The Stratospheric Ozone Layer

These human medical problems do not necessarily spell disaster—most skin cancers can be cured, eye damage can often be corrected, and we may come to regard the dry, wrinkled skin of a former tanning enthusiast as beautiful. However, the great unknown factor is the effect of increased UV-B and UV-C exposure on ecosystems at large. Already, it is a matter of common experience worldwide that frogs and other amphibians have suddenly become scarce although no causal connection with ozone layer 

Following the demonstration in 1970 by Paul Crutzen that nitrogen oxides destroy ozone catalytically, there has been much concern that the ozone layer could be depleted by introduction of excessive amounts of nitrogen oxides from supersonic commercial aircraft operating in the stratosphere  

Note that since the atomic chlorine consumed in reaction 8.5 is regenerated in 8.7, it acts as a catalyst for ozone decomposition. Reaction 8.6 represents the decomposition of ozone by ultraviolet light, so that reactions 8.6 and 8.2 form a dynamic system that accounts for the absorption of ultraviolet light by the ozone layer. 

Inevitably, the chemistry of the stratospheric ozone layer is more complicated than reactions 8.2-8.S suggest. First, it has been found that HCl and CIONO2 (chlorine nitrate) serve as chlorine reservoirs in stratospheric 

Second, reaction 8.9 and other relevant reactions appear to occur preferentially on available solid surfaces, which are often ice crystals but may also be particles of sulfate hazes from volcanic eruptions or human activity. Third, volatile bromine compounds are even more effective (via Br atoms) than chlorine sources at destroying ozone methyl bromide is released into the atmosphere naturally by forest fires and the oceans, but anthropogenic sources include the use of organic bromides as soil fumigants (methyl bromide, ethylene dibromide) and bromofluorocarbons as fire extinguishers (halons such as CFsBr, CF2BrCl, and C2F4Br2). 

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An important effect of air pollution on the atmosphere is change in spectral transmission. The spectral regions of greatest concern are the ultraviolet and the visible. Changes in ultraviolet radiation have demonstrable adverse effects e.g., a decrease in the stratospheric ozone layer permits harmful UV radiation to penetrate to the surface of the earth. Excessive exposure to UV radiation results in increases in skin cancer and cataracts. The worldwide effort to reduce the release of stratospheric ozone-depleting chemicals such as chlorofluorocarbons is directed toward reducing this increased risk of skin cancer and cataracts for future generations. 

Ozone has received increased attention for its occurrence and function in the Earth s atmosphere.For example the decreasing ozone concentration in the stratospheric ozone layer, becoming most obvious with the Antarctic ozone hole. 

C04-0064. The use of Freons is being phased out because of the damage these compounds do to the stratospheric ozone layer. One of the Freons, CCI2 F2, is manufactured from carbon... 

The Stratospheric Ozone Layer Its Photochemical Formation and Degradation... 

The simplest model for considering the stratospheric ozone layer is the Chapman oxygen-only mechanism (Figure 7.11), which describes the reactions steady-state ozone concentration as resulting from a... 

Due to its gaseous nature it may have an effect on the stratospheric ozone layer [281, 402, 404]. After injection into soil for fumigation, methyl bromide rapidly diffuses through the soil pore space to the soil surface and then into the atmosphere [159,162,163,405,406]. Since a plastic sheet typically covers the soil surface, the rate of emission into the atmosphere depends upon the thickness and density of the plastic, if other conditions are the same [159, 406]. Other routes of disappearance from soil include chemical hydrolysis, methylation to soil organic matter through free radical reactions, and microbial degradation [ 136,159,405,407]. Several reports appeared on the study of the microbial transformations of methyl bromide, summarized as follows ... 

This report deals primarily with the origins and effects of ozone and other photochemical oxidants. It is limited, more or less, to the problem of urban pollution and to such closely related topics as natural background in the earth s boundary layer. No consideration is given to the stratospheric ozone layer and the effects produced by supersonic transport (sst) emission or halocarbons. 

Volatile organic compounds (VOC) contribute to the formation of tropospheric ozone (summer smog). Certain halogenated hydrocarbons (e.g. CFCs) also destroy the stratospheric ozone layer. Chlorinated solvents are hazardous to water and, if disposed of incorrectly (e.g. burning), may emit highly toxic substances (e.g. dioxins). 

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. 

