Stratosphere ozone cycle

Methane can also impact the stratospheric ozone cycle through two processes having opposite effects on ozone destruction. First, methane can convert Cl and CIO to HCl, which does not directly deplete O3. Second, the oxidation of methane in the stratosphere produces water vapor, enhancing the formation of PSCs which facilitate O3 destruction. 

Perfluorinated ethers and perfluorinated tertiary amines do not contribute to the formation of ground level ozone and are exempt from VOC regulations (32). The commercial compounds discussed above have an ozone depletion potential of zero because they do not contain either chlorine or bromine which take part in catalytic cycles that destroy stratospheric ozone (33). 

Considering natural stratospheric ozone pro-duction/destruction as a balanced cycle, the NO.v reaction sequence is responsible for approximately half of the loss in the upper stratosphere, but much less in the lower stratosphere (Wennberg et al, 1994). Since this is a natural steady-state process, this is not the same as a long term O3 loss. The principal source of NO to the stratosphere is the slow upward diffusion of tropospheric N2O, and its subsequent reaction with O atoms, or photolysis (McElroy et ai, 1976). 

Theoretical interpretation of the experimental observations will help in determining the relative roles played by stratospheric injection, plant emission, background methane, and transport to surfaces in the natural portion of the tropospheric ozone cycle. 

This is a very broad conclusion, and additional measurements must be made. Some of this effort (which is current) should address the problem of other pollutants and condensation nuclei that accompany the nonurban oxidant. Interpretation of these measurements will increase the specificity of separating anthropogenic sources from natural background sources. Theoretical assessments of the existing observations will shed light on the relative roles played by stratospheric injection, plant emission, background methane, and diy deposition on surfaces in the natural portion of the tropospheric ozone cycle. 

The role of biomass in the natural carbon cycle is not well understood, and in the light of predictions of a future atmospheric energy balance crisis caused by carbon dioxide accumulation, in turn the result of an exponential increase in the consumption of carbon fuel, the apparent lack of concern by scientists and policy makers is most troubling. Yet there is no other single issue before us in energy supply which will require action long before the worst effects of excess production will be apparent. The only satisfactory model is the action taken by the R D community with respect to the SST in nitric oxide potential and chloro-halocarbon emissions, when it was realised that the stratospheric ozone layer was vulnerable to interference. Almost all other responses to pollution" have been after definitive effects have become apparent. 

An assessment of the effects of HSCTs on stratospheric ozone is given by Stolarski et al. (1995), and the interactions between NO, and CIO, cycles at various concentrations are treated by Kinnison et al. (1988), Johnston et al. (1989), and Considine et al. (1995). A discussion of some of the general issues involved in the development and possible future use of the HSCT is found in Zurer (1995). 

In short, the chemistry of the halogens, NOx, and HOx is intimately connected. As we saw earlier with respect to the HSCT, effects on one of these can affect the other cycles significantly as well, and indeed, the overall effects on stratospheric ozone may be due mainly to these secondary interactions involving other families of compounds. 

In addition to these indirect effects of volcanic emissions, there are a variety of nonvolcanic parameters that, of course, can change 03 as well, and these must be taken into account in assessing the role of the volcanic emissions alone. For example, there is a natural solar variability, part of which cycles on a time scale of about 11 years and part of which is on a much longer time scale (Lean, 1991 Lean et al., 1995a, 1995b Labitzke and van Loon, 1996). In addition, stratospheric ozone levels vary with the quasi-biennial oscillation (QBO), which is associated with a periodic variation in the zonal winds at the equator between 20 and... 

Similarly, Fig. 13.12 shows the percentage deviation in regionally averaged stratospheric ozone for North America, Europe, and the Far East after variations due to the solar cycle, seasonal variations, the QBO, and atmospheric nuclear tests were subtracted out. Negative deviations are consistently seen in recent years, suggesting a long-term trend on top of the natural variability (Stolarski et al., 1992). 

The cycle repeats itself, so that one Cl atom can destroy thousands of O3 molecules (Seinfeld Pandis 1998). It is estimated that so far about 10% of the stratospheric ozone has been depleted. Because of the Montreal Convention of 1987 and its Amendment of 1992, fully haloge-nated CFCs are no longer manufactured legally in the world. Unfortunately, these CFCs are very long lived (in the order of hundreds of years), so the ozone hole will only be slowly filled in by natural production of O3 in the stratosphere (IPCC 2001). 

