Changes in the atmosphere
If you were to compare the earth with an
apple from the supermarket, the atmosphere would be no thicker than the layer of
shiny wax applied to it. Yet this thin, finite wrapping of air around the planet
receives more than 700,000 metric tons of pollutants each day in the United
States alone. Pollutants are substances with which ecosystems have had no prior
evolutionary experience, in terms of kinds or amounts, and so have no mechanisms
for dealing with them. From the human perspective, pollutants are substances
that severely affect our health, activities, or survival.
Local air
pollution
Industrial smog is gray air that predominates in
industrialized cities with cold, wet winters. London, New York, Pittsburgh, and
Chicago are examples. These cities use fossil fuel for heating, manufacturing,
and producing electric power. The burning fuel releases airborne pollutants,
including dust, smoke, ashes, soot, asbestos, oil, bits of heavy metals, and
sulfur oxides (硫氧化合物). Industrial smog was the cause of London's 1952 air
pollution, in which 4,000 people died.
Photochemical (光化学的) smog is
brown and smelly trademark of large cities found in warm climates. When the
surrounding land forms a natural basin, as it does around Los Angeles and Mexico
city, photochemical smog can reach harmful concentrations. The main component of
photochemical is nitric oxide (一氧化氮) ,which is produced chiefly by cars and
other vehicles with internal combustion engines. Nitric oxide reacts with oxygen
in the air to form nitrogen dioxide(二氧化氮). When exposed to sunlight, nitrogen
dioxide can react with hydrocarbons to from photochemical oxidant(光化氧化物质). Other
components of smog are ozone and PANs ( short for peroxyacetyl
nitrates(硝酸过氧乙酰酯)). PANs are similar to tear gas; even traces can sting the eyes
and irritate the lungs.
Acid deposition 1
Oxides of sulfur(硫磺)
and nitrogen(氮) are among the most dangerous air pollutant. Coal-burning power
plants, factories, and metal smelters (金属冶炼厂)are the main sources of sulfur
dioxides. Vehicles, power plants that burn fossil fuels, and nitrogen
fertilizers are sources of nitrogen oxides.
Depending on climatic
conditions, tiny particles of these substances may be airborne for a while and
then fall to earth as dry acid deposition. Most sulfur and nitrogen dioxides
dissolve in atmospheric water to form weak solutions of sulfuric acid and nitric
acid. Winds can distribute them over great distances before they fall to earth
in rain or snow; this is called wet acid deposition. Acid rain can be four to
forty times more acidic than normal rainwater, sometimes as much as lemon juice.
The acid attack marble, metals, mortar, rubber, plastic, even nylon stockings.
And they are disrupting ecosystems.
Researchers confirmed years ago that
power plants, factories, and vehicles are the main sources of acid deposition,
and that the depositions are indeed damaging the environment. Not much has been
done about it. Also, some of the responses to local air pollution standards have
contributed to the problem, as when very tall smokestack(大烟囱)are added to power
plants and smelting plants. The idea is to dump acid-laden smoke high in the
atmosphere so winds can distribute it elsewhere－which winds readily do. The
world's tallest smokestack, in Sunbury, Ontario, accounts for about one per cent
of the annual worldwide emission of sulfur dioxide.
Damage to the ozone
layer
The ozone layer in the lower stratosphere (平流层) absorbs most of
the ultraviolet wavelengths (紫外线的波长) from the sun--a form of radiation that is
harmful to organisms. Yet this layer has been thinning since 1976. Each spring,
an ozone "hole" appears over the Antarctic; it extends over an area about the
size of the continental United States.
Satellites and high-altitude
planes have been monitoring the ozone hole since 1978 by 1987, ozone levels
above Antarctica had declined by fifty per cent--this compared to the previous
worst case of forty per cent in 1985.
The reduction in the ozone layer
is allowing more ultraviolet (紫外线的)radiation to reach the earth's surface, with
potentially serious and wide-ranging consequences. Already there has been a
dramatic increase in skin cancers, which almost certainly are related to
increases in ultraviolet radiation. Contracts (白内障) may become more common, and
it appears that ultraviolet radiation also can weaken the immune system, making
individuals more vulnerable to some viral and parasitic(寄生的)infections.
The cause of ozone reduction are hotly debated in the scientific community, To
be sure, large volcanic eruption and cyclic(周期的)changes in solar activity have
some effects. But the prime suspects are chlorofluorocarbons(CFCs氯氟碳), which are
compounds of chlorine and carbon. These odorless, invisible, and otherwise
harmless compounds are widely used as propellants in aerosol spray cans,
collants in refrigerators and conditioners, and industrial solvents; and they
also are used in making plastic foams, including the Styrofoam cups and cartons
used for packaging foods, drinks, and other consumer goods.
An
international group assembled by the United Nations Environment Program agreed
to a draft treaty to halve CFC emissions by the year 1999. Most nations seem
certain to ratify (承认,批准) its provisions. The treaty is a step in the right
direction, although some feel that it is too little and too late. CFCs already
in the air will be there for over a century, before natural processes eutralize
them. You, your children, and your grandchildren will be living with their
destructive effects. Think about that, the next time you carry a Styrofoam
container (泡沫塑料饭盒) from a fast-food restaurant.