The NOAA Ozone Depleting Gas Index:
Guiding Recovery of the Ozone Layer
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 have 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 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 the planet (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. 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.
Note that this 2018 update to NOAA’s Ozone-Depleting Gas Index (ODGI) includes improvements to the calculation that were first introduced in 2012. These improvements were made to more accurately reflect the concentration of ozone-depleting halogen in the stratosphere for the stated date. In previous years, the ODGI value provided 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 beginning in 2012, see the details at the end of this web page.
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, contained in chemicals with lifetimes longer than approximately 0.5 yr. 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 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 calculating the 2018 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 depend 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). Furthermore, the efficiency of inorganic bromine in depleting ozone relative to chlorine is taken to be 60 to 65 times larger than chlorine. When these factors are considered in combination with measured global mean abundances in the lower troposphere, a quantity called Equivalent Effective Stratospheric Chlorine (EESC) is derived, and the ODGI is based on this metric.
Because transport-related time lags are explicitly included in the calculation of EESC, the ODGI 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 mentioned above, 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 generally 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) . In addition, EESC-ML values have decreased relatively closer 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 changes observed for shorter-lived chemicals that have decreased quite rapidly in the lower atmosphere during the past two decades (e.g., CH3CCl3).
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 at the beginning of 2018 was 79 i.e., by that time we had progressed 21% (i.e., 100-79) 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 2014 WMO/UNEP Scientific Assessments of Ozone Depletion (Harris and Wuebbles et al., 2014) (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 reactive halogen 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 2018 value of the ODGI-ML in the beginning of 2018 was 55, i.e., by that time we had progressed nearly 45% (i.e. 100-55) 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 (Harris and Wuebbles et al., 2014) (see Figure 2). Past changes in ODGI-A and ODGI-ML are displayed in Figure 3.
Click on image to view full size figure.
Figure 5. The contribution of long-lived chlorine- and bromine-containing gases to reactive halogen in the mid-latitude stratosphere.
Click on image to view full size figure.
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 2018. The notable exceptions are 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 concentration has 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 from CFC-11 has slowed substantially in recently years, as emissions of this controlled substance have unexpectedly increased since 2012 despite negligible reported global production since the global phase out mandated for 2010. These increased emissions appear related to unreported new production, inconsistent with the phase-out of CFCs in the fully adjusted and Amended Montreal Protocol (Montzka et al., 2018). 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 atmospheric concentration has declined most years since 1998, when reported total human industrial production was reduced owing to the Montreal Protocol restrictions. Although concentrations of HCFC’s continue to increase in the background atmosphere (see figure and data at ftp://ftp.cmdl.noaa.gov/hats/hcfcs/), reported production peaked recently and is scheduled for a complete phase-out by 2030. The initial phase-down in HCFC production is apparently reflected in a slowdown in recent rates of increase compared to just a few years ago; HCFCs contribute relatively little (~5%) 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. Recent emission increases noted for CFC-11 will delay that recovery, but the extent of that delay will depend on the speed and effectiveness of efforts to eliminate new CFC-11 production, uses, and associated emissions. 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 ozone layer in mid-latitudes. 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.
Improvements in the calculation of the ODGI initiated 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).
NOAA observations that are used to derive the ODGI can be found within the directories at: ftp://ftp.cmdl.noaa.gov/hats/
or with the interactive data viewer at: http://www.esrl.noaa.gov/gmd/dv/iadv/
and global means are summarized in the file: ftp://ftp.cmdl.noaa.gov/hats/Total_Cl_Br/2016 update total Cl Br & F.xls
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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