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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, D10223, doi:10.1029/2006JD007963, 2007

Cloud droplet activation of secondary organic aerosol

Anthony J. Prenni

Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado, USA


Markus D. Petters

Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado, USA


Sonia M. Kreidenweis

Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado, USA


Paul J. DeMott

Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado, USA


Paul J. Ziemann

Air Pollution Research Center, University of California, Riverside, USA


Abstract

Measurements of hygroscopicity and cloud condensation nuclei (CCN) activity were conducted on secondary organic aerosol (SOA) formed in a smog chamber. SOA precursors included α-pinene, β-pinene, Δ3-carene, and toluene, representative of both naturally and anthropogenically emitted organic species. Measured CCN activation was comparable for all of the species studied and occurred at humidity conditions which are readily attained in the atmosphere. Further, there was little variation in hygroscopic growth between compounds. However, measured droplet activation conditions were inconsistent with hygroscopicity measured below water saturation and Köhler theory expressions based on Raoult’s law for several parameterizations for water activity. In the atmosphere, SOA may compose a large fraction of atmospheric particulate matter and will often exist internally mixed with inorganic species. Using the current results, we compare SOA to insoluble organic species to calculate CCN activation from mixed organic-sulfate particles for a range of atmospheric conditions. We find that droplet activation behavior of mixed particles containing SOA is the same as that of mixed particles for which the organic component is nonhygroscopic, except for cases in which there are low particle concentrations, low updraft velocities, and the aerosol composition is dominated by organics.

Received 24 August 2006; accepted 24 January 2007; published 26 May 2007.

Keywords: CCN; SOA; hygroscopicity.

Index Terms: 0320 Atmospheric Composition and Structure: Cloud physics and chemistry; 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801, 4906); 3311 Atmospheric Processes: Clouds and aerosols; 3310 Atmospheric Processes: Clouds and cloud feedbacks.

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Citation: Prenni, A. J., M. D. Petters, S. M. Kreidenweis, P. J. DeMott, and P. J. Ziemann (2007), Cloud droplet activation of secondary organic aerosol, J. Geophys. Res., 112, D10223, doi:10.1029/2006JD007963.