Section 9.9 Radicals and Stratospheric Ozone 353FOOD PRESERVATIVES
Radical inhibitors that are present in food are
known as preservativesor antioxidants. They pre-
serve food by preventing unwanted radical reactions. Vitamin E9.9 Radicals and Stratospheric Ozone
Ozone a major constituent of smog, is a health hazard at ground level. In the
stratosphere, however, a layer of ozone shields the Earth from harmful solar radiation.
The greatest concentration of ozone occurs between 12 and 15 miles above the Earth’s
surface. The ozone layer is thinnest at the equator and densest towards the poles.
Ozone is formed in the atmosphere from the interaction of molecular oxygen with
very short wavelength ultraviolet light.
The stratospheric ozone layer acts as a filter for biologically harmful ultraviolet radia-
tion that otherwise would reach the surface of the Earth. Among other effects, high-
energy short-wavelength ultraviolet light can damage DNA in skin cells, causing
mutations that trigger skin cancer (Section 29.6). We owe our very existence to this
protective ozone layer. According to current theories of evolution, life could not have
developed on land in the absence of this ozone layer. Instead, life would have had to
remain in the ocean, where water screens out the harmful ultraviolet radiation.
Since about 1985, scientists have noted a precipitous drop in stratospheric ozone
over Antarctica. This area of ozone depletion, known as the “ozone hole,”is unprece-
dented in the history of ozone observations. Scientists subsequently noted a similar
decrease in ozone over Arctic regions, and in 1988 they detected a depletion of ozone
over the United States for the first time. Three years later, scientists determined that
the rate of ozone depletion was two to three times faster than originally anticipated.
Many in the scientific community blame recently observed increases in cataracts and
skin cancer as well as diminished plant growth on the ultraviolet radiation that has
penetrated the reduced ozone layer. It has been predicted that erosion of the protec-
tive ozone layer will cause an additional 200,000 deaths from skin cancer over the
next 50 years.
Strong circumstantial evidence implicates synthetic chlorofluorocarbons (CFCs)—
alkanes in which all the hydrogens have been replaced by fluorine and chlorine, such
as and —as a major cause of ozone depletion. These gases, known
commercially as have been used extensively as cooling fluids in refrigera-
tors and air conditioners. They were also once widely used as propellants in aerosol
Freons®,CFCl 3 CF 2 Cl 2ozoneO 2 OO+OO+ 2 O 3h(O 3 ),In 1995, the Nobel Prize in chemistry
was awarded to Sherwood Rowland,
Mario Molina, and Paul Crutzen
for their pioneering work in
explaining the chemical processes
responsible for the depletion of the
ozone layer in the stratosphere. Their
work demonstrated that human
activities could interfere with global
processes that support life. This was
the first time that a Nobel Prize
was presented for work in the
environmental sciences.is a naturally occurring preservative found in vegetable oil.
BHA and BHT are synthetic preservatives that are added to
many packaged foods.food preservativesOCH 3OH
C(CH 3 ) 3 C(CH 3 ) 3OCH 3C(CH 3 ) 3OHCH 3OH
(CH 3 ) 3 Cbutylated hydroxyanisole
BHAbutylated hydroxytoluene
BHTF. Sherwood Rowlandwas born in
Ohio in 1927. He received a Ph.D.
from the University of Chicago and is
a professor of chemistry at the
University of California, Irvine.Mario Molinawas born in Mexico
in 1943 and subsequently became a
U.S. citizen. He received a Ph.D.
from the University of California,
Berkeley, and then became a
postdoctoral fellow in Rowland’s
laboratory. He is currently a
professor of earth, atmospheric, and
planetary sciences at the
Massachusetts Institute of
Technology.Paul Crutzenwas born in
Amsterdam in 1933. He was trained
as a meteorologist and became
interested in stratospheric chemistry
and statospheric ozone in particular.
He is a professor at the Max Planck
Institute for Chemistry in Mainz,
Germany.