The atmosphere subjects aerosol particles to an array of transport and transformation processes that alter their size, number, and composition. Advective and turbulent transport of particles is nearly identical to that of the interstitial gas. Transformation processes include condensation and evaporation, which result from diffusion of vapors between the particle and the interstitial gas, homogeneous nucleation to produce new particles from supersaturated vapors, coagulation, which combines two particles into one by collision and sticking, and chemical reactions occurring in individual particles (Seinfeld and Pandis, 1998). That a major portion of atmospheric aerosol mass is secondary in nature is indicative of the importance of gas-to-particle conversion.
The aqueous phase of atmospheric aerosols contains primarily strong electrolytes such as sodium chloride, nitric and sulfuric acids, and ammonium. At relative humidities much below saturation, the vast majority of water in the atmosphere is in the vapor phase, and therefore any liquid water associated with aerosol particles is too small to affect the ambient relative humidity. For relative humidities below saturation, water is in equilibrium between the vapor and aqueous phases because the characteristic time for water equilibration is relatively short compared to all other processes taking place. Other volatile aerosol species may or may not be in equilibrium depending on their equilibration characteristic time (Seinfeld and Pandis, 1998).
Much nucleation research relevant to the atmosphere has been focused, via measurement and theory, on the binary nucleation of sulfuric acid and water (Seinfeld and Pandis, 1998). Although a classical theory of binary nucleation of sulfuric acid and water exists (Jaecker-Voirol and Mirabel, 1989), substantial uncertainty still remains as to how accurately this classical theory represents the actual nucleation process. Measured nucleation rates can differ from theoretically predicted values by several orders of magnitude. From the point of view of atmospheric applications, significant nucleation rates can be defined as those exceeding 1 nucleus/cm3 s.
Coagulation is the process whereby two particles collide and stick to form a single particle. Atmospheric processes that may lead to particle collisions include Brownian motion, turbulent shear, and differential settling. The latter two can be shown to be much less effective in this regard than Brownian motion (Wexler et al., 1994). In atmospheric aerosol dynamics, coagulation does not play a significant role unless number concentrations are relatively high and/or residence times are relatively long.
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