Contrasting Vertical Circulation between Severe and Light Air Pollution inside a Deep Basin: Results from the Collaborative Experiment of 3D Boundary-Layer Meteorology and Pollution at the Sichuan Basin (BLMP-SCB)

Abstract

Abstract 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.