Abstract
Abstract. 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 FAME07 campaign. Most of the particles activate at ~0.6% supersaturation, characteristic of the aged nature of the aerosol sampled. Application of Köhler theory, using measurements of bulk composition, size distribution, and assuming that organics are insoluble resulted in agreement between predicted and measured CCN concentrations within 7±11% for all supersaturations, with a tendency for CCN underprediction (16±6%; r2=0.88) at the lowest supersaturations (0.21%). Including the effects of the water-soluble organic fraction (which represent around 70% of the total organic content) reduces the average underprediction bias at the low supersaturations, resulting in a total closure error of 0.6±6%. 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 salt fraction is sufficient for the description of the CCN activity in this area.
Highlights
The absorption and scattering of radiation by atmospheric aerosol particles, especially those from anthropogenic activities, is an important component of anthropogenic climate change (IPCC, 2007)
With appropriate extensions to account for the multicomponent nature of global aerosol, Kohler theory remains the theoretical basis for linking aerosol to cloud condensation nuclei (CCN) activity, as it determines the characteristic level of ambient water vapor supersaturation, Sc, required for particles to activate into cloud droplets
We report the CCN concentrations and the activation characteristics of atmospheric aerosols in a diversity of air masses sampled at an Eastern Mediterranean ground site during the Finokalia Aerosol Measurement Experiment-2007 (FAME-07) campaign
Summary
The absorption and scattering of radiation by atmospheric aerosol particles, especially those from anthropogenic activities, is an important component of anthropogenic climate change (IPCC, 2007). Roberts et al (2003) performed CCN measurements during the Cooperative LBA (Large-scale Biosphere-Atmosphere Experiment in Amazonia, 1998) using a static diffusion cloud chamber between 0.15 and 1.5% supersaturation, a scanning mobility particle sizer for size distribution and a multistage cascade impactor for chemical composition, concluding that the CCN activity of aerosol in the LBA strongly depended on the water-soluble fraction. The aforementioned studies give a representative but by no means complete subset of the published CCN closure literature; all point out that in Kohler theory sufficiently describes the activation of ambient aerosol, provided that the chemical composition, aerosol size distribution and mixing state are well constrainted. CCN prediction error most often arises from sampling or instrumentation limitations, especially when determining the aerosol chemical composition The latter is quite important if the aerosol is externally mixed, or contains a large fraction of organic compounds. The contribution of the aerosol organic fraction to CCN activity is assessed, as well as their potential impact on droplet activation kinetics
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