Ozone depletion described

Because of the expanded scale and need to describe additional physical and chemical processes, the development of acid deposition and regional oxidant models has lagged behind that of urban-scale photochemical models. An additional step up in scale and complexity, the development of analytical models of pollutant dynamics in the stratosphere is also behind that of ground-level oxidant models, in part because of the central role of heterogeneous chemistry in the stratospheric ozone depletion problem. In general, atmospheric Hquid-phase chemistry and especially heterogeneous chemistry are less well understood than gas-phase reactions such as those that dorninate the formation of ozone in urban areas. Development of three-dimensional models that treat both the dynamics and chemistry of the stratosphere in detail is an ongoing research problem. 

There now exist alternatives or sufficient quantities of controlled substances for almost all applications of ozone-depleting solvents. Exceptions have been noted for certain laboratory and analytical uses and for manufacture of space shuttle rocket motors. HCFCs have not been adopted on a large scale as alternatives to CFC solvents. In the near term, however, they may be needed as the conventional substances in some limited and unique applications. HCFC-141b is not a good replacement for methyl chloroform (1,1,1 -trichloroethane) because its ODP is three times higher. Alternatives for specific uses of ozone-depleting solvents are briefly described below. 

Although in this chapter we have focused on the potential effects of increased UV-B radiation on the Antarctic marine ecosystem, our results also have bearing on efforts to describe the effects of UV radiation on global marine productivity. However, here again, considerable uncertainties still remain in assessing the effects of ozone depletion on global production. Several authors have predicted a... 

The ozone hole would almost certainly be much worse if chemists had not studied the reactions of CFCs with atmospheric gases before ozone depletion was discovered. The 1995 Nobel Prize in chemistry was awarded to the three pioneers in this effort. A German chemist, Paul Crutzen, discovered how ozone concentration is regulated in a normal stratosphere, while two Americans, F. Sherwood Rowland and Mario Molina, showed that CFCs can destroy ozone. These studies of molecular reactions allowed quick determination that CFCs are a likely cause of ozone depletion and led to the international restrictions described above. 

A mechanism of action describes the molecular sequence of events (covalent or non-covalent) that lead to the manifestation of a response. The complete elucidation of the reactions and interactions among and between chemicals, include very complex and varied situations including biological systems (macromolecular receptors, physical phenomena (thermodynamics of explosions) or global systems (ozone depletion). Unfortunately, this level of mechanistic detail is often unavailable but recent advances in molecular toxicology and others hazards, at the molecular level, have provided valuable information that elucidates key steps in a mechanism or mode of action. ... 

The ozone-depleting reaction involves a rather complicated series of reactions, all of which occur in the gas phase. Equation (8.14) describes the rate-determining step ... 

Most people associate the military with environmental problems rather than solutions. However, Stephen Andersen, E. Thomas Morehouse, Jr., and Alan Miller describe how the military has sped the adoption of new, environmentally friendlier technologies and, working with U.S. indnstry, has eliminated its need for ozone-depleting substances (Andersen et al., 1994). 

Stratospheric Ozone depletion is largely due to chlorine and bromine radicals released from halogenated hydrocarbons. This paper describes properties, emission histories and budgets of relevant substances and outlines the pertinent photochemical processes, along with a comprehensive presentation of halocarbon measurements and global distributions. 

In more recent times, there have been discoveries of ozone depletion in the Arctic that occur by similar mechanisms as the ones described here (see Figure 28). The Arctic equivalent does not tend to be as dramatic owing to the fact the Artie stratosphere does not get as cold as the Antarctic, mainly owing to a less well-formed vortex, largely owing to northern hemisphere topography. 

We can conclude that there are accurate potential energy surfaces to describe the reaction O ( D) + H2, which plays an important role in the ozone depletion cycle. The most recent PESs correctly reproduce the molecular beam experimental results, namely, the differential cross sections and energy distribution of the products, including the contribution of the abstraction mechanism in the first excited PES, within the present experimental resolution. 

