Summary and Conclusions

The subset of particles from the overall aerosol population that can act as CCN is determined mostly by the number of soluble ions or molecules a particle contains, which in turn is a function of particle size and chemical composition. The complexity and variability of the composition of ambient aerosols make an explicit treatment of the chemical properties of CCN in atmospheric models difficult. Recent studies have shown, however, that for the purpose of assessing CCN activity, the influence of aerosol composition can be efficiently represented by a single parameter: the hygroscopicity parameter k. Knowledge of this parameter and of the ambient aerosol size distribution enables efficient calculation of CCN spectra (CCN concentration as a function of water-vapor supersaturation).

Measurements of CCN as a function of water-vapor supersaturation and particle size suggest that continental and marine aerosols tend to cluster on average into relatively narrow ranges of effective hygroscopicity (continental k = 0.3±0.1; marine k = 0.7±0.2), which should facilitate their treatment in atmospheric models. Thus the influence of aerosol chemical composition and hygroscopicity appears to be less variable and less uncertain than other factors that determine the effects of aerosols on warm cloud formation in the atmosphere (e.g., particle number concentration, size distribution, sources, sinks, and meteorological conditions) (see Anderson et al., Kinne, and Kreidenweis et al., all this volume).

Nevertheless, more detailed investigations and representations of the hygroscopic properties of aerosol particles as a function of chemical composition are needed before aerosol-cloud interactions—especially for low water-vapor supersaturations, low aerosol concentrations, and organic components—are fully explained. Even for simple and well-defined inorganic reference substances, such as ammonium sulfate and sodium chloride, the critical supersaturations or critical dry particle diameters of CCN activation calculated with different Köhler models can deviate by up to 20%. To ensure that measurement and model results can be properly compared, CCN studies should always report exactly which Köhler model equations and parameters have been applied.

Looking beyond equilibrium model calculations, potential kinetic limitations of water uptake appear to be one of the most crucial open questions of CCN properties and activation. Moreover, knowledge of the chemical composition of aerosol particles is essential to determine aerosol sources and production mechanisms and to assess the role of anthropogenic versus natural contributions to the atmospheric aerosol burden.

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