The NOAA Ozone Depleting Gas Index:
Guiding Recovery of the Ozone Layer

NOAA Earth System Research Laboratory, R/GMD, 325 Broadway, Boulder, CO 80305-3328
Stephen.A.Montzka@noaa.govGeoff.Dutton@noaa.gov  and James.H.Butler@noaa.gov

Fall 2019

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.

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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

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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 2019 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.

Locations across Earth's surface where regular measurements are taken
Figure 1.  Locations where ozone-depleting chemicals are regularly made by NOAA/ESRL. Different symbols indicate different sampling frequencies (circles≅weekly; stars≅daily). A subset of these results is used to derive hemispheric and global surface means. The chemicals are listed in Tables 1 and 2.

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 2019 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 Equivalent Chlorine as a function of time
Figure 2.  Past and projected future changes in reactive halogen concentrations in the atmosphere. Past concentrations are derived from NOAA measurements of both chlorine- and bromine-containing chemicals; “WMO scenarios” are from the WMO/UNEP 2018 Ozone Assessment, which are tied to NOAA observations in the past and, for the future, assume full adherence to controls on production and consumption of ODSs in the fully revised and amended Montreal Protocol (Carpenter and Daniel et al., 2018). Measured tropospheric changes are indicated with dashed curves and points, while inferred stratospheric changes are indicated as solid curves. Estimates are provided for different regions: the mid-latitude stratosphere and the Antarctic stratosphere. The down-pointing arrows represent the estimated dates that concentrations of stratospheric halogen will return to the benchmark levels present in 1980.
Click on image to view full size figure.

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 2019 was 78 (77.9) i.e., by that time we had progressed 22% (i.e., 100-77.9) 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 2018 WMO/UNEP Scientific Assessments of Ozone Depletion (Carpenter and Danial et al., 2018) (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 2019 value of the ODGI-ML was 53.4, i.e., by that time we had progressed nearly 47% (i.e. 100-53.4) 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 (Carpenter and Daniel et al., 2018) (see Figure 2). Past changes in ODGI-A and ODGI-ML are displayed in Figure 3.

The contribution to Equivalent Chlorine by long-lived gases
Figure 3. The Ozone Depleting Gas Index (ODGI) vs. time calculated for the Antarctic and mid-latitude stratosphere. As before, the ODGI derived directly from the Equivalent Effective Stratospheric Chlorine (EESC) determined from our atmospheric surface observations.
Click on image to view full size figure.
The Effective Equivalent Chlorine as a function of time Figure 4.  The contribution of long-lived chlorine- and bromine-containing gases to reactive halogen in the Antarctic stratosphere.
Click on image to view full size figure.

The Effective Equivalent Chlorine as a function of time 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 for which production and international trade is restricted by the Montreal Protocol, NOAA measurements show that atmospheric concentrations of nearly all were decreasing in the atmosphere in 2019. 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). A sustained decline in future years, however, will rely on sustained decreases in emissions and concentrations of CFCs in particular. Given this, it is particularly concerning that emissions of CFC-11 have increased since 2013. Even more striking is that the increase is most likely explained by renewed production of CFC-11 well past the 2010 global phase-out, in apparent violation of Montreal Protocol controls (Montzka et al., 2018; Rigby et al., 2019).

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 since 1998, when reported total human industrial production was reduced owing to the Montreal Protocol restrictions.

Although the concentration of HCFC-22, the most abundant HCFC, continues to increase in the background atmosphere (see figure and data at ftp://ftp.cmdl.noaa.gov/hats/hcfcs/), the concentration of HCFC-141b and HCFC-142b appear to have stabilized in recent years, likely because reported production of HCFCs summed together peaked recently and is scheduled for a complete phase-out by 2030. Even at their current maximum concentrations, these three HCFCs contribute relatively little (~5%) to the atmospheric burden of reactive halogen.

