CCN Activity of Aerosol Particles

The ability of an aerosol particle to act as a CCN ("CCN activity")—that is, its ability to induce water-vapor condensation and cloud droplet formation under a given set of conditions—depends primarily on (a) water-vapor supersaturation, (b) dry particle size, and (c) hygroscopicity (soluble particles) or wettability (insoluble particles) (Seinfeld and Pandis 1998; Pruppacher and Klett 2000; McFiggans et al. 2006; Andreae and Rosenfeld 2008).

In science and engineering, hygroscopicity is generally defined as the ability of a substance to absorb or adsorb water vapor. In atmospheric and aerosol research, however, the term hygroscopicity is usually reserved for the absorption of water by soluble substances; the term wettability is used for the adsorption of water on the surface of insoluble particles. In the atmosphere, most aerosol particles are at least partly soluble, and thus the following discussion will focus on the hygroscopicity of fully or partially soluble particles and particle components rather than on the wettability of insoluble particles.

In general, the hygroscopicity of an aerosol particle depends on its chemical composition and the following factors:

• the mass or volume fraction of soluble components (water-soluble fraction) and their actual solubility (saturation equilibrium concentration in aqueous solution);

• the efficiency of soluble components in reducing water activity ("Raoult efficiency"), which can be expressed by an equivalent number of ions or molecules that go into solution per unit mass or unit volume of the particle material (Raoult's law);

• the surface activity of soluble components (i.e., their ability to reduce the surface tension and Kelvin effect of an aqueous solution).

To describe and model the CCN activity of atmospheric particles, the influence of chemical composition and the factors listed above can be efficiently summarized and approximated using a single hygroscopicity parameter, as will be detailed below.

The actual solubility of water-soluble components (i.e., their saturation equilibrium concentration in aqueous solution) is usually of minor importance for the CCN activity of atmospheric aerosol particles. Most chemical components can effectively be regarded as either fully soluble or completely insoluble. In Köhler model calculations, the effects of limited solubility need to be considered only for a relatively narrow range of sparingly soluble organic compounds and appear to be negligible for real atmospheric multicomponent particles (Petters and Kreidenweis 2008; Kreidenweis et al., this volume).

Nevertheless, sparingly soluble organic compounds can influence CCN activation as surfactants that reduce droplet surface tension and/or form a surface film that inhibits water uptake (mass transfer kinetics). The practical importance of sparingly soluble as well as of fully soluble surface-active substances and their effects on the CCN activity of atmospheric aerosol particles has not yet been well characterized and needs to be further elucidated (McFiggans et al. 2006). Relative to particle size and the fraction and Raoult efficiency of water-soluble components, however, changes of surface tension appear to be a second-order effect. It is unlikely to exceed the ~10% relative uncertainty that is more or less inherent to most state-of-the-art field measurements and model calculations of atmospheric aerosol properties (McMurry 2000; Raes et al. 2000; Kanakidou et al 2005; Poschl 2005; Fuzzi et al. 2006; Textor et al. 2006; Rose et al 2008a,b; Kreidenweis et al., this volume).

In the scientifi c literature and discussion, aerosol particles that are more hygroscopic and CCN-active than others are frequently called "more soluble." This is, however, misleading because the solubility and the hygroscopicity of water-soluble compounds are not directly proportional. In fact, compounds with higher solubility can be less hygroscopic and less CCN-active than compounds with lower solubility. For example, NH4NO3 is very highly soluble (~27 mol kg1) but only moderately hygroscopic (k ~ 0.7), whereas NaCl is moderately soluble (~6 mol kg1) but very highly hygroscopic (k = 1.3; see discussion in next section). Solubility determines the relative humidity of deliquescence (i.e., the threshold for the transformation of a dry particle into a saturated aqueous solution droplet). The extent of water uptake (hygroscopic growth factor) and the critical supersaturation of CCN activation, however, are primarily governed by the hygroscopicity and Raoult efficiency of the soluble substance (effective molar density of soluble ions or molecules; see below). Accordingly, the deliquescence of NH4NO3 particles occurs at lower relative humidity than that of NaCl particles (~60% vs. ~75% RH; Mikhailov et al. 2004), but the critical supersaturation for the CCN activation of NH4NO3 particles is higher than that of NaCl particles (e.g., 0.20% vs. 0.15% for particles with a diameter of ~80 nm; cf. Equation 3.3).

In addition to the composition and properties of particulate matter, the gas phase composition can also influence the CCN activity of aerosol particles: Water-soluble gases like ammonia and nitric acid can facilitate droplet formation by dissolving in nascent droplets, adding to the solute amount in the droplet, and thus enhancing the Raoult effect. On the other hand, the times-cales of gas-particle interaction can influence the activation of CCN in the atmosphere (clouds) as well as in measurement instruments (HTDMA and CCN counters) and laboratory experiments (aerosol chambers and flow tubes) as a result of the kinetic limitations of mass transport and phase transitions (Mikhailov et al. 2004; McFiggans et al. 2006; Andreae and Rosenfeld 2008;

Ruehl et al. 2008). Potential kinetic limitations of water uptake go beyond equilibrium model calculations and appear to be one of the most crucial open questions of CCN activation and cloud droplet formation.

Note, however, that the understanding of surface and multiphase processes in aerosols and clouds is limited not only by the available kinetic and thermodynamic data but also by a "Babylonian confusion" of terms and parameters (e.g., inconsistency of mass accommodation coefficients for the uptake water vapor on liquid water: bulk vs. surface accommodation?). This problem will hopefully be resolved through the use of a comprehensive kinetic model framework with consistent and unambiguous terminology and universally applicable rate equations and parameters (Poschl et al. 2007).

Thus it follows that CCN are not a fixed, special type of particle, but rather a highly variable subset of the ambient aerosol population. Size, solute content, the presence of surface-active or slightly soluble substances, the wettability and shape of insoluble particles, the presence of soluble gases, and the timescales of gas-particle interaction all influence whether a particle can act as a CCN at a given supersaturation. Consequently, the abundance and properties of CCN are influenced by all of the mechanisms and processes that lead to the emission, formation, and transformation of atmospheric aerosols. Note, however, that the actual influence of aerosol chemical composition on CCN activation and cloud droplet growth appears to be limited by the dynamics of cloud formation and evolution under real atmospheric conditions (McFiggans et al. 2006).

The atmospheric abundance, sources, properties, and effects of CCN as well as the relations to anthropogenic pollution and global change have recently been reviewed by Andreae and Rosenfeld (2008). For a review of the characteristics and variability of atmospheric aerosol size distributions, the reader is referred to Heintzenberg et al. (2000) and Raes et al. (2000); recent results from remote sensing studies are presented by Kinne (see Part 1).

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