Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 7 5 2007 10.5194/acpd-7-14233-2007 http://www.atmos-chem-phys-discuss.net/7/14233/2007/ http://www.atmos-chem-phys-discuss.net/7/14233/2007/acpd-7-14233-2007.html http://www.atmos-chem-phys-discuss.net/7/14233/2007/acpd-7-14233-2007.pdf 14233 14264 2007-10-08 How quickly do cloud droplets form on atmospheric particles? C. R. Ruehl cruehl@ucsc.edu P. Y. Chuang A. Nenes Earth & Planetary Sciences, University of California, Santa Cruz, California, USA Earth & Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia, USA Chemical and Biomolecular Engineering, Georgia Inst. of Technology, Atlanta, Georgia, USA The influence of aerosols on cloud properties is an important modulator of the climate system. Traditional Köhler theory predicts the equilibrium concentration of cloud condensation nuclei (CCN); however, it is not known to what extent particles exist in the atmosphere that may be prevented from acting as CCN by kinetic limitations. We measured the rate of cloud droplet formation on atmospheric particles sampled at four sites across the United States during the summer of 2006: Great Smoky Mountain National Park, TN; Bondville, IL; Houston, TX; and the Atmospheric Radiation Measurement Program Southern Great Plains site near Lamont, OK. We express droplet growth rates with the mass accommodation coefficient (α), and report values of α measured in the field normalized to the mean α measured for lab-generated ammonium sulfate (AS) particles (i.e., α'=α/α_AS). Overall, 61% of ambient CCN grew at a rate similar to AS. We report the fraction of CCN that were "low-α'" (α'<10^−0.33). 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