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How quickly do cloud droplets form on atmospheric particles?

by C. R. Ruehl, P. Y. Chuang, A. Nenes
Atmospheric Chemistry and Physics Discussions ()
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The influence of aerosols on cloud properties is an important modulator of the climate system. Tra- ditional K tion 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 val- ues of α measured in the field normalized to the mean α measured for lab-generated ammonium sulfate (AS) parti- cles (i.e., α=α/αAS). Overall, 59% of ambient CCN grew at a rate similar to AS. We report the fraction of CCN that were low-α (α82% during at least one30 min period). Day to day variability was greatest in Tennessee and Illinois, and low-α particles were most prevalent on days when back trajectories suggested that air was arriving from aloft. The highest frac- tions of low-α CCN in Houston and Illinois occurred around local noon, and decreased later in the day. These results sug- gest that for some air masses, accurate quantification of CCN concentrations may need to account for kinetic limitations.

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