Strong evidence of surface tension reduction in microscopic aqueous droplets

The ability of airborne particles to take up water may be enhanced by surface-active components, but the importance of this effect is controversial because direct measurement of the surface tension of microscopic droplets has not been possible. Here we infer droplet surface tension from water uptake measurements of mixed organic-inorganic particles at relative humidities just below saturation (99.3-99.9%). The surface tension of droplets formed on particles composed of NaCl and a-pinene ozonolysis products was reduced by 50-75%, but only when enough organic material was present to form a film on the droplet surface at least 0.8 nm thick. This study suggests that if atmospheric particles are predominantly (greater than or similar to 80%) composed of surface-active material, their influence on cloud properties and thus climate could be enhanced, and their atmospheric lifetimes could be reduced. Citation: Ruehl, C. R., P. Y. Chuang, A. Nenes, C. D. Cappa, K. R. Kolesar, and A. H. Goldstein (2012), Strong evidence of surface tension reduction in microscopic aqueous droplets, Geophys. Res. Lett., 39, L23801, doi:10.1029/2012GL053706.