Tropospheric lifetimes removal processes

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. 

The final step in removal of any species from the atmosphere involves heterogeneous deposition to the Earth s surface. Removal processes include wet deposition via rain-out (following uptake into tropospheric clouds) and dry deposition to the Earth s surface, principally to the oceans. The rates of these processes are largely determined by the species chemistries in aqueous solution. Heterogeneous lifetimes of the parent HFCs, HCFCs and HFEs are of the order of hundreds of years because of their low aqueous solubility and reactivity. 

Methane is found throughout the troposphere in concentrations now exceeding 1.6 parts per million by volume (1 ppmv = 10" ), and is the most abundant source of C-H bonds in the atmosphere. Its primary atmospheric removal process is also reaction with HO radicals, as in (6). The atmospheric lifetimes for CHa and CILCCh can be connected through the relative rates of reactions (5) and (6), and the value observed in the laboratory... 

The other possibility for the discovery of such tropospheric sinks, either singly or in combination, lies through comparisons of the total amounts of CChF and CChF2 found in the atmosphere with the amounts expected still to be there if stratospheric photodecomposition is the only important removal process [Rowland and Molina, 1976]. An alternative to this integral approach is the differential, trend analysis method in which incremental changes in atmospheric burden are compared with incremental emissions to the atmosphere over a particular period of time. The most complete application of the trend analysis procedure has been carried out through the Atmospheric Lifetime Experiment (ALE) sponsored for its first several years by C.M.A. and now by N.A.S.A. [Cunnold et al., 1978, 1983a,b Prinn et al., 1983 Simmonds et al., 1983]. 

This reaction proceeds relatively slowly. The rate constant of 1.6 xlO"13 cm3molecule 1s 1 at room temperature8 corresponds to the lifetime for NH3 of 72 days with respect to its reaction with OH radicals at their typical average daytime atmospheric concentration of 1 x 106 radicals cm 3. The fate of NH2 radicals formed is uncertain. It was suggested, on thermochemical grounds, that the major removal processes for NH2 radicals in the troposphere are reactions with 02, NO, and N02.8... 

Using an estimated atmospheric hydroxyl radical concentration of 5.0x10 mol/cm (Atkinson 1985), the more recent rate constants translate to a calculated lifetime or residence time of 6 years. The estimated atmospheric lifetime of 1,1,1-trichloroethane, which incorporates all removal processes, was also estimated to be 6 years (Prinn et al. 1987 Prinn et al. 1992). This indicates that the predominant tropospheric sink of 1,1,1-trichloroethane is through its reaction with OH radicals. 

Because most of these halogen-containing gases have no significant removal processes in the troposphere, their atmospheric lifetimes can be quite long, for example ... 

The primary atmospheric removal processes for halocarbons are photolysis and reaction with tropospheric hydroxyl radicals (OH). For the fully halogenated CFCs and halons, photolysis is the only important sink and their atmospheric lifetimes are dependent on their absorption cross-sections, the solar flux, and the surface to stratosphere transport time. As a general rule, the greater the number of Cl, Br, or I atoms on any one carbon atom, the larger the cross-section and the shorter the lifetime. For example, the lifetimes of CCIF3 (CFC-13), CCI2F2... 

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. 

These data also demonstrate the impact of bromine chemistry on the stratosphere (see Chapter 12.D). The initial ODP for methyl bromide is 15, due primarily to the large a factor associated with bromine chemistry. However, since it is removed by reaction with OH in the troposphere as well as by other processes such as hydrolysis in the oceans and uptake by soils and foliage (see Chapter 12.D), it has a short atmospheric lifetime of 1.3 years and hence the ODP decreases rapidly with time, toward a long-term steady-state value. 

The rather fast reaction rate of halomethanes with Cl atoms suggests that this process may play a primary role in the removal of halomethanes from the troposphere and results in the formation of HC1 or 1C1 molecules. These degradation pathways do not lead to bromine or iodine atoms but to relatively stable molecules, which may initiate a different bromine and iodine cycles in the marine boundary layer. The atmospheric lifetime of IC1 is probably controlled by its sunlight photodissociation to iodine and chlorine atoms. Another possible degradation pathway of IC1 may be the hydrolysis to hypoiodous acid IOH, which may further be dissolved in seawater. 

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. 

The direct photolysis of compounds such as HONO, 03, HCHO, and N02 in the tropospheric gas phase is a very important source of reactive species, which are then involved in the transformation of organic compounds. Additionally, some organic molecules including organic pollutants undergo photolysis as a significant or even the main process of removal from the atmosphere. It is for instance the case for nitronaphthalenes, the atmospheric lifetime of which can be as low as a couple of hours because of direct photolysis [11, 12]. 

It is thought that rainout and washout may also be important removal mechanisms for soluble tropospheric gases with photochemical lifetimes longer than a few days. From the data for aerosols and fission debris, we can estimate a lifetime of at least one week, and possibly longer. It would also appear that this process is significant in only the first 5 km of the troposphere [Junge (128)]. 

The photolytic lifetimes for average tropospheric conditions were calculated on the basis of the cited quantum yields 4 days for MEK, 14 h for MEK, 22 h for MACR and 35 min for MGLY. These results indicate that photolysis processes in the troposphere dominate (in the case of MGLY) or are in competition with removal reactions initiated by OH radicals. 

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