Competing effects of rainfall on atmospheric pollutant levels in an urban semi-arid environment
Grace Betito1, Avelino Arellano1,2, Ali Behrangi1, Ellis Robinson2, and Armin Sorooshian1,2
1Department of Hydrology and Atmospheric Sciences, University of Arizona, Tucson, AZ, USA
2Department of Chemical and Environmental Engineering, University of Arizona, Tucson, AZ, USA
Abstract
Rainfall is an important pathway for removing atmospheric pollutants, but its effectiveness depends on rainfall characteristics and background conditions. In semi-arid regions, pollutant concentration responses to rainfall remain poorly studied, particularly for surface ozone. Here, we examine how rainfall affects surface levels of PM2.5 and ozone (O3), and column-integrated aerosol properties in Tucson, Arizona. Using hourly data of PM2.5, O3, and AERONET aerosol optical depth (AOD) and Angstrom exponent (AE) from 2015–2023, we quantify relative changes before and after individual rainfall events as a function of rainfall duration, intensity, and pre-rain pollutant levels. Results show that rainfall does not uniformly reduce pollutant concentrations. When pre-rain PM2.5 is low (< 5.0 µg m–3) and O3 is below 40 ppb, post-rain increases are frequently observed. In contrast, elevated pre-rain concentrations are strongly associated with substantial reductions, with median PM2.5 decreases of approximately 30–40% are observed across both short (≤ 1 hour) and long (> 1 hour) duration events, while O3 decreases by roughly 20–30%. Higher rainfall intensity further enhances removal. Column aerosol properties exhibit similar pre-rain loading dependence, though responses are more variable due to sampling limitations. Overall, pollutant removal in this semi-arid environment is highly conditional, with background pollutant levels exhibiting more influence on removal efficiency than rainfall characteristics alone.