| The organic fraction of ambient aerosols is a complex mixture of hundreds of organic compounds varying in chemical structure and physical properties. Determining the ability of these particles to activate into cloud droplets is necessary for understanding and quantifying the effects of aerosols on cloud formation. The cloud condensation nuclei (CCN) activities of single-component organic particles were experimentally investigated using a Tandem Differential Mobility Analyzer and a thermal diffusion Cloud Condensation Nucleus Counter. Studies were performed at supersaturations of 0.3 and 1% with dry particle diameters between 0.02 and 0.2 micrometers. Laboratory experiments were performed with sodium chloride, ammonium sulfate, glutaric acid, adipic acid, pinonic acid, glutamic acid, leucine, cholesterol, pinic acid, norpinic acid, hexadecane, hexadecanol, myristic acid, palmitic acid, and stearic acid. Experiments confirmed that Köhler theory works well in predicting activation of soluble inorganic species and organics that are wettable by water while an extension of the theory including limited solubility appears necessary to account for the low solubility organic species not wettable by water.; Internally mixed, multi-component particles as well as particles consisting of a core coated with a second species were investigated. An additive behavior assumption was sufficient for the prediction of the CCN activation diameter of the mixed particles. The results of the hydrophobic organic coating on various inorganic and organic particles showed that even films comprising 96% by volume of the final particle did not prevent activation of the core aerosol material. The ability of aerosols generated from the ozone oxidation of α-pinene, β-pinene, limonene, gasoline, and diesel fuel to act as CCN was investigated. Results indicated that secondary organic aerosol produced from oxidation could be a significant source of CCN material. The products covered a wide range of CCN activity. The CCN properties of ambient summertime aerosol sampled in Pittsburgh were also investigated. Results indicate that the ambient aerosol was excellent CCN material. The observed CCN activity ranged from being similar to the highly active inorganic salts to behaving similarly to many of the organic aerosol particles and some of the more active ozone oxidation products. |
