The NOAA Ozone Depleting Gas Index:
Guiding Recovery of the Ozone Layer
* Note that in this 2012 update to NOAA's Ozone-Depleting Gas Index (ODGI) improvements have been made to more accurately reflect the concentration of ozone-depleting halogen in the stratosphere. In previous years, the ODGI was an estimate of tropospheric changes that would be relevant for the stratosphere after a lag associated with mixing air into the stratosphere. For more on the improvements made to the calculation of the ODGI in 2012, see details at the end of this web page.
The stratospheric ozone layer, through absorption of solar ultraviolet radiation, protects all biological systems on Earth. In response to concerns over the depletion of the global ozone layer, the Clean Air Act, as amended in 1990, mandates NASA and NOAA to monitor stratospheric ozone and ozone-depleting substances.
SEC. 603. MONITORING AND REPORTING REQUIREMENTS
(d) Monitoring and Reporting to Congress
(2) The Administrators of the National Aeronautics and Space Administration and the National Oceanic and Atmospheric Administration shall monitor, and not less often than every 3 years following enactment of the Clean Air Act Amendments of 1990, submit a report to Congress on the current average tropospheric concentration of chlorine and bromine and on the level of stratospheric ozone depletion. Such reports shall include updated projections of -
(A) peak chlorine loading;
(B) the rate at which the atmospheric abundance of chlorine is projected to decrease after the year 2000; and
(C) the date by which the atmospheric abundance of chlorine is projected to return to a level of two parts per billion
This information is critical for assessing if the international Montreal Protocol on Substances that Deplete the Ozone Layer is having its intended effect of mitigating increases in harmful ultraviolet radiation. In order to provide the information necessary to satisfy this congressional mandate, both NASA and NOAA have instituted and maintained global monitoring programs to keep track of ozone-depleting gases as well as ozone itself. While data collected for the past 30 years has been used extensively in international assessments of ozone layer depletion science, the language of scientists often eludes the average citizen who has a considerable interest in the health of the Earth's protective ultraviolet radiation shield. Are the ozone-destroying chemicals declining in the lower atmosphere and stratosphere? When do we expect the ozone hole above Antarctica to disappear? Will the recovery be different for the ozone layer above mid-latitudes? In order to make the answers to these questions easier to understand, NOAA has developed an index, the Ozone Depleting Gas Index (ODGI). This index is derived from NOAA's measurements of chemicals that contain chlorine and bromine at multiple remote surface sites across Earth's surface (see the map in Figure 1). It is defined as 100 at the peak in ozone depleting halogen abundance as determined by NOAA observations, and zero for the 1980 level, which corresponds to when recovery of the ozone layer might be expected based on observations in the past.
Two different indices are calculated, one that is relevant for the ozone hole over Antarctica (the ODGI-A), and one that is relevant for the ozone layer at mid-latitudes (the ODGI-ML). While both indices are derived from NOAA measurements of halocarbon abundances at Earth's surface, separate indices for these different stratospheric regions are necessary to account for the unique nature of the Antarctic stratosphere compared to the stratosphere at mid-latitudes in both hemispheres. For more details on the improvements made to the calculation of the ODGI in this update, see details at the end of this web page. Though an index for the Arctic stratosphere is not explicitly calculated here, it is likely that its value would lie between the mid-latitude and Antarctic ODGI in any given year.
Observations of Ozone Depleting Gases
The 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 direct measure of the total number of chlorine and bromine atoms in the lower atmosphere, or troposphere. Because the lower atmosphere is quite well-mixed, these observations also provide an accurate estimate of the composition of air entering the stratosphere. The threat to stratospheric ozone from Ozone Depleting Substances (ODSs), however, is derived only after considering additional factors: the time it takes for air to be transported from the troposphere to different regions of the stratosphere, air mixing processes during that transport, and the rate at which ODSs photolytically degrade and liberate reactive forms of chlorine and bromine in the stratosphere.
In this improved calculation of the 2012 ODGI** photochemical degradation rates of ODSs specific to the stratospheric region of interest are used (based on Schauffler et al., 2003, see also Newman et al., 2007). Those degradation rates are dependent upon the length of time air in the stratosphere has been isolated from the troposphere (its “mean age”), which is about 3 years in the mid-latitude stratosphere and about 5.5 years for the Antarctic stratosphere). As before, the efficiency for inorganic bromine is taken to be 60 to 65 times larger than chlorine. When these factors are considered with observed tropospheric trends, a quantity called Equivalent Effective Stratospheric Chlorine (EESC) is derived, and the ODGI is based on this metric.
