Particles, atmospheric removal mechanisms

Once PAHs enter the atmosphere, they are distributed between gas and particle phases and subject to removal mechanisms, such as oxidative and... 

Figure 7.1 Major sources and modes of atmospheric aerosol particles and principal removal mechanisms (Whitby and Cantrell, 1976 Seinfeld and Pandis, 1998).

An example of a highly efficient interception system is the canopy of a coniferous forest. The large specific surface area of pine and spruce trees exceeds that of broadleaved deciduous trees, giving them a high scavenging efficiency for radioactive aerosol particles. Pine needles have been acknowledged as useful monitors of atmospheric pollution (Eriksson et al., 1989). However, radionuclides will be redistributed in the forest ecosystem due to various removal mechanisms such as rain... 

On a large scale, particles (as well as gases) are moved through the atmosphere by advection and turbulence, i.e., horizontal and vertical winds (Wexler et al. 1994 Seinfeld and Pandis 1998). Simultaneous with these large-scale motions are the smaller-scale processes that can transport particles across surface boundary layers (e.g., at the Earth s surface) and thus remove them. As discussed earlier, diffusion is the dominant removal mechanism for small particles because of their high diffusion coefficients and low gravitational settling velocities. Because of their very small sizes, nanoparticles can slip... 

A very significant peculiarity of the atmosphere is that so-called self-cleaning processes take place within the air. Due to these processes even those trace constituents which have no other chemical or biological sinks are relatively quickly removed from the air. Hence these removal mechanisms are of great importance for the control of the atmospheric pathways of some trace gases and aerosol particles (see Chapter 5). 

FIGURE 2.7 Idealized schematic of the distribution of particle surface area of an atmospheric aerosol (Whitby and Cantrell 1976). Principal modes, sources, and particle formation and removal mechanisms are indicated. 

Once particles are in the atmosphere, their size, number, and chemical composition are changed by several mechanisms until ultimately they are removed by natural processes. Some of the physical and chemical processes that affect the aging of atmospheric particles are more effective in one regime of particle size than another. In spite of the specific processes that affect particulate aging, the usual residence time of particles in the lower atmosphere does not exceed several weeks. Very close to the ground, the main mechanisms for particle removal are settling and dry deposition on surfaces whereas at altitudes above about 100 m, precipitation scavenging is the predominant removal mechanism. 

Some hazardous waste species in the atmosphere are removed by adsorption onto aerosol particles. Typically, the adsorption process is rapid so that the lifetime of the species is that of the aerosol particles (typically a few days). Adsorption onto solid particles is the most common removal mechanism for highly nonvolatile constituents such as benzo(a)pyrene. 

The atmospheric aerosol—that is, the particles that are normally found in oiu atmosphere—is a complex and dynamic mixture of solid and liquid particles from natural and anthropogenic sources. We consider first the natural background aerosol—the aerosol that would be present in the absence of human activity. At the other extreme the urban aerosol is dominated by anthropogenic sources. In both cases, primary particles are continuously emitted into, and secondary particles are formed in, the atmosphere. Both kinds of particles may undergo growth, evaporation, or chemical reactions and are subject to various removal mechanisms. The size distributions one finds in the atmosphere reflect the complex interaction of all these processes. We omit from this discussion the largest and most important atmospheric aerosol natural clouds of water droplets. 

Neat soap may or may not be blended with other products before flaking or powdering. Neat soap is sometimes filtered to remove gel particles and run into a reactor (crutcher) for mixing with builders. After thorough mixing, the finished formulation is run through various mechanical operations to produce flakes and powders. Because all of the evaporated moisture goes to the atmosphere, there is no wastewater effluent. 

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