Size-resolved CCN distributions and activation kinetics of aged continental and marine aerosol

We present size-segregated measurements of cloud condensation nucleus (CCN) activity of aged aerosol sampled at Finokalia, Crete, during the Finokalia Aerosol Measurement Experiment of summer 2007 (FAME07). From analysis of the data, hygroscopicity and activation kinetics distributions are derived. The CCN are found to be highly hygroscopic, (expressed by a size- and time- averaged hygroscopicity parameter κ ∼ 0.22), with the majority of particles activating at ~0.5–0.6 % supersaturation. Air masses originating from Central-Eastern Europe tend to be associated with higher CCN concentrations and slightly lower hygroscopicity (κ ∼ 0.18) than for other airmass types. The particles were always well mixed, as reflected by the high activation ratios and narrow hygroscopicity distribution widths. Smaller particles (~40 nm) were found to be more hygroscopic (~0.1κ units higher) than the larger ones (~100 nm). The particles with diameters less than 80 nm exhibited a diurnal hygroscopicity cycle (with κ peaking at ~14 h local time), consistent with photochemical aging and volatilization of less hygroscopic material from the aerosol. Use of bulk chemical composition and the aerosol number distribution results in excellent CCN closure when applying Köhler theory in its simplest form. Using asymptotic and threshold droplet growth analysis, the "aged" organics present in the aerosol were found not to suppress or delay the water uptake kinetics of particles in this environment.