| dc.description.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 mu m. 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 Koohler 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. | en_US |
