The Ozone Hole — A Crisis Averted?
by Bill Rizer, Editor-elect
As geologists we are often asked by friends or relatives to explain some earth-related topic or issue that has been on the news or in the science section of the local newspaper. One example that comes to mind and seems to reappear every so often is the ozone hole. Most of us know that ozone (O3) is a somehow important constituent of our atmosphere and that recently scientists have noted a worrysome “hole” in the ozone layer over Antartica. However, if you are like me, that is pretty much all that you know. This article represents my attempt at gaining some small degreee of understanding, sufficient to enable me to provide a somewhat reasonable explanation when I am asked to comment on the ozone layer and why it is important. In the course of this brief investigation, a story emerged that is at once scientifically engaging and emotionally encouraging because it illustrates how a potentially disasterous man-made environmental crisis can be averted when a responsible society takes appropriate action based on information provided by a watchful scientific community.
Most of us are familiar with the ozone watches and alerts that we hear on the radio, usually during those hot sultry days in the summer when there is little wind. Near the surface, ozone is a potent toxin that forms when nitrogen oxide gases from vehicle and industrial emissions react with volatile organic compounds (paint thinners and other carbon-containing chemicals that evaporate easily into the air). In the troposphere, that part of the atmosphere near the Earth’s surface, the natural concentration of ozone is about 10 parts per billion (ppb) which is 0.00001%. According to the Environmental Protection Agency, exposure to ozone levels greater than 80 ppb for 8 hours or longer is unhealthy (NASA, 2000). Such concentrations occur in or near cities during periods when the atmosphere is warm and stable. The harmful effects can include throat and lung irritation or aggravation of asthma or emphysema.
Fortunately for us, most atmospheric ozone is found in the stratosphere, the region of the atmosphere between about 10 and 50 km (32,000 -164,000 ft) where ultraviolet radiation is very high (Figure 1). Ozone is created when extreme ultraviolet radiation from the sun breaks an oxygen (O2) molecule into two free oxygen atoms. These then combine either with each other to form O2 or with O2 to form ozone (NASA, 2000). In general, ozone concentrations are low, a few molecules per million, even in the stratosphere. The peak concentration of ozone occurs at an altitude of roughly 32 km (20 miles) above the surface of the Earth. At that altitude, ozone concentration can be as high as 15,000 ppb (0.0015%). The standard measure for ozone is the Dobson Unit (DU).
A vertical profile (Figure 1) of O3 concentration with altitude shows that most of the ozone is found from 10-30 km. Despite its low concentration, this ozone layer is critical for life on earth as we know it. Stratospheric ozone absorbs all of the UV-c, most of the UV-b, and about half of the UV-a sunlight radiation incident on the atmosphere and prevents it from reaching the surface. Exposure to the highly energetic UV-c, and/or an increase in exposure to UV-b and to a lesser extent UV-a can lead to increased incidence of skin cancer, and can cause damage to our immune systems, marine organisms, and sensitive crops.
Normally, stratospheric ozone is produced and destroyed at a fairly constant rate, that is until modern society altered that balance by producing increasing levels of CFCs (chlorofluorocarbons). CFCs, found in older refrigerants, fire extinguishers, and certain solvents, reach into all levels of the stratosphere and decompose into ozone-depleting gases such as chlorine (Cl2) and bromine (Br2). One Cl2 or Br2 molecule can destroy 105 molecules of O3. Human activity contributes 82% of ozone-depleting chlorine to the atmosphere; explosive volcanic eruptions contibute only 3%.
The Ozone Hole
Farman et al. (1985) surprised atmospheric scientists when they announced a rapid decrease of 50% in total ozone that occurred over Halley Bay, Antarctica, each year over the period from 1975 to 1984, reaching the lowest annual values in early October. Furthermore, they found that spring total ozone values had decreased from 300+ DU in the late 1950s and early 1960s to around 200 DU in the early 1980s. Prior to this discovery, many scientists had recognized an annual variation in ozone levels over Antartica, but had not observed the dramatic annual decrease in total ozone over such a short time in early spring or the steady annual decline in total ozone that had been occurring since the late 1950s. Subsequent analyses of records dating back to the 1950s along with newer satellite data using the Total Ozone Mapping Spectrometer (TOMS) confirmed the Farman, et al. (1985) findings and demonstrated that the region of severe ozone depletion covered essentially the entire continent of Antarctica (Figure 2).
Each year the ozone hole develops in August and reaches a maximum in late September or early October before shrinking and disappearing in November or December. In a period of only a few weeks, the total amount of ozone can decrease by up to 50%, from 300 DU to 200 DU or less. The term ozone hole is defined as the area within which total ozone is 200 DU or less. Analysis of the historical data indicate a dramatic increase in the size of the ozone hole from the late 1970s to the present (Figure 3).
The largest Antarctic ozone hole measured to date occurred in 1998, and averaged 10.1 million square miles (Solomon, et al. 2005). This year’s ozone hole measured 9.4 million square miles at its peak between September and mid-October, slightly larger than in 2004. For 10 of the past 12 years, the Antarctic ozone hole has been larger than 7.7 million square miles. Before 1985, it measured less than 4 million square miles (NASA, 2005).
Causes of the Ozone Hole
The Antarctic ozone hole is primarily the result of destruction of stratospheric ozone by increasing levels of man-made chlorine and bromine combined with the pa