HR: 1340h
AN: A23B-1263    [Abstracts]
TI: Distinct Cloud Droplet Growth Kinetics Observed Above the Marine Boundary Layer
AU: * Ruehl, C R
EM: cruehl@ucsc.edu
AF: University of California, Santa Cruz, 1156 High St., Santa Cruz, CA 95064, United States
AU: Chuang, P Y
EM: pchuang@pmc.ucsc.edu
AF: University of California, Santa Cruz, 1156 High St., Santa Cruz, CA 95064, United States
AU: Nenes, A
EM: nenes@eas.gatech.edu
AF: Georgia Institute of Technology, 311 Ferst Drive, Atlanta, GA 30332, United States
AB: The influence of aerosols on cloud properties is an important modulator of the climate system, and remains one of the most uncertain components of the anthropogenic influence on the radiative budget of the atmosphere. Traditional Kohler theory predicts the ability of an atmospheric particle of known size and composition to act as a cloud condensation nucleus (CCN) at equilibrium. 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 at various sites across the United States during the summer of 2006. Our results suggested that kinetically-limited drops, with apparent mass accommodation coefficients (α) up to 10× lower than those observed for lab-generated ammonium sulfate particles, may be prevalent in the free troposphere. Here we report the results of similar measurements made in 2007 at a high-elevation site near the California coast that is typically above the marine boundary layer. We found that the ambient aerosol at this site was an external mixture of particles with distinct growth kinetics: approximately 50% of the time, we observed two or more modes in the α spectrum. The slower mode typically contained 25% to 50% of the total CCN at ~0.4% supersaturation. These results cannot be explained without incorporating kinetic effects into Kohler theory. By varying the chamber parameters (supersaturation ratio, residence time), we determine whether these kinetic limitations are more consistent with a model of lower α or a delay to activation.
DE: 0300 ATMOSPHERIC COMPOSITION AND STRUCTURE
DE: 0305 Aerosols and particles (0345, 4801, 4906)
DE: 0320 Cloud physics and chemistry
DE: 0368 Troposphere: constituent transport and chemistry
DE: 1622 Earth system modeling (1225)
SC: Atmospheric Sciences [A]
MN: 2007 Fall Meeting