Troposphere chemical removal processes

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). 

As Chapter 10 discusses in detail, chemical compounds in the atmosphere are partitioned between the gas and particle phases (Pankow, 1987 Bidleman, 1988), and the phase in which a chemical exists in the atmosphere can significantly influence its dominant tropospheric removal process(es) and lifetime (Bidleman, 1988 Atkinson, 1996). Gas/particle partitioning has been conventionally described by the Junge-Pankow adsorption model that depends on the liquid-phase (or sub-cooled liquid-phase) vapor pressure, Pu at the ambient atmospheric temperature, the surface area of the particles per unit volume of air, 9, and the nature of the particles and of the chemical being adsorbed (Pankow, 1987 Bidleman, 1988). The fraction of the chemical present in the particle phase, ( ), depends on these parameters through an equation of the form (Pankow, 1987 Bidleman, 1988) ... 

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. 

Sulfur dioxide (S02) reacts under tropospheric conditions via both gas-and aqueous-phase processes (see Section X) and is also removed physically via dry and wet deposition. With respect to chemical removal, reaction with the OH radical is dominant ... 

A chemical mechanism is the set of chemical reactions and associated rate constants that describes the conversion of emitted species into products. From the point of view of tropospheric chemistry, the starting compounds are generally the oxides of nitrogen and sulfur and organic compounds, and ozone is a product species of major interest. Chemical mechanisms are a component of atmospheric models that simulate emissions, transport, dispersion, chemical reactions, and removal processes (Seinfeld, 1986, 1988). 

A fairly general treatment of trace gases in the troposphere is based on the concept of the tropospheric reservoir introduced in Section 1.6. The abundance of most trace gases in the troposphere is determined by a balance between the supply of material to the atmosphere (sources) and its removal via chemical and biochemical transformation processes (sinks). The concept of a tropospheric reservoir with well-delineated boundaries then defines the mass content of any specific substance in, its mass flux through, and its residence time in the reservoir. For quantitative considerations it is necessary to identify the most important production and removal processes, to determine the associated yields, and to set up a detailed account of sources versus sinks. In the present chapter, these concepts are applied to the trace gases methane, carbon monoxide, and hydrogen. Initially, it will be useful to discuss a steady-state reservoir model and the importance of tropospheric OH radicals in the oxidation of methane and many other trace gases. 

The basic terms embodied in the differential equations of a model describe the transport properties of the troposphere, the rates of chemical reactions, and physical removal processes. Many models utilize anEulerian description of the troposphere by subdividing the airspace into an assembly of boxes that exchange air and trace constituents with adjacent boxes in accordance with the prevailing tropospheric flow field. Surface sources of trace constituents are prescribed by appropriate boundary conditions. The equations are solved numerically on fast computers. 

Within the Northern Hemisphere and within the Southern Hemisphere, tropospheric chemicals become well mixed within several weeks. Exchange of tropospheric chemicals across the equator, however, is much slower (Fig. 4.13) mixing between hemispheres requires 1-2 years. Transport of chemicals from the troposphere to the stratosphere is even slower on average, a molecule of an ideal tracer gas resides in the troposphere for 20-30 years before entering the stratosphere. Therefore, when chemicals are removed from the troposphere on shorter time scales by physical processes or by degradation, the chemicals tend not to mix appreciably into the stratosphere. 

The chlorofluorocarbons (CFCs) have very long lifetimes in the troposphere. This is a consequence of the fact that they do not absorb light of wavelengths above 290 nm and do not react at significant rates with 03, OH, or N03. In addition to the lack of chemical sinks, there do not appear to be substantial physical sinks thus they are not very soluble in water and hence are not removed rapidly by rainout. While laboratory studies have shown that some of the CFCs decompose on exposure to visible and near-UV present in the troposphere when the compounds are adsorbed on siliceous materials such as sand (Ausloos et al., 1977 Gab et al., 1977, 1978), the lifetimes for CFC-11 and CFC-12 with respect to these processes have been estimated to be 540 and 1800 years, respectively (National Research Council, 1979). Similarly, an observed thermal decomposition when adsorbed on sand appears to be an insignificant loss process under atmospheric conditions. 

The term "heterogeneous" as applied to the atmosphere refers to chemistry that occurs in or on ambient condensed phases that are in contact with the gas phase aerosols, clouds, surface waters, etc. It is important to distinguish between heterogeneous processes that occur on the surface of the solid, and multiphase chemical reactions that take place in the bulk of the liquid medium. In the latter case, it is assumed that the reaction takes place after the molecule has been incorporated in the bulk liquid medium, such as occurs by wet deposition, where a species is ultimately removed from the atmosphere, especially in the troposphere. 

In addition to photolysis (Chapter 15) and chemical reactions (see the next section), wet and dry deposition also can remove gas- and particle-phase chemical compounds from the troposphere (Eisenreich et al., 1981 Bidleman, 1988). Thus to completely characterize the atmospheric loss processes and overall lifetime of a chemical, we must understand its atmospheric lifetime due to dry and/or wet deposition. Wet deposition refers to the removal of the chemical (or particle-associated chemical) from the atmosphere by precipitation of rain, fog, or snow to earth s surface). Dry deposition refers to the removal of the chemical or particle-assodated chemical from the atmosphere to the Earth s surface by diffusion and / or sedimentation. 

Photochemistry of the OH radical controls the trace gas concentration. The photochemistry of the free hydroxyl radical controls the rate at which many trace gases are oxidized and removed from the atmosphere. Processes that are of primary importance in controlling the concentration of OH in the troposphere are indicated by solid lines in the schematic diagram those that have a negligible effect on OH levels but are important because they control the concentrations of associated reaction and products are indicated by broken lines. Circles indicate reservoirs of species in the atmosphere arrows indicate reactions that convert one species to another, with the reactant or photon needed for each reaction indicated along each arrow. Multistep reactions actually consist of two or more sequential elementary reactions. HX = HQ, HBr, HI, or HF. CxHy denotes hydrocarbons. (From Chameides and Davis, Chem. Eng. News 60 (40) 38-52, 1982. Copyright American Chemical Society.)... 

According to the above concept of ozone destruction, the troposphere is an inert medium concerning ozone chemistry. However, as Crutzen (1974) pointed out, there are several possible reaction steps for tropospheric 03. Thus ozone can be removed chemically from the air by transformation processes tabulated in Table 10. One reaction chain starts with the photolysis of 03, which is caused by radiations in the Hartley and Chappuis bands. The excited oxygen atoms, formed by Rl, are partly transformed to ground state atomic oxygen by R4. However, they also react with water vapour to give OH radicals (R5). The sum of reactions 1-5 can be written in the following way ... 

The following sections describe, in turn, particulate size distributions, coagulation and condensation processes, production mechanisms, chemical composition, removal from the atmosphere, and the tropospheric budget of the aerosol. 

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