Ozone Depleting GasesThe ODGI is estimated directly from observations at Earth’s surface of the most abundant long-lived, chlorine and bromine containing gases regulated by the Montreal Protocol (15 individual chemicals). These ongoing surface-based observations provide a measure of the total number of chlorine and bromine atoms in the atmosphere that are likely to reach the stratosphere and contribute to ozone depletion in the near future. Because air reaching the Antarctic stratosphere has been isolated from the troposphere for a long period (~6 years on average), nearly all of the halocarbons reaching the Antarctic stratosphere during springtime have degraded to inorganic forms that are potential ozone-depleting agents. When the enhanced efficiency of bromine to destroy ozone compared to chlorine is also considered, this total halogen amount is called the Equivalent Chlorine (ECl) burden of the atmosphere (Montzka et al., 1996).
The calculation of the ODGI for mid-latitudes of both hemispheres is different than for Antarctica primarily because air in the mid-latitude stratosphere has a younger mean ‘stratospheric age’ (~3 years) compared to air above Antarctica. As a result, halocarbons in the mid-latitude stratosphere have had less time to become degraded by high-energy solar radiation. By accounting for compound-dependent degradation rates in the stratosphere, a younger mean stratospheric air age, and the enhanced efficiency for bromine to destroy ozone compared to chlorine, a quantity known as the Equivalent Effective Chlorine (EECl) can be derived to represent how the burden of ozone-depleting halogenated gases is changing in the mid-latitude stratosphere (Daniel et al., 1995; Montzka et al., 1996).
Figure 2 shows ECl (for Antarctica) and EECl (for midlatitudes) vs time calculated primarily from NOAA’s surface-based measurements and compares them with future projections provided by the 2007 WMO/UNEP Scientific Assessment of Ozone Depletion baseline scenario (Daniel et al., 2007). Different lag times have been applied to observed and projected tropospheric changes (indicated as solid lines and points) to approximate stratospheric changes in different regions (dashed lines). While a lag time of 6 years is used here to account for the time it takes for gases at Earth’s surface to reach the Antarctic stratosphere, a mean lag of about 3 years is more appropriate when considering air transport to the stratosphere at mid-latitudes.
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