TABLE OF CONTENTS Although coagulation (Chapter 7) modifies the size distribution of an aerosol, it causes no change in the mass concentration. The other important internal process within a gas that shapes the size distribution, gas-to-particle conversion, results in an increase in the aerosol mass concentration. In studying this process we are interested in the mechanisms by which gases are converted to particles, the rates at which conversion takes place, and the distributions of the condensed matter with respect to particle size.
Gas-to-particle conversion may result from homogeneous gas-phase processes, or it may be controlled by processes in the particulate phase. Gas-phase processes, either physical or chemical, can produce a supersaturated state which then collapses by aerosol formation. Physical processes producing supersaturation include adiabatic expansion or mixing with cool air discussed in the last chapter or radiative or conductive cooling. Gas-phase chemical reactions such as the oxidation of SO 2 to sulfuric acid in the atmosphere or the oxidation of SiCl 4 to SiO 2 in industry also generate condensable products.
Once a condensable species has been formed in the gas phase, the system is in a nonequilibrium state. It may pass toward equilibrium by the generation of new particles (homogeneous nucleation) or by condensation on existing particles (heterogeneous condensation). If all collisions among condensable molecules are effective, the process resembles aerosol coagulation (Chapter 7). However, in certain important cases, small molecular clusters are unstable; an energy barrier must be surmounted before stable nuclei can form as discussed in this...
