Distinct Impacts of Light and Heavy Precipitation on PM2.5 Mass Concentration in Beijing

Using hourly observation data of precipitation and PM2.5 at 12 sites in Beijing from 2015 to 2017, this study investigates the impacts of different types of precipitation on PM2.5 mass concentration, along with the characteristics of precipitation and PM2.5. There were totally 91-123 precipitation events annually, 69.7-79.4% of which has precipitation amount less than 5 mm. By investigating the differences of PM2.5 mass concentration between 1 hr after and before the precipitation events, this study finds distinct impacts of different types of precipitation on PM2.5 mass concentration. For precipitation events with amount of 0.1-0.5 mm, PM2.5 mass concentration increased with precipitation amount with a rate of 0.85 mu g/m(3) per 0.1 mm. For precipitation events with amount of 0.5-10 mm, there was no clear relationship between precipitation amount and PM2.5 mass concentration. For precipitation events with amount larger than 10 mm, PM2.5 mass concentration decreased with precipitation amount with a rate of 0.17 mu g/m(3) per 1 mm. Further analysis shows that weak precipitation less than 10 mm increased PM10, and heavy precipitation larger than 10 mm decreased PM10. The aerosol amount also affects the response of PM2.5 to precipitation, with weak pollution prone to increase with precipitation and heavy pollution prone to decrease with precipitation. Likely mechanisms are discussed, which include the aerosol hygroscopic growth and gas-particle conversion that increase aerosol amount and precipitation scavenging that decreases aerosol amount. Shortly, the mechanisms that increase (decrease) aerosol amount more probably dominate when precipitation is light (heavy).