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In order to provide the information necessary to satisfy this congressional mandate, both NASA and NOAA have instituted global monitoring programs to keep track of ozone-depleting gases as well as ozone itself.  While the information that has been collected for the past 25 years has been used extensively in international assessments of the science of ozone layer depletion, 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 atmosphere? When do we expect the ozone layer above Antarctica to fully recover?  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 atmospheric measurements of chemicals that contain chlorine and bromine at sites across the globe (Figure 1).  It is defined here as being 100 at the time that NOAA’s observations indicated a maximum in ozone-depleting halogen gases, and zero for the level we anticipate will correspond to recovery of the ozone layer. 

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.

Figure 1.  Locations across Earth’s surface where regular measurements of the atmospheric abundance of ozone-depleting gases are conducted by NOAA/ESRL.  The chemicals measured are listed in the Tables.

The Antarctic Ozone Depleting Gas Index (ODGI-A)

The Antarctic ODGI is estimated directly from observations of all long-lived, chlorine and bromine containing gases.  These 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 springtime above Antarctica.  Because air reaching the Antarctic stratosphere has been isolated from the troposphere for a long period (~6 years), 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 burden of the atmosphere (or ECl).

Figure 2 shows ECl vs time from NOAA’s surface-based measurements and compares it with a future projection provided by the WMO/UNEP Scientific Assessment of Ozone Depletion baseline scenario (blue points and thick green line).  A lag time of 6 years is added to the baseline scenario to account for the approximate 6 years it takes for gases at Earth’s surface to reach the Antarctic stratosphere (thick blue line).  The range of ECl values relevant for the conditions in the Antarctic stratosphere over which the NOAA Antarctic Ozone Depleting Gas Index (ODGI-A) is defined is indicated in Figure 2.

Figure 2.  The Equivalent Chlorine, including both chlorine and bromine compounds, as a function of time showing NOAA measurements and a projection for the future provided by a WMO/UNEP scenario, which assumes that regulations of the Montreal Protocol will be followed.  The Ozone Depleting Gas Index relevant for the Antarctic Stratosphere (red scale on the right) had a value of 86.2 for the year 2006.
Click on image to view full size figure.

ODGI-A is defined here as being 100 at the time that NOAA’s surface-based observations indicated a maximum in ECl (1994).  The zero point of the scale is defined as the ECl level that existed when the Antarctic ozone hole first became easily detectable (about 2170 ppt ECl in about 1980), which is the ECl level when full recovery of the ozone hole is expected.  On this scale, the current value of ODGI-A is about 86, i.e., we have progressed about 14% of the way along the path toward a stratospheric halogen level that should allow an ozone-hole-free Antarctic stratosphere.  The latter has been projected to occur sometime in the around 2080 range as indicated by the recent WMO/UNEP Scientific Assessments of Ozone Depletion scenarios.  Though the recovery of the ozone layer depends critically on continued declines in halocarbon abundances in the future, the exact date for full recovery also depends upon future changes in other factors that influence ozone and that influence the efficiency of bromine and chlorine to destroy ozone, for example, climate change.

Figure 3. The contribution to Equivalent Chlorine by all long-lived chlorine- and bromine-containing gases.
Click on image to view full size figure.

In order to see which gases are causing the decline in ECl, Table 1 and Figure 3 delineates the contributions of individual gases to ECl.  Of the ozone depleting gases restricted by the Montreal Protocol, the NOAA results show that nearly all were decreasing in the atmosphere by 2006.  The notable exceptions include the halons and HCFCs, which are used as replacements for CFCs in many applications.  It is clear from this Figure that most of the decline in ECl has been due to the relatively rapid phase out and atmospheric decline of short-lived chemicals such as methyl chloroform (CH3CCl3) and methyl bromide.  The decline related to CFC-11 and CFC-12, the two major ECl components, has been relatively small.  Though emissions of these two CFCs have declined substantially over the past 15 years, their atmospheric decay has been slow 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 components.  Halons continue to increase slowly in the atmosphere because of large banks or reserves that are slowly being emitted to the atmosphere.  Though HCFC’s continue to increase and production is not scheduled for a complete phase-out until 2040, they currently contribute relatively little to the atmospheric burden of ECl.

Notes:  ECl is derived from ground-based measurements of Ozone Depleting Substances, with consideration given to the total number of chlorine and bromine atoms in each ODS, and the higher efficiency for bromine relative to chlorine in destroying ozone (a factor of 60 is used).  The Antarctic ODGI is derived from the measured ECl burden in the lower atmosphere, but it is made relevant for the stratosphere based upon ECl inferred for the stratosphere when the ozone hole was first observed (1980) and a lag time required for air at the surface to reach the polar stratosphere of 6 years.  “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.

