Environmental Engineering Reference
In-Depth Information
lower troposphere, their residence time in the
atmosphere is relatively short. Their eventual
distribution is therefore less widespread than
volcanic emissions—which are concentrated in
the upper troposphere and stratosphere. Aerosols
produced in the industrial areas of the
northeastern United States have almost entirely
dropped out of the westerly air-stream before it
reaches Europe, but aerosols from European
sources spread over most of North Africa in
January, and may drift out over the Atlantic also,
depending upon the strength of the continental
high pressure system (Lockwood 1979). Such
patterns may help to explain the rising turbidity
levels in the Alps and the Caucasus, and even in
the Sinai. In the southern hemisphere, the
particulate contribution from industrial activity
is negligible, and there is only limited cross-
equatorial flow from the north. Aerosols
produced during the burning of tropical forests
and grasslands will offset the reduction in
industrial aerosols (Bach 1976), but the
proportion of aerosols of direct anthropogenic
origin is usually considered to be much less than
in the northern hemisphere. The absence of
anthropogenic aerosol sources may explain the
lack of change in turbidity over the Antarctic,
but the presence of high pressure at the surface
and the strong westerlies of the circumpolar
vortex aloft may combine to prevent the
transport of aerosols into the area. This is
certainly the case with natural aerosols.
Particulate matter from Pinatubo and Mount
Hudson was unable to penetrate the Antarctic
circumpolar vortex during the first winter
following its emission (Deshler
et al.
1992). The
exact contribution of anthropogenic sources of
aerosols to atmospheric turbidity is difficult to
determine, and is likely to remain so until the
number of measuring sites is increased and their
current uneven distribution is improved.
continued to burn for several months, kept alight
by oil and gas brought to the surface under
pressure from the underlying oil fields. During
that time they added massive amounts of smoke,
sulphur dioxide, carbon dioxide, unburned
hydrocarbons and oxides of nitrogen to the
atmosphere. Most of these products were
confined to the lower half of the troposphere,
with the top of the plume never exceeding 5 km.
At the height of the fires it was estimated that
sulphur dioxide was being added to the
atmosphere at an equivalent rate of 6.1 million
tonnes per year, and soot at 6.4 million tonnes
per year (Johnson
et al.
1991). Although most of
the original emissions were retained in the region
as the result of stable air masses, those aerosols
that had reached higher in the atmosphere were
carried northeastwards bringing acid rain to Iran
and black snow to the mountains of Pakistan.
Several months later, unexpectedly high levels of
carbon soot in the upper troposphere above Japan
were also identified as products of the oil fires
(Okada et al. 1992).
With particulate mass densities of 500-1,000
µgm-
3
, the impact of the pollution cloud on
incoming solar radiation was spectacular.
Beneath the centre of the plume the shortwave
radiation flux was measured at zero (Johnson
et
al.
1991). This led to daytime temperature
reductions beneath the cloud of as much as 5.5°C
(Seager 1991), and although some infrared
radiation was returned from the cloud it did little
to ameliorate the cooling. Mean monthly
temperatures between March and September
were reduced by 0.8 to 2.4°C, and record low
mean monthly temperatures were established in
July and August (Shaw 1992). With large
amounts of energy being intercepted by the plume
and absorbed by the soot particles, the plume
itself warmed up. In earlier assessments of the
impact of the oil fires it was suggested that such
internal heating would be sufficient to cause
lofting of aerosols into the stratosphere, which
would in turn increase the climatic consequences
of the fires (Pearce 1991a). This did not happen,
however, perhaps because of the low altitude of
the initial plume, and the rapidity with which
THE KUWAIT OIL FIRES
In the final stages of the Gulf War in February
1991 between 500 and 600 oil wells were set
alight by the retreating Iraqi army. These wells
