(e). The Ozone Layer

The ozone layer is a region of concentration of the ozone molecule (O₃) in the Earth's atmosphere. The layer sits at an altitude of about 10-50 kilometers, with a maximum concentration in the stratosphere at an altitude of approximately 25 kilometers. In recent years, scientists have measured a seasonal thinning of the ozone layer primarily at the South Pole. This phenomenon is being called the ozone hole.

The ozone layer naturally shields Earth's life from the harmful effects of the Sun's ultraviolet (UV) radiation. A severe decrease in the concentration of ozone in the ozone layer could lead to the following harmful effects:

• An increase in the incidence of skin cancer (ultraviolet radiation can destroy acids in DNA).
• A large increase in cataracts and Sun burning.
• Suppression of immune systems in organisms.
• Adverse impact on crops and animals.
• Reduction in the growth of phytoplankton found in the Earth's oceans.
• Cooling of the Earth's stratosphere and possibly some surface climatic effect.

Ozone is created naturally in the stratosphere by the combining of atomic oxygen (O) with molecular oxygen (O₂). This process is activated by sunlight. Ozone is destroyed naturally by the absorption of ultraviolet radiation,

O₃ + UV >>> O₂ + O

and by the collision of ozone with other atmospheric atoms and molecules.

O₃ + O >>> 2O₂


O₃ + O₃ >>> 3O₂

Since the late 1970s, scientists have discovered that stratospheric ozone amounts over Antarctica in springtime (September - November) have decreased by as much as 60%. Satellite measurements (NIMBUS 7 - Total Ozone Mapping Spectrometer) have indicated a 2% decrease in ozone between 65 degrees North - 65 degrees South per decade since 1978 (Figure 7e-1). A reduction of about 3% per year has been measured at Antarctica where most of the ozone loss is occurring globally. During the late 1990s, large losses of ozone were recorded above Antarctica year after year in the months of September and August. In some years, spring levels of stratospheric ozone were more than 60% lower than the levels recorded months prior to the seasonal development of the hole.

Figure 7e-1: This image shows the global (from 65° N latitude to 65° S latitude) monthly average total ozone amount from 1979 to 2001. The green line shows measurements from Nimbus-7 TOMS instrument. The red line shows the results from the Meteor-3 TOMS instrument. The blue line shows the results from the Earth Probe TOMS instrument. (Source: NASA, TOMS Multimedia).

 

The following animation describes the change in ozone levels at the South Pole during the period 1978 to 1992:

Figure 7e-2: Quicktime Animation of Antarctic Ozone Levels 1978-1992.

The following two animations describe 1979 and 1992 ozone levels at the South Pole for the period September 15 to November 15. View these images for comparison:

Figure 7e-3: Quicktime Animation of Antarctic Ozone Levels 1979; and

Figure 7e-4: Quicktime Animation of Antarctic Ozone Levels 1992.

It appears that human activities are altering the amount of stratospheric O₃. The main agent responsible for this destruction was human-made chlorofluorocarbons or CFCs. First produced by General Motors Corporation in 1928, CFCs were created as a replacement to the toxic refrigerant ammonia. CFCs have also been used as a propellant in spray cans, cleaner for electronics, sterilant for hospital equipment, and to produce the bubbles in styrofoam. CFCs are cheap to produce and are very stable compounds, lasting up to 200 years in the atmosphere. By 1988, some 320,000 metric tons of CFCs were used worldwide.

In 1987, a number of nations around the world met to begin formulating a global plan, known as the Montreal Protocol, to reduce and eliminate the use of CFCs. Since 1987, the plan has been amended in 1990 and 1992 to quicken the schedule of production and consumption reductions. By 1996, 161 countries were participating in the Protocol. The Montreal Protocol called for a 100 % reduction in the creation and use of CFCs by January 1, 1996 in the world's more developed countries. Less developed countries have until January 1, 2010 to stop their production and consumption of these dangerous chemicals.

CFCs created at the Earth's surface drift slowly upward to the stratosphere where ultraviolet radiation from the Sun causes their decomposition and the release of chlorine (Cl). Chlorine in turn attacks the molecules of ozone chemically converting them into oxygen molecules.

Cl + O₃ >>> ClO + O₂


ClO + O = Cl + O₂

A single chlorine atom removes about 100,000 ozone molecules before it is taken out of operation by other substances. Chlorine is removed from the stratosphere by two chemical reactions:

ClO + NO₂ >>> ClONO₂


CH₄ + Cl >>> HCl + CH₃

Normally, these two reactions would quickly neutralize the chlorine released into the stratosphere. However, the presence of polar stratospheric clouds, rich in nitrogen, and sunlight facilitates a series of reactions which prolongs the reactive life of chlorine in the atmosphere. Interestingly, these polar stratospheric clouds require very cold air (approximately -85° Celsius) for their formation. Stratospheric air of this temperature occurs normally every year above Antarctica in the winter and early spring months. Destruction of the ozone begins in Antarctica in the spring as this region moves from 24 hours of night to 24 hours of day. These clouds are less frequent in the Arctic stratosphere because winter cooling of the air in the stratosphere is less severe.

NASA's Earth Probe -Total Ozone Mapping Spectrometer home page has the latest images describing the current status of global stratosphere ozone levels in the atmosphere.

The average areal coverage of the Antarctic ozone hole has now leveled off at about 24 million square kilometers (Figure 7e-5). Scientists believe that the ozone hole over Antarctica will maintain this size till about 2018. After this date, the ozone hole should begin to recover and be completely gone by about 2070.

Figure 7e-5: Average areal coverage of the ozone hole from 1979 to 2004. In this analysis, the ozone hole is defined as the area for which ozone is less than 220 Dobson Units. The ozone hole has grown from a few million square kilometers in 1981 to an area larger than North America (about 24 million square kilometers) in the year 2000. (Source: NASA, TOMS Multimedia).

 

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