Nucleation condensable species

Nucleation is the growth of clusters of molecules that become a thermodynamically stable nucleus. This process is dependent on the vapor pressure of the condensable species. The molecular clusters undergo growth when the saturation ratio, S, is greater than 1, where saturation ratio is defined as the actual pressure of the gas divided by its equilibrium vapor pressure. S > 1 is referred to as a supersaturated condition (14). 

The prediction made by the model calculations should be taken with some care for two reasons 1) H2O and H2SO4 are considered to be the condensing species, whereas other species may be active in experimental or domestic environments 2) the model uses classical nucleation theory, which is the only workable theory, but which is also to be criticized because it applies macroscopic entities to clusters that contain only a few molecules (3). 

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) ... 

The reactant species impinging from the vapor phase is attracted to the surface of the substrate mainly due to the instantaneous dipole moment of the substrate surface atoms. Adsorption of the condensed species is accompanied by lateral diffusion, i.e., the species moves along the surface of the substrate for a particular distance before it resides on the surface. An aggregate of condensed reactant species that reside on the surface of the substrate is termed a "nucleus". This could vary from a single atom or molecule to a cluster of species, and the process is referred to as nucleation. Nuclei are usually three dimensional island type structures formed on the surface of the substrate and nucleation is usually the initial stage of the film formation. It is worthwhile to note that, while simple condensation is usually exothermic, CVD adsorption and subsequent film formation is usually an endothermic reaction with few exceptions. 

J The total surface area of the Los Angeles smog aerosol is of the order of 1000 Estimate the maximum rate of formation of a condensable species by chemical reaction that can be sustained wUhoia homogeneous nucleation taking place. Express your answer in hr as... 

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. 

Colloidal dispersions can be formed either by nucleation with subsequent growth or by subdivision processes [1-6]. The nucleation/condensation process requires a phase change, such as condensation of vapour to yield liquid or solid, or precipitation from solution. The colloidal species are built up by deposition on nuclei that may be of the same or different chemical species. If the nuclei are of the same chemical species, the process is referred to as homogeneous nucleation if the nuclei are of different chemical species, the process is referred to as heterogeneous nucleation. Tadros reviews nucleation/condensation processes and their control [7]. Some mechanisms of such colloid formation are listed in Table 7.1. 

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... 

Although the data presented here are limited to a single coal burned in two combustor operating modes, several important observations can be made about the fine particles generated by pulverized coal combustion. The major constituents of the very small nucleation generated particles vary with combustion conditions. High flame temperatures lead to the volatilization of refractory ash species such as silica and alumina, probably by means of reactions which produce volatile reduced species such as SiO or Al. At lower flame temperatures which minimize these reactions other ash species dominate the fine particles. Because the major constitutents of the fine particles are relatively refractory, nucleation is expected to occur early in the combustion process. More volatile species which condense at lower temperatures may also form new particles or may condense on the surfaces of the existing particles. Both mechanisms will lead to substantial enrichment of the very small particles with the volatile species, as was observed for zinc. 

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... 

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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