The effect of anthropogenic sulfate aerosols on marine cloud droplet concentrations

Nonseasalt sulfate (nss SO42-) mass concentrations, cloud condensation nuclei (CCN) number concentrations, and cloud droplet concentrations in warm cumulus and stratocumulus clouds were simultaneously measured in situ in marine air masses on El Yunque peak in Puerto Rico. Our results show that CNN number concentrations (measured at 0.5% supersaturation) and nss SO42- mass concentrations (in the range of ~ 400–1700 ng m-3) are significantly correlated at this site. Droplet concentrations in the cumulus clouds studied do not show a discernible trend with nss SO42- mass concentrations (in the range of ~ 300–1400 ng m-3). In stratocumulus clouds, a small increase in droplet concentration with nss SO42- mass concentrations in the range of ~ 300–1100 ng m-3 was observed. We attribute the low sensitivities of the droplet number concentrations to nss SO42- mass concentrations to the entrainment/mixing processes in these clouds. The magnitudes of the empirically derived sensitivities are considerably lower than those assumed in recent assessments of the effect of anthropogenic sulfate aerosols on cloud albedo.