Stratospheric ozone depletion, Chapter

Because of the gaseous nature of many of the important primary and secondary pollutants, the emphasis in kinetic studies of atmospheric reactions historically has been on gas-phase systems. However, it is now clear that reactions that occur in the liquid phase and on the surfaces of solids and liquids play important roles in such problems as stratospheric ozone depletion (Chapters 12 and 13), acid rain, and fogs (Chapters 7 and 8) and in the growth and properties of aerosol particles (Chapter 9). We therefore briefly discuss reaction kinetics in solution in this section and heterogeneous kinetics in Section E. 

UV at the earth s surface can also be affected by concentrations of tropospheric gases that absorb in the same region as 03. These include, of course, tropospheric ozone, as well as S02. Briihl and Crutzen (1989), for example, have shown that increased tropospheric ozone can lead to decreases of UV at the earth s surface. Sabziparvar et al. (1998) calculated that at low latitudes, increases in tropospheric O-, occurring since preindustrial times (see Chapter 14.B.2d) may have decreased surface UV in the 280- to 320-nm range by up to 5% and the erythemally weighted UV by up to 9%. Similarly, increased emissions of S02, which absorbs light in the 300-nm region as well (see Chapter 4.K), can counteract UV increases due to stratospheric ozone depletion (De Muer and De Backer, 1992). 

Over the past several decades, there has been increasing recognition in a number of areas of the environmental impacts, both realized and potential, of human activities not only on local and regional scales but also globally. This is particularly true of changes to the composition and chemistry of the atmosphere caused by such anthropogenic activities. One example, for which there is irrefutable evidence, is stratospheric ozone depletion by chlorofluorocarbons, discussed in detail in Chapters 12 and 13. 

Understanding the chemical and physical processes discussed throughout this book is key to the development of cost-effective and health-protective air pollution control strategies. Application of atmospheric chemistry to reducing stratospheric ozone depletion was discussed in Chapter 13. Here we focus on its key role in strategies for controlling tropospheric pollutants, including ozone, acids, particles, and hazardous air pollutants. 

Some HAPs impact not only the troposphere but also the stratosphere. The most obvious example is highly toxic methyl bromide, CH3Br, used as a soil fumigant as well as for treatment of buildings for termites. As discussed in Chapter 12, this is a significant source of stratospheric bromine and hence contributes to stratospheric ozone depletion. Its continued use has been controversial and is being phased out (e.g., see Thomas, 1996 Ristaino and Thomas, 1997 and Duafala, 1996). 

Atmospheric chemistry is a vast subject, and it was one of the first areas of environmental chemistry to be developed with some scientific rigor. Part of the motivation for this field was early problems with smog in Los Angeles and with stratospheric ozone depletion. This chapter presents only a quick survey of some of these areas for more details, one should consult the excellent textbooks by Seinfeld and Pandis1 or by Finlayson-Pitts and Pitts.2... 

Nitrous oxide (N2O) is a potent greenhouse gas (approximately 200 times more effective than CO2 on a molar basis) that has also been implicated in stratospheric ozone depletion (Kim and Craig, 1990 Yoshida et al., 1989) (see Bange, Chapter 2, this volume). Currendy, N2O accounts for about 5.5% of the enhanced radiative forcing attributed to all gases in the atmosphere (IPCC, 2007). Furthermore, while the atmospheric inventory of N2O is increasing, its sources are not well understood causing a renewed interest in the role of marine ecosystems as a potential source for N2O. 

This chapter has outlined the history and conceptual understanding of the processes responsible for ozone depletion by chlorofluorocarbons in the stratosphere. In brief, the long lifetimes of chlorofluorocarbons are reflected in their observed worldwide accumulation in the atmosphere. Their role in stratospheric ozone depletion depends critically on... 

The stratospheric ozone-depleting potential of a compound emitted at the Earth s surface depends on how much of it is destroyed in the troposphere before it gets to the stratosphere, the altitude at which it is broken down in the stratosphere, and chemistry subsequent to its dissociation. Halocarbons containing hydrogen in place of halogens or containing double bonds are susceptible to attack by OH in the troposphere. (We will consider the mechanisms of such reactions in Chapter 6.) The more effective the tropospheric removal processes, the less of the compound that will survive to reach the stratosphere. Once halocarbons reach the stratosphere their relative importance in ozone depletion depends on the altitude at which they are photolyzed and the distribution of halogen atoms, Cl, Br, and F, contained within the molecule. 