This would create a tremendous boost to the work of the product innovators, as well as to the work of researchers who seek to develop theory and correlations between molecular structure and properties. The searchers in product innovations also need better search engines, in the form of databases that are designed and compiled to be reverse searchable, so that one can state a set of desired properties and find a set of materials that have them. This would address the modern Thomas Midgley problem, of finding the set of all compounds that boil between -30 and 0 °C, that are nonflammable and nontoxic, that do not harm the stratospheric ozone layer, and that do not cause global warming. 

Methane is removed continually from the atmosphere by reaction with OH radicals (Section 8.3). In contrast, chlorofluorocarbons and related volatile compounds are inert under the conditions of the lower atmosphere (troposphere), so atmospheric concentrations of these refrigerants and solvents will tend to increase as long as releases continue. The chief concern over chlorofluorocarbons is that they are a major factor in destruction of the stratospheric ozone layer (Section 8.3). They have been banned under the Montreal Protocol of 1988, but it is important that whatever substitutes (inevitably greenhouse active) are introduced to replace them degrade relatively quickly in the troposphere to minimize any contribution they may be capable of making to greenhouse warming. 

Atom for atom, bromine is even more efficient at destroying ozone than is chlorine. There has therefore been much concern that releases of volatile bromine compounds such as methyl bromide may contribute disporpor-tionately to thinning of the stratospheric ozone layer. Whereas there is no longer any doubt over the role of human activity in stratospheric pollution by CFCs, which are exclusively anthropogenic, attempts to assess the importance of human activity in pollution by methyl bromide have been confused by large natural releases of CH3Br from oceans and forest fires. Besides, unlike the case of CFCs released into the environment, a major fraction of the methyl bromide injected into soils to kill pests is destroyed in the ground. 

Paul Crutzen, Sherwood Rowland, and Mario Molina Chemistry Damage to the stratospheric ozone layer... 

Hydrolysis of bromodifluoromethyl tnphenylphosphonium bromide, yielding bromodifluoromethane and triphenylphosphine oxide, proceeds via difluorocar-bene rather than by the bromodifluoromethyl carbanion [46 (equation 46). Bromodifluoromethane is a candidate for the replacement of Halon 1301 (CFjBr), a fire extinguishant presumed to cause damage to the stratospheric ozone layer... 

At the surface of the Earth sunlight contains little UV radiation thanks to the absorption properties of the stratospheric ozone layer. Major sources of pollution are therefore chemical species which absorb in the VIS region, for example N02 (Figure 6.28). It is formed in combustion processes and undergoes a photodissociation reaction with VIS light to form a pair of radicals which lead to further reactions. 

Thickness of the stratospheric ozone layer over Halley Bay, Antarctica, measured each October over a 40-year period. Notice the rapid decline that begins in the mid-1970s. 

Long term observations indicate that UV-B radiation reaching the earth s surface may have decreased by 5-18% since the industrial revolution in the industrialised midlatitudes of the Northern Hemisphere (NH). However, on a global basis, this may have been offset by the stratospheric ozone layer reduction. It is not possible to estimate the net effect from both, attenuation and increase, because of the limited amount of spatial and temporal coverage of measurements (Liu et al., 1991). In an attempt to present calculated and modelled effects of aerosol on UV flux the authors used the Discrete Ordinate Radiative Transfer Model (DISORT Stammes et al. 1988) for different visual ranges and boundary layer depths (Figure 1). The decrease at 310 nm is 18% and 12 % for a 2km and 1km PBL respectively. 

TABLE 1. Perhalocarbons and Hydrohalocarbons relevant for the stratospheric ozone layer. Abundances are compiled for 1996. ... 

At this point it is worth distinguishing between good and bad ozone. Tropospheric ozone occurs from 0 to 10 miles above the earth s surface, and is harmful. Stratospheric ozone, located about 30 miles above the earth s surface, is responsible for filtering out incoming UV radiation and thus is beneficial. It is the decrease in the stratospheric ozone layer that has been of much concern recently. It is estimated that a 1% decrease in stratospheric ozone will increase the amount of UV radiation reaching the earth s... 

Figure 26.4 Abiotic and biotic interactions leading to the indirect toxicity of chlorofluorocarbons to amphibians. Atmospheric release of chlorofluorocarbons causes the depletion of the stratospheric ozone layer (abiotic-abiotic interaction). Depleted ozone allows for increased penetration of UV-B radiation (abiotic-abiotic interaction). UV-B radiation alone and in combination with fungus (abiotic-biotic interaction) causes increased mortality of amphibian embryos.

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