Agriculture Reference
In-Depth Information
makes them a ready source. Complete combustion results in carbon dioxide
and water, but anything short of combustion will be a source of hydrocar-
bons, including some of the original hydrocarbons in the fuels, as well as new
ones formed during combustion. The compounds that become free radicals,
such as the oxides of nitrogen, are also readily available from internal combus-
tion engines, since three-fourths of the troposphere is made up of molecular
nitrogen (N
2
). Although N
2
is not relatively chemically reactive, under the
high-temperature, high-pressure conditions in an engine, it combines with
the O
2
from the fuel-air mix to generate oxides that can provide electrons to
the photochemical reactions.
Ozone is not always bad and is absolutely essential as a component of the
stratosphere. The less O
3
there is in the stratosphere, the more harmful ultravi-
olet (UV) waves there are that find their way to Earth's surface. This illustrates
that even though our exposure is to the physical insult (i.e., the UV), the ex-
posure was brought about by chemical contamination. Chemicals released into
the atmosphere, in turn, react with ozone in the stratosphere, decreasing the
ozone concentration and increasing the amount of UV radiation at earth's sur-
face. This has meant that the mean UV dose in the temperate zones of the world
has increased, which has been associated with an increase in the incidence of
skin cancer, especially the most virulent form, melanoma. Thus, to prevent
this type of cancer requires a comprehensive viewpoint and an understanding
of the complexity of the factors that have led to increased UV exposure.
There is much uncertainty about the effects of the presence of these radiant
gases (see Table 7.4), but the overall effect of the composite of gases is well un-
derstood. The effectiveness of CO
2
as a global warming gas has been known for
over 100 years. However, the first useful measurements of atmospheric CO
2
were
not taken until 1957. The data from Mauna Loa show that even in the 1950s,
the CO
2
concentration had increased from the baseline 280 ppm to 315 ppm;
and this has continued to climb over the last 50 years at a nearly constant rate
of about 1.6 ppm per year. The most serious problem with CO
2
is that the
effects on global temperature are delayed due to its greenhouse effect. Even in
the completely impossible scenario of not emitting any new CO
2
into the atmo-
sphere, CO
2
concentrations will continue to increase from our present 370 ppm,
with some estimates of possibly higher than 600 ppm.
Methane (CH
4
) is the product of anaerobic decomposition and human food
production. Methane is also emitted during the combustion of fossil fuels and
the cutting and clearing of forests. The concentration of CH
4
in the atmosphere
has been steady at about 0.75 ppm for over a thousand years, and then increased
to 0.85 ppm in 1900. Since then, in only a hundred years, it has skyrocketed to
1.7 ppm. Methane is removed from the atmosphere by reaction with the
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