Removal of aerosol particles

The reduction in PAEC is considerably higher than can be accounted for by filtration directly (Jonassen, 1984a, Jonassen and McLaughlin, 1984), because the filtration as suggested, not only removes radioactive material, but also through removal of aerosol particles increases the unattached fractions and thus enhances the plate out- removal of airborne radioactivity. On the other hand the filtration-induced increase in unattached fractions will increase the average radiological dose per unit of PAEC and this is responsible... 

Gebhart, J. and J. Heyder, Removal of Aerosol Particles from Stationary Air within Porous Media, J. Aerosol Sci. 16 175-187 (1985). 

The observations by Radke et al. (1982) of efficient removal of aerosol particles produced by the TITAN satellite launch (P.V. Hobbs, personal communication) may not be applicable, as the aerosol from the TITAN launch must have been strongly hygroscopic because of the presence of high concentrations of the combustion product HCl, which with ambient NH3 will produce yield NH4CI aerosol. 

The problem of the removal of aerosol particles from gas streams has become of increasing importance from the standpoint of public health and the recovery of valuable products. Technology of controlling the aerosol particles or improving the liquid phase of aerosol is very important in many industrial processes such as oil and petroleum, electronic, mining, and food, as well as waste products like noxious emission of aerosol in chemical plants. There are several ways for this purpose among which fibrous filters are more popular so that it is obvious to try to improve their efficiency. The efficiency of collection and the pressure drop are the most important practical considerations in the design of these fibrous filters [2], Various... 

The problems related to the water cycle will also not be considered in spite of the fact that, taking into account its quantity and atmospheric effects, water is one of the most important trace materials. This omission is explained by a historical precedent. The study of the atmospheric cycle of the water as well as the measurement of its concentration were included in the past in the program of other branches of atmospheric science. Thus, the formation of clouds and precipitation, the subject of the cloud physics (e.g. Mason, 1957, Fletcher, 1962), will only be discussed in relation with the wet removal of aerosol particles and water-soluble gases. 

In the upper troposphere the size distribution of large and giant particles was investigated by Soviet (Kondratyev et al.. 1969) and American (Blifford. 1970) research workers. Particles were collected by impactors in both cases. Figure 29 shows Blifford s size distributions for different altitudes, obtained over Nebraska, U.S.A. An interesting feature emerging from the distributions presented is the decrease in the steepness of the slope in the distributions (that is the value of fi in equation [4.12] decreases). It is very difficult to explain this peculiarity of aerosol behaviour. However, it is believed that the removal of aerosol particles by cloud elements (Chapter 5) plays an important role in control of the size distribution of aerosol particles in the troposphere. 

The aim of this chapter is to present briefly our ideas on the removal of aerosol particles and water soluble gases (e.g. S02, NH3, N02), both during dry weather conditions (dry removal) and during periods with cloud and precipitation formation (wet removal). Because of wet removal, precipitation water contains many soluble (and insoluble) materials, as we will see at the end of this chapter. 

The removal of aerosol particles under dry weather conditions is caused by turbulent diffusion and gravitational sedimentation, which transport particles to the Earth s surface, as well as by impaction on vegetation, buildings and other objects. Turbulent diffusion itself does not remove particles (Twomey, 1977). Soil and other surfaces are bordered by a thin laminar layer ( 1 mm thick) across which particles must be transported by other processes (e.g. phoretic forces, molecular diffusion, sedimentation). 

Disregarding impaction, dry removal of aerosol particles can be described formally by the following equation (e.g. Makhonyko, 1966) ... 

Finally, it is to be noted that in the foregoing discussion of dry removal we considered the total ensemble of particles. If size ranges are taken into account separately, additional sinks have to be mentioned. Thus, thermal coagulation of particles with very small size, as well as the condensation (below a relative humidity of 100 %) of vapours with low saturation vapour pressure provide effective removal for Aitken particles. It is believed (Hidy, 1973) that these processes are dominant in the removal of aerosol particles in the size range below 0.1 /im radius. 

As for other constituents of the atmosphere, it is possible to set up a mass budget of the aerosol and to calculate its residence time. The main problem is to characterize the global distribution of particulate matter in order to determine its total mass in the troposphere. One may then apply the emission estimates of Table 7-11 to calculate the tropospheric residence time ta with the help of Eq. (4-11). This approach will be discussed in the first part of this section. Subsequently, we consider an independent method for estimating the residence time, which results from the use of radioactive tracers. Finally, the removal of aerosol particles by sedimentation and impactation at the Earth surface will be discussed. 

Rhode, H., and J. Grandell (1972). On the removal of aerosol particles from the atmosphere by precipitation scavenging. Tellus 24, 442-454. 

The quintessential engineered system for the removal of aerosol particles is the spray found in the containments of many pressurised water reactors and in the drywells of many boiling water reactors. Sprays remove particles by ... 

