Interactions between rainfall and fine particulate matter investigated by simultaneous chemical composition measurements in downtown Beijing

Abstract

Rainfall can directly remove atmospheric pollutants through the below-cloud scavenging process, and the atmospheric particles also affect the chemical composition of rainfall. Therefore, the interactions between rainfall properties and fine particulate matter (PM2.5) need deep investigation. Based on simultaneous in situ measurements of PM2.5 and rainfall samples during May–October in 2017 and 2018, their chemical composition dynamics and the relationship were identified. It was found that NO3− and SO42− were predominant in PM2.5, whereas NO3− and Ca2+ were the main ionic species in rainfall. The characteristics of ions in rainfall and PM2.5 showed that Beijing was heavily affected by mobile sources and by anthropogenic pollution. The PM2.5 was most effectively removed from the atmosphere by rainfall and the responses of the ionic compositions of PM2.5 to the washing effect were much different. Simultaneously, PM2.5 posed a significant impact on Ca2+ and K+ in rainwater. The effect of PM2.5 1 day before rain on the Ca2+ in rainwater was more prominent than the effect of the chemical species in PM2.5 on the day of the rainfall, while the trend was the opposite for K+. The key factors affecting the characteristics of ions in PM2.5 and rainfall were rainfall amount, duration and relative humidity. The wind direction also have big impacts on NH4+. The findings provide more scientific supports the rainfall pollution and PM2.5 management in the urban.