Atmos. Chem. Phys. Discuss., 5, 6263-6293, 2005
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Closure between measured and modeled cloud condensation nuclei (CCN) using size-resolved aerosol compositions in downtown Toronto
K. Broekhuizen1, R. Y.-W. Chang1, W. R. Leaitch2, S.-M. Li2, and J. P. D. Abbatt1
1Department of Chemistry, University of Toronto, 80 St. George St., Toronto, ON M5S 3H6, Canada
2Meteorological Service of Canada, 4905 Dufferin St., Toronto, ON M3H 5T4, Canada

Abstract. Measurements of cloud condensation nuclei (CCN) were made in downtown Toronto during August and September, 2003. CCN measurements were performed at 0.58% supersaturation using a thermal-gradient diffusion chamber, whereas the aerosol size distribution and composition were simultaneously measured with a TSI SMPS and APS system and an Aerodyne Aerosol Mass Spectrometer (AMS), respectively. Aerosol composition data shows that the particles were predominately organic in nature, in particular for those with a vacuum aerodynamic diameter of <25 μm. In this study, the largest contribution to CCN concentrations came from this size range, suggesting that the CCN are also organic-rich. Using the size and composition information, a detailed CCN closure analysis was performed. In all analyses, the particles were assumed to be internally mixed, the organic fraction was assumed to be insoluble, and the inorganic fraction was assumed to be ammonium sulfate. The AMS time-of-flight data were used for Köhler theory predictions for each particle size and composition to obtain the dry diameter required for activation. By so doing, this closure analysis yielded an average value of CCNpredicted/CCNobserved= 1.04 (R2=0.87). Several other closure analyses were performed to mimic other methods of aerosol compositional analysis. In all cases, by assuming uniform aerosol composition across a wider range of particle sizes, significant overprediction of CCN concentrations resulted.

Citation: Broekhuizen, K., Chang, R. Y.-W., Leaitch, W. R., Li, S.-M., and Abbatt, J. P. D.: Closure between measured and modeled cloud condensation nuclei (CCN) using size-resolved aerosol compositions in downtown Toronto, Atmos. Chem. Phys. Discuss., 5, 6263-6293, doi:10.5194/acpd-5-6263-2005, 2005.
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