Chlorine-containing compounds 2 Ozone

An understanding of the sources and ensuing reactions of chlorine-containing compounds in the stratosphere is essential for the evaluation of their impact on the ozone layer. Whilst phosgene should be largely removed in the troposphere, it is anticipated that oniy slow photolysis will occur in the stratosphere. 

In addition, stratospheric ozone concentrations appear to be declining, due to photolysis by shortwave UV of volatile chlorine-containing compounds such as CFCI3 and its relatives (Reaction 1.48) the Cl atoms produced in this reaction scavenge O atoms (Reaction 1.49) and... 

FIGURE 7.5 A simplified summary of how chlorofluorocarbons and other chlorine-containing compounds can destroy ozone in the stratosphere faster than it is formed. Note that chlorine atoms are continuously regenerated as they react with ozone. Thus, they act as catalysts, chemicals that speed up chemical reactions without being used up by the reaction. Bromine atoms released from bromine-containing compounds that reach the stratosphere destroy ozone by a similar mechanism. 

The single most important use of chlorine-containing compounds is water disinfection. About 98% of the drinking water in the US and 96% of the waste water is treated with chlorine. There are four technologies that could replace chlorination membrane filtration, ultraviolet irradiation, filtration on activated carbon bed and treatment with ozone (ozonolysis). All of them are more expensive than chlorination, and none of them were studied in as much detail as chlorination was. If ozone is used, the by-products formed in the reactions of ozone with organic compounds have to be removed in a separate step using activated carbon. Overall, there is no viable alternative to chlorination today. 

Heinzle E, Geiger F, Fahmy M, Kut O M (1992) Integrated Ozone-Biotrealment of Pulp Bleaching Effluents Containing Chlorinated Phenolic Compounds, Biotechnology Progress 8 67-77. 

Ozone depletion Destruction of the stratospheric ozone layer that protects the Earth from harmful effects of ultraviolet radiation. Depletion of the ozone layer is due to the breakdown of certain chlorine- or bromine-containing compounds (chlorofluorocarbons or halons), which break down when they reach the stratosphere and then catalytically destroy ozone molecules. 

Oxidizer Chemical substance that causes oxygen to combine with another chemical substance examples include oxygen and hydrogen peroxide Ozone depletion Destruction of the stratospheric ozone layer that protects the Earth from harmful effects of ultraviolet radiation. Depletion of ozone layer is due to the breakdown of certain chlorine- and/or bromine-containing compounds (chlorofluorocarbons or halons), which break down when they reach the stratosphere and then catalytically destroy ozone molecules Ozone layer Protective layer in the atmosphere, about 15 miles above the ground. The ozone layer absorbs some of the sun s ultraviolet rays, thereby reducing the amount of potentially harmful radiation that reaches the Earth s surface PAHs Polycyclic aromatic hydrocarbons... 

Bromine-containing compounds have the potential to released Br atoms upon degradation in the atmosphere. Once in the stratosphere, Br atoms are about 45 times more effective than chlorine in destroying stratospheric ozone [22]. An important example of Br-containing compounds is 1-bromopropane, which is currently utilized as a industrial solvent and have been proposed as a replacement for CFCs, controlled under the agreements of fhe Montreal Protocol. 

While there is a total chlorine compound mixing ratio in the stratosphere of approximately 3400 ppt, that for bromine gases is only 20 ppt (Figure 5.1). Remarkably, with 150 times less abundance than chlorine, bromine is approximately as important as chlorine in overall ozone destruction. Methyl bromide (CH3Br) constitutes about half the source of bromine in the stratosphere (see Section 2.5). The H-atom-containing bromine compounds, CH3Br, CH2Br2, and CHBr, release their Br almost immediately on entry into the stratosphere the... 

The research has shown that not all HFCs and HCFCs are equally environmentally acceptable from a gas-phase atmospheric chemistry standpoint. Most are only slowly removed, allowing the possibility that a fraction of what is released at the earth s surface reaches the stratosphere. For HCFCs, subsequent photolysis releases chlorine, making these compounds less attractive as CFC substitutes. Even if the parent HCFC undergoes tropospheric degradation, some of the chlorine-containing by-products can reach the stratosphere and release Cl atoms. It is up to the atmospheric modelers to ascertain the ozone depletion potentials implied by these chemical processes. 

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