Particles heterogeneous condensation

Heterogeneous condensation is secondary aerosol formation by the scavenging of the low-vapor-pressure products onto preexisting particles. If the concentration of particles is sufficiently high, this dominates over the formation of new nuclei via homogeneous nucleation (e.g., Friedlander, 1978, 1980). 

The rate of heterogeneous condensation depends on the exchange of matter and heat between a particle and the continuous phase. The extreme cases of a particle much larger or much smaller than the mean free path of the suspending gas are easy to analyze. In the continuum range (dp ip), diffusion theory can be used to calculate the transport rate. For a single sphere in an infinite medium, the steady-state equation of diffusion in spherical coordinates takes the form... 

By homogeneous reactions, vapour phase sulfuric acid is formed. This vapour condenses in the air by bimolecular condensation (H20 molecules also take part in the phase transition). The rate of this process depends, among other things, on the concentration of H 20 and H 2S04 molecules as well as on the temperature (Kiang et al., 1973). In the presence of suitable aerosol particles the condensation is heterogeneous, that is vapour molecules condense on the surface of aerosol particles (Cox, 1974). 

It is therefore mainly limited to the cocondensation of sulfuric acid and water. The acid in this case is produced in the atmosphere by gas phase oxidation of SO2. A special case is the production of sea salt aerosols over the oceans, which are effectively produced by the disintegration of air bubbles on the ocean. The atmospheric aerosol is a dynamic system the initially produced particles undergo various processes like heterogeneous condensation, coagulation, and deposition which lead to a change in their size and chemical composition and gives rise to the universality of the aerosol size-distribution. 

If the formation of drops occurs on condensation nuclei, then the process is called heterogeneous condensation. Such nuclei can be particles of impurities, pollution, droplets, etc. If the formation of drops occurs as a result of vapor condensation on spontaneously forming germs, then the process is called homogeneous condensation. 

Modeling of the gas phase chemistry and particle growth shows that homogeneous nucleation occurs early on in the process followed by heterogeneous condensation processes. Molecular dynamics computation have indicated that the iron oxide clusters will phase segregate and migrate toward the inside edge of the silica cluster consistent with experimental observation. 

Finally, atmospheric chemical transformations are classified in terms of whether they occur as a gas (homogeneous), on a surface, or in a liquid droplet (heterogeneous). An example of the last is the oxidation of dissolved sulfur dioxide in a liquid droplet. Thus, chemical transformations can occur in the gas phase, forming secondary products such as NO2 and O3 in the liquid phase, such as SO2 oxidation in liquid droplets or water films and as gas-to-particle conversion, in which the oxidized product condenses to form an aerosol. 

Heterogeneous reactions these various processes are cyclic and continuous, taking place in all sections of the boiler from the economizer to the condensate lines and including suspended ferric oxide and cupric oxide particles. Unless the hydrazine overfeed is extremely substantial (say, six to seven times theoretical), these latter reactions tend to predominate ... 

For condensable precursors, particle formation may occur by homogeneous or heterogeneous nucleation. It is generally accepted that heterogeneous processes are most likely in the atmosphere, because of the large number of nuclei present. 

The chemistry in the midlatitude stratosphere follows that discussed throughout this chapter. As seen in the previous sections, the heterogeneous chemistry that was once thought to be unique to PSCs also occurs in and on the liquid solutions characteristic of sulfate particles distributed globally, with their relative importance being determined by the temperature, composition, and phase of the condensed phase. 

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