Environmental Engineering Reference
In-Depth Information
main mass of particulates is concentrated in two
peaks, one between 0.01 micrometre (µm) and 1
µm, centred at 0.1 µm and the other between 1
µm and 100 µm (see Figure 5.3), centred at 10
µm (Shaw 1987). The smaller particles in the first
group are called secondary aerosols since they
result from chemical and physical processes
which take place in the atmosphere. They include
aggregates of gaseous molecules, water droplets
and chemical products such as sulphates,
hydrocarbons and nitrates. As much as 64 per
cent of total global aerosols are secondary
particulates, 8 per cent of them anthropogenic
in origin from combustion systems, vehicle
emissions and industrial processes. The other 56
per cent are from natural sources such as
volcanoes, the oceans and a wide range of organic
processes (Fennelly 1981). Some estimates
suggest that sulphate particles are now the largest
group of atmospheric aerosols, accounting for
as much as 50 per cent of all secondary particles
(Toon and Pollack 1981). Most of the
tropospheric sulphates are of anthropogenic
origin, and contribute to the problems of acid
rain (see Chapter 4) whereas those found in the
stratosphere are most likely to be the products
of volcanic eruptions. The larger particles with
diameters between 1 and 100 µm are called
primary aerosols, and include soil, dust and solid
industrial emissions, usually formed by the
physical breakup of material at the earth's surface
(Fennelly 1981). There is some evidence that these
groupings are a direct result of the processes by
which the aerosols are formed. Mechanical
processes are unable to break substances into
pieces smaller than 1 µm in diameter, whereas
the growth of secondary particles appears to
cease as diameters approach 1 µm. (Shaw 1987).
Los Angeles, photochemical action on vehicle
emissions causes major increases in secondary
particulate matter; over the oceans, 95 per cent
of the aerosols may consist of coarse sea-salt
particles. Such variability makes it difficult to
establish the nature of the relationship between
atmospheric aerosols and climate. It is clear,
however, that the aerosols exert their influence
on climate by disrupting the flow of radiation
within the earth/atmosphere system, and there
are certain elements which are central to the
relationship. The overall concentration of
particulate matter in the atmosphere controls the
amount of radiation intercepted, while the optical
properties associated with the size, shape and
transparency of the aerosols determines whether
the radiation is scattered, transmitted or absorbed
(Toon and Pollack 1981). The attenuation of
solar radiation caused by the presence of aerosols
provides a measure of atmospheric turbidity, a
property which, for most purposes, can be
considered as an indication of the dustiness or
dirtiness of the atmosphere.
Several things may happen when radiation
strikes an aerosol in the atmosphere. If the
particle is optically transparent, the radiant
energy passes through unaltered, and no
change takes place in the atmospheric energy
balance. More commonly, the radiation is
reflected, scattered or absorbed—in
proportions which depend upon the size, colour
and concentration of particles in the
atmosphere, and upon the nature of the
radiation itself (see Figure 5.3). Aerosols which
scatter or reflect radiation increase the albedo
of the atmosphere and reduce the amount of
insolation arriving at the earth's surface.
Absorbent aerosols will have the opposite
effect. Each process, through its ability to
change the path of the radiation through the
atmosphere, has the potential to alter the
earth's energy budget. The water droplets in
clouds, for example, are very effective in
reflecting solar energy back into space, before it
can become involved in earth/atmosphere
processes. Some of the energy scattered by
aerosols will also be lost to the system, but a
AEROSOLS AND RADIATION
Atmospheric aerosols comprise a very
heterogeneous group of particles, and the mix
within the group changes with time and place.
Following volcanic activity, for example, the
proportion of dust particles in the atmosphere
may be particularly high; in urban areas, such as
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