Because transport-related time lags are now explicitly included in the calculation of EESC, the improved ODGI now provides a measure of changes in the present-day stratosphere, as opposed to being an estimate of tropospheric changes relevant for the stratosphere three to six years in the future, as was done in earlier (pre-2012) versions of the ODGI.
As before, the ODGI is calculated for different stratospheric regions: mid-latitudes and the Antarctic. Different trends in EESC are observed in these regions because of differences in transport and chemistry. The ODGI in the Antarctic stratospheric (ODGI-A) is derived from values of EESC in the Antarctic stratosphere (EESC-A), and the ODGI in the mid-latitude stratospheric (ODGI-ML) is derived from values of EESC in the mid-latitude stratosphere (EESC-ML).
In the Antarctic springtime, air has been isolated from the troposphere for 5 to 6 years on average, so nearly all of the halocarbons reaching the Antarctic stratosphere during springtime have degraded to inorganic forms that are potential ozone-depleting agents. As a result, concentrations of ODSs (EESC-A) are higher over Antarctica (Figure 2). Furthermore, progress has been slower in reducing EESC-A back to 1980 values compared to EESC-ML in mid-latitudes because the most recent tropospheric changes have yet to reach the Antarctic stratosphere (Figure 3). Antarctic changes in EESC-A are also delayed because of mixing processes.
The concentration of reactive halogen in the mid-latitude stratosphere (EESC-ML) is currently smaller than in the Antarctic stratosphere because halocarbons have had less time to become degraded by high-energy solar radiation in the younger mid-latitude stratosphere (the mean age of mid-latitude stratospheric air is ~3 years) . But EESC-ML values have decreased relatively further back to 1980 levels primarily because they more closely track tropospheric trends given the shorter transport times for moving air from the troposphere to mid-latitude stratosphere. Another factor contributing to the larger relative decrease in EESC-ML arises because reactive halogen levels in mid-latitudes are more sensitive to short-lived chemicals that have decreased quite rapidly in the lower atmosphere in the past decade (e.g., CH3CCl3).
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The Ozone Depleting Gas Index (ODGI)
The ODGI-A is defined by the observed decline in halogen abundance (as EESC-A) from its peak in Antarctica (ODGI = 100) relative to the drop needed for EESC-A to reach its value in 1980, which is about when the Antarctic ozone hole was readily detected (Figure 2, dotted green line). Though some halogen-catalyzed ozone depletion was occurring before 1980, return of EESC-A back to the 1980 level would represent a significant milestone for the Montreal Protocol (Figure 2). On the ODGI scale, the value of ODGI-A in 2012 was 87.2 i.e., we have progressed about 13% (i.e., 100-87.2) of the way along the path toward the 1980 benchmark halogen level (Figure 3, green line and points). The latter has been projected to occur over Antarctica sometime around 2070 considering the updated future scenarios in the 2011 WMO/UNEP Scientific Assessments of Ozone Depletion (Daniel and Velders et al., 2011) (see Figure 2).
Similar to ODGI-A, the ODGI-ML is defined as 100 at the peak in EESC-ML, and zero at the 1980 benchmark EESC-ML level, corresponding to when substantial ozone-layer recovery might be expected in the mid-latitude stratosphere if all other factors were to remain constant. Based upon halocarbon abundances inferred for the mid-latitude stratosphere in 1980, we expect this recovery level to occur as EESC-ML drops below approximately 1200 ppt EESC (Figure 2, dotted blue line). On this scale, the 2012 value of the ODGI-ML is about 65.4, i.e., we have progressed about 35% of the way along the path toward a stratospheric halogen level that would allow a near-normal ozone layer in mid-latitudes, all other factors being constant (Figure 3). The latter has been projected to occur in mid-latitudes sometime around 2045 (Daniel and Velders et al., 2011) (see Figure 2). Past changes in ODGI-A and ODGI-ML are displayed in Figure 3.
Figure 3. The Ozone Depleting Gas Index (ODGI) vs. time calculated for the Antarctic and mid-latitude stratosphere. Two estimates are given in this 2012 update for both regions; they reflect the magnitude of the difference between the older methodology that provided estimates of tropospheric changes, and the newer methodology that provides an estimate of stratospheric changes. As before, the ODGI derived directly from the Equivalent Effective Stratospheric Chlorine (EESC) determined from our atmospheric surface observations.
