Closure between measured and modeled cloud condensation nuclei (CCN) usingsize-resolved aerosol compositions in downtown Toronto

[摘要] Measurements of cloud condensation nuclei (CCN) were made in downtownToronto during August and September, 2003. CCN measurements were performedat 0.58% supersaturation using a thermal-gradient diffusion chamber,whereas the aerosol size distribution and composition were simultaneouslymeasured with a TSI SMPS and APS system and an Aerodyne Aerosol MassSpectrometer (AMS), respectively. Aerosol composition data shows that theparticles were predominately organic in nature, in particular for those witha vacuum aerodynamic diameter of <0.25 µm. In this study, the largestcontribution to CCN concentrations came from this size range, suggestingthat the CCN are also organic-rich. Using the size and compositioninformation, detailed CCN closure analyses were performed. In the firstanalysis, the particles were assumed to be internally mixed, the organicfraction was assumed to be insoluble, and the inorganic fraction was assumedto be ammonium sulfate. The AMS time-of-flight data were used for Köhlertheory predictions for each particle size and composition to obtain the drydiameter required for activation. By so doing, this closure analysis yieldedan average value of CCN_predicted/CCN_observed=1.12±0.05.However, several sample days showed distinct bimodal distributions, and aclosure analysis was performed after decoupling the two particle modes. Thisanalysis yielded an average value of CCN_predicted/CCN_observed=1.03±0.05.A sensitivity analysis was also performed to determine theaerosol/CCN closure if the organic solubility, droplet surface tension, orchamber supersaturation were varied.

[学科分类] 大气科学