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.

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.

Global Equivalent atmospheric chlorine (all chlorine and bromine compounds), now decreasing because of the Montreal Protocol.

While the data collected 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 atmosphere? 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 atmospheric measurements of chemicals that contain chlorine and bromine at multiple remote surface sites across the globe.

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

GMD has launched balloon-borne Frost Point Hygrometers (FPHs) from Boulder, Colorado since 1980 to monitor the amount of water vapor in the stratosphere. The long-term measurement record depicts a 20% increase in stratospheric water vapor since 1980. Besides implications for climate change, this increase can affect the rate of chemical ozone loss, for example, by increasing the incidence of polar stratospheric clouds.

Increasing trend in stratospheric water vapor over Boulder, Colorado, as measured by the balloon-borne NOAA Frost Point Hygrometer. Mean water vapor mixing ratios between 16 and 26 km for individual balloon flights (black dots) are presented with their uncertainties (error bars are one standard deviation). The increasing trend (red curve) is clearly visible against the long-term mixing ratio average (4.37 ppmv, purple line).

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. Click on image to see full size figure.

Ozone - UV Correlation