Aerosol-Limited Effects on Cloud Microphysical Properties in Clean Conditions in the Tropical South Atlantic Boundary Layer

Aerosol and cloud 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) and the effective radius of droplets at cloud base. Accumulation-mode particles accounted for ~70% of CCN at supersaturations <0.3% in clean and smoky conditions. For particle concentrations <400 cm^-3, the Hoppel minimum diameter dependence on accumulation-mode number concentration indicated an aerosol-limited regime controlling the lower diameter cutoff for which particles serve as CCN. At higher concentrations, the contributions of Aitken particles increase but do not change the activation of accumulation-mode particles or the cloud critical diameter. The droplet effective radius at cloud base showed negative correlation with the number concentration of accumulation mode particles in clean conditions for optically thin single-layer clouds (50-250 g m^-2 liquid water path). These results provide the first multi-month in situ quantitative constraints on the contributions of aerosol number size distributions to CCN and their impacts on aerosol indirect effects in the clean tropical South Atlantic boundary layer.