Solid Aerosol Particles

Aerosol particles in the atmosphere contain a variety of volatile compounds (ammonium, nitrate, chloride, volatile organic compounds) that can exist either in the particulate or in the gas phase. We estimate in this section the timescales for achieving thermodynamic equilibrium between these two phases and apply them to typical atmospheric conditions. The problem is rather different compared to the equilibration between the gas and aqueous phases in a cloud discussed in the previous section. Aerosol particles are solid or concentrated aqueous solutions (cloud droplets are dilute aqueous solutions), they are relatively small, and aqueous-phase reactions in the aerosol phase can be neglected to a first approximation because of the small liquid water content. 

If the air surrounding an aerosol population is supersaturated with a compound A, the compound will start condensing on the surface of the particles in an effort to establish thermodynamic equilibrium. The gas-phase concentration of A will decrease, and its particulate-phase concentration will increase until equilibrium is achieved. The characteristic time for the two phases to equilibrate due to the depletion of A in the gas phase will be inversely proportional to the total flux of A to the aerosol phase. 

For solid-phase aerosol particles the equilibrium concentration of A is constant and does not change as A is transferred to the aerosol phase. However, if the aerosol contains water and A is water-soluble, the equilibrium concentration of A will increase as condensation proceeds, and equilibration between the two phases will be accelerated. This change in equilibrium concentration is a result of changes in the chemical composition of the particle as A is transferred to the particulate phase. The cases of solid and aqueous phases will be examined separately in subsequent sections, based on the analysis of Wexler and Seinfeld (1990). 

If the aerosol population is monodisperse and consists of N particles per cm3, then the total flux J from the gas phase to the aerosol phase will be 

The characteristic time for gas-phase concentration change and equilibrium establishment X, can be estimated by nondimensionalizing (12.130). The characteristic concentration is ceq and the characteristic timescale is xs, so we define 

The above flux will be equal to the rate of change of the concentration Cq dcc 

Once more, all the physical information about the system has been included in the dimensionless parameter u. Setting 1, we find that 

We can express this timescale in terms of the aerosol mass concentration nip given by 

---

Particle size selective OEL have already been established to address the problems associated with specific health effects, especially within fields where solid aerosol particles are to be measured. A number of filter cassette designs have been tailored to sample specific size ranges of the total aerosol fraction, in order not to overestimate the exposure causing health effects by inhalation. Since only particles less than approximately 100 /rm can be inhaled and the inhaled aerosol fraction is most certainly the main contributor to specific health effects among workers exposed to isocyanates, it seems reasonable to use sampling devices designed for sampling of the inhalable... 

Langer, G. and G. Yamate. Encapsulation of bquid and solid aerosol particles to form dry powders. Journal of Colloid and Interface Science 29(3) (1969) 450—455. 

Allen, M. D. and Raabe, O. G. Slip correction measurements of spherical solid aerosol particles in an improved Millikan apparatus. Aerosol Sci. Tech., 4, 269-286, 1985. 

At present there are more than fifteen widely used different test methods to evaluate smoke, each employing its own unique set of heating conditions, sample size and orientation, gas flow and means of smoke measurement. The most frequently used tests are those based on optical methods, i.e. attentuation of a light beam due to the sample formulations burning. There are also mechanical methods, those based on separation of liquid and solid aerosol particles from the smoke gases, the Arapahoe method and electrical methods (generation of electrical charges in an ionization chamber). 

Liquid aerosol particles are nearly always spherical. Solid aerosol particles usually have complex shapes, as shown in Figs. 1.1—1.5. In the development of the theory of aerosol properties, it is usually necessary to assume that the particles are spherical. Correction factors and the use of equivalent diameters enable these theories to be applied to nonspherical particles. An equivalent diameter is the diameter of the sphere that has the same value of a particular physical property as that of an irregular particle. For approximate analysis, shape can usually be ignored, as it seldom produces more than a twofold change in any property. Particles with extreme shapes, such as long, thin fibers, are treated as simplified nonspherical shapes in different orientations. The complex shape of some fiime and smoke particles can be characterized by their fractal dimension. (See Section 20.2.)... 

Allan MD, Raabe OG. Slip eorreetion measurements of spherieal solid aerosol particles in an improved Milikan apparatus. Aerosol Sci Teehnol 1985 4 69 86. Gebhart J, Heyder J, Stahihofen W. Use of aerosols to estimate pulmonary air-spaee dimensions. J Appl Physiol 1981 51 465-476. 

