Chemistry

Ozone Depletion



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The abundance of ozone in a column of air having a diameter of one centimeter varies seasonally and with latitude. In the equatorial region of the Earth, the ozone content of the atmosphere is about 250 Dobson units (DU), where:

1 DU = 2.69 x 10^16 molecules of ozone per square centimeter.

The DU is defined by the statement that 100 DU corresponds to a layer of ozone 10 mm thick at a pressure of 1.0 atmosphere and a temperature of 0 degrees C.

The amount of ozone in the polar regions varies seasonally. In the Arctic the range is from about 300 DU in the early fall up to about 460 DU in the springs. In the Antarctic the ozone levels in the spring reach only about 400 DU under natural conditions. However, the measurements by scientists showed that the average ozone concentration declined from about 330 DU in 1957 to about 200 DU in 1984. The monthly average ozone content has continued to decline since then and fell to just over 120 DU in 1989.

The cause for this alarming phenomenon was ultimately deduced from the following observations: (1) Direct measurements by a high-flying ER-2 plane on September 21, 1987, demonstrated a strong anti-correlation between the concentrations of ClO and O3 in the stratosphere over Antarctica at latitudes greater than about 70 degrees south; (2) A balloon launched from McMurdo station in October of 1987 detected a significant decrease in the ozone profile in the lower stratosphere between altitudes of about 12 and 21 km; (3) balloon launches demonstrated a 20-degree increase in the temperature at 21 km in the stratosphere over McMurdo between August 29 and October 27, 1987. These direct observations indicated that the reduced ozone concentrations in Antarctica occur in the early austral spring, coincide with elevated concentrations of ClO, and are associated with an increase in the temperature of the lower stratosphere. These are the principal clues that were used to explain the development of the ozone hole over Antarctica.

During the uastral winter months, the atmosphere over Antarctica is isolated from the global circulation by the polar vortex and the stratosphere receives no direct sunlight. As a result, the temperature of the stratosphere over Antarctica declines to 80 C. This permits the formation of ice crystals composed of a mixture of water and nitric acid. The absence of UV radiation caused by the absence of solar radiation retards the reactions that produce and destroy ozone in the stratosphere. Under these conditions, chlorine nitrate (ClONO2) is incorporated into the ice crystals and is absorbed on their surfaces where it reacts with HCl to form Cl2 and HNO3:

ClONO2 + HCl -> Cl2 + HNO3

The Cl2 molecules are released into the air of the stratosphere, whereas the HNO3 remains in the ice crystals. When sunlight returns to Antarctica in the early spring, the ice crystals sublime and the chlorine molecules are dissociated by UV, thus releasing the Cl catalyst:

Cl2 + hv -> 2 Cl

Consequently, the return of the Sun to Antarctica triggers rapid catalytic destruction of ozone by the Cl stored in the stratosphere. However, as the stratosphere warms up, the circumpolar circulation breaks down and the ozone-depleted air over Antarctica mixers with ozone-rich air at lower latitudes. This phenomenon causes the "ozone hole" to spread to populated areas in the southern hemisphere until normal transport processes in the stratosphere restore the global distribution of ozone.

The potential hazard to the biosphere of increased exposure to UV-B caused by the large-scale destruction of ozone by CFCs was considered in 1987 by an international conference in Montreal. The protocol arising from that conference called for a voluntary 50% reduction in the manufacture of CFCs by 1998. This recommendation was strengthened at a subsequent conference in 1990 in London by specifying maximum allowable production of CFCs relative to 1986: 80% by 1993; 50% by 1995; 15% by 1997; and 0% by 2000. In addition, the production of methylchloroform (CH3CCl3) and carbon tetrachloride (CCl4) ended in 2005. Although the production of CFCs in Europe and North America has been reduced, the targets have not been met by some major industrial nations.

Model calculations by atmospherics scientists predicted a significant 28% reduction of stratospheric ozone (relative to 1965) in the southern hemisphere by the year 2030 even if the production targets of London were actually realized. However, if CFC production continues at 1974 levels, the ozone depletion of the southern hemisphere will reach 48% by 2030. If the recommendations for reduced production of CFCs are not implemented and production rises above 1974 levels, the ozone depletion will be greater than 48% and the resulting health hazard to humans may become quite serious.

For this reason, the feasibility of reducing the chlorine content of the stratosphere over Antarctica by injecting ethane (C2H6) and propane (C3H8) was considered. These gases react with Cl to form HCl, thus sequestering it and preventing the destruction of ozone.

Unfortunately, a fleet of high-flying aircraft would be required to disperse about 50,000 tons of the hydrocarbons annually. In addition, the gas must be dispersed widely within the atmosphere encompassed by the polar cortex whose position and integrity vary unpredictably. It is probably better to stop production of chlorine-bearing gases worldwide than to attempt to repair the atmosphere. However, even in the unlikely event that production of these gases is actually stopped, ozone depletion in the southern hemisphere will continue for many decades until the atmosphere gradually cleans itself up.

Ozone depletion by CFCs is, of course, a world-wide phenomenon that is aggravated in the Southern Hemisphere because of the isolation of Antarctica during the winter months by the polar vortex. In the Northern Hemisphere the problem is not as severe because the Arctic region does not develop a polar vortex. Consequently, the ozone content of the stratosphere in the Arctic is reduced by only 10-20% of normal levels when the sun returns at the end of the winter.

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