The Mid-latitude Ozone Depleting Gas Index (ODGI-ML)

ODGI-ML is also estimated directly from observations of all long-lived, chlorine and bromine containing gases.  The calculation, however, is different from ODGI-A primarily because, owing to the closer proximity to the main chlorine and bromine gas sources (which are located predominantly in mid-latitudes of the Northern Hemisphere), air in the mid-latitude stratosphere has a younger mean ‘stratospheric age’ compared to air above Antarctica.  As a result, halocarbons in the mid latitude stratosphere have had less time to become degraded by Sun’s high-energy rays.  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, we derive a quantity known as the Effective Equivalent Chlorine (EECl) to represent how the burden of ozone-depleting halogenated gases are changing in the mid-latitude stratosphere. 

Figure 4.  The Effective Equivalent Chlorine, including both chlorine and bromine compounds, as a function of time showing NOAA measurements and a projection for the future provided by a WMO/UNEP scenario, which assumes that regulations of the Montreal Protocol will be followed.  The Ozone Depleting Gas Index relevant for the mid-latitude stratosphere (red scale on the right) had a value of 74.6 for the year 2006.
Click on image to view full size figure.

Figure 4 shows EECl vs time from NOAA’s surface-based measurements and compares it with a future projection provided by the WMO/UNEP Scientific Assessment of Ozone Depletion scenario (points and thick green line).  A lag time of 3 years is added to the surface-based observations to account for the approximate 3 years it takes for gases at Earth’s surface to reach the stratospheric ozone layer in mid-latitudes (thick blue line).

The range of EECl values relevant for the conditions in the mid-latitude stratosphere over which the NOAA Antarctic Ozone Depleting Gas Index (ODGI-ML) is defined is indicated in Figure 4.  Similar to ODGI-A, the ODGI-ML is defined as 100 at its peak, and zero at the level corresponding to when recovery might be expected in the mid-latitude stratosphere.  Based upon halocarbon abundances inferred for the mid-latitude stratosphere in 1980, we expect this recovery level to be approximately 1825 ppt EECl, or somewhat less than required for full recovery in Antarctica.  On this scale, the current value of the ODGI-ML is about 75, i.e., we have progressed about 25% of the way along the path toward a stratospheric halogen level that would allow a normal ozone layer.  The latter has been projected to occur in mid-latitudes sometime around the 2050 range (see Figure 4) in recent WMO/UNEP Scientific Assessments of Ozone Depletion scenarios.  Though the recovery of the ozone layer depends critically on continued declines in halocarbon abundances in the future, the exact date for full recovery also depends upon future changes in other factors that influence ozone and that influence the efficiency of bromine and chlorine to destroy ozone, such as climate change.

Figure 5.  The contribution to Effective Equivalent Chlorine by all long-lived chlorine- and bromine-containing gases.
Click on image to view full size figure.

As in Figure 3, Figure 5 and Table 2 delineate the contributions of individual gases to EECl.  The same gases that drive the declines in ECl and the ODGI-A also cause the decrease in EECl and the ODGI-ML.  Some interesting differences can be noted, however.  For example, because of its relatively rapid degradation in the mid-latitude stratosphere, CFC-11 contributes more than CFC-12 to EECl and the ODGI-ML.  Furthermore, HCFCs are only partially degraded in the mid-latitude stratosphere and, as a result, account for an even smaller increase in the ODGI-ML than in the ODGI-A.  As was the case for the ODGI-A, most of the decline in EECl has been due to the relatively rapid phase out and atmospheric decline of short-lived chemicals such as methyl chloroform (CH3CCl3) and methyl bromide.  Also, the decline related to CFC-11 and CFC-12, the two major EECl components, has been relatively small.  Though emissions of these two CFCs have declined substantially over the past 15 years, their atmospheric decline has been slow because their lifetimes are very long (50-100 years).

Notes:  EECl is derived from ground-based measurements of Ozone Depleting Substances (ODS), with consideration given to the number of chlorine and bromine atoms in each ODS, the rate at which these ODS photolytically decompose in the mid-latitude stratosphere, and the higher efficiency for bromine relative to chlorine in destroying ozone (a factor of 60 is used).  The mid-latitude ODGI is derived from the measured EECl burden in the lower atmosphere, but it is made relevant for the stratosphere based upon EECl inferred for the stratosphere when the ozone hole was first observed (1980) and a lag time required for air at the surface to reach the polar stratosphere of 3 years.  “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.

Sustained declines in atmospheric chlorine and bromine in future years hinges upon continued adherence to the production and consumption restrictions outlined in the Montreal Protocol on Substances that Deplete the Ozone Layer.  Recovery of the ozone layer is expected during this period as the ODGI approaches zero, though the timing of ozone layer recovery is difficult to determine exactly because other chemical and physical factors such as climate change 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 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.