![]() ![]() ![]() ![]() At one point, an ODP of 0.2 was considered the threshold of acceptability, at least within the US. The ODP values are thus crucial in the negotiations leading to the different Montreal Protocol Amendments. In principle, any substance with a nonzero ODP has the potential to destroy ozone if emitted in sufficiently large quantities. Scientifically, it is impossible to determine an “acceptable” ODP. The largest ODPs correspond to the halons this is due to a combination of their relatively long lifetimes and high ozone removal efficiency of bromine. We can see in this table the high ODP of the CFCs and chlorinated hydrocarbons relative to the HCFCs. Most of the ODPs shown have been calculated by models, unless otherwise specified. Table 1 lists the ODPs for the most important halogen source gases, taken from the last international compilation ( WMO, 2003 it should be noted that an updated report will be available at the end of 2006). However, recent model evaluations (Weisenstein, private communication Sinnhuber, private communication Wuebbles, private communication) indicate that this factor is closer to 60, or even higher. Previous studies ( WMO, 2003) have used a value of 45 for α. Finally, the factor α indicates the removal efficiency of a halogen atom X relative to chlorine. We can then use the slopes of these scatter plots to estimate an average for the ratio of fractional release factors this method tends to agree fairly well with more complete model calculations, although there are some exceptions ( WMO, 2003). In practice, one can take advantage of the fact that, for most compounds, the scatter plot of F X versus F CFC-11 forms a compact curve, which is furthermore almost linear over the altitude regime where most of the ozone destruction occurs ( Schauffler et al., 1999, 2003). The angular brackets indicate that the ratio inside needs to be averaged over the whole stratosphere. Where ρ X( z) denotes the mixing ratio of X at a given point z, and “entry” denotes the entry point of X into the stratosphere, i.e., the tropical tropopause. (36) F X ( z ) = ρ X ( entry ) − ρ X ( z ) ρ X ( entry ) ![]()
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