Geoscience Reference
In-Depth Information
in the atmosphere. “Good” (Dr. Jeckel) ozone occurs naturally in the stratosphere approximately
10 to 30 miles above the Earth's surface and forms a layer that protects life on Earth from the
sun's harmful rays. In the Earth's lower atmosphere, ground-level ozone is considered “bad” (Mr.
Hyde).
VOC + NO
x
+ Heat + Sunlight = Ozone
Motor vehicle exhaust and industrial emissions, gasoline vapors, and chemical solvents are some
of the major sources of NO
x
and VOCs that help to form ozone. Sunlight and hot weather cause
ground-level ozone to form in harmful concentrations in the air. As a result, it is known as a sum-
mertime air pollutant. Many urban areas tend to have high levels of “bad” ozone, but even rural
areas are also subject to increased ozone levels because wind carries ozone and pollutants that form
it hundreds of miles away from their original sources.
15.5.1.2 Nitrogen Oxides (NO
x
)
Nitrogen oxides (NO
x
) is the generic term for a group of highly reactive gases all of which contain
nitrogen and oxygen in varying amounts. This group includes NO, NO
2
, NO
3
, N
2
O, N
2
O
3,
N
2
O
4
,
and N
2
O
5
—but only two are important in the study of air pollution: nitric oxide (NO) and nitrogen
dioxide (NO
2
). Many of the nitrogen oxides are colorless and odorless; however, nitrogen dioxide
(NO
2
), a common pollutant, along with particles in the air can often be seen as a reddish-brown
layer over many urban areas. Nitrogen oxides form when fuel is burned at high temperatures, as in
a combustion process. The primary sources of NO
x
are motor vehicles, electric utilities, and other
industrial, commercial, and residential sources that burn fuels.
15.5.1.3 Particulate Matter
Particulate matter (PM) is the term for particles found in the air, including dust, dirt, soot, smoke,
and liquid droplets. Particles can be suspended in the air for long periods of time. Some particles
are large or dark enough to be seen as soot or smoke. Others are so small that individually they can
only be detected with an electron microscope. Some particles are directly emitted into the air. They
come from a variety of sources such as cars, trucks, buses, factories, construction sites, tilled fields,
unpaved roads, stone crushing, and burning of wood. Other particles may be formed in the air from
the chemical change of gases. They are indirectly formed when gases from burning fuels react with
sunlight and water vapor. These can result from fuel combustion in motor vehicles, at power plants,
and in other industrial processes.
15.5.1.4 Sulfur Dioxide (SO
2
)
Sulfur dioxide (SO
2
) belongs to the family of sulfur oxide gases (SO
x
). These gases dissolve easily
in water. Sulfur enters the atmosphere in the form of corrosive sulfur dioxide gas, a colorless gas
possessing the sharp, pungent odor of burning rubber. Also, sulfur is prevalent in all raw materials,
including crude oil, coal, and ore that contains common metals such as aluminum, copper, zinc,
lead, and iron. SO
x
gases are formed when sulfur-containing fuel, such as coal and oil, is burned,
and when gasoline is extracted from oil or metals are extracted from ore. SO
2
dissolves in water
vapor to form acid, and interacts with other gases and particles in the air to form sulfates and other
products that can be harmful to people and their environment.
Over 65% of SO
2
released to the air, or more than 13 million tons per year, come from electric
utilities, especially those that burn coal. Other sources of SO
2
are industrial facilities that derive
their products from raw materials such as metallic ore, coal, and crude oil, or that burn coal or oil
to produce heat. Examples are petroleum refineries, cement manufacturing, and metal processing
facilities. Also, large ships, locomotives, and some nonroad diesel equipment currently burn high
sulfur fuel and release SO
2
emissions to the air in large quantities.
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