When a liquid or solid substance is emitted to the air as particulate matter, its properties and effects may be changed. As a substance is broken up into smaller and smaller particles, more of its surface area is exposed to the air. Under these circumstances, the substance, whatever its chemical composition, tends to combine physically or chemically with other particles or gases in the atmosphere. The resulting combinations are frequently unpredictable. Very small aerosol particles (from 0.001 to 0.1 Im) can act as condensation nuclei to facilitate the condensation of water vapor, thus promoting the formation of fog and ground mist. Particles less than 2 or 3 [Lm in size (about half by weight of the particles suspended in urban air) can penetrate the mucous membrane and attract and convey harmful chemicals such as sulfur dioxide. In order to address the special concerns related to the effects of very fine, iuhalable particulates, EPA replaced its ambient air standards for total suspended particulates (TSP) with standards for particlute matter less than 10 [Lm in size (PM, ). 

There is a large variety of atmospheric sulfur compounds, in the gas, solid, and liquid phases. Table 7-3 lists a number of gaseous compounds, range of concentration, source, and sink (where known). As this list illustrates, a significant number of these gases contribute to the existence of oxidized sulfur in the forms of SO2 and sulfate aerosol particles. Table 7-4 lists the oxy-acids of sulfur and their ionized forms that could exist in the atmosphere. Of these the sulfates certainly are dominant, with H2SO4 and its products of neutralization with NH3 as the most frequently reported forms. 

H2CO3 (carbonic acid) aqueous phase HC2H3O2 (acetic acid) aqueous, gas phases HCHOz (formic acid) aqueous, gas phases H2C2O4 (oxalic acid) aerosol particles solid phase RCCX3H (many carboxylic acids)... 

Aerosols, like solutions, are mixtures. Unlike solutions, however, they are not single phases. Instead, an aerosol is a suspension in a gas of tiny particles of a condensed phase, either liquid or solid. The particles that make up an aerosol can have a range of diameters between 10 nm and 10 // m. 

Fig. 5. Retention of 144Ce in lung, liver, skeleton, and soft tissue remainders of Beagle dogs after inhalation of l44Ce chloride in Cs chloride aerosol particles. Average values and total ranges of data are shown in the upper figure along with solid line curves which were projected from the biological model, all of which include physical decay. The lower figure shows the same model projections only corrected for physical decay.

When estimating air-aerosol partitioning of gas phase substances such as PAHs, most of which are solids, it is usual to use the liquid state vapor pressure as the correlating parameter. This is because the PAH is effectively in a liquidlike state on or in the aerosol particle. It does not exist in crystalline form. 

In the case of S02, oxidation in the aqueous phase, present in the atmosphere in the form of aerosol particles, clouds, and fogs, is also important. Thus S02 from the gas phase dissolves in these water droplets and may be oxidized within the droplet by such species as H202, 03, 02, and free radicals. Oxidation of S02 on the surfaces of solids either present in the air or suspended in the water droplets is also possible. On the other hand, it is believed that HN03 is formed primarily by reaction (10) in the gas phase and subsequently dissolves in droplets. 

Because of the gaseous nature of many of the important primary and secondary pollutants, the emphasis in kinetic studies of atmospheric reactions historically has been on gas-phase systems. However, it is now clear that reactions that occur in the liquid phase and on the surfaces of solids and liquids play important roles in such problems as stratospheric ozone depletion (Chapters 12 and 13), acid rain, and fogs (Chapters 7 and 8) and in the growth and properties of aerosol particles (Chapter 9). We therefore briefly discuss reaction kinetics in solution in this section and heterogeneous kinetics in Section E. 

FIGURE 12.30 Comparison of predicted ratio NO /NO,. as a function of latitude at 19.5-20.5 km using only gas-phase chemistry (dotted line) or with the N205 hydrolysis on aerosol particles (solid line) compared to measured values shown as unfilled circles (from Fahey et al., 1993). 

FIGURE 13.9 Trend in column O, (a) from November 1984 to May 1991 based on SAGE II measurements in the stratosphere (at pressures above 82.5 mbar, 0 and dashed lines) and TOMS measurements (solid line) and (b) from January 1988 to May 1991, which was a period of relatively small and stable aerosol particle concentrations. Note change in vertical scale. (Adapted from Cunnold et al., 1996.)... 

There are other aerosol methods which can yield uniform powders, such as by dispersing aqueous dispersions of particles (e.g. of latex) and evaporating the water (12). In this case each droplet should contain only one particle, a task not easily accomplished. Alternatively, it is possible to nebulize solutions of electrolytes or other substances, which on removal of the liquid result in solid particles, dispersed in the carrier gas (13,14). This process has been expanded to include sintering of resulting solid aerosols in a continuous process to produce powders for various applications (15-18). 

Purification consists of removal of gaseous impurities by means of absorption or adsorption processes (such as by activated charcoal) Refs 1) F. Fraas O.C. Ralston, I EC 32, v 600—04(1940) (Ekectrostatic sepn of solids from gases) 2) H.F. Johnstone M,H. Roberts, IEC 41, 2417(1949) (Deposition of aerosol particles from moving gas streams)... 

---