Effects of Atmospheric Surfactants on the Microphysics of Individual Cloud Droplets

Abstract:

Clouds contribute one of the largest uncertainties in estimates of the Earth’s changing energy budget. Current findings suggest that organics and other surfactants in cloud droplets can affect the rate at which water condenses onto and evaporates from droplets. Varying the concentrations and properties of surfactants in individual droplets may alter the sign and magnitude of these rates, which are necessary to understand in order to accurately represent the microphysical properties of clouds in models. Surfactants extracted from atmospheric aerosol particles can considerably reduce the surface tension of water, making them important factors in cloud droplet growth that were until recently considered to be negligible. The concentrations and composition of surfactants and their abilities to depress surface tension varies in atmospheric samples. Using anthropogenic and biological surfactants extracted from atmospheric samples and other compounds with similar compositions, we create model cloud droplets in the laboratory. We built an aerosol optical trap combined with Raman spectroscopy to hold and study the evaporation of water from individual cloud droplets. In this study, we have observed competing effects of surfactants in single water droplets, both reducing the surface tension of the overall droplet allowing water to condense onto the droplets and also creating a coating or barrier that impedes evaporation.