Impact of the Manaus Plume on the Amazon Green Ocean Atmosphere: Aerosol-Cloud Interaction during the Wet Season with fully coupled online chemistry in the WRF model
Authors: Herdies Dirceu , Reis Andre , Nascimento Janaina , Vara-Vela Angel 
Published: March 2024

In addition to dynamic and thermodynamic processes, aerosols' chemical and aerodynamic characteristics play a significant role in cloud microphysics and convective development. The region surrounding Manaus, located in the Northern region of Brazil, constitutes a unique environment worldwide for studying the impact of anthropogenic and biogenic emissions on aerosol concentration and, consequently, cloud microphysics. This study investigates chemical-aerosol-cloud interactions during the wet season, employing the Weather Research and Forecasting model with coupled chemistry (WRF-Chem) version 3.9.1.1 and observed data from the GoAmazon2014/5 experiment. The model was configured with a 3 km grid, utilizing the Regional Atmospheric Chemistry Mechanism (RACM) for gas-phase chemistry, and adopting the Modal Aerosol Dynamics Model for Europe (MADE) with parameterization for Secondary Organic Aerosol (SOA) production based on the Volatility Basis Set (VBS) approach for aerosol treatment. The Abdul-Razzak and Ghan option was employed to relate aerosol properties to the 2-moment Morrison cloud microphysics parameterization. The model is initialized with ERA5 and Copernicus Atmosphere Monitoring Service (CAMS) data, incorporating anthropogenic emissions from a regional inventory and biogenic emissions using the Model of Emissions of Gases and Aerosols from Nature (MEGAN). Results indicate that the model effectively represents meteorology and chemistry in the Manaus region. The fully coupled model successfully reproduces plume dispersion and aging. The aerosol concentration peak in the accumulation mode is observed approximately 100 km from Manaus, returning to background concentrations beyond 300 km. The increase in aerosol concentrations is associated with the formation of biogenic and anthropogenic Secondary Organic Aerosol (SOA) and sulfate-derived aerosols with mass peaks at 4, 100, and 1.4 times the background concentration. This aerosol concentration increase significantly correlates with Cloud Condensation Nuclei (CCN) concentration at 0.5% supersaturation. Despite the elevated aerosol concentration in the plume and higher CCN concentration, the CCN/aerosol ratio decreases to 0.02, in contrast to 0.3 in the background region. The distinct chemical and aerodynamic characteristics of aerosols in the background and urban plume regions modulate the Droplet Number Concentration (DNC), Liquid Water Content (LWC), and Effective Radius (Re). Clouds in the plume exhibit higher DNC and LWC, and lower Re. Approximately 40% of clouds in the plume have LWC above 2.5 g/m&#179;, compared to only 10% in background regions. The average Re is 8 and 13 &#956;m in the plume and background regions, respectively. Sensitivity simulations also show that both anthropogenic and biogenic emissions influence cloud processes in the Amazon region. Our results suggest that a more accurate representation of aerosols, often simplified in numerical models, is necessary for enhanced weather and climate modeling.