Chevrolet TrailBlazer, Service Manual

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Working with Federal, State and Local mandated regulations and by carefully monitoring
emissions, big business has greatly reduced the amount of pollutant introduced from its
industrial sources, striving to obtain an acceptable level. Because of the mandated
industrial emission clean up, many land areas and streams in and around the cities that
were formerly barren of vegetation and life, have now begun to move back in the direction
of nature's intended balance.
Automotive Pollutants
The third major source of air pollution is automotive emissions. The emissions from the
internal combustion engines were not an appreciable problem years ago because of the
small number of registered vehicles and the nation's small highway system. However,
during the early 1950's, the trend of the American people was to move from the cities to
the surrounding suburbs. This caused an immediate problem in transportation because the
majority of suburbs were not afforded mass transit conveniences. This lack of
transportation created an attractive market for the automobile manufacturers, which
resulted in a dramatic increase in the number of vehicles produced and sold, along with a
marked increase in highway construction between cities and the suburbs. Multi-vehicle
families emerged with a growing emphasis placed on an individual vehicle per family
member. As the increase in vehicle ownership and usage occurred, so did pollutant levels
in and around the cities, as suburbanites drove daily to their businesses and employment,
returning at the end of the day to their homes in the suburbs.
It was noted that a smoke and fog type haze was being formed and at times, remained in
suspension over the cities, taking time to dissipate. At first this "smog,'' derived from the
words "smoke'' and "fog,'' was thought to result from industrial pollution but it was
determined that automobile emissions shared the blame. It was discovered that when
normal automobile emissions were exposed to sunlight for a period of time, complex
chemical reactions would take place.
It is now known that smog is a photo chemical layer which develops when certain oxides of
nitrogen (NO
x
) and unburned hydrocarbons (HC) from automobile emissions are exposed
to sunlight. Pollution was more severe when smog would become stagnant over an area in
which a warm layer of air settled over the top of the cooler air mass, trapping and holding
the cooler mass at ground level. The trapped cooler air would keep the emissions from
being dispersed and diluted through normal air flows. This type of air stagnation was given
the name "Temperature Inversion.''
Temperature Inversion
In normal weather situations, surface air is warmed by heat radiating from the earth's
surface and the sun's rays. This causes it to rise upward, into the atmosphere. Upon rising
it will cool through a convection type heat exchange with the cooler upper air. As warm air
rises, the surface pollutants are carried upward and dissipated into the atmosphere.
When a temperature inversion occurs, we find the higher air is no longer cooler, but is
warmer than the surface air, causing the cooler surface air to become trapped. This warm
air blanket can extend from above ground level to a few hundred or even a few thousand
feet into the air. As the surface air is trapped, so are the pollutants, causing a severe smog
condition. Should this stagnant air mass extend to a few thousand feet high, enough air