Aerosol particles that act as cloud condensation nuclei (CCN) play a central role in the formation of clouds. Here, the basic concepts and key aspects that link the CCN activity of aerosol particles to their size, chemical composition, and hygroscopicity (i.e., their ability to absorb water vapor) are discussed. Literature data and recent field measurements suggest that the influence of chemical composition can be efficiently described by a single effective hygroscopicity parameter that relates the dry particle diameter to the so-called critical water-vapor supersaturation (i.e., the minimum supersaturation required to form a cloud droplet). This hygroscopicity parameter, k, can be easily calculated from chemical composition data and is typically in the range of ~0.1 for pyrogenic and secondary organic aerosols to ~1 for sea spray aerosols. Continental and marine boundary layer aerosols tend to cluster into relatively narrow ranges of effective hygroscopicity (continental k = 0.3±0.1; marine k = 0.7±0.2).

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). Nevertheless, more detailed investigations and representations of the hygroscopic properties of aerosol particles, as a function of chemical composition, are needed to elucidate fully aerosol-cloud interactions, especially for low water-vapor supersaturations, low aerosol concentrations, and organic components. 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 as much as 20% from the most accurate available models. To ensure that measurement and model results can be compared properly, CCN studies should always report exactly which Köhler model equations and parameters have been applied. In addition, potential kinetic limitations of water uptake appear to be one of the most crucial open questions of CCN properties and activation.

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