Gas-to-particle conversion

The aerosol particles are formed either by coagulation and condensation processes or by gas-to-particle conversion. Analytically  

In the atmosphere, all three processes take place concurrently. The generation of new particles requires conditions that allow the growth of molecular clusters by condensation in the 

Aitken, J. (1923). In Knott, C.G. (Ed.), Collected Scientific Papers of John Aitken. Cambridge University Press, London, 591 pp. 

Whitby, K.T. (1967). Concentration and size distribution measurements of atmospheric aerosols and a test of the theory of self-preserving size distributions. J. Atmos. Sci. 24,677-687. 

Dankelraan, V., Reineking, A., Porstendorfer, J. (2001). Determination of neutralisation rates of Po ions in air. Radiat. Prot. Dosim. 94, 353-357. 

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The sources of particulate matter in the atmosphere can be primary, directly injected into the atmosphere, or secondary, formed in the atmosphere by gas-to-particle conversion processes (13). The primary sources of fine particles are combustion processes, e.g., power plants and diesel... 

The secondary source of fine particles in the atmosphere is gas-to-particle conversion processes, considered to be the more important source of particles contributing to atmospheric haze. In gas-to-particle conversion, gaseous molecules become transformed to liquid or solid particles. This phase transformation can occur by three processes absortion, nucleation, and condensation. Absorption is the process by which a gas goes into solution in a liquid phase. Absorption of a specific gas is dependent on the solubility of the gas in a particular liquid, e.g., SO2 in liquid H2O droplets. Nucleation and condensation are terms associated with aerosol dynamics. 

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. 

Particles in the atmosphere come from different sources, e.g., combustion, windblown dust, and gas-to-particle conversion processes (see Chapter 6). Figure 2-2 illustrates the wide range of particle diameters potentially present in the ambient atmosphere. A typical size distribution of ambient particles is shown in Fig. 2-3. The distribution of number, surface, and mass can occur over different diameters for the same aerosol. Variation in chemical composition as a function of particle diameter has also been observed, as shown in Table 4-3. 

Vohra, K.G., Gas-to-Particle Conversion in the Atmospheric Environment by Radiation-Induced and Photochemical Reactions, Radiation Research. Acad. Press. New York, 1314—1323 (1975). 

Jenkin, M. E., Clement, C., and Ford, I. Gas-to-particle conversion pathways, First annual report on the contract Met2a/1053/Project2, AEA Technology, 1996. 

Assuming that there is 100% gas to-particle conversion, that the formed condensable species is the one with the lowest vapor pressure (carboxylic acid), and that there is no vapor-pressure lowering by condensable species pcdymerization or other effect. 

Assuming that the rate of gas-to-particle conversion of any condensable species is greater than its rate of formation in the gas phase (which is the case for heten eneous nucleation predominant in the atmosphere, but may not be valid for homogeneous nucleation in clean-air smog-chamber studies) ... 

Heisler, S. L. Gas-to-Particle Conversion in Photochemical Smog Growth Laws and Mechanisms for Organics. Ph.D. Thesis. Pasadena California Institute of Technology, 1976. 215 pp. 

Kiang, C. S., D. Stauffer. V. A. Mohnen, J. Bricard, and D. Vigla. Heteromolec-uiar nucleation theory applied to gas-to-particle conversion. Atmos. Environ. 7 1279-1283, 1973. 

With emission source chemical signatures and corresponding aerosol or rainwater sample measurements PLS can be used Co calculate a chemical element mass balance (CEB). Exact emission profiles for the copper smelter and for a power plant located further upwind were not available for calculation of source contributions to Western Washington rainwater composition. This type of calculation Is more difficult for rainwater Chan for aerosol samples due Co atmospheric gas to particle conversion of sulfur and nitrogen species and due Co variations In scavenging efficiencies among species. Gatz (14) has applied Che CEB to rainwater samples and discussed Che effect of variable solubility on the evaluation of Che soil or road dust factor. 

A different approach which also starts from the characteristics of the emissions is able to deal with some of these difficulties. Aerosol properties can be described by means of distribution functions with respect to particle size and chemical composition. The distribution functions change with time and space as a result of various atmospheric processes, and the dynamics of the aerosol can be described mathematically by certain equations which take into account particle growth, coagulation and sedimentation (1, Chap. 10). These equations can be solved if the wind field, particle deposition velocity and rates of gas-to-particle conversion are known, to predict the properties of the aerosol downwind from emission sources. This approach is known as dispersion modeling. 

In the general case, individual particles have differing compositions and refractive indices and to take this into account in detail is not possible from a practical point of view. To allow for a variation of refractive index, a convenient model is that of a mixture of aerosols from the several sources, each with its own extinction cross-section. The particles are assumed not to coagulate so that the aerosol is not mixed on the individual particle basis. Such an aerosol is known as an external mixture. This model would also be applicable, approximately, to an aerosol mixture whose particles are growing in size by gas-to-particle conversion. 

When the size distribution changes by gas-to-particle conversion, the following analysis is applicable Returning to the fundamental relationship, Eq. (5), it is clear that another condition under which y. is constant is given by... 

On the other hand, particles from fossil fuel combustion and gas-to-particle conversion are generally much smaller (< 2.5-/Am diameter) and fall in the respirable size range. These particles can reach the alveolar region where gas exchange occurs. This region is not coated with a protective mucus layer, and here the clearance time for deposited particles is much greater than in the upper respiratory tract hence the potential for health effects is much greater (Phalen, 1984). 

Remote continental -104 Three modes centered at Dp - 0.02, 0.12, and 1.8 p, m includes products of gas-to-particle conversion and biological sources, e.g., pollens... 

Urban aerosol 105 Three modes nuclei, accumulation, and coarse larger particles contain crustal elements (Fe, Si, etc.), smaller contain nitrate, sulfate, ammonium, and elemental and organic carbon and are formed by combustion processes and gas-to-particle conversion... 

Polar O V) 1 V) J Typical Dp - 0.15 p,m contain gas-to-particle conversion products such as sulfate and ammonium... 

Biomass burning -104 close to source 103 downwind Two modes often seen Dp 0.1-0.4 and Dp > 2 p,m smaller mode contains gas-to-particle conversion products (sulfate, nitrate, ammonium, organics) while larger mode has soil and ash particles... 

Similar data for sulfate have been reported in many studies. Figure 9.36, for example, shows overall average sulfate distributions measured in marine areas as well as at continental sites (Milford and Davidson, 1987). The marine data show two modes, a coarse mode associated with sea salt and a fine mode associated with gas-to-particle conversion. Sulfate in seawater, formed, for example, by the oxidation of sulfur-containing organics such as dimethyl sulfide, can be carried into the atmosphere during the formation of sea salt particles by processes described earlier and hence are found in larger particles. The continental data show only the fine particle mode, as expected for formation from the atmospheric oxidation of the S02 precursors. 

Depends on assumptions concerning gas-to-particle conversion in the plume. 

A schematic illustration of the gas-to-particle conversion route is shown in Figure 7.37. Precursor vapors react at high temperatures to form molecules of intermediate... 

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