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Ozone was first discovered in 1839 by German
scientist Christian Friedrich Schonbein. It is a pale blue,
relatively unstable molecule made up of three oxygen atoms.
The ozone molecule is angular, polar, and diamagnetic. Both
oxygen bond lengths (1.28 angstroms) are identical. It is
formed from molecular oxygen (O2) by ultraviolet
and extreme ultraviolet photolysis followed by recombination
of atomic oxygen (O) with O2.
It may also be formed by passing an electrical
discharge through gaseous oxygen. It is characterized by a
unique odor that is often noticed during electrical storms
and in the vicinity of electrical equipment. In fact, the
term ozone is derived from the Greek word ozein which
means "to smell." The density of ozone is about 2.5 times
that of O2. At -112°C it condenses to a deep
blue liquid. It is a powerful oxidizing agent and, as a concentrated
gas or a liquid, is highly explosive.
Excess oxygen atoms, also known as free radicals,
oxidize materials that they contact and are associated with
the aging process.
Destructive Ozone
Depending on where ozone resides, it can
protect or harm life on Earth. When it is close to the planet's
surface, in the air we breathe, ozone is a harmful pollutant
that causes damage to lung tissue and plants, and is considered
to be "bad ozone." It is a powerful photochemical oxidant that
damages rubber, plastic, and all plant and animal life. It also
reacts with hydrocarbons from automobile exhaust and evaporated
gasoline to form secondary organic pollutants such as aldehydes
and ketones. The peroxyacyl nitrates are especially damaging
photochemical oxidants that are very irritating to the eyes
and throat.
Ozone pollution originating in urban areas
can extend into surrounding rural and forested areas that
are hundreds of kilometers downwind. Episodes of elevated
ozone concentrations are associated with warm, slow moving
high pressure systems and contain between 30 and 50 parts
of ozone per billion by volume. Concentrations 3 to 8 times
greater than natural background levels have been observed.
During the summer heat wave of 1988, record ozone concentrations
were recorded in the United States. Even Acadia National Park
in Maine and the Shenandoah mountains of Virginia were affected
by dangerous levels of ozone pollution. These rural areas
are far removed from industrial regions and polluted cities.
The ozone pollution recorded in Acadia most likely originated
in New York City. That in Virginia may have migrated from
refineries on the Gulf Coast.
Photochemical oxidants are the most significant
cause of agricultural loss in the United States. Their damaging
effects on vegetation and crops have been confirmed in the
eastern United States, adjacent areas in Canada, and much
of Europe. Ozone alone, or in combination with sulfur dioxide
(SO2) and nitrogen dioxide (NO2), accounts
for 90% of the annual crop losses in the U.S. that are caused
by air pollution.
Protective Ozone
Most ozone is concentrated in the stratosphere,
at about 25 km in altitude, and is considered to be "good ozone."
In this region, ozone acts as a shield to protect Earth's surface
by absorbing harmful ultraviolet radiation. Without this shield,
we would be more susceptible to skin cancer, cataracts, and
impaired immune systems. A 1% decrease in total column ozone
causes the amount of transmitted UV radiation, in the spectral
region that damages deoxyribonucleic acid (DNA), to increase
by about 2%. Although good ozone only represents a tiny fraction
of the atmosphere, it is crucial for life on Earth.
The proportion of good and bad ozone
in the atmosphere depends on the balance between processes
that create ozone and those that destroy it. An upset in this
balance can have serious consequences for life on Earth, and
scientists are finding evidence that the balance has changed.
Concentrations within the protective ozone shield are decreasing,
while levels in the air we breathe are increasing.
Ozone amounts in the stratosphere are small,
rarely exceeding 10 parts per million by volume. Ozone is
measured in Dobson Units. One Dobson Unit (DU) corresponds
to 2.69 x 1016 molecules per square centimeter, which is equivalent
to the amount of gas in one square centimeter at 1 atmosphere
of pressure. Average ozone levels are 300 DU, which would
be equivalent to a layer three millimeters thick if compressed
to the planet's surface. Levels may range from less than 100
DU to over 500 DU globally.
Text, images and videos
courtesy of Distributed Active Archive Center at NASA's Goddard
Space Flight Center.
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