Aerosol-Correlated Cloud Activation for Clean Conditions in the Tropical Atlantic Boundary Layer During LASIC

Aerosol measurements during the DOE ARM Layered Atlantic Smoke Interactions with Clouds (LASIC) campaign were used to quantify the differences between clean and smoky cloud condensation nuclei (CCN) budgets. Accumulation -mode particles accounted for similar to 70% of CCN at supersaturations <0.3% in clean and smoky conditions. Aitken-mode particles contributed <20% and sea-spray-mode particles <10% at supersaturations <0.3%, but at supersaturations >0.3% Aitken particles contributions increased to 30%-40% of clean CCN. For clean conditions, the Hoppel minimum diameter was correlated to the accumulation -mode number concentration, indicating aerosol-correlated cloud activation was controlling the lower diameter cutoff for which particles serve as CCN. For smoky conditions, the contributions of Aitken particles increase and the correlation of cloud activation to accumulation -mode particles is masked by the lower-hygroscopicity smoke. These results provide the first multi-month in situ quantitative constraints on the role of aerosol number size distributions in controlling cloud activation in the tropical Atlantic boundary layer.