Environmental Engineering Reference
In-Depth Information
The World Health Organization states that 2.4 million people die each
year from causes directly attributable to air pollution. Worldwide, more
deaths per year are linked to air pollution than to automobile accidents,
but those that stem from pollution are more subtle, often overlooked,
and not worthy of news headlines.
Air Pollution and Ultraviolet Radiation
Two types of anthropogenic pollution in the air thousands of feet above
our heads affect human lives: the effect on precipitation, which is of
lesser importance, and the effect on the ozone layer, which is very
serious.
Soot and Rainfall
Water vapor in the air normally condenses to liquid rain at elevations
between 10,000 and 30,000 feet. Most of the condensation results from
decreasing temperatures as elevation increases, an activity that is unaf-
fected by human activities. The water droplets must grow to a certain
size before they drop to the ground. Within the past ten years, it has
been found that particles suspended in the air lower the amount of pre-
cipitation by preventing large droplets from forming.
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The particles thus
have a dampening effect on rainfall, which can have effects that extend
far beyond suppressing local precipitation. For example, the incessant
rainfall in the tropics produces much of the energy needed for global air
circulation. Any change in rainfall there is certain to affect global climate,
but no numbers are yet available. The quantitative effect of soot on
tropical rainfall is an active fi eld of current research.
The Ozone Layer
High above our heads around the globe at an elevation of 10 to 22 miles
is a zone of air with a naturally occurring concentration of ozone about
ten times as much as the amount in ground-level smog in Los Angeles.
It is essential for the maintenance of life on earth. This zone has been
known to meteorologists since 1913 but was brought to public notice
only in 1985 when its thinning was recognized; it is known as the ozone
layer. In public consciousness the word
ozone
is nearly always followed
by the word
hole
, which meteorologists defi ne as a depletion of ozone
in the layer by at least one-third.
The amount of ozone in the 12-mile-thick zone in the stratosphere is
determined by the balance between ultraviolet (UV) radiation from the
