Dry aerosols

Dry aerosols, or particulate matter, differ so much from the carrying gas stream that their removal should present no major difficulties. The aerosol is different physically, chemically, and electrically. It has vastly different inertial properties than the carrying gas stream and can be subjected to an electric charge. It may be soluble in a specific liquid. With such a variety of removal mechanisms that can be applied, it is not surprising that particulate matter, such as mineral dust, can be removed by a filter, wet scrubber, or electrostatic precipitator with equally satisfactory results. 

A second factor in the deposition process is the physical size of the radioaerosol particles deposited during dry deposition. Particle size affects the efficiency of foliar collection of dry aerosol particles and may also be important in determining the rate of solution of soluble constituents within the aerosol particle. Lockhart et al. (7) indicated that nuclear debris in surface level air is generally associated with aerosol particle sizes about 0.5-1.0p in diameter. Martell (8) stated that high yield detonations causing condensation of "Sr at stratospheric altitudes leads to the formation of particles with diameters less than 1 fi. Hence,... 

The limits to the areal density of deposit for filters are set by clogging of the filter that sets in at typically 100 xg/cm2. The limit of areal density for impactors is set by the problem of particle bounce. This is a serious problem for coarse, dry aerosols but less so for fine, wet, secondary aerosols. Nevertheless, sticky substrates are universally used (19), and deposits are generally limited to a few monolayers of particles for a 2.5- xm particle. This limit amounts to no more than 7 xm of deposit, or, for 1.5- xg/m3 aerosols (per stage), about 1000 xg/cm2 of deposit. 

For special applications direct current plasma (DCP) (Leis et al., 1989) and micro-wave-induced plasma (MIP) may be used. The MIP first became widely used as a spectroscopic radiation source after a stable discharge at atmospheric pressure had been obtained (Beenakker, 1977 Beenakker et al., 1978). The MIP is not capable of taking up wet aerosols, but is useful for the excitation of dry aerosols, produced by electrothermal evaporation from a graphite furnace (Aziz et a ., 1982). Direct sample insertion has been discussed recently by Blain and Savin (1992). 

To analyze metals and alloys directly without dissolution, both spark ablation [349] and laser ablation [61,211] dry aerosol generation systems have been used to introduce samples into an ICP-MS. These approaches often require matrix-matched standards, although several active research groups are focusing on techniques to reduce that requirement. The amount of material ablated depends on the sample type. Fractionation of elements can also be a problem, depending on the sample, the laser fluence, the laser wavelength, and the number of laser pulses used to sample from a fixed location. Volatile elements that are segregated in the samples appear to be most prone to fractionation problems [61],... 

Estimated retained dose (20-30%) using dry aerosol of T-2 toxin, all animals died within 18 h, Cresia and Lambert (1989)... 

Fig. 6.4 Horizontal distribution of the dry aerosol mass (a) (at 20 m above ground) and the according shortwave radiation balance (b), both for 20.08.2005, 12 UTC...

Fraction of light incident upon an aerosol that is removed from the primary beam per unit length by scattering from aerosol particles. Highly sensitive to RH for hygroscopic materials, but often measured and reported for dried aerosol depends on wavelength A and complex index of refraction... 

Figure 28-6 Processes leading to atoms, molecules, and ions with continuous sample introduction into a plasma or flame. The solution sample is converted into a spray by the nebulizer. The high temperature of the flame or plasma causes the solvent to evaporate, leaving dry aerosol particles. Further heating volatilizes the particles, producing atomic, molecular, and ionic species. These species are often in equilibrium, at least in localized regions.

The relative importance of wet and dry deposition in removing PAHs from the atmosphere varies with the individual PAH. For example, Perwak et al. (1982) estimated that a total of 23% of benzo[a]pyrene released to the atmosphere is deposited on soil and water surfaces. Dry deposition of benzo[a]pyrene adsorbed to atmospheric aerosols accounts for most of the removal wet deposition is less significant by a factor of 3-5. In a mass balance study of the atmospheric deposition of PAHs to Siskiwit Lake, which is located on a wilderness island in northern Lake Superior, dry aerosol deposition of particulate phase PAHs was found to be the predominant form of input to surface waters by an average ratio of 9 1 over wet deposition (McVeety and Hites 1988). 

The NO /SO ratio in fog is higher than in the dry aerosol proceeding the fog, which suggests that gaseous HNO3 is incorporated into the fog. 

Figure 9.5 Dry aerosol antiperspirant formulation (a cyclomethicone, ex Dow Corning Int., Brussels, Belgium b cyclomethicone and dimethiconol, ex Dow Corning)...

Strontium present in the atmosphere is in the form of wet or dry aerosols. The principal chemical species in the air is strontium oxide. Strontium oxide can undergo transformation to strontium ions by reaction with water. Strontium is dispersed by atmospheric cycling and subsequently deposited by wet deposition on the earth s surface. In surface and groundwater, strontium exists primarily as a hydrated ion. 

In this book, except for the discussion of removal process in Chapter 5 we always deal with unsaturated air, that is. with "dry aerosol. ... 

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