UC San Diego

Low-level cloud microphysical properties are very sensitive to changes in the properties of aerosol, particularly in clean marine regions where small relative number concentration changes are amplified in low background conditions. Aerosol-cloud measurements from Ascension Island, St. Helena, a remote marine site in the tropical South Atlantic, were used to evaluate the effects of clean and smoky marine aerosol on low-level cloud microphysical properties during the 2016-2017 DOE ARM Layered Atlantic Smoke Interactions with Clouds (LASIC) campaign. Measured aerosol size distributions were fit with three modes representing Aitken, accumulation, and sea spray aerosol, with the sea spray mode retrieved from a new technique combining submicron mass size distributions and constraints on supermicron mass from three-wavelength nephelometer scattering using Mie Theory. To investigate cloud supersaturation, the “Hoppel minimum” diameter was retrieved from the modal fits and hygroscopicity estimated from submicron composition. Accumulation-mode aerosol contributed 75% of the cloud condensation nuclei number at supersaturation <0.3%. Three ensemble-based and two parcel-based approaches were employed to further estimate supersaturation using measured aerosol and droplet concentrations and cloud base updrafts across clean and smoky conditions. Ensemble-based supersaturation agreed well with a parcel-based parameterization constrained by the observed accumulation-mode number and updraft. Increases in accumulation-mode number increased droplet number and decreased droplet size and cloud supersaturation, illustrating the supersaturation response to aerosol through competition. Aerosol-related changes in cloud albedo and optical depth showed that aerosol competition can dampen the Twomey effect and its radiative forcing by approximately 20%, resulting in a net local cooling that has strong sensitivity to smoky aerosol perturbations to the clean marine background.