In this paper I will describe some aspects of important environmental problems from the point of view of the chemical reactions that occur in the atmosphere. An overview of the processes involved in stratospheric ozone depletion is given in the papers by Professor Rowland and Professor Anderson, in acid precipitation by Dr. Phillips, and in tropospheric photochemistry by Professor Chameides. It is not practical for me to discuss the details of all of these complicated systems, so I will concentrate on a few issues which are of current interest to me and which I believe touch on some of the key uncertainties in our understanding of the environmental problems. I will also limit my discussion to gas phase reactions, although we know that many liquid phase or heterogeneous reactions are taking place, especially in the troposphere. 

Quantitative understanding of the sources, sinks and atmospheric lifetime for CHa is an important future goal for several reasons. The direct increase in tropospheric CHa concentrations adds another important infrared absorbing contributor to the greenhouse effect. The calculated contribution from a CHa increase of 0.18 ppmv in a decade is a tropospheric temperature increase of 0.04 C [N.A.S., 1983], about 1/3 as large as that calculated for the observed 12 ppmv increase for CO2 over the decade from 1970-1980. As described earlier, increasing concentrations of CHa in the stratosphere have an influence on ozone-depletion by ClOx through diversion of Cl into HCl, and should in addition after oxidation increase the upper stratospheric concentrations of H2O. Methane is also a participant in tropospheric chemical reaction sequences which lead under some conditions to the formation of ozone. 

While many other fundamentally important measurements can be described, we move on to review the most recent appraisals of ozone depletion levels because recent advances in the area have significantly changed our outlook on this subject. 

With little doubt, the most significant change in our understanding of fluorocarbon induced ozone destruction, both qualitatively and quantitatively, since the Rowland-Molina proposition of radical catalyzed recombination of ozone by Cl and CIO in 1974, is described in a recent paper in Nature by Prather et al. [20]. What that paper defines is the dramatically non-linear dependence of ozone depletion on added chlorine at high levels of total CL = Cl + CIO + HCl + CIONO2 + HOCl. The critical point is that at levels of CL approaching 15 ppbv, CIO titrates NO2 out of the system via the previously described thermolecular reaction... 

Recognizing Cause and Effect Environmental chemistry has a number of cause-and-effect relationships. Describe these relationships in a) the formation of acid rain, b) ozone depletion. 

J.J. Cullen, P.J. Neale (1997). Biological weighting functions for describing the effects of ultraviolet radiation in aquatic ecosystems. In D.-P. Hader (Ed.), The Effects of Ozone Depletion on Aquatic Ecosystems (pp. 97-117). R. G. Landes. 

Annual report a copy of the annual report and the CER were received. As well as presenting the financial statements the annual report contains a business review in which some environmental highlights are reported. These mention certification of several sites to ISO 14001 and the conclusion of a marketing agreement for a low ozone-depleting solvent. There is also a separate section on HS E issues, which includes safety figures, reports on environmental management and describes Elf Atochem s approach to product stewardship. 

Special Topic 7-1 Green Chemistry—Substitutes for Chlorofluorocarbons describes substitutes for chlorofluorocarbons, and Special Topic 7.2 Other Ozone-Depleting Chemicals describes other ozone-depleting chemicals. 

Observations show that about 25% of the observed mid-latitude ozone column depletion occurs above about 25 km, in the altitude range where gas-phase photochemistry is rapid and it is difficult for dynamics to compete (see e.g., SPARC, 1998). A further contribution due to PSC processing and to vortex breakdown is highly likely. Thus, a substantial chemical contribution of at least half of the trend in the column seems difficult to dispute even if locally-driven chemical ozone depletion in the mid-latitude lower stratosphere were to be substantially smaller than suggested by modelling studies and by the post-Pinatubo measurements described above. Thus, the evidence suggests that chlorine chemistry has played an important and very likely dominant role in the observed trends in mid-latitude ozone over the past two decades (for a recent review, see WMO/UNEP, 2003). 

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