Sizes of Atmospheric Particles

Atmospheric aerosols consist of particles ranging in size from a few tens of angstroms (A) to several hundred micrometers. Particles less than 2.5 pm in diameter are generally referred to as fine and those greater than 2.5 pm diameter as coarse. The fine and coarse particle modes, in general, originate separately, are transformed separately, are removed from the atmosphere by different mechanisms, require different techniques 

Transient Nuclei or I Accumulation Mechanically Generated Aitken Nuclei Range I Range Aerosol Range 

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. 

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The atmosphere, whether in remote or urban areas, always contains significant concentrations of particles, up to 10x cm These may have diameters anywhere within the entire range from molecular clusters to 100 /xm. Because the size of atmospheric particles plays such an important role in both their chemistry and physics in the atmosphere as well as in their effects, it is important to know the distribution of sizes. We thus consider first how these size distributions are characterized. 

Since the size of atmospheric particles covers several orders of magnitude (see Subsection 4.1.1) the concentration alone is not sufficent to characterize atmospheric particles. For more complete aerosol characterization the size... 

The size of atmospheric particles ranges from molecular dimensions up to a diameter of above 10 m. Since particles with diameters below 0.1 fim (molecules and Aitken s nuclei) pre.sent no important problem and particles with diameters exceeding 10 fim are simply removed from the atmosphere by gravitational sedimentation, particles with diameters between 0.1 and 10 m are the main focus of interest. They can affect the reflection and scattering of the incident solar light, the local cloudiness and precipitation. 

The particle size, volume concentration, and number are probably the most important physical properties used in describing the atmospheric aerosol. The size of atmospheric particles covers a range of five orders of magnitude, from a few nanometers up to several hundred micrometers in diameter. The total particle volume can differ from 1 to 300 pm cm depending on the origin of the aerosol. 

Fig. 5. Size distributions of atmospheric particles ia (—) urban, (------) mral, and (------) remote background areas.

AA-gpjj. Conditionally, the ionic atmosphere is regarded as a sphere with radius r. The valnes of approach the size of colloidal particles, for which Stokes s law applies (i.e., the drag coefficient 9 = where r is the liquid s viscosity) when they... 

A role for iron has been suggested in pathological effects derived from reactive oxygen following the inhalation of other particulate matter in the atmosphere. As with asbestos, the size of the particles is especially important, and particles with an aerodynamic diameter of less than 10 pm have been associated with increased mortality and morbidity. 

The interplay of these two basic rates determines the size of the resulting particles. For instance, the reason that snow flakes reach sizes of several cm at lower latitudes but arrive as extremely small crystals, called diamond dust in Antarctica, is that the nuclei that are formed in a cloud, will grow during their voyage to earth by adsorbing water molecules. Obviously, this growth will be more important in the moist atmosphere at low latitudes than in the extremely dry atmosphere above Antarctica. The same interplay of nucleation and growth determine the size of metal particles that are formed on a support by chemical reduction of adsorbed precursors, such as metal ions. Here... 

Move 1] Currently atmospheric particulate matter is regulated based on various size categories because of the apparent association between particle size and adverse health effects. [Move 2] However, the current size-based understanding of atmospheric particles is relatively crude because it does not account for differences in the chemical composition of these particles. Presumably a chemically reactive particle has a greater potential for damage than a chemically inert particle of comparable size. Of the metals potentially... 

The condensation of low-volatility vapors on preexisting particles depends on a number of factors, including the rate of collisions of the gas with the surface, the probability of uptake per collision with the surface, i.e., the mass accommodation coefficient (see Chapter 5.E.1), the size of existing particles, and the difference in partial pressure of the condensing species between the air mass and the particle surface. While some of these parameters are reasonably well known, others are not. For example, mass accommodation coefficients for the complex surfaces found in the atmosphere are not well known. Indeed, the exact nature of the surfaces themselves, which determines the uptake and the partial pressures of gases at the surface, remains a research challenge. 

Upon reconsidering this subject, it occurred to me that I had never contemplated the effect of dijference of size in the particles of elastic fluids. By size I mean the hard particle at the centre and the atmosphere of heat taken together. If, for instance, there be not exactly the same number of atoms of oxygen in a given volume of air, as of azote in the same volume, then the sizes of the particles of oxygen must be different from those of azote. And if the sizes be different, then on the supposition that the repulsive power is heat, no equilibrium can be established by particles of unequal sizes pressing against each other. 

The fact that fine atmospheric particles are enriched in a number of toxic trace species has been known since the early 1970s. Natusch and Wallace (20, 21) observed that the fine particles emitted by a variety of high-temperature combustion sources follow similar trends of enrichment with decreasing particle size as observed in the atmosphere, and they hypothesized that volatilization and condensation of the trace species was responsible for much of the enrichment. Subsequent studies of a number of high-temperature sources and fundamental studies of fine-particle formation in high-temperature systems have substantiated their conclusions. The principal instruments used in those studies were cascade impactors, which fractionate aerosol samples according to the aerodynamic size of the particles. A variety... 

Wiedensohler A et al (2012) Particle mobility size spectrometers harmonization of technical standards and data structure to facilitate high quality long-term observations of atmospheric particle number size distributions. Atmos Meas Tech 5 657-685... 

Deposition velocities depend on the atmospheric stability, nature of the surface, nature of the chemical (for a gas-phase chemical), and (for a particle or particle-phase chemical) the size of the particle. For particle deposition, the deposition velocity is a minimum for particles with mean diameter in the range 0.3-0.5 pm, and it increases with both increasing and decreasing particle size (Eisenriech et al., 1981 Bidleman, 1988). 

Tin may be transported in the atmosphere by the release of particulate matter derived from the combustion of fossil fuels and solid wastes. The vapor pressure of elemental tin is negligible (Cooper and Stranks 1966). Tin in aerosol samples that existed in particulate-carbon masses was removed from the atmosphere predominantly by gravitational settling (Byrd and Andreae 1986). The half- life of airborne particles is usually on the order of days, depending on the size of the particle and atmospheric conditions (Nriagu 1979). Removal by washout mechanisms (such as rain) is thought to be unimportant. 

The nucleation mode (ii ie < 0.1 pm) accounts for the majority of particles by number but because of their small size, these particles rarely account for more than a few percent of the total mass of atmospheric particles. These particles originate from condensation of supersaturated vapors from combustion processes and from the nucleation of atmospheric particles to form fresh particles (Seinfeld and Pandis, 1998 Horvath, 2000). 

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