Nucleation, Condensation, and Coagulation

The condensation of a low-vapor-pressure species to form a new particle is known as homogeneous nucleation. Recall that the vapor pressure of a substance over the curved surface of a droplet is greater than over a flat surface of the same substance (e.g., see 

TABLE 9.9 Some Reported Contributions (%) of Various Particle Compounds to Light Scattering and 

Location so42 no3 Scattering Organics Soil coarse particles Absorption Reference 

Fraser Valley, British Columbia, Canada 29-39 16-35 22-33 1 — Pryor et al., 1997 

The term binary homogeneous nucleation is used to describe the formation of particles from two different gas-phase compounds such as sulfuric acid and water such nucleation can occur when their individual concentrations are significantly smaller than the saturation concentrations needed for nucleation of the pure compounds. It is believed that in the atmosphere, formation of particles from low-volatility gases occurs not by condensation of a single species but rather by the formation and growth of molecular clusters involving at least two, and as described shortly, probably three or more different species. 

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Binder K, Stauffer D (1976) Statistical theory of nucleation, condensation and coagulation. Adv Phys 25 343-396... 

In Figure 10, we show the interrelationship between the three microphysical processes of nucleation, condensation, and coagulation, which result in the precipitation and growth of particles during RESS. As described below, arrows 1-6 illustrate how a given process (at the head of the arrow) affects a second process (at its tail). Arrow 1 Only when nucleation has produced a sufficient number of particles does condensation become an effective precipitation process. 

Figure 3 Measured volume-size distribution showing fine-mode and coarse-mode particles and the nuclei and accumulation modes within the fine-particle mode. DGV, geometric mean diameter by volume equivalent to volume median diameter, and Og, geometric standard deviation, are shown for each mode. Also shown are transformation and growth mechanisms (e.g., nucleation, condensation, and coagulation). (From Ref 138.)...

The aerosol module treats condensation, evaporation, nucleation, deposition and coagulation of aerosols (Baklanov 2002) as shown in Fig. 16.3a. The numerical evolution of aerosols is solved by treating the aerosol size distributions as normal distributions. However, the aerosol module as a part of the modelling system has not been routinely tested yet in the 3D version, i.e. only in OD (see Gross and Baklanov 2004) and although it was evaluated in the 3D Enviro-HILAM research version of the model (see Korsholm et al. 2008a, b). 

After formation of ultrafine particles (ufp) by nucleation, subsequent growth of supercritical clusters occurs by condensation and coagulation. Coagulation is discussed in this section dealing with evolution of aerosols of ufp. The discussion is limited to Brownian coagulation which is the principal mode of coagulation for ufp. 

The accumulation mode (0.1 < d ic < 1pm) particles included in this mode originate from coagulation of particles in the nucleation mode and from condensation of vapors onto existing particles. These particles usually accounts for a substantial part of the aerosol mass and for most of the aerosol surface area (Seinfeld and Pandis, 1998). 

The current version of GEM-AQ has five size-resolved aerosols types, viz. sea salt, sulphate, black carbon, organic carbon, and dust. The microphysical processes which describe formation and transformation of aerosols are calculated by a sectional aerosol module (Gong et al. 2003). The particle mass is distributed into 12 logarithmically spaced bins from 0.005 to 10.24 pm radius. This size distribution leads to an additional 60 advected tracers. The following aerosol processes are accounted for in the aerosol module nucleation, condensation, coagulation, sedimentation and dry deposition, in-cloud oxidation of SO2, in-cloud scavenging, and below-cloud scavenging by rain and snow. 

Figure 5.63. Schematic diagram of the physical and chemical processes related to the formation, growth, transport, and destruction of atmospheric aerosols C = coagulation, Ch = chemistry, D = diffusion, E = evaporation, Em = emission, G = condensation and growth, I = injection, N = nucleation, P = photolysis, S = sedimentation, W = washout and rainout. From Turco et al. (1979).

UFP can be emitted directly from anthropogenic sources such as vehicles, furnaces or other technologies involving the combustion of fossil fuels. They can also form through a series of secondary chemical reactions in the atmosphere. There are five main processes that govern the formation, transformation and removal of UFP nucleation, condensation or evaporation of semi-volatile species, coagulation, deposition, and dilution. The effect of each of these processes on the total number of particles in the atmosphere and their size is summarized in Table 1. Each process affects the number and size of UFP and operates over vastly different characteristic times. Further, these processes have varying importance for different sized particles. 

It is necessary to supplement the listed equations with further equations obtained in Section 16.3, which describe the dynamics of a drop and the change n(y,x) due to condensation growth and coagulation. Suppose that in the course of a nucleation only propane is condensed, while methane remains unaffected. 

The twin mechanisms of coagulation and heterogeneous nucleation (condensation of one materiai to another) tend to accumulate submicrometre aerosol particle mass in this mode (Whitby and Cantrell, 1976 Willeke and Whitby, 1975). Because of the sharp decrease in particles larger than 0.3 pm in diameter, little mass is transferred from the accumulation mode to the coarse particle size range. Sedimentation and impaction tend to increase the relative concentration of the smallest mechanically produced particles, and then accumulate in this mode. Salt from sea spray is typically present as particles in the 1-5 pm size range, outside the normal accumulation mode. 

Aerosol Dynamics. Inclusion of a description of aerosol dynamics within air quaUty models is of primary importance because of the health effects associated with fine particles in the atmosphere, visibiUty deterioration, and the acid deposition problem. Aerosol dynamics differ markedly from gaseous pollutant dynamics in that particles come in a continuous distribution of sizes and can coagulate, evaporate, grow in size by condensation, be formed by nucleation, or be deposited by sedimentation. Furthermore, the species mass concentration alone does not fliUy characterize the aerosol. The particle size distribution, which changes as a function of time, and size-dependent composition determine the fate of particulate air pollutants and their... 

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