Tropospheric Fates

Figure 4.33 shows the absorption cross sections of HC1 and HBr at room temperature (DeMore et al., 1997 Huebert and Martin, 1968). Neither absorb above 290 nm, so their major tropospheric fates are deposition or reaction with OH. Even in the stratosphere, photolysis is sufficiently slow that these hydrogen halides act as temporary halogen reservoirs (see Chapter 12). 

Kanakidou et al. (1995) have carried out three-dimensional modeling studies of the global tropospheric fates of HFC-134a and other HCFCs using projected emissions and the chemistry described earlier. Table 13.9 shows their calculated lifetimes for some of the CFC replacements with respect to oxida-... 

The tropospheric fate of hydrohalomethanes, following reaction with OH radicals and leading ultimately to formation of carbonyl halides, is summarized by the mechanism shown in Figure 1. 

Collectively, these examples illustrate the diversity of transformations of xenobiotics that are photochemically induced in aquatic and terrestrial systems. Photochemical reactions in the troposphere are extremely important in determining the fate and persistence of not only xenobiotics but also of naturally occurring compounds. A few illustrations are given as introduction ... 

Considerable attention has been given to the persistence and fate of organic compounds in the troposphere, and this has been increasingly motivated by their possible role in the production of ozone by reactions involving NO. ... 

The major fate mechanism of atmospheric 2-hexanone is photooxidation. This ketone is also degraded by direct photolysis (Calvert and Pitts 1966), but the reaction is estimated to be slow relative to reaction with hydroxyl radicals (Laity et al. 1973). The rate constant for the photochemically- induced transformation of 2-hexanone by hydroxyl radicals in the troposphere has been measured at 8.97x10 cm / molecule-sec (Atkinson et al. 1985). Using an average concentration of tropospheric hydroxyl radicals of 6x10 molecules/cm (Atkinson et al. 1985), the calculated atmospheric half-life of 2-hexanone is about 36 hours. However, the half-life may be shorter in polluted atmospheres with higher OH radical concentrations (MacLeod et al. 1984). Consequently, it appears that vapor-phase 2-hexanone is labile in the atmosphere. 

Singh, H. B M. Kanakidou, P. J. Crutzen, and D. J. Jacob, High Concentrations and Photochemical Fate of Oxygenated Hydrocarbons in the Global Troposphere, Nature, 378, 50-54 (f995). 

Second, in bi- and termolecular reactions, tl/2 and r depend on the concentration of other reactants this is particularly important when interpreting atmospheric lifetimes. For example, as discussed earlier, reaction with the OH radical is a major fate of most organics during daylight in both the clean and polluted troposphere. However, the actual concentrations of OH at various geographical locations and under a variety of conditions are highly variable for example, its concentration varies diurnally since it is produced primarily by photochemical processes. Finally, the concentration of OH varies with altitude as well, so the lifetime will depend on where in the troposphere the reaction occurs. 

The reactions of R02 with NO and with R02 generate alkoxy radicals (RO). Alkoxy radicals have several possible atmospheric fates, depending on their particular structure. These include reaction with 02, decomposition, and isomerization as we shall see, reactions with NO and N02 are unlikely to be important under most tropospheric conditions. Atkinson et al. (1995b) and Atkinson (1997b) have reviewed reactions of alkoxy radicals and /3-hydroxyalkyl radicals ... 

The simplest phenoxy radical, Cf)H,0, does not react with 02 (k < 5 X 1CT21 cm3 molecule-1 s-1) but does react with NO (k = 1.9 X 1CT12 cm3 molecule-1 s-1) and with N02 (k = 2.1 X 10-12 cm3 molecule-1 s-1), suggesting that reactions with NOx will be its primary fate in the troposphere (Platz et ai, 1998b). However, this may not be the case for the larger, hydroxylated phenoxy radicals from the OH-aromatic-02-NO reaction sequence. 

The peroxynitrate CH3C0CH200N02 formed from the N02 reaction thermally decomposes, with a rate constant of 3 s-1 at 700 Torr and 295 K. Sehested et al. (1998b) suggest that its lifetime with respect to thermal decomposition is sufficiently small even at the lower temperatures of the upper troposphere that this species cannot participate in long-range transport of NOx, as is the case for PAN, and that in the upper troposphere, reaction of CH3C0CH200N02 with OH and photolysis will be major fates. 

Formation and Fates of Inorganic and Organic Acids in the Troposphere... 

To treat the chemistry of oxides of nitrogen, which play such a central role in the chemistry of both the polluted and remote troposphere, in a consistent manner, we have discussed the formation and fates of... 

Fate Urban Remote marine Free troposphere Nonurban continental... 

In short, exchange of air between the Northern and Southern Hemispheres is slow, as is that between the troposphere and stratosphere, both being on the time scale of about a year (Warneck, 1988). The mechanisms of stratosphere-troposphere exchange are complex but a detailed understanding of these is critical to the assessment of the atmospheric fates of many species, particularly those emitted in the lowermost stratosphere. For reviews of these processes, see Holton et al. (1995), Salby and Garcia (1990), and Mahlman (1997) and for some relevant studies, Langford et al. (1996) and Folkins and Appenzeller (1996). 

Reaction (8), reaction of CF302 with NO, is also sufficiently fast that this is a major fate of CF302 in the troposphere. Thus the rate constant at 298 K for reaction (8) is 1.6 X 10-M cm3 molecule-1 s-1 (e.g., see Sehested and Nielsen, 1993 Bevilacqua et al., 1993 Turnipseed et al., 1994 and Bhatnagar and Carr, 1994), which gives a lifetime for CF302 of 4 min at 10 ppt NO. At small NO concentrations, reaction with H02 (or ROz) can also occur ... 

The subsequent reactions of CF3 are as discussed earlier. In the troposphere, the most likely fate of Cl is reaction with organics (see Chapter 6). 

Barbara J. Finlayson-Pitts is Professor of Chemistry at the University of California, Irvine. Her research program focuses on laboratory studies of the kinetics and mechanisms of reactions in the atmosphere, especially those involving gases with liquids or solids of relevance in the troposphere. Reactions of sea salt particles to produce photochemically active halogen compounds and the subsequent fates of halogen atoms in the troposphere are particular areas of interest, as are reactions of oxides of nitrogen at aqueous and solid interfaces. Her research is currently supported by the National Science Foundation, the Department of Energy, the California Air Resources Board, the Dreyfus Foundation, and NATO. She has authored or coauthored more than 80 publications in this area, as well as a previous book, Atmospheric Chemistry Fundamentals and Experimental Techniques. 

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