Long-Term Links Between Precipitation Regimes and PM2.5 in an Urban Area of Eastern Amazonia (Belém, Brazil), 1980–2024

Air pollution remains a major global environmental risk, and exposure to fine particulate matter (PM2.5) is associated with adverse health outcomes even at low concentrations. Meteorological conditions influence PM2.5 variability, and precipitation is often expected to reduce particle loads through wet removal. However, humid and wet conditions may coincide with elevated PM2.5 under specific atmospheric and compositional conditions. Here, we investigate long-term relationships between precipitation regimes and PM2.5 concentrations in the Metropolitan Region of Belém (Eastern Amazonia) over the period 1980–2024. We combined PM2.5 from the MERRA-2 reanalysis (including a bias-corrected product) with in situ precipitation records, and classified precipitation conditions using the Standardized Precipitation Index (SPI). We find statistically significant positive long-term tendencies in both precipitation and PM2.5. Stratified analyses show that PM2.5 concentrations are significantly higher under wet conditions, with a weak but significant positive relationship between SPI and PM2.5 (r = 0.23 for the full period; r = 0.24 for the wet class, p-value < 0.01). These findings indicate that increased precipitation in a strong humid tropical urban environment does not necessarily lead to improved air quality. Instead, wet conditions may favor processes such as hygroscopic growth and secondary aerosol formation, contributing to higher PM2.5 concentrations on a monthly scale. Overall, this study highlights the importance of considering precipitation regimes and associated atmospheric processes when assessing air quality in tropical urban environments.