Publication Type:Journal Article
Source:Bulletin of the American Meteorological Society (2013)
In addition to their direct radiative forcing, long-lived gases containing chlorine and bromine also influence RF indirectly through destruction of stratospheric ozone. Production of many halocarbons has been substantially reduced in recent years through amendments and adjustments to the 1987 Montreal Protocol on Substances that Deplete the Ozone Layer. As a result, atmospheric mole fractions of many of the most potent ozone-depleting gases have been declining at Earth's surface (Figs. 2.30d, 2.31). While mole fractions of many ozone-depleting substances (ODS) are declining, those of some halogenated gases continue to increase globally (Fig. 2.31). The most rapid increases are observed for hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs), which are common replacements for chlorofluorocarbons (CFCs), and halons. HCFCs contain chlorine and contribute to ozone depletion, while HFCs do not. Progress towards stratospheric halogen declining back to the 1980 level, often used as a benchmark, can be assessed with the NOAA Ozone-Depleting Gas Index (ODGI; Table 2.6; Hofmann and Montzka 2009). This index is derived from surface measurements of ozone-depleting substances together with an estimate of their potential to destroy stratospheric ozone, for example bromine is 60-65 times more effective than chlorine (Montzka et al. 2011). Factors also include transit time from Earth's surface to the stratosphere, mixing during transit, and photolytic reactivity (Schauffler et al. 2003; Newman et al. 2007). The weighted sum of chlorine and bromine, combined with these factors yields a quantity known as Equivalent Effective Stratospheric Chlorine (EESC), which is an estimate of reactive halogen mole fraction in the stratosphere (Fig. 2.33a). The ODGI is the change in EESC from its peak, relative to that needed to reach the 1980 value (Fig. 2.33b). An updated NOAA ODGI was released in January 2013. The update includes improved methods of accounting for transport and reactions with solar radiation to form reactive halogen in the stratosphere (see http://www.esrl.noaa.gov.libproxy.mit.edu/gmd/odgi/).
The EESC is calculated for two stratospheric regions: Antarctica and midlatitudes. EESC was -1670 ppt and -3900 ppt at the beginning of 2012 for the midlatitudes and Antarctica, respectively. Current EESC values in the midlatitudes are smaller than those in the Antarctic because the liberation of reactive halogen from halocarbons is substantially less in the midlatitude stratosphere compared to the Antarctic. The ODGI for Antarctica was 87.2 at the beginning of 2012 (Fig. 2.33b). Thus, reactive halogen levels in the Antarctic stratosphere have progressed 13% of the way towards the 1980 benchmark. By contrast, the ODGI for midlatitudes was 65.4, indicating substantial progress towards the 1980 benchmark.