ESRL Global Monitoring Division

Ozone Depletion

Research by GMD has been critical in determining the degree of the depletion of stratospheric ozone and the trends of the compounds causing this depletion. GMD determines the vertical extent of depletion over Antarctica (the ozone hole), makes a significant contribution to world-wide, ground-based measurements of total-column ozone, and monitors the gases responsible for depleting stratospheric ozone. Understanding the production and fate of ozone and the compounds that deplete it has been and remains a focal point of GMD research.

The discovery of a major stratospheric ozone layer event (the ozone hole) over Antarctica in 1985 fueled interest in ozone depletion as a potential health and ecological threat related to increased solar ultraviolet radiation. Intensified research of this phenomenon followed and led ultimately to the strengthening of the Montreal Protocol, restricting or banning industrial production of chlorinated and brominated compounds causing the depletion.

Stratospheric Ozone Measurements

Measurements of total-column ozone have been made for over 40 years with the Dobson spectrophotometer. The 16-station GMD Cooperative Dobson Network is a significant portion of the global Dobson network making ground-based, column-ozone measurements. All GMD network stations and almost all of the global network stations are linked to the world calibration standard maintained by GMD. Six of the Dobson instruments are automated to provide ozone vertical profiles using the Umkehr technique and eight balloon-borne ozonesonde stations, including the South Pole, provide ozone profiles to an altitude of ~32 km.

From this strong complement of ozone-measuring techniques, it has been possible to measure the decline in ozone over the past two decades at mid-latitudes of the northern hemisphere and the tropics and to characterize the dramatic ozone depletion over Antarctica. GMD also monitors spectral UV at several sites and has shown the expected anti-correlation between ozone and UV.

[South Pole Ozone Hole Page]

[GMD South Pole ozone/H2O balloon launch.]

Ozone-Depleting Gases

Three gases that make a significant contribution to stratospheric ozone depletion, CFC-11, CFC-12 and N2O, have been monitored by GMD since the mid-1970s. Since then, numerous additional CFC's, HCFC's, and other halogenated gases have been incorporated into the measurement program as the number of monitoring sites increased. Most of the gases that are responsible for depleting stratospheric ozone are anthropogenic, but some, such as CH3Br and CH3Cl, have natural contributions as well.

Not only do scientists at GMD monitor the trends and distributions of these gases in the atmosphere, but they also investigate their sources and sinks. GMD's research extends to the troposphere, the stratosphere, the ocean, the polar snowpack, and terrestrial ecosystems in an effort to understand and predict the atmospheric behavior of these gases.

Stratospheric Aerosols

Ozone depletion, through halogen-related chemistry, is facilitated by increased stratospheric particles as provided by stratospheric clouds in the polar regions and globally by volcanic eruptions. In order to study the volcanic aerosol-ozone interaction, GMD monitors stratospheric aerosol with lidars at Boulder and Mauna Loa with an additional system being implemented in Samoa. Modeling suggests that with present halogen levels, a major eruption such as that of Pinatubo in 1991 could decrease total column ozone by as much as 10% at mid-latitudes.

Lidar measurements at Mauna Loa Observatory showing the integrated laser light backscattered from the 15-33 km region of the stratosphere. Volcanic eruptions which perturbed the stratosphere and increased the stratospheric aerosol loading are indicated.

Stratospheric Water Vapor

Utilizing balloon-borne frost-point hygrometers, GMD has detected an approximately 1% per year increase in stratospheric water vapor at Boulder, Colorado, since 1980. Besides implications for climate change, increased water vapor can affect the rate of chemical ozone loss, for example, by increasing the incidence of polar stratospheric clouds. Satellite measurements of water vapor, although not of adequate length for accurate trend determination, suggest that the increase may extend to other latitudes.

Increasing water vapor trend at 16-24 km over Boulder, Colorado, determined with the GMD balloon-borne frost-point hygrometer. The crosses are measurements, the continuous smooth curve is a statistical fit to the data, and the dotted curves are plus and minus one standard deviation.

Scientific Questions Related to Ozone Depletion

Will stratospheric ozone recover as anticipated?
Will total equivalent atmospheric chlorine continue to decline?
What are the contributions to inorganic bromine in the stratosphere?
How will future ozone variability affect UV radiation at the earth's surface?
Will further increases in stratospheric water vapor exacerbate ozone depletion?
Will the next major volcanic eruption decrease stratospheric ozone?

Actions and Impacts

Maintain and improve the current total ozone and ozone profiling network.
Impact: GMD total column, Umkehr, and ozonesonde profile records go back a minimum of 15 years and will provide the basis for detecting the anticipated recovery of stratospheric ozone.
Continue monitoring the distributions and trends of gases involved in stratospheric ozone depletion.
Impact: Regulations are in place on some ozone-depleting substances and new regulations will come into effect on replacement compounds in 20 years. The total equivalent chlorine from these gases has been declining but must be monitored to ensure that the regulations continue to have the desired effect.
Continue measurements of the oceanic and terrestrial fluxes of methyl halides and short-lived halocarbons.
Impact: Continued surveillance will allow the detection of feedbacks from climate change that may take place and alter oceanic fluxes and concentrations of the methyl halides and short-lived gases that contribute to stratospheric bromine.
Continue cooperative work with NASA toward understanding the processes depleting ozone in the stratosphere.
Impact: GMD designed and built instruments used for measurements from a NASA ER-2. The instrumental designs developed in these programs are used in replacing old technology at GMD's surface observing sites.
Improve water vapor instrumentation and expand the measurement program.
Impact: Simplification of production of the current balloon-borne cryogenic hygrometer will allow for more frequent measurements. Expansion of measuring sites will allow the determination of whether the increase in stratospheric water vapor observed at Boulder is global in nature.
Continue monitoring UV radiation and stratospheric aerosols at existing sites.
Impact: Combined with ozone measure-ments, the correlation between ozone and UV under differing atmospheric conditions can be studied. Stratospheric aerosol measurements following major volcanic eruptions will allow for the determination of their affects on ozone and climate.

Stratospheric ozone depletion from several Dobson instruments at northern mid-latitudes.

Ozone - UV Correlation