Atmospheric Physical Removal Processes

Compounds emitted into the atmosphere are removed from it by a variety of physical and chemical processes. The rates of removal depend on the chemical nature of the compound, its susceptibility to chemical attack, its solubility in water, and its physical state (vapor or particulate 

This chapter is devoted to chemical removal processes in the troposphere. In this section, however, we briefly discuss physical removal processes. Gases and particles are physically removed from the atmosphere by deposition at the earth s surface (so-called dry deposition) and by absorption into droplets followed by transfer of the drops to the surface in the form of precipitation (so-called wet deposition). 

Because the processes by which a molecule, or particle, is transported to the surface of the earth and absorbed by the surface are quite complex, the dry deposition process is represented by an overall transfer coefficient. It is generally assumed that the dry deposition flux is proportional to the local pollutant concentration [at a known reference height (zr), typically 10 m], resulting in the expression F = — vdC, where F represents the dry deposition flux (the amount of pollutant depositing to a unit surface area per unit time) and C is the local pollutant concentration at the reference height. The proportionality constant, vd, has units of length per unit time and is known as the deposition velocity. 

Dry deposition velocities for gases to a variety of surfaces are available in the literature (see, for example, Dolske and Gatz, 1985 Colbeck and 

Harrison, 1985 Huebert and Robert, 1985 Shepson et al., 1992). Gases that are removed readily by dry deposition have deposition velocities of order 1 cm s 1 or larger. Examples include nitric acid (HN03) and sulfur dioxide (S02). Ozone deposition velocities are about 0.5 cm s-1. With a 1 km deep atmospheric boundary layer, a gas with a deposition velocity of 1 cm s-1 has a timescale for dry deposition of the order of 1 day. Dry deposition will be an important removal process for species whose timescale for dry deposition is comparable to or smaller than that for chemical transformation. 

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In the air, both photolysis and physical removal processes such as gravitational settling of aerosols and wet deposition by rain and snow will probably determine the fate of 2-nitrophenol and 4-nitrophenol. The atmospheric half-lives of these compounds are not known. In water, both photolysis and biodegradation will be important fate processes. Photolysis will be more important in near-... 

The phase (gas or particle) in which the PCBs, PCDDs and PCDFs occur in the atmosphere greatly affects their tropospheric removal processes and lifetimes. Analogous to other organic compounds, the PCDDs, PCDFs and PCBs in the atmosphere can be removed and/or transformed by a number of physical and chemical processes.71,72,76,77 While the present chapter focuses on the chemical transformations of the PCBs, PCDDs and PCDFs, the physical removal processes are also discussed for completeness and to assess the relative importance of the various tropospheric removal processes. 

In the ocean, elements that form insoluble hydroxides have relatively short residence times (e.g., A1 and Fe have residence times in the ocean of 100 and 200 years, respectively). Cations, such as Na (aq) and K (aq), and anions, such as Cl (aq) and Br (aq), have longer residence times in the ocean ( 7 x 10 to 10 years). In the atmosphere, the very stable gas nitrogen has a residence time of a million years or so, while oxygen has a residence time of 5,000-10,000 years. Sulfur dioxide, water, and carbon dioxide, on the other hand, have residence times in the atmosphere of only a few days, 10 days, and 4 years, respectively. Of course, residence times may be determined by physical removal processes (e.g., scavenging by precipitation) as well as chemical. 

In this chapter, we made an attempt to provide a comprehensive review of the current state-of-the-art on sources, chemical nature, and physical properties of organic aerosols. This review begins with an overview of few basic concepts on atmospheric aerosols, followed by a description of the major constituents of atmospheric aerosols. The sources, transformations, and removal processes of organic aerosols are outlined and followed by an overview of the major environmental and human health issues associated with organic aerosols. The chemical and physical characterization of organic aerosols is then reviewed and is finally followed by a list of uncertainties and suggestions that require further studies. 

