Halocarbons ozone depleting potential

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]. 

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. 

The stratospheric ozone-depleting potential of a compound emitted at the Earth s surface depends on how much of it is destroyed in the troposphere before it gets to the stratosphere, the altitude at which it is broken down in the stratosphere, and chemistry subsequent to its dissociation. Halocarbons containing hydrogen in place of halogens or containing double bonds are susceptible to attack by OH in the troposphere. (We will consider the mechanisms of such reactions in Chapter 6.) The more effective the tropospheric removal processes, the less of the compound that will survive to reach the stratosphere. Once halocarbons reach the stratosphere their relative importance in ozone depletion depends on the altitude at which they are photolyzed and the distribution of halogen atoms, Cl, Br, and F, contained within the molecule. 

Ozone-Depleting Potential of Halocarbons 212 Effect of Aircraft Emissions on Stratospheric Ozone 215 Carbonyl Sulfide (OCS) and the Stratospheric Aerosol Layer 216... 

HCFCs are allowed as stop-gap replacement for halons and CFCs. Ultimately, they will also be phased out of production due to their GWP and small OOP. Halocarbon gas systems are formd to be more convenient in suppressing fires in place of halons. A number of fire extinguishing halocarbon gases with zero ozone depletion potential (ODP) have been developed. These include both HFCs such as pentafluoroethane, 2H-heptafluoropropane and... 

Numerical models have been used to predict the potential ozone depletion in response to the emission of halocarbons, based on different plausible scenarios. All of these models indicate that the time required for the middle atmosphere to respond to surface emissions of these halocarbons is very long (several decades). In particular, even with the measures taken to reduce or phase-out the emissions of the CFCs and other halocarbons, it is expected that the Antarctic ozone hole will be observed each spring (September-October) at least until the year 2040. It should also be noted that the halocarbons are active in the infrared, and contribute to the greenhouse effect. 

Table 1 Atmospheric lifetime, ozone depletion and global warming potentials of the major atmospheric halocarbons ...

Under the agreements of the Montreal Protocol on Substances that Deplete the Ozone Layer, the production of chlorofluorocarbons (CFCs), halons and several other halocarbons has been prohibited [1,2]. Consequently, there is an interest in replacing these compounds [3]. As part of the development of such replacing compounds, it is necessary to consider and evaluate the potential environmental effects of their use, especially on stratospheric ozone [2],... 

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