Urban pollution effects on warm-cloud microphysics in the central Amazon Basin: A WRF-Chem and GoAmazon2014/5 analysis

The impact of emissions from large urban centers on atmospheric chemistry and regional climate is a central issue in atmospheric sciences. This study uses WRF-Chem and GoAmazon2014/5 data to assess how urban and forest emissions interact to influence cloud properties in the Amazon. The region around Manaus, in the central Amazon, provides a unique environment for this study worldwide. During the wet season, clean air masses transported by trade winds interact with urban emissions, altering the atmosphere’s chemical composition and impacting clouds’ microphysical and radiative properties. The interaction between urban emissions from Manaus and emissions from the Amazon rainforest modulates aerosol concentration and size. Polluted clouds exhibit a smaller effective radius (De), while liquid water content (LWC) and cloud droplet number concentration (DNC) are higher. In background areas, the DNC is approximately 50 cm -3 and exceeds 150 cm -3 under highly polluted conditions. The higher LWC observed in polluted clouds is associated with the increased DNC, while the smaller De may delay the onset of rainfall. The amount of water vapor available in the atmosphere and the level of supersaturation determine how urban emissions influence the microphysical processes of cloud formation. Under low LWC conditions, warm precipitation is suppressed, while for high LWC (above 0.75 g m -3 ), the De increases until reaching levels associated with precipitation. These results highlight that the spatial and temporal variation in CCN concentration, modulated by the interaction between urban emissions and those from the tropical rainforest, plays a significant role in cloud microphysical processes in the central Amazon. • - Urban-biogenic interactions increase CCN and suppress warm rain over the Amazon Rainforest. • - High CCN reduces droplet size and delays precipitation despite higher LWC. • - CCN rises ∼300 cm -3 from aging plume due to urban-biogenic chemical interaction. • - Elevated CCN enhances DNC but limits De, affecting convective cloud development. • - CCN variability explains shifts in cloud lifetime and regional rainfall patterns.