Page 194 - Fundamentals of Air Pollution

160 11. Long-Term Effects on the Planet

In 1975, Rowland and Molina (12) postulated that chlorofluorocarbons
(CFCs) could modify the steady- state concentrations of stratospheric ozone,
CFCs are chemically very stable compounds and have been used for over
50 years as refrigerants, aerosol propellants, foam blowing agents, cleaning
agents, and fire suppressants. Because of their stability in the troposphere,
they remain in the troposphere for long periods of time, providing the
opportunity for a portion of these chemicals to diffuse into the stratosphere.
Rowland and Molina suggested that CFCs in the stratosphere would up-
set the balance represented by Eqs. (11-2) to (11-3). In the stratosphere,
CFCs would be exposed to shortwave UV radiation with wavelengths
X <220 nm and undergo photodissociation, releasing chlorine atoms (Cl),
and Cl would interfere with the ozone balance in the following manner:

The chlorine atoms would provide another destruction pathway for ozone
in addition to (11-3), shifting the steady-state ozone to a lower value.
Because of the catalytic nature of Eqs. (11-5) and (11-6), one chlorine atom
destroys many ozone molecules.
The discovery of ozone holes over Antarctica in the mid-1980s was strong
observational evidence to support the Rowland and Molina hypothesis.
The atmosphere over the south pole is complex because of the long periods
of total darkness and sunlight and the presence of a polar vortex and polar
stratospheric clouds. However, researchers have found evidence to support
the role of CIO in the rapid depletion of stratospheric ozone over the south
pole. Figure 11-3 shows the profile of ozone and CIO measured at an
altitude of 18 km on an aircraft flight from southern Chile toward the south
pole on September 21, 1987. One month earlier the ozone levels were fairly
uniform around 2 ppm (vol).
Ozone holes are considered by many as a harbinger of atmospheric
modification. Investigators have found a similar but less intense annual
decrease in ozone over the Arctic region of the globe. Additional studies
are providing evidence for stratospheric ozone depletion over the northern
temperate regions of the globe. These observations have prompted a world-
wide phaseout of the manufacture and use of chlorofluorocarbons and
halogens over the next decade. These chemicals will be present at elevated
levels for many years to come because of their stability.
CFCs represent only one class of chemicals being released to the atmo-
sphere which have long-term effects. Replacement chemicals will be re-