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Figure 5. The contribution of long-lived chlorine- and bromine-containing gases to reactive halogen in the mid-latitude stratosphere.
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In order to identify the gases primarily responsible for the decline in the abundance of reactive halogen to date, Table 1 and Figure 4 delineate the contributions of individual gases to total reactive halogen with weightings relevant for the Antarctic stratosphere. Table 2 and Figure 5 give similar data with weightings relevant for the mid-latitude stratosphere. Of the ozone depleting gases restricted by the Montreal Protocol, NOAA measurements show that atmospheric concentrations of nearly all these gases were decreasing in the atmosphere by 2012. The notable exceptions include halon-1301 (a bromine-containing compound used mainly in fire extinguishers) and HCFCs, which are used as replacements for CFCs in many applications. It is clear from Figures 4 and 5 that most of the decline in reactive halogen concentrations have been due primarily to the relatively rapid phase-out and atmospheric decline of shorter-lived chemicals such as methyl chloroform (CH3CCl3) and methyl bromide (Montzka et al., 1999; 2003). The decline related to CFC-11 and CFC-12, the two major components of reactive halogen, has been less dramatic because of continuing emissions and because their lifetimes are very long (50-100 years). Methyl bromide and methyl chloride (CH3Br, CH3Cl) are unique among ozone-depleting gases because they have substantial natural sources. Despite the large natural source of CH3Br, its mixing ratio has declined almost every year since 1998, when reported total human industrial production was reduced owing to the Montreal Protocol restrictions. Though HCFC's continue to increase and production is not scheduled for a complete phase-out until 2030, they currently contribute relatively little (~5% or less) to the atmospheric burden of reactive halogen.
While the Montreal Protocol on Substances that Deplete the Ozone Layer must be considered a success and a model for future efforts to stem climate change (Montzka et al., 2011), ozone layer recovery is expected only with sustained declines in atmospheric chlorine and bromine in future years and continued adherence to the production and consumption restrictions outlined in the Protocol. Recovery of the ozone layer is expected as the ODGI approaches zero, though the timing of complete ozone layer recovery is difficult to determine exactly because other chemical and physical factors related to climate change and continuing anthropogenic emissions of long-lived gases also influence stratospheric ozone abundances and the efficiency for chlorine and bromine to destroy stratospheric ozone.
The ODGI-A and ODGI-ML represent important components of NOAA's effort to guide the recovery of the ozone hole over Antarctica and the mid-latitude ozone layer. These indices provide a means by which adherence to international protocols can be assessed and they allow the public and policy makers to discern if policy measures are having their desired effect. Because ozone depletion is still near its peak, continued monitoring of ozone and ozone depleting gases is critical for ensuring that the recovery proceeds as expected through the 21st century.
Notes for tables 1 and 2: “Halons” represents the aggregate of H-1211 and H-1301; “HCFCs” represents the aggregate of HCFC-22, HCFC-141b, and HCFC-142b; “WMO minor” represents CFC-114, CFC-115, halon 2402 and halon 1201 (Daniel and Velders et al., 2011). SUM* is a mixing ratio of reactive halogen weighted by fractional release factors but without transport lag times considered, whereas EESC includes consideration of lag times for transport and mixing associated with transport.
**Improvements in the calculation of the ODGI in 2012:
- The ODGI now includes updated estimates of halocarbon decomposition rates in the stratosphere for most ODSs. These revised “fractional release” factors are derived from the stratospheric observations of Schauffler et al. (2003) and have been used subsequently in deriving stratospheric trends of ozone-depleting halogen in Newman et al. (2007) and in the WMO Scientific Assessment of Ozone Depletion Reports (Montzka and Reimann et al., 2011; Daniel and Velders et al., 2011).
- The ODGI now expresses compositional changes relevant for the present day in the mid-latitude and Antarctic stratosphere. Previously the ODGI reflected tropospheric changes without explicit consideration of transport times. Previously the ODGI also reflected changes through mid-year of the year indicated. It now is an estimate of changes at the beginning of the year indicated.
- The ODGI is derived from EESC, which includes consideration of stratospheric mixing processes that act to dampen tropospheric changes (as in Waugh and Hall et al., 1992 and Newman et al., 2007).
We gratefully acknowledge all those involved in sampling and analysis of air samples both within NOAA and within the cooperative air sampling network. We particularly thank Brad Hall for his attention to detail in preparation and maintenance of accurate standard scales for these trace gases. This research was supported by the NOAA Climate Program Office.
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