Geoscience Reference
In-Depth Information
chlorine to chlorine oxide that destroys ozone, with the peak
loss taking place in September and October. As a result of this
decrease in ozone, the amount of surface exposure to UV radia-
tion at 55° S has increased about 10% per decade since the late
1970s. In addition to the Antarctic ozone hole, a similar but
weaker hole develops over the Arctic region in the Northern
Hemisphere winter. Significant depletions of ozone have also
been measured in the midlatitudes of the Northern Hemisphere.
As a result, the average amount of annual exposure to UV radia-
tion at 55° N has increased about 7% per decade since the late
1970s. These increases in UV exposure are of great concern
for several reasons, including damage to some forms of aquatic
life, reduced crop yields, and increases in the incidence of skin
cancer in humans.
Although scientists have legitimate concerns about ozone
depletion and its impact on life and human health, the threat
may diminish in your lifetime. Shortly after the ozone issue
emerged, the world community began working diligently to
solve the problem of CFC emissions. These efforts culminated
in the 1987 Montreal Protocol, an environmental treaty spon-
sored by the United Nations and signed by 23 nations. This
treaty called for developed countries such as the United States
and the nations of the European Union to cut CFC emissions by
50% by 1999 and for developing countries such as various Af-
rican nations and India to halt CFC use by 2010. The effect of
this treaty has been generally positive, with stratospheric CFC
concentrations peaking in 1997 and slowly falling since that
time. If the current rate of decline continues, the ozone layer
could be completely restored by the latter part of this century.
10
°
S
Darwin
20
°
S
AUSTRALIA
Brisbane
30
°
S
Figure 4.14 Dust storm in
new south Wales, Australia.
The dust in this cloud was
derived from poorly vegetated
soils that were severely erod-
ed by the wind.
Perth
Griffith
Sydney
Melbourne
150
°
E
40
°
S
120
°
E
TASMANIA
140
°
E
130
°
E
3.
The atmosphere consists largely of nitrogen (78%) and,
to a lesser extent, oxygen (21%).
4.
Although variable gases and particulates compose
less than 1% of the atmosphere, they nevertheless
affect atmospheric processes and climate in important
ways. A good example of the impact of these gases
is the greenhouse effect and the way it moderates
temperature on Earth.
Particulates
The last important variable component of the atmosphere is
particulates, which are microscopic bodies carried in the air,
existing in both liquid and solid form. The liquid variety comes
in the form of clouds and rain, which develop when water va-
por changes its physical state due to temperature changes in
the atmosphere. Solid particulates take an assortment of forms,
including snow, hail, pollutants, wind-blown soil (dust), smoke
from wildfires, volcanic ash, pollen grains from plants, and salt
spray from breaking waves. Most of the time particulate con-
centrations are densest near their place of origin, such as during
an intense dust storm (Figure 4.14) or near a volcano. Overall,
however, they make up less than 1% of the Earth's atmosphere.
Despite their small proportion of the atmosphere,
particulates nevertheless play an important role in weather and
climate. Precipitation would not occur if dust particles were
not present in the atmosphere because they provide a nucleus
around which water condenses in the first step of cloud forma-
tion. You can see this relationship the next time it rains right
after you wash your car, because your car will be coated with
fine dust. Other particulates, such as smoke and volcanic ash,
are important because they either absorb or reflect solar energy.
These combined processes influence local weather and regional
climate by moderating the temperature of the atmosphere.
Although most particulates are a positive component of the
atmosphere, some can cause negative environmental and health
effects, especially the toxic air pollutants associated with hu-
man activities. According to the U.S. Environmental Protection
Agency, over 100 pollutants are of particular concern. Benzene
and perchlorethylene, for example, are pollutants derived from
gasoline and dry cleaning facilities, respectively. Other toxic
pollutants include dioxin, asbestos, toluene, and metals such
KEY CONCEPTS TO REMEMBER ABOUT
THE COMPOSITION OF THE ATMOSPHERE
1.
Although the atmosphere is technically about 10,000 km
(6000 mi) thick, the vast majority of the air is found in the
lowermost 30 km (10 mi).
2.
The atmosphere contains constant gases (primarily
nitrogen and oxygen), variable gases (mainly carbon
dioxide and water vapor), and particulates.
