Cloud condensation nuclei measurements in the marine boundary layer of the Eastern Mediterranean: CCN closure and droplet growth kinetics
Measurements of cloud condensation nuclei (CCN) concentrations (cm(-3))\nbetween 0.2 and 1.0% supersaturation, aerosol size distribution and\nchemical composition were performed at a remote marine site in the\neastern Mediterranean, from September to October 2007 during the FAME07\ncampaign. Most of the particles activate at similar to 0.6%\nsupersaturation, characteristic of the aged nature of the aerosol\nsampled. Application of Kohler theory, using measurements of bulk\ncomposition, size distribution, and assuming that organics are\ninsoluble resulted in agreement between predicted and measured CCN\nconcentrations within 7 +/- 11% for all supersaturations, with a\ntendency for CCN underprediction (16 +/- 6%; r(2)=0.88) at the lowest\nsupersaturations (0.21%). Including the effects of the water-soluble\norganic fraction ( which represent around 70% of the total organic\ncontent) reduces the average underprediction bias at the low\nsupersaturations, resulting in a total closure error of 0.6 +/- 6%.\nUsing threshold droplet growth analysis, the growth kinetics of ambient\nCCN is consistent with NaCl calibration experiments; hence the presence\nof aged organics does not suppress the rate of water uptake in this\nenvironment. The knowledge of the soluble salt fraction is sufficient\nfor the description of the CCN activity in this area.