Effect of COVID19 Aerosol Reduction on Rainfall in the Western United States

The COVID-19 lockdown in 2020 caused a marked decline in industrial and traffic emissions, which lowered aerosol levels in many regions. This study examined how these changes affected rainfall in the Western United States. Data from 95 meteorological stations and MODIS satellite products were used for March–December 2020 and compared with the same months in 2018, 2019, and 2021. A linear regression model and a rainfall anomaly index were applied to analyze the relation between aerosol optical depth and rainfall. Results show that aerosol optical depth and nitrogen dioxide fell by 15–25% in large urban areas, while rainfall changes were small and uneven. Valleys with warm-rain processes recorded increases of up to 5%, while mountain regions with mixed-phase clouds showed changes within ±3%. Most aerosol–rainfall relations were weak and not significant at the 95% level. These results indicate that reduced aerosols may slightly increase warm-rain efficiency, but circulation and terrain remain the main drivers. The lockdown acted as a short-term case to study air quality and rainfall, and the findings are useful for climate studies, water management, and rainfall forecasting.