Enclosed-terrain urbanized areas require vertically stratified air quality management: Insights from particulate matter vertical observations in China's Sichuan Basin.

Urbanized basins are widely recognized as hotspots‌ of particulate matter (PM) pollution. Characterizing the vertical stratification of PM in these regions is essential to elucidate the influence of regional air pollutant transport, dynamics of planetary boundary layer, and pollution-meteorology feedbacks. Using Mount Emei (500-3100 m a.s.l.) as a natural observation tower, this study investigated the composition and sources of PM, ‌along with‌ the six criteria pollutants (PM, PM, O, SO, CO, and NO) across three atmospheric layers within the Sichuan Basin (SCB). The monitoring sites are located in the planetary boundary layer (M-base: 550 m a.s.l.), ‌the‌ cloud/fog-active layer (M-upper: 2400 m a.s.l.), and ‌the‌ free troposphere (M-summit: 3100 m a.s.l.). The results revealed that PM at M-base exhibited typical urban pollution characteristics, with SNA (i.e., SO, NO, and NH) accounting for ∼60 % of total suspended particles (TSP) mass annually, ‌primarily driven‌ by anthropogenic emissions. Autumn/wintertime PM extremes at M-base resulted from the synergistic effects of increased emissions, stagnant meteorology, and enhanced aqueous-phase reactions under high relative humidity (>80 %)‌. As altitude increased, PM concentrations declined, primarily due to more effective wet scavenging and reduced anthropogenic influence. At M-upper and M-summit, the contributions of SNA reduced to ∼35 % of TSP mass annually, ‌whereas‌ contributions from biomass burning and crustal dust increased. ‌Unlike M-base‌, PM concentrations at M-summit peaked in spring, primarily due to transboundary influences, particularly the biomass burning in neighbouring countries and ‌dust from Gobi/desert in northwestern China‌. Overall, the deep-basin topography imposed three regulatory mechanisms‌ on PM pollution, including ground-level PM enhancement through chemical-meteorological interactions‌, ‌upper-altitude cloud/fog/precipitation and PM interactions, and dominance of transboundary pollutant transport in the upper/free-troposphere‌. This study provides a mechanistic basis for vertically stratified air quality management in the SCB, ‌as well as‌ other similar terrain regions worldwide.