Unexpected deep mixing layer in the Sichuan Basin, China

Abstract In the Sichuan Basin (SCB), one of the four major basins in China, a one-year continuous observation study was performed using a ceilometer in Chengdu (2018.11–2019.12). The results show that the mixing layer height (MLH) in the SCB exhibits a bimodal seasonal variation pattern, with peaks in June (842 m) and October (704 m) and valleys in September (535 m) and January (607 m). Stable atmospheric conditions in September were the main reason for the decrease in MLH. Through comparison to other regions in China, it is found that the seasonal evolution of the MLH in the SCB is similar to that in the Yunnan-Guizhou Plateau region and more pronounced than that in the central and eastern plain regions of China. This indicates that the change in MLH is influenced by both the Western Pacific Subtropical High and South Asian High. Combined with an analysis of the fine-particulate matter concentration, it is found that the main influence factor of the air pollution in the SCB is not the atmospheric dilution capability, and the local contribution should be paid more attention to. The MLH as a meteorological index for air quality prediction varies from place to place. This study is of great importance to the understanding of the mixing layer structure in the basin and its influence on air pollution.