The CAA of 1990 contains six titles and related provisions designed to encourage air pollution abatement and reduction. These provisions address several environmental pollution problems that affect us all, such as tropospheric ozone, hazardous pollution, mobile emissions, urban pollution, acid deposition, and stratospheric ozone depletion. Because the scope of this chapter is on solvents and the regulations that impact their use, only Titles I, III, V, and VI and their relevance to solvents will be discussed. 

This chapter covers the most significant areas where solvents can have an environmental impact, stratospheric ozone depletion (restricted to certain halogenates), tropospheric ozone creation in areas of nitrogen oxide pollution and solvents in water and solid waste. It gives an overview of these issues, and looks at the options available to reduce overall environmental impact. 

Clearly, the focus of our attention in this chapter should be on stratospheric ozone depletion and tropospheric ozone formation. 

Stratospheric chemistry is described in detail by Brasseur and Solomon (2005), including the mesosphere, and is also given in the textbooks by Wameck (1988), Brasseur et al. (1999), Finlayson-Pitts and Pitts (2000), Wayne (2000), McElroy (2002), Seinfeld and Pandis(2006). A review on the reactions of halogen radicals in the stratosphere has been given by Bedjanian and Pullet (2003), and an updated review on stratospheric ozone depletion has been provided periodically by WMO (2011). In the present chapter, chemical reaction system is described exclusively among the stratospheric chemistry in which transport and reaction are coupled together. 

In summary, the chemistry of the stratosphere and the effects of anthropogenic perturbations on it have a rich history, with new chemistry that continues to unfold. For reviews of various aspects of the chemistry and history, see Cicerone (1981, 1987), Rowland (1989, 1992,1993), Molina (1991), Rowland and Molina (1994), Toohey (1995), Brasseur et al. (1995), chapters by Li et al. (1995a), Anderson, and Sander et al. in the book edited by Barker (1995), chapters by Brune, Middle-brook and Tolbert, Wilson, and Brasseur et al. in the book edited by Macalady (1998), and the World Meteorological Organization (WMO) 1995 and 1999 reports Scientific Assessment of Ozone Depletion. ... 

It is clear from the data presented in this chapter that the effects of control strategies developed for CFCs and halons are already measurable. Although loss of stratospheric ozone with accompanying increases in ultraviolet radiation in some locations have clearly occurred, the tropospheric concentrations of CFCs are not increasing nearly as fast as in the past. Indeed, the concentrations of CFC-11 and CFC-113 appear to have peaked and have started to decline. The equivalent effective stratospheric chlorine concentrations are predicted to have peaked about 1997 and to return to levels found around 1980 at about the year 2050 (World Meteorological Organization, 1995). The significance of the 1980 level is that these levels resulted in detectable Antarctic ozone depletion. 

In the winter of 1984, massive losses of stratospheric ozone were detected in Antarctica over the South Pole (Halley Bay). This ozone depletion is known as the ozone hole. We know now that it also forms over the Arctic, although not as dramatically as in the Antarctic. Stratospheric ozone protects life on the surface of the Earth by screening harmful UV radiation coming from the sun through a photodissociation mechanism (see Chapter 4). 

The Antarctic "ozone hole" is one of the most dramatic indications of anthropogenic environmental change. Depletion of stratospheric ozone via the catalytic mechanisms described in Chapters 5, 10, and 12 was first detected in a surprising fashion by British observers in Antarctica (Farman et al., 1985). They measured increases of springtime solar UV radiation penetrating the atmosphere at wavelengths that normally are absorbed by O3. The results showed almost a factor of 2 depletion... 

Ravishankara, A.R., and T.G. Shepherd (Lead Authors), M.P. Chipperfield, P.H. Haynes, S.R. Kawa, T. Peter, R.A. Plumb, R.W. Randel, D.W. Waugh, and D.R. Worsnop, Lower Stratospheric Processes, Chapter 7 in Scientific Assessment of Ozone Depletion 1998, Global Ozone Research and Monitoring Project-Report No. 44, World Meteorological Organization/United Nations Environment Programme (WMO/UNEP), Geneva, 1999. 

There are no sinks for nitrous oxide in the troposphere. Instead, all of it rises eventually in the stratosphere where each molecule absorbs UV light and decomposes, usually to N2 and atomic oxygen (90%) or reacts with atomic oxygen (10%). More details about the role of N2O as ozone-depleting species are contained in Chapter 6. 

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