Bromine compounds, ozone depletion

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). 

Several of the commercially available 16,000 chlorinated and brominated compounds have already been regulated or harmed, CFCs, DDT and chlorinated biphenyls are typical examples. Many others are being phased out according to the Montreal Protocol on Substances that Deplete the Ozone Layer. This includes chlorinated solvents, methyl bromide and halons (e.g. CF3Br). The milder ozone destroyers, hydrochlorofluorocarbons (HCFCs) will also, eventually, be phased out. 

Supporting a seawater source for the halogens is the observation by Shepson and co-workers of significant amounts of as yet unidentified photolyzable chlorine as well as bromine compounds in the spring in the Arctic (Impey et al., 1997a, 1997b). In addition, Platt and co-workers have detected both BrO and CIO at the surface during ozone depletion events (Platt and Haus-mann, 1994 Hausmann and Platt, 1994 Tuckermann et al, 1997). 

Indeed, these reactions play an important role in the Antarctic ozone hole and they have important implications for control strategies, particularly of the bromi-nated compounds. For example, Danilin et al. (1996) examined the effects of ClO -BrO coupling on the cumulative loss of O-, in the Antarctic ozone hole from August 1 until the time of maximum ozone depletion. Increased bromine increased the rate of ozone loss under the denitrified conditions assumed in the calculations by converting CIO to Cl, primarily via reactions (31b) and (31c) (followed by photolysis of BrCl). Danilin et al. (1996) estimate that the efficiency of ozone destruction per bromine atom (a) is 33-55 times that per chlorine atom (the bromine enhancement factor ) under these conditions in the center of the Antarctic polar vortex, a 60 calculated as a global average over all latitudes, seasons, and altitudes (WMO, 1999). 

Perfluorocarbons are a class of organic compounds in which all of the hydrogen atoms are replaced with fluorine atoms. They possess unique properties that make them very useful as dispersants, carrier solvents, and processing solvent additives. Their lack of chlorine or bromine atoms results in zero-ozone-depletion potential. 

If handled responsibly, PFCs can be excellent choices to replace ozone-depleting compounds in many demanding, high-performance applications. Perfluorinated liquids are colorless, odorless, essentially nontoxic, and nonflammable. In addition, since they are not precursors to photochemical smog, PFCs are exempt from the U.S. EPAs volatile organic compounds (VOC) definition. Most importantly, these materials do not contain the carbon-bound chlorine or bromine, which can cause ozone depletion. 

Bromine containing species, introduced from Man s release of halons, are also believed to play a significant role in the polar ozone depletion, despite the fact that the total inorganic bromine concentration in the stratosphere is typically two orders of magnitude lower than the inorganic chlorine. This is manifested in the presence of Br0N02 and HOBr, which however are less stable than the chlorine reservoirs, so that relatively more BrO, is in the active fotm [36,37]. There is a synergism between the chlorine and bromine species the oxides radicals GO and BrO react with each other to produce a series of products, G, Br, BrCl and OCIO. The latter compound is an indicator of the elevated levels of both BrO and GO [38]. 

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. 

BrO] show a pulse in the first two hours after dawn ascribed to the photolysis of inorganic bromine compounds produced either by the bromine explosion mechanism s or the photolysis of mixed bromo/ iodo-organohalogens S built up overnight. Using measured concentrations of BrO, 10 and HO2, the data in Table 7 show the ozone depletion cycle (Cycle type I) involving the BrO and lO cross-reaction is the most important with an O3 depletion rate of 0.3 ppbv h ... 

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... 

These catalytic cycles are largely responsible for the depletion of ozone. One chlorine atom may destroy more than 100,000 ozone molecules before it is transformed into a non-reactive species. Despite the substantial reduction of chlorine and bromine compounds released into the atmosphere as a result of the Montreal Protocol, this has not shown any significant impact on the reduction of the size of the ozone hole. If such a trend continues, it may still take some half a century for the recovery of ozone to the levels it had prior to 1984. 

Long lived compounds in the troposphere are transported into the stratosphere, where they decompose providing a source of inorganic stratospheric bromine [25]. Even short-lived halocarbons such as bromopropane can have significant ozone depletion potentials (ODP) due to rapid transport to the stratosphere by tropical convection [26]. Bromopropane molecules release bromine atoms 2-3 times more effectively than some CFCs would release chlorine atoms in the lower stratosphere [27]. 

Other substances that can find their way into the stratosphere can increase the rate of ozone depletion as well. Halons, which are compounds consisting of bromine, fluorine, and carbon, can end up in the upper atmosphere where the halogens found in the compounds catalyze the ozone consuming reactions. Methyl bromide... 

In 1995 the Nobel Prize for chemistry was awarded to F. Sherwood Rowland and Mario Molina, physical chemists from the University of California, Irvine. In then-published ozone depletion hypothesis [1], they proposed that chlorine atoms could form high in the stratosphere. As an offshoot of this work as well as some other work, the Montreal Protocol on Substances That Deplete the Ozone Layer was signed in 1987. The treaty took effect on January 1, 1989, and has since undergone several revisions. The treaty addresses ozone-depleting compounds that contain chlorine or bromine. Fluorine is not included in the treaty because it has not been shown to harm the ozone layer. 

International agreements (Montreal Protocol in 1987 and subsequent amendments), as well as national regulations, have strongly limited the production and the use of the CFCs. These chemical compounds have been gradually replaced by partially halogenated hydrocarbons, and specifically by hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs). These alternative products are relatively easily destroyed in the troposphere and hence their lifetimes are substantially shorter than those of the CFCs (typically 1-10 yrs as opposed to 10-100 yrs). The ozone depletion potential of the HCFCs is about an order of magnitude smaller than that of the fully halogenated halocarbons. HFCs are not a threat to ozone because they do not contain any chlorine or bromine atoms. 

Halogens Chlorine, bromine, fluorine and iodine are all present in the atmosphere due to several natural and anthropogenic processes. The most important compounds are the chlorofluorocarbons (CFCs). Besides being greenhouse gases, CFCs were responsible also for the rapid depletion of the Antarctic ozone layer in the 1980s. As a result of international action, the concentration of atmospheric CFCs has declined, which should stabilize the ozone depletion. 

Environmental thinking and claims have very much influenced the use of bromine. Methyl bromide was for a long time used as a biocide and insecticide. In the 1990 Clean Air Act the compound was however characterized as a Class 1 ozone-depleting substance and will be phased out before 2005. 

Other than chlorine, halogen compounds containing bromine, iodine and fluorine are present in the stratosphere. Among these, the most important from the point of ozone depletion is bromine. Although iodine also forms an ozOTie-destructing chain cycle, its impact is limited since its mixing ratio is low. In contrast, fluorine does not constitute such a chain cycle due to its reactivity, and does not contribute to ozone destruction. 

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