While the Montreal Protocol on Substances that Deplete the Ozone Layer is considered a success and could be a model for future efforts to stem climate change (Montzka et al., 2011), ozone layer recovery is not a forgone conclusion. Full 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 many factors such as the speed and effectiveness of efforts to eliminate new CFC-11 production, uses, and associated emissions. It will also depend on the magnitude of renewed production, which, given the likely uses of the newly produced CFC-11 in foams, could be many times larger than the emission increase documented so far. Recovery of the ozone layer is expected as the ODGI approaches zero, although 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 greenhouse 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.

 

Table 1: The contributions of ozone depleting chemicals and groups of chemicals to ozone-depleting halogen in the Antarctic atmosphere (Equivalent Chlorine, in parts per trillion or ppt), and the Ozone-Depleting Gas Index for Antarctica (ODGI-A) (both the original and revised methodologies are provided here for comparison; note that EESC and ODGI(new) are stratospheric values).
Year CFC-12 CFC-11 CH3Cl CH3Br CCl4 CH3CCl3 halons CFC-113 HCFCs WMO Minor SUM* (ppt) EESC SUM (ppt) ODGI(old) Antarctic ODGI(new) Antarctic
1992 861 789 500 583 431 400 292 215 41 72 4185 3544 97.7 69.6
1993 876 795 500 583 429 393 313 219 44 75 4228 3666 99 75.7
1994 885 796 501 583 425 370 336 224 48 77 4246 3784 100 81.6
1995 895 793 508 583 422 333 357 226 53 80 4250 3888 99.4 86.8
1996 907 789 500 585 418 296 367 227 59 82 4228 3974 99.2 91.1
1997 915 786 492 582 414 254 381 227 65 82 4198 4042 98.3 94.4
1998 921 782 505 594 411 214 391 225 70 83 4196 4089 98.8 96.8
1999 926 778 513 596 407 179 404 224 76 84 4187 4120 97.5 98.4
2000 930 773 506 572 402 149 414 222 81 84 4134 4141 95.4 99.4
2001 932 768 496 545 397 124 419 221 87 83 4072 4153 93 100
2002 934 763 491 529 393 103 424 220 92 83 4032 4152 91.9 100
2003 934 756 494 525 388 86 428 218 96 83 4008 4139 91 99.3
2004 934 749 491 515 384 72 434 216 100 82 3977 4118 89.9 98.3
2005 932 743 492 505 380 60 441 214 103 81 3952 4094 89 97.1
2006 931 737 492 496 376 50 442 212 107 80 3923 4069 87.9 95.8
2007 927 731 495 488 371 42 442 210 111 79 3897 4043 87.3 94.5
2008 923 724 497 481 366 35 440 208 116 79 3869 4017 85.8 93.2
2009 919 719 496 463 359 29 437 206 121 77 3826 3990 84.3 91.9
2010 914 713 493 454 356 25 435 204 125 77 3794 3963 83.5 90.5
2011 908 707 490 456 351 21 431 202 129 75 3771 3933 89
2012 904 702 491 453 346 17 427 200 134 74 3748 3902 87.5
2013 899 696 495 444 341 14 423 198 138 74 3723 3872 86
2014 893 691 493 430 337 12 417 197 141 73 3684 3843 84.5
2015 888 688 499 429 332 10 411 195 145 70 3669 3815 83.1
2016 883 684 506 437 327 8 404 194 147 70 3660 3786 81.7
2017 877 681 502 431 324 7 398 192 149 69 3629 3759 80.3
2018 872 679 498 426 319 6 393 190 150 67 3602 3734 79.1
2019 866 676 497 421 316 5 387 189 151 66 3574 3710 77.9
Click here to download this table as comma separated values (csv).
Table 2: The contribution of ozone depleting chemicals and groups of chemicals to the ozone-depleting halogen in the mid-latitude stratosphere (Equivalent Effective Chlorine, in parts per trillion or ppt), and the Ozone-Depleting Gas Index relevant for mid-latitudes (ODGI-ML) (both the original and revised methodologies are provided here for comparison; note that EESC and ODGI(new) are stratospheric values).
