Studying the impact of precipitation on PM_2.5 mass concentration in different regions can provide an important scientific support for the air quality assessment and forecast as well as pollution prevention in this region. Based on the hourly precipitation observation data and PM_2.5 mass concentration monitoring data in five typical cities (Chengdu, Leshan, Yibin, Mianyang and Dazhou) of Sichuan Basin from 2016 to 2021, the scavenging effect of precipitation processes on PM_2.5 was analyzed in different cities from some aspects including the occurrence time, duration, intensity of precipitation and the initial mass concentration of PM_2.5. The results show that the proportion of positive scavenging processes of precipitation on PM_2.5 increases with the increase of precipitation intensity or initial mass concentration of PM_2.5 in Sichuan Basin, and the scavenging rate rises. Under the condition of air pollution, the scavenging effect of precipitation with intensity exceeding 1 mm·h^-1 on PM_2.5 improves obviously in Sichuan Basin, and the scavenging rate reaches 35.0%. The scavenging effect is positively correlated with the duration of precipitation processes, and the scavenging rate of precipitation processes with the duration more than 3 hours is 9.0%-18.0% higher than that of precipitation processes with the duration less than or equal to 3 hours. The probability of positive scavenging processes is higher in the early morning and afternoon in Sichuan Basin, and the precipitation processes in the early morning have better scavenging effect on PM_2.5. In comparison, the proportion of positive scavenging processes is higher in Leshan and Yibin after the precipitation, and under different initial mass concentrations of PM_2.5, the scavenging rate is significantly higher than that in other cities with the increase of precipitation duration.

Based on the monthly cloud dataset of ISCCP D2 from January 1994 to October 2009, the temporal and spatial distribution characteristics of total cloud amount, cloud water path and cloud optical depth over five regions of China were analyzed in detail. The results are as follows: (1) The most liquid-phase cloud distributed over Sichuan Basin to southeast coast of China, while the most ice-phase cloud distributed over northern China and Tibet Plateau. The coverage of cirrus was the widest, and its cloud amount was the biggest over China, followed by cirrostratus, liquid-phase altostratus, liquid-phase cumulus, and the low cloud with ice-phase was the smallest. (2) The total cloud amount, cloud water path and cloud optical depth of stratocumulus, nimbostratus with liquid-phase and deep convective cloud were bigger, which indicated that the cloud water contents were rich, their contributions were bigger to precipitation over Sichuan Basin to southeast coast. (3) The seasonal variations of total cloud amount with different types were obvious over China, and their performances were different over five regions of China. The total cloud amount of most liquid-phase clouds especially nimbostratus was more in summer and less in winter over northern China and Tibet Plateau, while it was less in summer and more in winter over southwestern China and southeastern China. However, the regional difference of cloud amount with ice-phase was little, and the cloud amount of ice-phase altocumulus and altostratus was more in winter and less in summer over most regions, while that of ice-phase cirrostratus and deep convective cloud was more in summer and less in winter, which indicated that the temperature declining and convection weakenning were benificial to gather of ice-phase clouds and development to medium and low layers in winter, while the temperature increasing and convection strengthening were benificial to gather of liquid-phase clouds and development to high layer in summer. (4) The cloud water path of liquid-phase stratus and nimbostratus had obvious seasonal variations over China, and the regional characteristics were obvious, especially the monthly distribution appearred double peaks pattern over southeastern China, the peaks were in February and November. However, the peak of cloud water path of ice-phase clouds occurred in summer over northern China and winter in southern China.