Environmental Engineering Reference
In-Depth Information
2
The atmosphere
The atmosphere is a thick blanket of gases,
containing suspended liquid and solid particles,
which completely envelops the earth, and
together with the earth forms an integrated
environmental system. As part of this system, it
performs several functions which have allowed
mankind to survive and develop almost anywhere
on the earth's surface. First, it provides and
maintains the supply of oxygen required for life
itself. Second, it controls the earth's energy
budget through such elements as the ozone layer
and the greenhouse effect, and—by means of its
internal circulation—distributes heat and
moisture across the earth's surface. Third, it has
the capacity to dispose of waste material or
pollutants generated by natural or human
activity. Society has interfered with all of these
elements, and, through ignorance of the
mechanisms involved or lack of concern for the
consequences of its action, has created or
intensified problems which are now causing
concern on a global scale.
develops in these lower layers, but certain
elements in the upper reaches of the atmosphere
are also involved, and some appear as important
components in the global environmental issues
to be examined here.
Oxygen and nitrogen
Ignoring for the moment the liquids and solids
always present, the gaseous mixture which makes
up the atmosphere has a remarkably uniform
composition in the troposphere where most of
the air is located. Two gases, oxygen and
nitrogen, account for 99 per cent of the total by
volume (see Table 2.1). Oxygen (21 per cent by
volume) participates readily in chemical
reactions, and provides one of the necessities of
life. It is also capable of absorbing solar radiation.
In contrast, nitrogen (78 per cent by volume) is
basically inert, seldom becoming directly involved
in atmospheric chemical or biological processes
except under extraordinary circumstances.
During thunderstorms, for example, the
enormous energy flow in a lightning stroke may
cause nitrogen to combine with oxygen to
produce oxides of nitrogen. On a less spectacular,
but ultimately more important level, certain soil
bacteria—such as
Clostridium
and
Azobacter
—
along with those found in the root nodules of
leguminous plants, are capable of fixing the
atmospheric nitrogen essential for the creation
of the complex nitrogen compounds found in all
forms of life on earth (Steila 1976).
Efficient recycling processes maintain the
volume of both gases, and turbulent mixing
THE ATMOSPHERIC GASES
The constituents of the atmosphere are
collectively referred to as air, although air itself
is not a specific gaseous element, rather it is a
mixture of individual gases each of which retains
its own particular properties. Although traces of
atmospheric gases have been detected well out
into space, 99 per cent of the mass of the
atmosphere lies within 30 km of the earth's
surface, and 50 per cent is concentrated in the
lowest 5 km. Most of the world's weather