Year CFC-12 CFC-11 CH3Cl CH3Br CCl4 CH3CCl3 halons CFC-113 HCFCs WMO Minor SUM* (ppt) EESC SUM (ppt; new) ODGI(old) Mid Latitude ODGI(new) Mid-Lat
1992 230 374 242 326 242 271 140 69 13 39 1946 1795 98.8 81.9
1993 234 377 242 326 240 266 150 71 14 41 1962 1847 99.7 88.5
1994 237 378 242 326 238 250 161 72 16 42 1962 1889 100 94
1995 239 377 246 326 236 225 171 73 18 44 1954 1917 98.4 97.6
1996 242 375 242 327 234 200 177 73 20 44 1934 1933 97.4 99.6
1997 245 373 238 326 232 172 184 73 22 45 1909 1936 95.5 100
1998 246 371 244 333 230 145 190 73 24 45 1900 1929 95.3 99.1
1999 248 369 248 333 228 121 196 72 26 46 1887 1919 92.7 97.8
2000 249 367 245 320 225 101 201 72 28 46 1852 1908 89.3 96.4
2001 249 364 240 305 222 84 204 71 30 46 1815 1891 85.5 94.2
2002 250 362 238 296 220 70 207 71 32 45 1789 1867 83.5 91.1
2003 250 359 239 294 218 58 208 70 33 45 1774 1840 81.9 87.7
2004 250 356 238 288 215 48 211 69 35 45 1755 1817 79.9 84.6
2005 249 353 238 283 213 40 214 69 36 44 1738 1795 78.2 81.9
2006 249 350 238 277 210 34 214 68 37 44 1721 1775 76.3 79.3
2007 248 347 239 273 208 28 214 68 39 43 1706 1757 75 76.9
2008 247 344 240 269 205 24 212 67 40 43 1691 1739 72.6 74.7
2009 246 341 240 259 201 20 210 66 42 42 1667 1723 70.1 72.5
2010 244 339 238 254 199 17 208 66 43 42 1650 1704 68.8 70.2
2011 243 336 237 255 196 14 206 65 45 41 1638 1686 67.7
2012 242 333 237 253 194 12 203 65 46 40 1625 1669 65.6
2013 240 330 239 249 191 10 201 64 48 40 1612 1654 63.6
2014 239 328 239 241 189 8 197 63 49 39 1591 1639 61.8
2015 238 327 241 240 186 7 193 63 50 38 1583 1623 59.7
2016 236 325 244 245 183 6 189 62 51 38 1579 1609 57.8
2017 235 323 243 241 181 5 186 62 51 37 1564 1598 56.4
2018 233 322 241 239 179 4 183 61 52 36 1550 1587 55
2019 232 321 240 236 177 4 180 61 52 35 1537 1575 53.4
Click here to download this table as comma separated values (csv).

Notes for tables 1 and 2: “Halons” represents the aggregate of H-1211, H-1301 and H-2402; “HCFCs” represents the aggregate of HCFC-22, HCFC-141b, and HCFC-142b; “WMO minor” represents CFC-114, CFC-115, halon 2402 and halon 1201 (Harris and Wuebbles et al., 2014). “SUM* (ppt)” represents the amount of reactive halogen weighted by fractional release factors but without transport lag times considered, whereas reactive halogen expressed as “EESC SUM (ppt)” includes consideration of lag times for transport and mixing associated with transport (this was added in 2012 along with the new ODGI calculation). In both Tables, values are derived directly from measured lower tropospheric global mean abundances without any adjustments.

Improvements in the calculation of the ODGI initiated in 2012:

  1. 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).
  2. 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.
  3. 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).

Note:

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: https://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

Acknowledgements

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.

References

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