Particles condensation processes

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. 

Nucleation A cleaning process using a humidification and cooling cycle, causing water or another fluid to condense on sub-micrometer particles. This process increases particle size until impingement on packing is possible. 

In the case of organic constituent formation, growth of particles is governed by physical laws of condensation, provided that the precursors are formed in the gas phase. For a diffusion-limited condensation process, the rate of volume change in particles is... 

As exhaust gases and fly ash particles are vented from the furnace, they quickly begin to cool, leading to the condensation and adsorption of volatilized elements onto the surfaces of fly ash particles entrained in the gas stream (Kaakinen etal. 1975). Under high-temperature combustion conditions certain elements, including S, are enriched on the surface of particles (Davison et al. 1974 Smith 1980). A vaporization-condensation process is the primary mechanism... 

Ullrafine particles (UFPs) of metal and semiconductor nitrides have been synthesized by two major techniques one is the reactive gas condensation method, and the other is the chemical vapor condensation method. The former is modified from the so-called gas condensation method (or gas-evaporation method) (13), and a surrounding gas such as N2 or NII2 is used in the evaporation chamber instead of inert gases. Plasma generation has been widely adopted in order to enhance the nitridation in the particle formation process. The latter is based on the decomposition and the subsequent chemical reaction of metal chloride, carbonate, hydride, and organics used as raw materials in an appropriate reactive gas under an energetic environment formed mainly by thermal healing, radiofrequency (RF) plasma, and laser beam. Synthesis techniques are listed for every heal source for the reactive gas condensation method and for the chemical vapor condensation method in Tables 8.1.1 and 8.1.2, respectively. 

Colloidal dispersions can be formed either by nucleation with subsequent growth or by subdivision processes [12,13,16,25,152,426], The nucleation process requires a phase change, such as condensation of vapour to yield liquid or solid, or precipitation from solution. Tadros reviews nucleation/condensation processes and their control [236], Some mechanisms of such colloid formation are listed in Table 7.1. The subdivision process refers to the comminution of particles, droplets, or bubbles into smaller sizes. This process requires the application of shear. Some of the kinds of devices used are listed in Table 7.2 [228]. 

The CF-CVC (combustion flame-chemical vapor condensation) process developed by Kear and co-workers (Skandan et al., 1996 Tompa et al., 1999) is a continuous process using the equipment shown in Fig. 1. The starting materials are metal complexes that can be vaporized and fed into a flat flame, which immediately converts the compounds to nanostructured metal oxides. The particle dilution is controlled to prevent agglomeration in a hot state... 

Interparticle transport may be possible either by surface diffusion across the support or by vapor phase transport. Depending on the supported systems and on the sintering conditions the particles may grow predominantly via one of these possible routes. Supported PcFe deposits have been sintered in experimental conditions close to that of the condensation process (T = 235°C, residual pressure 10 torr). The surface areas of PcFe after 2 and 3 hours of sintering are shown in Table IV. It is seen that the sintering effect is more pronounced with a homogeneous carbon support. 

In cases where the particle surfaces are not wettable, condensation proceeds with much more difficulty. This is because the condensing liquid tends to pull into small spheres on the particle surface, and only when the entire surface is covered with these spheres is a liquid coating formed. Fletcher (1958a, b) has treated this problem by considering the contact angle between an embryo sphere formed on the particle and the particle surface. His results correspond to what has been observed experimentally—it is very difficult to get condensation to take place on nonwettable particles unless high supersaturations are used. The role of insoluble nuclei in the condensation process is still in question and remains another problem for future investigators to solve. 

The Si02 walls are built up by condensation processes, for which the anchored silicate species act as nucleation points. The enrichment of the oxide at the templating crystals occurs by an Ostwald ripening mechanism the particles grow in size at the tube walls and highly soluble very small silica particles re-dissolve to the reaction solution. The polycondensation reaction is finished within about I2h. 

The laser ablation or laser-heated gas phase condensation process could produce NPs less than 50 nm with broad-size distributions. For example. Filers and Tissue reported that the EU2O3 NPs could be made by a laser-heated gas phase condensation process. The particles are polydis-perse in a range of 2-30 nm (Filers and Tissue, 1995). 

Our understanding of stratospheric aerosols is still far from being satisfactory. In particular, the gas-to-particle conversion processes involving SO2 oxidation and condensation nuclei formation are still poorly understood. Future research, therefore, should include in situ measurements of aerosols, trace gases and condensation nuclei involved in aerosol formation. Accompanying laboratory studies of relevant chemical processes are also needed. 

Gas-to-particle conversion processes in the atmosphere consist of formation of low-vapor-pressure gases formed by atmospheric chemical reactions followed by new particle formation (nucleation), condensation of the low-vapor-pressure material on existing particles, or both. Chemical reaction producing the low-vapor-pressure product also occurs in cloud droplets, with the product remaining in the condensed phase of clear-air aerosol particles following cloud evaporation. Substances of intermediate vapor pressure may reversibly... 

Other clear-air, gas-to-particle conversion processes include uptake of acidic gases by basic aerosol (e.g., SO2 uptake by sea salt or carbonate dust) and uptake of ammonia by acidic aerosol. Some such processes are reversible. An example is the co-condensation of HNO3 and NH3 to form ammonium nitrate, for which the equilibrium constant is rather strongly temperature dependent. An example of the release of species from the condensed to the gas phase is the uptake of HNO3 by sea salt, resulting in release of HCl into the gas phase. 

Condensation processes may play a decisive roll by incorporating salt aerosol into larger droplets. Under these conditions the upper size limit of particles being sampled will determine the observed salt load. Thus, we... 

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