Physical Removal of Chemicals from the Atmosphere

A chemical may be removed from the atmosphere either by physical processes or by chemical transformation. If a chemical exists in the atmosphere in the form of sufficiently large particles or liquid droplets, physical deposition of the chemical to soil, vegetation, or water bodies may occur by gravitational settling. Smaller particles with negligible settling velocities also may be deposited to surfaces by impaction or diffusion. Chemicals in gaseous form may either sorb onto surfaces directly or sorb onto airborne particles that are subsequently removed. 

Many of these processes are analogous to processes that deposit aquatic chemicals to the sediments of a lake or a river. Chemical transformation in the atmosphere typically results in the production of more oxidized species, and the complete mineralization of many organic chemicals is possible. In this section, physical processes that result in the removal of chemicals from the atmosphere without changes to their chemical structure are discussed. In Section 4.6, chemical transformation processes are presented. 

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PHYSICAL REMOVAL OF CHEMICALS FROM THE ATMOSPHERE TABLE 4.9 Washout Ratios for Several Chemicals ... 

The fates and behaviour of the organic compounds emitted into the atmosphere are markedly dependent on the physical and chemical properties of the individual organic compound. VOCs are removed by photochemical and deposition processes on timescales varying from minutes to months [19,20]. Removal of VOCs from the atmosphere is mainly initiated by reaction with an OH radical, although reactions with O3 and NO3 may also be significant for certain species under specific conditions. Through oxidation, the VOCs are converted into more polar and hydrophilic forms, which make these photooxidation products more susceptible to wet removal by rain, formation of SOA, or dry deposition on surfaces. 

Because ILs can absorbed significant amount of water from the atmosphere and trivial water is difficult to be removed, water becomes the most common impurity in ILs (Huddleston et al, 2001 Seddon et al, 2000 Takamuku et al., 2009). The existence of water in ILs may affect many of their physical and chemical properties, such as polarity, viscosity, conductivity, and reactivity as well as solvation and solubility properties (Brown et al., 2001 Cammarata et al., 2001 Najdanovic-Visak et al., 2003 Schroder et al., 2000 Widegren et al., 2005). 

It is well established that in non-arid regions, precipitation is the primary means by which contaminating aerosols are removed from the atmosphere. Many chemical, physical, and meteorological parameters affect the micro, meso, and synoptic scale processes through which precipitation transports radioactive aerosols from atmosphere to ground. These parameters include the radioactivity component of the natural aerosols, the processes by which water vapor condenses and grows to raindrops, and the incorporation of the radioactive aerosol into the precipitation. Thus, the prediction of specific deposition from fundamental considerations has proved to be difficult because of the many uncertainties yet prevalent in these processes. Many attempts have been made to evaluate the deposition of these aerosols by empirical studies. 

The high volatility of benzene is the controlling physical property in the environmental transport and partitioning of this chemical. Benzene is considered to be highly volatile with a vapor pressure of 95.2 mm Hg at 25 °C. Benzene is slightly soluble in water, with a solubility of 1,780 mg/L at 25 °C, and the Henry s law constant for benzene (5.5 10"3 atm-m3/mole at 20 °C) indicates that benzene partitions readily to the atmosphere from surface water (Mackay and Leinonen 1975). Mackay and Leinonen (1975) estimated a volatilization half-life for benzene of 4.81 hours for a 1-meter-deep body of water at 25 °C. Even though benzene is only slightly soluble in water, some minor removal from the atmosphere via wet deposition may occur. A substantial portion of any benzene in rainwater that is deposited to soil or water will be returned to the atmosphere via volatilization. 

Once emitted into the atmosphere, the VOCs involve in several chemical and physical processes, leading to transformation and removal from the atmosphere [6]. Chemical transformation of VOCs in the atmosphere occurs in sev-... 

Cupitt (1980), however, considered the loss of 1,2-dichloroethane from the atmosphere by dissolution into rain drops or adsorption onto aerosols insignificant compared with loss from chemical degradation based on mathematical calculations. Since 1,1-dichloroethane has higher volatility and lower aqueous solubility than the 1,2-isomer, physical removal of 1,1 -dichloroethane from the atmosphere would be even less likely to be important (EPA 1985). Pellizzari et al. (1979) measured actual concentrations of airborne contaminants in the vicinity of known emission sources of 1,1-dichloroethane, making aerial transport the logical source of downwind concentrations. 

The escape of Rn from soils is the source of 99% of the Rn in the atmosphere. Typical radon escape rates are on the order of 1 atomcm s from the land surface, which result in a radon inventory of the global atmosphere of 1.5Xl0 Bq. Atmospheric radon itself is a chemically inert and unscavenged, i.e., not removed from the atmosphere by physical or chemical means. Because its half-life is much less than the mixing time of the atmosphere, it is a tracer of atmospheric transport and can be used in a synoptic approach to identify air masses derived from continental boundary layers or in a climatological manner to verify the predictions of numerical models of transport. 

In addition to being removed from the atmosphere by physical processes, atmospheric chemicals can be removed by chemical transformations. Chemical transformations also can be sources of atmospheric pollutants a notorious example is the production of urban smog by reactions involving hydrocarbons, nitrogen oxides, and oxygen. 

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). 

During transport through the atmosphere, all except the most inert substances are likely to participate in some form of chemical reaction. This process can transform a chemical from its original state, the physical (gas, liquid, or solid) and chemical form in which it first enters the atmosphere, to another state that may have either similar or very different characteristics. Transformation products can differ from their parent substance in their chemical properties, toxicity, and other characteristics. These products may be removed from the atmosphere in a manner very different from that of their precursors. For example, when a substance that was originally emitted as a gas is transformed into a particle, the overall removal is usually hastened since particles often tend to be removed from the air more rapidly than gases. 

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