Smoke is the Dominant Cloud-Nucleating Aerosol for Boundary Layer Clouds in the Wintertime Western Tropical North Atlantic

Aerosol size and chemistry significantly impact the formation of cloud droplets is important for understanding aerosol-cloud interactions, particularly in the tropical North Atlantic marine boundary layer where marine cumulus clouds obscure the low-albedo ocean surface. Previous research linking aerosol size, composition, and water uptake properties in the tropical North Atlantic have focused mainly on dust and have been conducted primarily during the boreal summer, when African dust transport to the western Atlantic region is at its peak. In this study, we focus on the boreal winter, and compare size-resolved aerosol composition to water uptake properties derived from size-resolved cloud condensation nuclei (CCN) measurements collected at the Barbados Atmospheric Chemistry Observatory at Ragged Point, Barbados during two field campaigns: Elucidating the Role of Clouds-Circulation Coupling in Climate (EUREC⁴A) and Atlantic Tradewind Ocean-Atmosphere Mesoscale Interaction Campaign (ATOMIC). Observations collected from January through February 2020 revealed unexpected wintertime long-range transport of African smoke and dust to Barbados. Upon the arrival of smoke and dust particles, accumulation-mode aerosol particles increased, altering water uptake properties and number concentrations of accumulation-mode CCN. This indicates that the long-range transported aerosols were affecting cloud droplet formation. Size-resolved chemical analysis revealed that the accumulation-mode particle loading was primarily composed of smoke, suggesting smoke, not dust, was primarily responsible for the observed increase in CCN number concentrations. These findings indicate the importance of African smoke particles in aerosol-cloud interactions, and further indicates a need for greater consideration of the impacts of smoke in the tropical marine boundary layer.