Closure between measured and modeled cloud condensation nuclei (CCN) using size-resolved aerosol compositions in downtown Toronto K. Broekhuizen3, 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 3Department of Chemistry, Colgate University, 13 Oak Dr., Hamilton, NY 13346, USA
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 <0.25 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, detailed CCN closure analyses were performed. In the first
analysis, 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=1.12±0.05.
However, several sample days showed distinct bimodal distributions, and a
closure analysis was performed after decoupling the two particle modes. This
analysis yielded an average value of CCNpredicted/CCNobserved=1.03±0.05.
A sensitivity analysis was also performed to determine the
aerosol/CCN closure if the organic solubility, droplet surface tension, or
chamber supersaturation were varied.
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., 6, 2513-2524, doi:10.5194/acp-6-2513-2006, 2006.