Cloud albedo enhancement by surface-active organic solutes in growing droplets

Understanding the properties of clouds in the Earth's atmosphere is current ly limited by difficulties at the fundamental level of adequately describin g the processes of cloud droplet nucleation and growth. Small changes in dr oplet population may significantly influence cloud albedo(1) as well as for mation of precipitation. Models of cloud formation based on laboratory stud ies with idealized composition of nuclei suggest that organic solutes signi ficantly lower surface tension(2)-one of the parameters determining droplet population-but the lack of data on composition and properties of the organ ic material in the atmosphere precludes realistic laboratory or model studi es. Here, we report measurements on vacuum-evaporated samples of cloud wate r from the Po Valley, Italy, that show a large decrease in surface tension, by up to about one-third relative to pure water, for realistic concentrati ons of organic solutes expected to exist in growing droplets. Such large su rface-tension changes, if they occur in cloud droplets near the critical si ze for nucleation, lead to an increase in droplet population and hence in c loud albedo. The error produced in ignoring this effect is estimated to be comparable to other calculated direct and indirect influences of aerosols o n scattering and absorption of solar radiation(3).