Understanding the properties of clouds in the Earth's atmosphere is currently limited by difficulties at the fundamental level of adequately describing the processes of cloud droplet nucleation and growth. Small changes in droplet population may significantly influence cloud albedo as well as formation of precipitation. Models of cloud formation based on laboratory studies with idealized composition of nuclei suggest that organic solutes significantly lower surface tension-one of the parameters determining droplet population but the lack of data on composition and properties of the organic material in the atmosphere precludes realistic laboratory or model studies. Here, we report measurements on vacuum-evaporated samples of cloud water 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 concentrations of organic solutes expected to exist in growing droplets. Such large surface-tension changes, if they occur in cloud droplets near the critical size for nucleation, lead to an increase in droplet population and hence in cloud albedo. The error produced in ignoring this effect is estimated to be comparable to other calculated direct and indirect influences of aerosols on scattering and absorption of solar radiation.

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