were higher in Chengdu (winter: 79.4 ± 27.7 μg m‒3, summer: 29.1 ± 10.2 μg m‒3) than in Kangding (winter: 15.2 ± 5.1 μg m‒3, summer: 8.1 ± 2.4 μg m‒3), whereas water-soluble inorganic ions (WSIIs) were more important in Chengdu and carbonaceous components contributed more substantially in Kangding. Seasonal ion patterns showed a shift from SO42- in summer to NO3- in winter, with stronger secondary inorganic formation in Chengdu. The positive matrix factorization analysis showed that the PM2.5 in Chengdu was comprehensively influenced by combustion (26.0%), traffic-related (32.4%), and secondary (35.3%) sources, while Kangding was dominated by combustion emissions (47.9%). Wintertime single-particle analysis further revealed contrasting particle-scale mixing characteristics between the two cities: Chengdu (29.0%) showed a higher number fraction of secondary inorganic aerosol (SIA) particles than Kangding (17.6%), whereas Kangding had a higher organic matter (OM)-SIA to SIA particles (OM-SIA/SIA) ratio (1.15) than Chengdu (0.57), suggesting that secondary inorganic species in Kangding were more frequently internally mixed or associated with organic-rich particles. These findings suggested that differences in emissions, meteorology, and atmospheric processing between basin and plateau transition environments influenced not only PM2.5 mass and composition, but also the particle-scale mixing states of aerosol.