The effectiveness of an aerosol particle as a CCN depends on its size and response to water. Atmospheric aerosol particles are either hydrophobic (i.e. will not activate in cloud under any circumstances), water-insoluble but possess hydrophyllic sites that allow the particles to wet and activate at higher supersaturations, or have some water-soluble component and will activate at lower supersaturations given sufficient time to achieve their critical radius. Only particles with some water-soluble species are significant for the indirect forcing (e.g., Kulmala et al., 1996; Eichel et al., 1996). However, there are many water-soluble compounds in the atmospheric aerosol with widely varying degrees of solubility.

Sulphates, sodium chloride, and other water-soluble salts and inorganic acids are common to the atmospheric aerosol (Section 5.2) and to CCN (e.g., Hudson and Da, 1996). The abilities of these species to serve as CCN are relatively well known, whereas our understanding of the ability of organic species to act as CCN is relatively poor. This is a critical area of uncertainty for the global-scale modelling of cloud droplet nucleation.

The water-soluble fraction of organic species in aerosols can be high relative to sulphate (e.g., Li et al., 1996; Zappoli et al., 1999), and organics may be important sources of CCN in at least some circumstances (Novakov and Penner, 1993; Rivera-Carpio et al., 1996). Aerosols from biomass burning are primarily composed of organics and may act as CCN, but some of their CCN activity may actually be due to co-resident inorganic constituents (Van-Dinh et al. 1994; Novakov and Corrigan, 1996; Leaitch et al., 1996a). Measurements from a forested area suggest that some of the products of terpene oxidation may serve as CCN (Leaitch et al., 1999), and Virkkula et al. (1999) found that particles from pinene oxidation absorbed some water at an RH of about 84%. Cruz and Pandis (1997) examined the CCN activity of particles of adipic acid and of glutaric acid (products of alkene oxidation) and found reasonable agreement with Köhler theory, whereas Corrigan and Novakov (1999) measured much higher activation diameters than predicted by theory. Volatile organic acids (formic, acetic, pyruvic, oxalic) may also contribute to the formation of CCN in areas covered with vegetation and in plumes from biomass burning (Yu, 2000). Pösfai et al. (1998) suggest that organic films can be responsible for relatively large water uptake at low RH.

The wide range of solubilities of the organic species may help to explain why observations do not always provide a consistent picture of the uptake of water. The water solubility of the oxygenated organic species tends to decrease with increasing the carbon number (Saxena and Hildemann, 1996). Shulman et al. (1996) showed that the cloud activation of organic species with lower solubilties might be delayed due to the increased time required for dissolution, and delays of 1-3 seconds have been observed (Shantz et al., 1999). For a mixture of an inorganic salt or acid with an organic that is at least slightly soluble, the presence of the organic may contribute to some reduction in the critical supersaturation for activation (Corrigan and Novakov, 1999) especially if the organics reduce the surface tension (Shulman et al. 1996) as demonstrated in natural cloud water (Facchini et al., 1999).

Historically, much of the interest in organics has focused on the inhibition of CCN activation in cloud by surface-active organics. Recently, Hansson et al. (1998) found that thick coatings of either of two insoluble high molecular weight organics (50 to 100% by mass of tetracosane or of lauric acid) reduced the hygroscopic growth factor for particles of NaCl. On the other hand, studies by Shulman et al. (1996), Cruz and Pandis (1998), and Virkkula et al. (1999) suggest that coatings of organic acids do not necessarily inhibit the effect of the other species on the condensation of water. The ability of the hydrophobic coating to form a complete barrier to the water vapour is a critical aspect of this issue. It is unlikely that such coatings are common in the atmosphere.

Although the CCN activities of inorganic components of the aerosol are well known, there are other aspects to the water activity of these species that need to be considered. Highly soluble gases, such as HNO₃, can dissolve into a growing solution droplet prior to activation in cloud. The addition of this inorganic substance to the solution can decrease the critical supersaturation for activation (Kulmula et al., 1993; Laaksonen et al., 1998). The result is an increase in cloud droplet number but this is tempered somewhat by an enhanced condensation rate that contributes to a slight reduction in the cloud supersaturation. The importance of this effect will depend on the mixing ratio of such gases relative to the CCN solute concentrations and this has not been properly evaluated. An important consideration for the development of the CCN spectrum is the in-cloud oxidation of SO₂. Current models indicate that the fraction of secondary sulphate that is due to SO₂ oxidation in cloud can be in the range of 60 to 80% (Table 5.5). While the uncertainty in this estimate does not greatly impact the model sulphur budgets, it has significant consequences for the magnitude of the predicted indirect forcing (Chuang et al., 1997; Zhang et al., 1999).

Large-scale models must be able to represent several factors related to CCN in order to better assess the indirect effect: the size distribution of the mass of water-soluble species, the degree of solubility of the represented species, and the amount of mixing of individual species within a given size fraction. The most critical species are sulphates, organics, sea salt and nitrates.

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