Ozone Hole

The so-called ozone hole sometimes is confused with the problem of global warming. Even though there is a connection between the two environmental issues, because ozone contributes to the greenhouse effect, the ozone hole is a separate issue. This article briefly addresses how ozone depletion is measured.

Ozone in Earth's Atmosphere

Ozone is a colorless, gaseous form of oxygen found in the Earth's atmosphere, primarily in the upper region known as the stratosphere, where it is naturally produced and destroyed. The chemical element oxygen normally forms a molecule containing two atoms (O₂). But in the presence of ultra-violet light or an electrical spark in the air, oxygen can form a molecule containing three atoms (O₃). The molecule of three oxygen atoms is called ozone.

Within the stratosphere is a layer between 20 and 40 kilometers (km) above Earth's surface that is known as the ozone layer. Here ozone takes up a greater proportion of the atmospheric column than at any other height. In the stratosphere, the concentration of ozone is 1,000 times greater than in the lower region of Earth's atmosphere known as the troposphere. Ozone in the stratosphere is beneficial because it protects Earth's inhabitants from the Sun's harmful ultraviolet radiation.

Measuring Ozone Levels

Scientists assess ozone by calculating how much there would be if all the ozone over a particular spot on Earth were compressed to a standard atmosphere of pressure—that is, the average pressure of air at sea level. On average, this would result in a column of ozone no more than 3 millimeters (mm) thick.

The unit of measure used to represent the amount of ozone above a particular position on the surface is the Dobson unit (DU), with one unitrepresenting 0.01 mm of ozone compressed to one standard atmosphere. Therefore, there is typically 300 DU in a column of the normal atmosphere.

G. M. B. Dobson was a British physicist who initiated the first regular monitoring of atmospheric ozone using spectrographic instruments in the 1920s. He was able to derive the vertical distribution of ozone from a series of measurements of the relative intensities of two particular wavelengths of light scattered in the zenith sky. One wavelength is more strongly absorbed by ozone than the other wavelength. As the Sun's zenith angle varies, a reversal occurs in the variation of the ratio of the intensities. Dobson compared the intensities of these two wavelengths with an instrument he constructed using a photomultiplier and an optical wedge; this instrument is now known as a Dobson spectrophotometer.

Dobson found that the ozone in the atmosphere is far from uniformly spread. The lowest concentrations, around 250 DU, were consistently found at the equator, although the polar winters resulted in periods where their concentrations might fall below the equatorial level. The highest concentrations were found in higher latitudes, where the variation fluctuated from as high as 460 DU to 290 DU in the upper latitudes of the Northern Hemisphere and between 400 DU and 300 DU in the Southern Hemisphere.

The International Geophysical Year* of 1957–1958 witnessed the World Meteorological Organization (WMO) take responsibility for establishing uniform and high quality ozone measurement world wide. The WMO subsequently established 160 ground-based ozone observation stations.

*The International Geophysical Year (July, 1957 through December, 1958) consists of eighteen months of a period of maximum sunspot activity. It was designated for cooperative study of the solar-terrestrial environment by the scientists of sixty-seven nations.

From the 1920s to the 1970s ozone was measured from the ground. Since the late 1970s scientists have used satellites, aircraft, and balloons to measure ozone levels from above Earth. The National Aeronautics and Space Administration (NASA) has also launched many scientific studies to investigate ozone. The figure below is one example of the results of this data monitoring.

Recording Low Ozone Levels

In the 1970s a research group with the British Antarctic Survey (BAS) was monitoring the atmosphere above Antarctica when the scientists first noticed a loss of ozone in the lower stratosphere. At first they believed their instruments to be faulty, and new instruments were sent to ensure that the readings were accurate.

By 1985 the BAS was reporting a dramatic decline of 50 percent in springtime ozone levels above Halley Bay Station when compared to the previous decade. At the most affected altitude, 14 to 19 km above the surface, more than 99 percent was lost. This was an unsettling discovery because NASA had been monitoring ozone levels globally since 1979 with the Total Ozone Mapping Spectrometer (TOMS) aboard the Nimbus 7 satellite.

The standard TOMS data-processing procedure was to automatically neglect ozone levels below a fixed value of 180 DU, considering such data to be unreliable. Hence, the Antarctic springtime data had been ignored. Only after the British survey team's report were the TOMS data reprocessed; the ozone depletion was verified and the geographical extent of the hole was determined. This lowering of the amount of ozone over the Antarctic became known as the ozone hole.

Ozone Hole

Ozone in Earth's Atmosphere

Measuring Ozone Levels

Recording Low Ozone Levels

Bibliography

Internet Resources

OZONE CLOSE TO EARTH