Wet removal processes are further controlled by precipitation types and rates. Dry deposition processes on surfaces are affected by atmospheric transport rates that mix fresh pollutant into the surface boundary layers and by the physical properties of particles. For the Eastern U.S., the approximate annual deposition rates of sulfate can be compared as follows (Table III), considering that deposition flux is the product of a concentration and a velocity of deposition (Vd) (20) ... 

X 10 g(O)/year. Most of this oxygen is removed rather quickly by respiration/decomposition and thus photosynthesis by itself does not account for the net production of oxygen. The production of oxygen results from the physical removal of some of the reduced carbon from contact with oxygen before it has a chance to decompose. This process is known as carbon burial and it represents the difference between photosynthesis and respiration/decomposition. Carbon burial is the removal of fixed carbon to anaerobic sediments where reaction with atmospheric oxygen does not occur until the sediments are returned to the surface by tectonic processes. The rate of carbon burial corresponds to 3.2 x 10 g(O)/ year, less than 1% of the amount of oxygen formed by photosynthesis. 

TWO types of physical loss processes should be considered In the external removal of a chemical species. First, the species of Interest may be lost to the atmosphere through water-air exchange. This process depends on the Henry s Law constant for the chemical species, as well as the atmospheric concentration and the structure of the surface microlayers (. Wind stress and turbulence of the water body surface have a pronounced effect, especially for surface-active materials for which bubble scavenging and surface film ejection as aerosol takes place. Transfer rates at the air-water interface are complex problems In themselves and are not dealt with In this Chapter. 

These documents provide descriptions of the physical processes associated with spray removal of aerosols from contaimnent atmospheres and the expected efficacy of removal processes under accident conditions. 

Lead exits the atmosphere through dry and wet deposition processes. Each mechanism for lead removal from ambient air has its own set of characteristics and differs in relative importance for impact on receiving environmental compartments and lead-exposed populations. The removal processes are reasonably well understood, particularly in terms of the physics of dry deposition (Friedlander, 1977 U.S. EPA, 1986). There are three zonal or layer elements in the dry precipitation process for lead removal the main airstream, the boundary surface, and the receiving surface. Each of these zones is viewed in terms of aerodynamic resistance, boundary layer resistance, and surface resistance. 

The physical deactivation processes can be reversed to a certain extent, but only after the damage has been done. Cyclic treatment of crystallites in an oxidizing atmosphere and then a reducing atmosphere has been found to redisperse large crystallites. Dissolved metal atoms can be made to migrate back to the support surface by proper heat treatment. The deposited particulates can be removed either by combustion or by dissolving them with a suitable solvent. 

The principal mechanisms for the removal of inorganic lead particulates from the atmosphere are dry and wet deposition [54]. The removal efficiency depends on the physical characteristics of the suspended material, atmospheric conditions, and the nature of the receiving surface. Dry deposition occurs by sedimentation, diffusion, and inertial mechanisms such as impaction [73]. Wet deposition removal processes include rainout and washout [71]. 

As indicated in previous chapters, the atmosphere serves as the medium through which air pollutants are transported and dispersed. While being transported, the pollutants may undergo chemical reactions and, in addition to removal by chemical transformations, may be removed by physical processes such as gravitational settling, impaction, and wet removal. 

Figure 7-12 depicts the main physical pathways by which aerosol particles are introduced into and removed from the air. Processes that occur within the atmosphere also transform particles as they age and are transported. This form of distribution of mass with size was originally discovered in polluted air in Los Angeles, but it is now known to hold for remote unpolluted locations as well (Whitby and Sverdrup, 1980). In the latter case, the... 

Fig. 7-12 Schematic of an atmospheric aerosol size distribution. This shows the three mass modes, the main sources of mass for each mode, and the principal processes involved in inserting mass into and removing mass from each mode (m = mass concentration. Dp = particle diameter). (Reproduced with permission from K. T. Whitby and G. M. Sverdrup (1983). California aerosols their physical and chemical characteristics. In "The Character and Origin of Smog Aerosols" (G. M. Hidy, P. K. Mueller, D. Grosjean, B. R. Appel, and J. J. Wesolowski, eds), p. 483, John Wiley, New York.)...

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