Environmental Engineering Reference
In-Depth Information
proportion will be scattered forward towards
its original destination. Most aerosols,
particularly sulphates and fine rock particles
scatter solar radiation very effectively.
The most obvious effects of scattering are
found in the visible light sector of the radiation
spectrum. Particles in the 0.1 to 1.0 µm size range
scatter light in the wavelengths at the blue end
of the spectrum, while the red wavelengths
continue through. As a result, when the aerosol
content of the atmosphere is high, the sky
becomes red (Fennelly 1981). This is common in
polluted urban areas towards sunset when the
path taken by the light through the atmosphere
is lengthened, and interception by aerosols is
increased. Natural aerosols released during
volcanic eruptions produce similar results. The
optical effects which followed the eruption of
Krakatoa in 1883, for example, included not only
magnificent red and yellow sunsets, but also a
salmon pink afterglow, and a green colouration
when the sun was about 10° above the horizon
(Lamb 1970). As well as being aesthetically
pleasing, the sequence and development of these
colours allowed observers to calculate the size
of particles responsible for such optical
phenomena (Austin 1983).
Among the atmospheric aerosols, desert dust
and soot particles readily absorb the shorter solar
wave lengths (Toon and Pollack 1981; Lacis
et
al.
1992) with soot a particularly strong absorber
across the entire solar spectrum (Turco
et al.
1990). The degree to which a substance is capable
of absorbing radiation is indicated by its specific
absorption coefficient. For soot, this value is 8-
10 m
2
g
-1
, which means that 1 g of soot can block
out about two-thirds of the light falling on an
area of 8-10 m
2
(Appleby and Harrison 1989).
Individual soot particles in the atmosphere are
approximately 0.1 µm in diameter and tend to
link together in branching chains or loose
aggregates. With time these clusters become more
spherical and their absorption coefficient
declines, but even when the aggregate diameters
exceed 0.4 µm, the specific absorptivity may
remain as high as 6 m
2
g
-1
(Turco
et al.
1990).
Thus, the injection of large amounts of soot into
the atmosphere has major implications for the
earth's energy budget.
In addition to disrupting the flow of incoming
solar radiation, the presence of aerosols also has
an effect on terrestrial radiation. Being at a lower
energy level, the earth's surface radiates energy
at the infrared end of the spectrum. Aerosols—
such as soot, soil and dust particles—released into
the boundary layer absorb infrared energy quite
readily, particularly if they are larger than 1.0
µm in diameter (Toon and Pollack 1981), and as
a result will tend to raise the temperature of the
troposphere. However, the absorption efficiency
of specific particles varies with the wavelength
of the radiation being intercepted. The absorption
coefficient of soot at infrared wavelengths, for
example, is only about one-tenth of its value at
the shorter wavelengths of solar radiation. In
addition, since they are almost as warm as the
earth's surface, tropospheric aerosols are less
efficient at blocking the escape of infrared
radiation than colder particles, such as those in
the stratosphere (Bolle
et al.
1986). Thus, longer-
wave terrestrial radiation can be absorbed by
particles in the stratosphere, and re-radiated back
towards the lower atmosphere where it has a
warming effect. Much depends upon the size of
the stratospheric aerosols. If they are smaller in
diameter than the wavelengths of the outgoing
terrestrial radiation, as is often the case, they tend
to encourage scattering and allow less absorption
(Lamb 1970). The net radiative effects of
particulate matter in the atmosphere are difficult
to measure or even estimate. They include a
complexity which depends upon the size, shape
and optical properties of the aerosols involved,
and upon their distribution in the stratosphere
and troposphere.
Any disruption of energy fluxes in the earth/
atmosphere system will be reflected ultimately
in changing values of such climatological
parameters as cloudiness, temperature or hours
of bright sunshine. Although atmospheric
aerosols produce changes in the earth's energy
budget, it is no easy task to assess their
climatological significance. Many attempts at
that type of assessment have concentrated on
