problems occur due to the presence and movement of pollutants within and
among the layers within the atmosphere. The location of the pollutants
in a certain layer is an important factor in determining what type of
air pollution problem may occur. For the most part, the polluting molecules
are heavier than air and circulate in the troposphere. A general description
of these are given earlier. The different species of molecules remain
in the troposphere for different amounts of time depending upon their
amount, reactivity, and local weather patterns. It is the non-reactivity
of chlorofluorocarbons that result in their drifting through the troposphere
and finding their way tot he stratosphere. Most of the CFC's do remain
in the upper part of the troposphere due to their weight. However very
little ultraviolet reaches here because of the stratospheric ozone layer.
Recall that their ability to disrupt the ozone layer occurs due to ultraviolet
knocking off a chlorine atom. So this does not happen in the troposphere.
Otherwise we might have other problems stemming from free highly reactive
chlorine in the troposphere!
transport of gases in the atmosphere occurs in the troposphere. This is
the region where the circulation patterns leading to the daily weather
and eventually the climate conditions occurs. The water cycle (described
in the Materials System) occurs between the earth (particularly oceans
and other bodies of water) and the lower half of the troposphere. The
troposphere extends to about 9.5 miles (15 km) from the Earth's surface.
When pilots announce the altitude of an airplane flight and you are above
the clouds, it is usually at 30000 - 35000 feet which is about 5 - 6 miles.
So the cloud activity is generally in the lowest part of the atmosphere.
Even with the faster circulation in the troposphere, on the average, a
water molecules spends about 9 days in the atmosphere once it gets released
from the water bodies of the earth. This is called the residence time
of the molecule in the water. Molecules like CFC's on the other hand have
residence times varying from 60 years to hundreds of years!
the effects of pollutants in the atmosphere are global, some regional,
and some local, depending on the layer at which they primarily circulate,
which in turn depends on how heavy the molecule is, its reactivity, and
what the circulation patterns are.
change, indicated by the so-called greenhouse gases and stratospheric
ozone depletion are global in nature. Acid precipitation (or acid rain)
due to release of oxides of sulfur and nitrogen from fossil fuel combustion
is regional, and affects areas up to hundreds of miles from the sources.
Tropospheric (or ground-level) ozone concentrations, air pollution from
CO, NO2, and SO2, and heat island effects arising
from the interaction of pollution with sunlight or with local circulation
patterns set up by buildings are local in nature and vary daily. These
effects are now described in more detail.