Rainfall scavenging of PM2.5 chemical components in Nanjing, China

Atmospheric aerosols, particularly PM 2.5 , profoundly influence air quality, human health, and climate, especially in industrial regions. Nanjing, as a representative industrial megacity in eastern China, is still influenced by heavy air pollution from time to time, although PM 2.5 has been decreasing remarkably in the last decade. There is still lack of a full understanding how PM 2.5 is accumulated in the air and how the individual chemical components of PM 2.5 respond to rainfall scavenging. This study investigates the wet scavenging effects of precipitation on PM 2.5 and its chemical components, utilizing high-resolution data collected from December 2020 to November 2021 in Nanjing. A total of 140 precipitation events fall into categories of weak (0–0.5 mm), moderate (0.5–10 mm), or strong (>10 mm). Rainy days reduce PM 2.5 mass concentrations by 19% compared to non-rainy days, with secondary organics (29%) showing the greatest impacts. Weak precipitation often elevates concentrations through aerosol secondary formation in the liquid phase, whereas moderate-to-heavy rain boosts scavenging efficiency, though this diminishes under heavy rain due to influences like wind and relative humidity. Secondary organics, chloride, and black carbon exhibit high removal rates in heavy rain, while secondary inorganic aerosols prove less sensitive owing to enhanced formation in humid conditions. A case study of 37-h heavy rain event (159.9 mm) demonstrates rapid initial scavenging followed by PM 2.5 mass concentration rebounds. These findings illuminate the intricate balance between wet removal, secondary formation, and meteorological factors, offering valuable insights for air quality management and atmospheric modeling in subtropical monsoon regions.