Surface tension of Rax cloud water and its relation to the concentration of organic material -: art. no. 4752

[1] During an intensive field campaign at a midlevel (1640 m above sea level) mountain in central Europe, cloud water samples were collected using a cloud water sampler and subsequently analyzed for organic and inorganic ions, total carbon (TC), and black carbon (BC). Cloud water surface tension (sigma) was measured using the ring method. Generally, the cloud water samples had surface tensions lower than that of pure water. The average value of sigma was 95.2% of the sigma of water, while the lowest value was 83.3%. A trend of lower sigma with higher concentrations of organic material (OC, obtained as TC-BC) was found at lower OC concentrations than those previously described [Charlson et al., 2001; Facchini et al., 1999a]. The effect of the lower sigma on cloud droplet activation was modeled using Kohler theory. For an assumed dry sulphate nucleus with 0.05 mum diameter, the critical supersaturation would be decreased by 7% (average sigma) and by 22% for minimum sigma.