Disparities in precipitation effects on PM2.5 mass concentrations and chemical compositions: Insights from online monitoring data in Chengdu

Abstract

Precipitation plays a pivotal role in wet deposition, significantly affecting aerosol purification. The efficacy of precipitation in removing aerosols depends on its type and the characteristics of the particulates involved. However, further research is necessary to fully understand how precipitation impacts PM_2.5 components. This study utilized high-temporal-resolution data on PM_2.5, its components and meteorological factors to examine varying responses influenced by precipitation intensity and duration. The findings indicate that increased rainfall intensity and duration enhance PM_2.5 and its constituents removal efficiency. Specifically, longer precipitation periods significantly improve PM_2.5 purification, especially with drizzle and light rain. Moreover, there is a direct correlation between pre-precipitation PM_2.5 levels and its scavenging rates, with drizzle potentially exacerbating PM_2.5 pollution under cleaner conditions (≤ 35 µg/m³). Seasonally, the efficacy of removing PM_2.5 components varies notably in response to drizzle and light rain. In spring, higher PM_2.5 levels after drizzle were primarily due to increased organic carbon concentrations favored by higher relative humidity and lower pH conditions compared to other seasons, conducive to secondary organic aerosol production. Lower wind speeds and higher temperatures further contribute to water-soluble organic carbon accumulation. Daytime and nighttime precipitation exerted differing influences on PM_2.5 components, particularly in spring where daytime drizzle and light rain significantly increased PM_2.5 and its constituents, notably NO₃^-, potentially associated with phase distribution changes between gas and aerosol phases in low-temperature, high-RH conditions compared to nighttime. These results propose a dual-impact mechanism of precipitation on PM_2.5 and provide scientific basis for designing effective control strategies.