Linking ATTO Black Carbon to Rainfall Patterns in the South America-Atlantic Tropical Zone

During the rainy season, black carbon (BC) particles exhibit strong variability in concentration. At the Amazon Tall Tower Observatory (ATTO), located in central Amazonia, elevated BC concentrations have been previously identified as originating from the African continent. However, BC mass concentrations approach zero during certain periods, characterizing pristine episodes. This study aims to identify the primary factors influencing BC concentration in the Amazon. The first analytical approach involved evaluating air mass back trajectories during episodes of high BC concentration (BC > 0.46 µg/m³, with the day of maximum concentration selected from neighboring days) and low BC concentration (BC < 0.08 µg/m³, with the day of minimum concentration chosen). Ensemble back trajectories, analyzed across multiple atmospheric levels, revealed minimal differences between the air trajectories associated with these two contrasting scenarios. The second approach examined accumulated rainfall at ATTO during the three days preceding the selected high- and low-concentration days. The results indicate that precipitation plays a dominant role in modulating BC concentrations. A histogram of precipitation data revealed two distinct patterns: one corresponding to high rainfall during pristine events and another to low or negligible rainfall during more polluted days. Using ERA-5 reanalysis data, this precipitation variability was observed to extend across the Intertropical Convergence Zone (ITCZ) over the Atlantic. Simulations were conducted using the ECHAM/MESSy Atmospheric Chemistry (EMAC) model to investigate this phenomenon further. The simulations demonstrated that rainfall variability influences the transport from Africa to the Amazonas of particles such as BC, dust, and gases, including CO₂ and O₃. Composite analyses of hemispheric synoptic patterns were performed by selecting days with high and low BC concentrations from January–February from 2015 to 2022. These composites revealed that the variability is driven by oscillations in the western hemisphere synoptic patterns linked to the positioning of cold fronts in both hemispheres. This variability has significant implications for transporting vital nutrients to the Amazon rainforest. Understanding the relationship between rainfall, synoptic patterns, and BC transport is crucial, particularly in the context of climate change, which could alter these patterns and profoundly impact the ecological systems of the Amazon basin.

This study was supported by FAPESP 2022/07974-0