The study of air pollution in a valley is a classic research subject. Compared with flat terrain, the formation and development of haze pollution are more complicated and unique within a deep basin. How a basin or valley plays a role in the horizontal and vertical distribution of air pollutants is poorly understood in highly industrialized deep basins in China due to scarce field observations. We conducted a collaborative experiment of three-dimensional (3D) boundary layer meteorology and pollution at the western Sichuan Basin (SCB) close to the Tibetan Plateau (TP). Generally, the concentrations of PM₁ (particulate matter smaller than 1.0 μm), NO, and NO₂ largely decline with elevation, while O₃ shows a slight increasing trend inside the SCB. Three different types of pollutant profiles and the formation mechanisms are described below. The high PM₁ near the surface layer corresponds to the vertical clockwise circulation, i.e., a wind shift with increasing altitude in a clockwise direction. The air pollutants at the central and eastern SCB can be transported to the eastern foothills of the TP by southeasterly winds and then are trapped within the western SCB by a strong surface temperature inversion. The pollutants over the eastern TP also can be dispersed to above the SCB by westerly winds. More aerosol particles are concentrated at about 2.0 km MSL by jointly ascending and descending motion below and above the layer over the valley. The relative uniform PM₁ in the vertical direction correlates to the counterclockwise circulation. The trapped pollutants in the western SCB can be transported to the eastern region by westerly winds and then are dispersed to the upper air by unstable stratification.

Scientific knowledge about light absorption by aerosols is extremely limited at the eastern slope of the Tibetan Plateau (ESTP). We conducted the first aerosol field experiment at six sites (Chengdu, Sanbacun, Wenchuan, Lixian, Maerkang, Hongyuan) along the ESTP, ranging in elevation from 500 to 3500 m. The fraction of light absorption by brown carbon (BrC) to total carbon increases from 20 % to 50 % with altitude, and the mass absorption efficiency (MAE) of BrC over the Tibetan Plateau (TP) is 2–3 times higher than that inside the Sichuan Basin (SCB), especially in winter. In contrast, the MAE of elemental carbon (EC) in winter decreases with altitude. The contrasting variation of EC and BrC MAE with altitude is mainly attributed to source differences between the TP and SCB. Emissions from the more urban sources (motor vehicles, industries, etc.) inside the SCB fail to be transported to the TP due to the stable air in winter inside the basin, which is also favourable for aerosol ageing to enhance absorption efficiency. The radiative forcing of BrC relative to EC varies from 0.10 to 0.42 as altitude increases with the higher organic carbon (OC) to EC ratio over the TP than SCB. Thus, the reason for the enhanced relative BrC to EC radiative forcing from polluted SCB to pristine TP is that the BrC concentration decreases more slowly than the EC concentration with altitude. This study contributes to the understanding of the difference in light absorption by EC and BrC with altitude, from polluted lower-altitude basins to the pristine TP, and provides a data set for regional climate model validation.