Nitrous oxide is nontoxic—it used as the propellant in whipped-cream spray cans—and so might seem to be an unlikely pollutant. However, as noted earlier, it may contribute significantly to greenhouse warming. Furthermore, on diffusing to the stratosphere, N20 becomes involved in the ozone cycle (reactions 8.2, 8.3, and 8.6) following its conversion to nitric oxide (NO) ... 

Molina and Rowland (711, 843) were the first to predict the possible j destruction of the stratospheric ozone by a catalytic cycle involving Cl atoms released from the photolysis of chlorofluoromethanes by sunlight. Since chlorofluoromethanes are unreactive with atoms and radicals in the troposphere, they eventually reach the stratosphere where they are photodis-sociated into Cl atoms by solar radiation below 2300 A. 

In this volume of Issues we address the sources, environmental cycles, uptake, consequences and control of many of the more important chlorinated organic micropollutants. Under this heading we have included a range of semi-volatile persistent compounds, notably polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) as well as a number of chlorinated pesticides. We have not sought to include volatile species such as CFCs which cause environmental problems of an entirely different nature. The compounds included in this volume cause no threat to the stratospheric ozone layer, but have given widespread cause for concern in relation to their environmental persistence and high toxicity, and their potential for adverse effects on humans and wildlife. 

Molina and Rowland [82] were the first to recognize the threat of the released chlorine to the stratospheric ozone layer. The threat originates from the catalytic cycle ... 

Halons such as Halon-1211 (CF2BrCl) and Halon-1301 (CF3Br) are bromi-nated CFCs which are used as fire extinguishers. Like CFCs, Halons are chemically inert in the troposphere but photolyze in the stratosphere. Photolysis releases bromine atoms, which can remove stratospheric ozone in a cycle that is analogous to the chlorine-based cycle above. The bromine cycle can couple... 

A British scientist Sydney Chapman suggested the basic ideas of stratospheric ozone in the 1930s, which have become known as the Chapman cycle. Short wavelength UV hv) can dissociate molecular oxygen and the atomic oxygen fragments produced react with oxygen molecules to make ozone. 

Figure 11 shows an idealised nitrogen cycle. The numbers in boxes represent quantities of nitrogen in the various reservoirs, while the arrows show fluxes. It is interesting to note that substances involving relatively small fluxes and burdens can have a major impact upon people. Thus nitrogen oxides, NO, NO2, and N2O are very minor constituents relative to N2 but play major roles in photochemical air pollution (NO2), acid rain (HNO3 from NO2), and stratospheric ozone depletion (N2O). 

In some respects, the oxygen cycle in the atmosphere is most concerned with ozone chemistry which will be discussed later today. The effect of nitrogen fertilizers on the atmosphere would be most strongly felt through perturbation of stratospheric ozone. The general consensus now seems to be that the effect of nitrogen fertilizers upon the atmosphere will be small compared to the magnitude of other sources. 

This cycle was proposed by Barrie et al. (1988) for the Arctic. Earlier it had been proposed by Wofsy et al. (1975) for the stratosphere, but shown to be unimportant there. Note that in the troposphere only the BrO self-reaction is of importance, whereas in the stratospheric ozone hole the CIO self-reaction (forming the CI2O2 dimer) dominates O3 destruction. Halogen oxide cross-reactions are... 

Approximately 90% of the ozone in the atmosphere is found in the stratosphere (15-50 km), with only —10% in the troposphere (0-15 km). Stratospheric ozone is very important because it absorbs ultraviolet radiation (200-300 nm) from the Sun and shields the surface from this biologically lethal radiation. Stratospheric ozone is destroyed via a series of chemical reactions involving NO, OH, Cl, and Br. These species destroy stratospheric ozone through the following catalytic cycle where X may be any of the following NO, OH, Cl, or Br (Wayne, 1991)... 

It has been suggested127 that oxides of chlorine, C10x, constitute an important sink for stratospheric ozone. The proposed photochemical scheme predicts that CIO is the dominant chlorine-containing constituent of the lower and middle stratosphere. The efficiency of 03-destruction of the C10x catalytic cycle appears to be greater than that of the NOx cycle. 

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