Unveiling the dual role of rainfall in governing ambient aerosol particles:From washout-dominated scavenging to production dynamics

Wet scavenging is a critical determinant of aerosol–precipitation interactions, yet its net effect on atmospheric aerosol burdens remains poorly constrained globally. Here, we analyzed 146 rainfall events in total based onfield observations in a subtropical near coastal site, revealing that light rainfall has paradoxically amplified aerosol burdens by +6.1 % averagely, with approximate +40 % for hydrophobic species (e.g., HOA/COA) and +5 %–19 % for secondary hygroscopic components (e.g., sulfate, nitrate, OOA). This contradicts conventional knowledge of light rain wet scavenging, indicating critical flaws in model parameterizations of wet scavenging efficiency. Our study also illustrates a pollution-dependent wet scavenging mechanism and shows efficient removal of aerosol particles under polluted conditions (PM_2.5 >30 μg m⁻³), whereas in very clean atmosphere (PM_2.5 <10 μg m⁻³), precipitation elevates aerosol concentrations by +13 %–100 %, implying that precipitation may shift from a clearing sink to a source of aerosols with the continuous improvement of air pollution in future. Furthermore, we identify a nonlinear size-dependent scavenging threshold of ∼100 nm, where the precipitation shows a transition from production to scavenging of fine aerosols; the 20–30 nm particles show peak increases (+23 %–41 %), while particles of around 600 nm exhibit maximum scavenging (−20 %), yet scavenging declines unexpectedly for larger particles (e.g. >1 μm). These findings highlight rainfall's dual role in aerosols removal vs. enhancement, offering critical constraints for improving air quality forecast and refining model simulations of aerosol-cloud-precipitation interactions.