How quickly do cloud droplets form on atmospheric particles?
- ISSN: 16807324
- DOI: 10.5194/acpd-7-14233-2007
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 limita- tions. 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 At- mospheric 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.