Particle generation mechanisms

Fig. 6. Size distribution of an urban aerosol showing the three modes containing much of the aerosol mass. The fine mode contains particles produced by condensation of low volatility gases. The mid-range, or accumulation mode, results from coagulation of smaller aerosols and condensation of gases on preexisting particles. Coarse particulates, the largest aerosols, are usually generated mechanically.

DUST Solid particles generated by mechanical action, present as airborne contaminant (e.g. <75 pm in size). 

The particle generation rate was calculated by a step mechanism, namely formation of primary precursor particles by homogeneous nucleation (JLQ.) followed by coagulation to latex particles (8-9). This homogeneous nucleation mechanism is often referred to as the HUFT mechanism for its originators Hansen, Ugelstad, Fitch, and Tsai. 

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 direct destruction of 03 by its reaction on soot particles generated by aircraft at midlatitudes has also been proposed, but given the large uncertainties in the mechanism and kinetics of this reaction, it is not clear that this will prove to be significant (Bekki, 1997 Lary et al., 1997). 

Dry Powder A tenn applied to the extinguishing agent suitable for use on combustible metals. Dusts Solid particles generated by mechanical processes such as crushing and grinding, without any chemical change from the paren material. Their size range is typically between 0.1 and 100 microns. 

THE AMBIENT ATMOSPHERIC AEROSOL consists of liquid and solid particles that can persist for significant periods of time in air. Generally, most of the mass of the atmospheric aerosol consists of particles between 0.01 and 100 xm in diameter distributed around two size modes a coarse or mechanical mode centered around 10- to 20- xm particle diameter, and an accumulation mode centered around 0.2- to 0.8- xm particle diameter (1). The former is produced by mechanical processes, often natural in origin, and includes particles such as fine soils, sea spray, pollen, and other materials. Such particles are generated easily, but they also settle out rapidly because of deposition velocities of several centimeters per second. The accumulation mode is dominated by particles generated by combustion processes, industrial processes, and secondary particles created by gases converting to par-... 

Microemulsion polymerizations follow a different mechanism from the conventional emulsion polymerizations. The most probable locus of particle nucle-ation was suggested to be the microemulsion monomer droplets [27], although homogeneous nucleation was not completely ruled out. The particle generation rate in microemulsion polymerization is given by an expression similar to Eq. (21), which was used for the miniemulsion polymerization of styrene [28] ... 

To maximize drug particle dispersion, mechanical means may be introduced into the flow path to generate a turbulent airflow that exerts... 

The coarse mode (c/ae > 1pm) include particles generated by mechanical processes and introduced directly into the atmosphere from anthropogenic and natural sources (Horvath, 2000). A few examples include sea spray, erosion, resuspension, and industrial and agricultural activities. 

In both processes of initiation and chemical induction active particles are generated. This is a general feature for all types of initiated and conjugated reactions. As shown in Chapter 2, the basic difference is observed from the very moment of active particle influence on the target reaction (in the case of chemical induction on the secondary reaction). Investigation of the active particle action mechanism on the target reaction gives an opportunity to determine the cases in which initiation or chemical induction is displayed. 

Present knowledge of the atmospheric aerosol and the limits of this review do not permit a detailed comparative analysis of the available variety and types of atmospheric particles, whose mechanisms of generation are both natural and anthropogenic. Therefore, proceeding from comparative data on the power of the sources of different kinds of aerosols... 

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