Pathways for venting the containment atmosphere may be provided for a number of reasons, and these pathways may be equipped with filters to remove iodine from the vented gas. Filters for iodine removal can be present in both passive systems (in which flow continues only as long as there is a pressme difference) and active systems (in which there is a continuous forced flow at a controllable rate). Dry filters intended for the removal of aerosol particles are not likely to be effective for the removal of gaseous forms of iodine, especially organic iodides. Even if gaseous iodine will absorb on the filter mediiun, heat loads on the filter medium caused by radioactive decay can lead to revaporization of the absorbed iodine. Filters that involve water must be maintained at high pH to avoid the formation of volatile forms of iodine by processes identical to those that occur in reactor containment smnps. 

In the diffusion battery, the gaseous ions are sorbed by the wall of the diffusion tube while en route through it, analogous to the removal of aerosol particles by the battery wall. The fraction penetrating the battery, F, is given by the equation (Townsend, 1900)... 

Other lesser mechanisms that result in aerosol removal by filters are (1) gravitational settling due to the difference in mass of the aerosol and the carrying gas, (2) thermal precipitation due to the temperature gradient between a hot gas stream and the cooler filter medium which causes the particles to be bombarded more vigorously by the gas molecules on the side away from the filter element, and (3) Brownian deposition as the particles are bombarded with gas molecules that may cause enough movement to permit the aerosol to come in contact with the filter element. Browruan motion may also cause some of the particles to miss the filter element because they are moved away from it as they pass by. For practical purposes, only the three mechanisms shown in Fig. 29-1 are normally considered for removal of aerosols from a gas stream. 

As a rule, simulations consider emissions of heavy metals from anthropogenic and natural sources, transport in the atmosphere and deposition to the underlying surface (Figure 6). It is assumed that lead and cadmium are transported in the atmosphere only as a part of aerosol particles. Besides, chemical transformations of these metals do not change removal properties of their particles-carriers. On the contrary, mercury enters the atmosphere in different physical and chemical forms and undergoes numerous transformations during its pathway in the atmosphere (Ilyn et al., 2002 2004 Ilyin and Travnikov, 2003). 

Fig. 13.9a. Cunnold et al. (1996) indicate that these errors in accurately extracting the aerosol contribution were responsible for about half of the discrepancy and increases in tropospheric ozone the other half. Similar discrepancies between SAGE measurements and the results from other techniques such as ozonesondes have also been reported and attributed largely to the problem of accurately removing the aerosol particle contributions (e.g., Veiga et al., 1995 Steele and Turco, 1997b).

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)... 

Distortion of the particle size during the sampling process is a concern in the use of this probe on an aircraft. Compressional heating due to deceleration of the particles may distort the size distribution, because evaporation of water from aerosol particles reduces their diameters. Likewise, particle sizes can be reduced by use of a heater, incorporated into some models of this probe, to prevent icing when supercooled clouds are being flown through. One study (88) indicated that the probe heater removes most of the water from aerosol particles sampled at relative humidities of 95%. Thus, size distributions of aerosol particles measured with the probe heater on correspond to that of the dehydrated aerosol. These results were confirmed by a later study (90) in which size distributions of aerosols measured with a nonintrusive probe were compared to size distributions measured with a de-iced PCASP probe. Measurement of the aerosol size distribution with the probe heater on may be an advantage in certain studies. 

Particle-Particle Interactions. Loss of strong acid content of aerosol particles can also occur because of reactions between co-collected acidic and basic particles impacted together on the collection surface. This phenomenon most frequently occurs as the result of interaction of coarse (>2.5 xm diameter), alkaline, soil-derived particles with fine (<2.5 xm diameter) acidic sulfate particles (66). Particle-particle interactions with net neutralization can be reduced in many cases by sampling with a virtual impactor or a cyclone to remove coarse particles, although this procedure does not prevent the effect if external mixtures of fine particles of different acid contents are sampled. In situ methods with shorter sampling times can be used such that these topochemical reactions are less likely to occur. 

Ranada, M. B., Adhesion and Removal of Fine Particles on Surfaces, Aerosol Sci. Techno. (1986) preprint. 

By their very nature, impactors are high-pressure drop sampling devices. Air is drawn at high velocity through a fairly small nozzle to remove small aerosol particles. The particle diameter which can be collected with a 50 percent cut efficiency for a specific set of operating conditions can be determined by recalling that... 

As discussed previously, terminal settling velocities of aerosol particles are generally quite small. Under normal circumstances it is unreasonable to expect that simple sedimentation as such will be an effective removal mechanism. 

The principal processes that govern the concentration and properties of atmospheric aerosols are emission of aerosol particles and precursor gases, gas-to-particle conversion and other pertinent atmospheric chemical reactions, transport, and processes by which particles are removed from the atmosphere. There is a substantial hterature on the characterization of these processes from laboratory studies and field measurements (cf. Section 4.04.1), so only a brief overview is provided here. 

---