Cloud condensation nuclei measurements in the eastern Mediterranean marine boundary layer: CCN closure and droplet growth kinetics

Measurements of cloud condensation nuclei (CCN) concentrations (cm−3) between 0.2 and 1.0% supersaturation, aerosol size distribution and chemical composition were performed at a remote marine site in the eastern Mediterranean, from September to October 2007 during the FAME-07 campaign. Virtually all the particles activate at 0.8% supersaturation, consistent with the very aged nature of the aerosol sampled. Application of Köhler theory, using measurements of bulk composition and size distribution, and assuming that organics are insoluble resulted in agreement between predicted and measured CCN concentrations within 3.4±11% for all supersaturations, with a tendency for CCN underprediction (15±8%; r2=0.92) at lower supersaturations (0.2-0.4%). Including the effects of the water-soluble organic fraction (which represents around 70% of the total organic content) reduces the underprediction bias at low supersaturations, but introduces a slight overprediction (around 5±15%) bias at higher supersaturations (0.6–0.8%), likely from size-dependent variations of the sulfate to organic ratio. Using threshold droplet growth analysis, the growth kinetics of ambient CCN is consistent with NaCl calibration experiments; hence the presence of aged organics does not suppress the rate of water uptake in this environment. The knowledge of the soluble fraction is sufficient for the description of the CCN activity in this area.