New Insights Into Scavenging Effect of Aerosol Species During Summer Rainfall Process in Beijing

Precipitation is a critical factor in changing aerosol life cycle, yet its impact on aerosol species with different properties in megacities remains unclear. Here we characterized the changes of PM2.5 aerosol species during rainfall processes in five summers (2018-2022) in Beijing using highly time-resolved measurements of aerosol chemical speciation monitor along with precipitation. Average pattern of 233 rainfall processes showed that over 30% decreases in aerosol species before rain start were due to the increased wind speed, and the subsequent decreases were caused by the combined effects of precipitation and winds with an average scavenging of 62%-100% in 1 hr. We also observed very different responses of aerosol species to precipitation depending on intensities, duration and formation mechanisms. During the rainfall processes, aerosol composition showed decreased contributions of organics and sulfate, particularly from late night to morning, while increased contributions of nitrate and chloride due to enhanced gas-particle partitioning associated with the increase of relative humidity and the decrease of temperature. The scavenging rates of aerosol species significantly increased as the increase of rainfall intensity (>5 mm hr(-1)) and duration (>4 hr). However, the scavenging effect of light rainfall was negligible although the cumulative contribution was similar to 50% due to high frequency, and even caused increases in nitrate and chloride. The case analysis of aerosol evolution during weak and heavy rainfall events further illustrated the dual impacts of precipitation on aerosol species through wet scavenging and secondary formation.

Automated summary

The impact of precipitation on different aerosol species in megacities, particularly in Beijing, was investigated using highly time-resolved measurements of aerosol chemical speciation monitor and precipitation. It was found that precipitation processes can have significant effects on aerosol composition, with different responses depending on the intensity, duration, and formation mechanisms of rainfall.