Environmental Engineering Reference
In-Depth Information
Figure 5.7
A comparison of natural and anthropogenic sources of particulate matter
Note:
The volcanic contributions appear small. Their major impact is produced by their ability to make a
rapid and intense contribution to the aerosol content of the atmosphere.
rainforests must make a significant contribution
to turbidity. Some authorities would also include
soil erosion—created by inefficient or destructive
agricultural practices—among anthropogenic
sources of aerosols (Lockwood 1979).
Anthropogenically generated particulate matter
tends to remain closer to the earth's surface than
the natural variety, as is readily apparent in most
large urban areas. One of the few exceptions is
the direct injection of soot particles into the
stratosphere by high-flying commercial aircraft.
The annual production of elemental carbon from
that source is estimated at 16,000 tonnes, of
which 10 per cent is added directly to the
stratosphere. With the introduction of a new
generation of supersonic aircraft—which would
spend a greater proportion of their flight time in
the stratosphere—stratospheric soot loading
would probably double (Pueschel
et al.
1992).
Various attempts have been made to estimate
the impact of human activities on background
levels of atmospheric turbidity. One approach to
this is to examine turbidity levels in locations such
as the high Alps and the Caucasus (renowned
for their clean air) or mid-ocean and polar
locations (far removed from the common sources
of aerosols). These are seen as reference points
from which trends can be established. As it
stands, the evidence is remarkably contradictory.
There are data which support the view that
human activities are enhancing global aerosol
levels. A set of observations from Davos in
Switzerland suggests an 88 per cent increase in
turbidity in the thirty years up to the mid-1960s
(McCormick and Ludwig 1967), and dustfall in
the Caucasus Mountains showed a rapid increase
during a similar time span (Bryson 1968). At
Mount St Katherine, over 2,500 m up into the
