Distinct impacts of light and heavy precipitation on PM2.5 mass concentration in Beijing

Using hourly observation data of precipitation and M_2.5 over 12 sites in Beijing from 2015 to 2017, this study investigates the characteristics of precipitation and PM_2.5, and emphasizes the impacts of different types of precipitation on PM_2.5 mass concentration. There were totally 91~123 precipitation events annually, with the probability of less than 5 mm precipitation between 69.7% ~ 79.4%. By investigating the differences of PM_2.5 mass concentration between 1 hour after and before the precipitation events, this study finds distinct impacts of different types of precipitation on PM_2.5 mass concentration. For precipitation events with amount of 0.1~0.5 mm, PM_2.5 mass concentration increased with precipitation amount with a rate of 0.85 μg/m³ per 0.1 mm. For precipitation events with amount of 0.5~10 mm, there was no clear relationship between precipitation amount and PM_2.5 mass concentration. For precipitation events with amount larger than 10 mm, PM_2.5 mass concentration decreased with precipitation amount with a rate of 0.17 μg/m³ 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 particle amount also affects the response of PM_2.5 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 dominants when precipitation is light (heavy).