Atmospheric Chemistry and Physics www.atmos-chem-phys.net 1680-7316 1680-7324 8 18 2008 10.5194/acp-8-5649-2008 http://www.atmos-chem-phys.net/8/5649/2008/ http://www.atmos-chem-phys.net/8/5649/2008/acp-8-5649-2008.html http://www.atmos-chem-phys.net/8/5649/2008/acp-8-5649-2008.pdf 5649 5667 2008-09-26 The influence of chemical composition and mixing state of Los Angeles urban aerosol on CCN number and cloud properties M. J. Cubison B. Ervens G. Feingold K. S. Docherty I. M. Ulbrich L. Shields K. Prather S. Hering J. L. Jimenez jose.jimenez@colorado.edu Cooperative Institute for Research in the Environmental Sciences (CIRES), University of Colorado, Boulder, CO, USA Atmospheric Science Department, Colorado State University, Fort Collins, CO, USA NOAA Earth System Research Laboratory, Boulder, CO, USA Dept. of Chemistry and Biochemistry, University of Colorado, Boulder, CO, USA University of California at San Diego, CA, USA Aerosol Dynamics, Inc., Berkeley, CA, USA The relationship between cloud condensation nuclei (CCN) number and the physical and chemical properties of the atmospheric aerosol distribution is explored for a polluted urban data set from the Study of Organic Aerosols at Riverside I (SOAR-1) campaign conducted at Riverside, California, USA during summer 2005. The mixing state and, to a lesser degree, the average chemical composition are shown to be important parameters in determining the activation properties of those particles around the critical activation diameters for atmospherically-realistic supersaturation values. Closure between predictions and measurements of CCN number at several supersaturations is attempted by modeling a number of aerosol chemical composition and mixing state cases of increasing complexity. It is shown that a realistic treatment of the state of mixing of the urban aerosol distribution is critical in order to eliminate model bias. Fresh emissions such as elemental carbon and small organic particles must be treated as non-activating and explicitly accounted for in the model. The relative number concentration of these particles compared to inorganics and oxygenated organic compounds of limited hygroscopicity plays an important role in determining the CCN number. 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