Environmental Engineering Reference
In-Depth Information
5
Atmospheric turbidity
One of the more obvious indications of
atmospheric pollution is the presence of solid or
liquid particles, called aerosols, dispersed in the
air. These aerosols are responsible for
phenomena as diverse as the urban smogs that
bedevil the world's major cities, and the
spectacular sunsets which often follow major
volcanic eruptions. The concentration and
distribution of particulate matter in the
atmosphere is closely linked to climatic
conditions. Some local or regional climates
encourage high aerosol concentrations, as in
Los Angeles, for example, with its combination
of high atmospheric pressure, light winds and
abundant solar radiation. On a global scale, the
mid-latitude westerlies and their associated
weather systems, already implicated in the
distribution of acid rain are responsible for the
transportation of aerosols over long distances
in the troposphere. The jet streams in the upper
atmosphere are also involved in the distribution
of aerosols, carrying particles around the world
several times before releasing them. Knowledge
of such relationships has important practical
implications. At the local level, the success of
pollution abatement programmes often
depends upon an understanding of the impact
of climate on aerosol distribution. At a
continental, or even hemispheric scale, the
relationship between atmospheric circulation
patterns and the spread of particulate matter
can be used to provide an early warning of
potential problems following catastrophic
events such as volcanic eruptions or nuclear
accidents. In such situations, the atmospheric
aerosols are responding to existing climatic
conditions. There has been growing concern in
recent years that they may do more than that;
they may also be capable of initiating climatic
change.
AEROSOL TYPES, PRODUCTION AND
DISTRIBUTION
The total global aerosol production is presently
estimated to be between 2-3×10
9
tonnes per
annum, and on any given day perhaps as many
as 1×10
7
tonnes of solid particulate matter is
suspended in the atmosphere (Bach 1979;
Cunningham and Saigo 1992; Ahrens 1993).
Under normal circumstances, almost all of the
total weight of particulate matter is
concentrated in the lower 2 km of the
atmosphere in a latitudinal zone between 30°N
and 60°N (Fennelly 1981). The mean residence
time for aerosols in the lower troposphere is
between 5 and 9 days, which is sufficiently
short that the air can be cleansed in a few days
once emissions have stopped (Williamson
1973). The equivalent time in the upper
troposphere is about one month, and in the
stratosphere the residence time increases to two
to three years (Williamson 1973). As a result,
anything added to the upper troposphere or
stratosphere will remain in circulation for a
longer time, and its potential environmental
impact will increase.
Aerosols can be classified in a number of ways,
but most classifications include such elements as
origin, size and development, sometimes
individually, sometimes in combination (see
Figure 5.1). Most of the atmosphere's aerosol
