ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-8-5869-2008 The effect of organic compounds on the growth rate of cloud droplets in marine and forest settings Shantz N. C. 1 Leaitch W. R. 2 Phinney L. 3 Mozurkewich M. 4 Toom-Sauntry D. 2 Department of Chemistry, University of Toronto, Toronto, Ontario, Canada Climate Research Division, Environment Canada, Toronto, Ontario, Canada Air Quality Sciences, Meteorological Service of Canada, Dartmouth, Nova Scotia, Canada Department of Chemistry and Centre for Atmospheric Chemistry, York University, Toronto, Ontario, Canada 13 10 2008 8 19 5869 5887 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/8/5869/2008/acp-8-5869-2008.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/8/5869/2008/acp-8-5869-2008.pdf

Organic matter represents an important fraction of the fine particle aerosol, yet our knowledge of the roles of organics in the activation of aerosol particles into cloud droplets is poor. A cloud condensation nucleus (CCN) counter is used to examine the relative growth rates of cloud droplets for case studies from field measurements on the North Pacific Ocean and in a coniferous forest. A model of the condensational growth of water droplets, on particles dissolving according to their solubility in water, is used to simulate the initial scattering of the droplets as they grow in the CCN counter. Simulations of the growth rates of fine particles sampled in the marine boundary layer of the North Pacific Ocean shows no evidence of natural marine organic material contributing to the CCN water uptake but there is an indication of an influence from organics from diesel ship emissions on the size distribution of sulphate and the ability of these particles to act as CCN. Simulations of the observations of water uptake on biogenic organic aerosol particles sampled in a coniferous forest indicate an impact of the organic on the water uptake rates, but one that is still smaller than that of pure sulphate. The existence of organics becomes important in determining the water uptake as the organic mass increases relative to sulphate. The values of the organic component of the hygroscopicity parameter κ that describes the CCN activity were found to be negligible for the marine particles and 0.02–0.05 for the forest particles.

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