21% oxygenStudies of air samples show that up to 55 miles above sea level the
composition of the atmosphere is substantially the same as at ground level;
continuous stirring produced by atmospheric currents counteracts the tendency of
the heavier gases to settle below the lighter ones. In the lower atmosphere, ozone
is normally present in extremely low concentrations. The atmospheric layer 12 to
30 miles up contains more ozone that is produced by the action of ultraviolet
radiation from the Sun. In this layer, however, the percentage of ozone is only
0.001 by volume. Human activity adds to the ozone concentration in the lower
atmosphere where it can be a harmful pollutant.
The ozone layer became a subject of concern in the early 1970s when it was
found that chemicals known as fluorocarbons, or chlorofluoromethanes, were
rising into the atmosphere in large quantities because of their use as refrigerants
and as propellants in aerosol dispensers. The concern centered on the possibility
that these compounds, through the action of sunlight, could chemically attack and
destroy stratospheric ozone, which protects Earth’s surface from excessive
ultraviolet radiation. As a result, U.S. industries and the Environmental Protection
Agency phased out the use of certain chlorocarbons and fluorocarbons as of the
year 2000. There is still ongoing concern about both these environmental
problems: the greenhouse effect and the deterioration of the ozone layer as it
relates to possible global warming.
SOME REPRESENTATIVE GASES
Oxygen
Of the gases that occur in the atmosphere, the most important one to us is oxygen.
Although it makes up only approximately 21% of the atmosphere, by volume, the
oxygen found on Earth is equal in weight to all the other elements combined.
About 50% of Earth’s crust (including the waters on Earth, and the air
surrounding it) is oxygen. (Note Figure 15.)