Reaction aerosol phase chemical

Aerosol growth laws are expressions for the rate of change in particle size as a function of particle size and the appropriate chemical and physical properties of the system. Such expressions are necessary for the calculation of changes in the size distribution function with time as shown in thi.s and the next chapter. In this section, transport-limited growth laws based on the previous section are discussed first followed by growth laws determined by aerosol phase chemical reactions. 

Potential reactant trace gases may, as in the stratosphere, include molecules possessing large proton affinities or large gas phase acidities. A major difference, however, arises from the fact that trace gases which can be depleted by heterogeneous interaction with aerosols may have small and possibly strongly variable abundances. Such interactions may involve condensation, dissolution, or surface as well as liquid phase chemical reactions. 

Examples of growth laws including those limited by chemical reaction in the aerosol phase are summarized in Table 10.3. The growth rate dvjdt is proportional to dp for diffusion in the continuum range and to dp for droplet phase chemical reaction. Different... 

Processing of accumulation and coarse mode aerosols by clouds (Chapter 17) can also modify the concentration and composition of these modes. Aqueous-phase chemical reactions take place in cloud and fog droplets, and in aerosol particles at relative humidities approaching 100%. These reactions can lead to production of sulfate (Chapter 7) and after evaporation of water, a larger aerosol particle is left in the atmosphere. This transformation can lead to the formation of the condensation mode and the droplet mode (Hering and Friedlander 1982 John et al. 1990 Meng and Seinfeld 1994). 

Measurements of the urban aerosol mass distribution have shown that two distinct modes often exist in the 0.1 to 1.0 pm diameter range (Hering and Friedlander 1982 McMurry and Wilson 1983 Wall et al. 1988 John et al. 1990). These are referred to as the condensation mode (approximate aerodynamic diameter 0.2 pm) and the droplet mode (aerodynamic diameter around 0.7 pm). These two submicrometer mass distribution modes have also been observed in nonurban continental locations (McMurry and Wilson 1983 Hobbs et al. 1985 Radke et al. 1989). Hering and Friedlander (1982) and John et al. (1990) proposed that the larger mode could be the result of aqueous-phase chemical reactions. Meng and Seinfeld (1994) showed that growth of condensation mode particles by accretion of water vapor or by gas-phase or aerosol-phase sulfate production cannot explain existence of the droplet mode. Activation of condensation mode particles, formation of cloud/fog drops, followed by aqueous-phase chemistry, and droplet evaporation were shown to be a plausible mechanism for formation of the aerosol droplet mode. 

The dimensionless time has once more been defined relative to the characteristic time for vapor deposition, o is the ratio of the mass transfer rate to the gas-phase deposition rate, oj is the ratio of the initial aerosol concentration of AB to the corresponding concentration of the rest of the aerosol species, o is the ratio of the deposition velocities of the two gas-phase species, 04 is the ratio of the aerosol deposition velocity to the deposition velocity of A(g), o is the ratio of the emission (or gas-phase chemical reactions) of A to its initial deposition rate, 6 is the ratio of the emission rates of A and B, (77 is the ratio of the initial gas-phase concentrations of A and B, and finally o is the ratio of the initial concentrations of gas species A and aerosol AB. 

Finally, a number of gas-phase chemical reactions, such as the oxidation of selected organic compounds, generate nonvolatile condensable products that associate with aerosol particles. A large fraction of the aerosol consists of ammonium sulfate, which derives fi om the oxidation... 

Landgrebe, J. D. and S. E. Pratsinis, A Discrete Sectional Model for Particulate Production by Gas Phase Chemical Reaction and Aerosol Coagulation in Free Molecular Regime, J. Colloid Inter/ Sci., 139, 63-86 (1990). 

Landgrebe, J. D., and Pratsinis, S. E., Gas Phase Manufacture of Particulates Interplay of Chemical Reaction and Aerosol Coagulation in the Free-Molecular Regime." Ind. Eng. Chem. Res., 28 1474-1481 (1989)... 

An additional complexity that has not been modeled is the simultaneous inhalation, absorption, and chemical reaction in the gas or liquid phase of two or more gases (e.g., sulfur dioxide and ozone). For sufficiently dilute mixtures, Henry s law can be used for each gas. If droplet aerosols and one or more reactive gases are simultaneously present, absorption with or without chemical conversion in the droplets must be considered. 

Fig. 6. Chemical reaction rate data for NH3 reacting with H3PO4 solution droplets, from Rubel and Gentry (1984a). The data are compared with theory for surface reaction control (S) and gas-phase diffusion control (D). Reprinted with permission from J. Aerosol Sci. 15,661-671, Rubel, G. O., and Gentry, J. W., Copyright 1984, Pergamon Press pic.

In this chapter, we treat metallic fine particles whose size is less than micrometers in many cases down to nanometers, produced by physical methods in the gas phase (aerosol technique). Physical methods have a great advantage for producing fine particles because of their versatility and universality for application to many sorts of substances, rather than chemical methods, although they have a weak point in size control and mass production. It should be emphasized that chemically clean surfaces can be obtained by a physical method without any sophisticated techniques. If chemical reaction takes place, the surfaces of metallic particles are generally covered with unknown by-products. It is difficult to remove these contaminating species, once they have occurred, to reach the desired purity level. 

The term "heterogeneous" as applied to the atmosphere refers to chemistry that occurs in or on ambient condensed phases that are in contact with the gas phase aerosols, clouds, surface waters, etc. It is important to distinguish between heterogeneous processes that occur on the surface of the solid, and multiphase chemical reactions that take place in the bulk of the liquid medium. In the latter case, it is assumed that the reaction takes place after the molecule has been incorporated in the bulk liquid medium, such as occurs by wet deposition, where a species is ultimately removed from the atmosphere, especially in the troposphere. 

Heterogeneous chemistry involves a number of processes that combine the overall rale of transport and chemical conversion between the gas and condensed phases. These processes include (a) gas diffusion to the surface of the aerosol, (b) accommodation ("sticking") at the surface, (c) diffusion within the condensed phase, (d) chemical reaction in the condensed phase, and (e) diffusion of the resultant products to the surface and evaporation from the interface. 

Conde-Gallardo, A., Guerrero, M., Fragoso, R. and Castillo, N. (2006). Gas-phase diffusion and surface reaction as limiting mechanisms in the aerosol-assisted chemical vapor deposition of Ti02 films from titanium diisopropoxide. J. Mater. Res. 21(12), 3205-3209. 

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