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McMurry et al., 1996

" Percentage contributions to b only.

" Percentage contributions to b only.

Adamson, 1973, and Chapter 14.C.2). The smaller the radius of the droplet, the higher is the vapor pressure over the droplet surface. For example, for pure water at 25°C, the vapor pressure is only 0.1% greater over a droplet of f-^m radius compared to that for a flat surface but is If % greater if the radius is 0.01 /jl m. (see Problem f). This raises the question as to how homogeneous nucleation of even a single species can occur at all, since the first very small droplets formed would tend to evaporate rapidly. The explanation lies in the formation of molecular clusters of molecules that occur as molecules collide in the gas phase. When the system becomes supersaturated, the concentration of the condensable species increases, as does that of the clusters. The clusters grow by the sequential attachment of molecules until they reach a critical diameter (D*) above which the droplets are stable and grow and below which they evaporate (Friedlander, 1977, 1983). The critical diameter is given by

Ayv where y is the surface tension of the chemical forming the particle, v is the molecular volume, k is the Boltz-mann constant, T is the temperature (K), and 5 is the saturation ratio, defined as the ratio of the actual vapor pressure to the equilibrium vapor pressure at that temperature.

The term binary homogeneous nucleation is used to describe the formation of particles from two different gas-phase compounds such as sulfuric acid and water; such nucleation can occur when their individual concentrations are significantly smaller than the saturation concentrations needed for nucleation of the pure compounds. It is believed that in the atmosphere, formation of particles from low-volatility gases occurs not by condensation of a single species but rather by the formation and growth of molecular clusters involving at least two, and as described shortly, probably three or more different species.

A great deal of work on nucleation in the atmosphere has focused on sulfuric acid, since the oxidation of S02 is such a common atmospheric process and the acid product has low volatility. Similar considerations apply to methanesulfonic acid, which is formed by the oxidation of some organic sulfur compounds (see Chapter 8.E) and which is also found in particles because of its low vapor pressure (e.g., see Kreidenweis and Seinfeld, 1988a, 1988b). Even for the ostensibly simple case of sulfuric acid and water, the predicted and observed binary homogeneous nucleation rates are not in good agreement. For example, Fig. 9.30 shows both the theoretically predicted (Jaecker-Voirol and Mirabel, 1989) and the experimentally determined (Wyslouzil et al., 1991) concentrations of sulfuric acid needed to have

100 80

ra 40

a cc

0 0

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