DEFINITION OF OZONE HOLE

Ozone is a poisonous gas (O3) that is present

in trace amounts in much of the atmosphere but reaches a

maximum concentration in a stratospheric layer at 9–25

miles (15–40 km) above the Earth, with a peak at 15.5 miles

(25 km). The presence of ozone in the stratosphere is essential

for most life on Earth, since it absorbs the most carcinogenic

part of the solar spectrum with wavelengths between

0.000011 and 0.0000124 inches (280 and 315 nm). If these

ultraviolet rays were to reach the Earth they would cause

many skin cancers and possibly depress the human immune

system. These harmful rays would greatly reduce photosynthesis

in plants and reduce plant growth to such an extent

that the global ecosystems would crash.

Ozone naturally changes its concentration in the stratosphere

and is also strongly affected by human or anthropogenically

produced chemicals that make their way into the

atmosphere and stratosphere. Ozone is produced by photochemical

reactions above 25 kilometers mostly near the equator

and moves toward the poles where it is most abundant,

and where it is gradually destroyed. The concentration of

ozone does not vary greatly in equatorial regions, but at the

poles it tends to be the greatest in the winter and early spring.

Stratospheric circulation in the winter over Antarctica is

characterized by the formation of a strong vortex that isolates

the stratospheric air over the pole during the night in the

Antarctic winter.

Atmospheric and stratospheric flow dynamics can

change the distribution of ozone, solar flare and sunspot

activity can enhance ozone, and volcanic eruptions can add

sulfates to the stratosphere that destroy ozone. In the 1970s

it was realized that some aerosol chemicals and refrigerants,

the chlorofluorocarbons (CFCs), could make their way into

the stratosphere and be broken down by ultraviolet light to

release chlorine, which can destroy ozone. The use of CFCs

was subsequently curtailed, but the aerosols and chlorine

have very long residence times in the stratosphere, and each

chlorine ion is capable of destroying large amounts of ozone.

Since the middle 1980s, a large hole marked by large depletions

of ozone in the stratosphere has been observed above

Antarctica every spring, its growth aided by the polar vortex.

The hole has continued to grow, but the relative contributions

to the destruction of ozone by CFCs, other

chemicals (such as supersonic jet and space shuttle fuel), volcanic

gases, and natural fluctuations is uncertain. In 1999

the size of the Antarctic ozone hole was measured at more

than 9,650,000 square miles (25 million km2), more than

two and half times the size of Europe. However, the appearance

of ozone depletion above arctic regions has added credence

to models that show the ozone depletion being largely

caused by CFCs. Many models suggest that the CFCs may

lead to a 5–20 percent reduction in global ozone, with consequent

increases in cancers, disease, and loss of crop and

plant yield.

See also ATMOSPHERE.

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