Geoscience Reference
In-Depth Information
aerosol mass is typically POM and sulphate. However,
there are important regional differences. For example,
the sulphur content of fuels used in California is lower
than in the eastern United States and Europe, and NO 2
emissions greatly exceed those of SO 2 in California. The
production of the Los Angeles smog, which, unlike
traditional city smogs, occurs characteristically during
the daytime in summer and autumn, is the result of
a very complex chain of chemical reactions termed the
disrupted photolytic cycle (Figure 12.22). Ultraviolet
radiation dissociates natural NO 2 into NO and O.
Monatomic oxygen (O) may then combine with natural
oxygen (O 2 ) to produce ozone (O 3 ). The ozone in turn
reacts with the artificial NO to produce NO 2 (which goes
back into the photochemical cycle forming a dangerous
positive feedback loop) and oxygen. The hydrocarbons
produced by the combustion of petrol combine with
oxygen atoms to produce the hydrocarbon free radical
HcO*, and these react with the products of the O 3 -NO
reaction to generate oxygen and photochemical smog.
This smog exhibits well-developed annual and diurnal
cycles in the Los Angeles basin (see Figures 12.19C and
D). Annual levels of photochemical smog pollution
in Los Angeles (from averages of the daily highest
hourly figures) are greatest in late summer and autumn,
when clear skies, light winds and temperature inversions
combine with high amounts of solar radiation. The
diurnal variations in individual components of the dis-
rupted photolytic cycle indicate complex reactions. For
example, an early morning concentration of NO 2 occurs
due to the buildup of traffic and there is a peak of O 3
when incoming radiation receipts are high. The effect of
smog is not only to modify the radiation budget of cities
but also to produce a human health hazard.
Evolving state and city regulations in the United
States have given rise to considerable differences in the
type and intensity of urban pollution. For example,
Denver, Colorado, situated in a basin at 1500-m altitude,
regularly had a winter 'brown cloud' of smog and high
summer ozone levels in the 1970s and 1980s. By the
beginning of this century, substantial improvements had
been achieved through the mandatory use of gasoline
additives in winter, restrictions on wood burning, and
scrubbers installed on power plants.
c Pollution distribution and impacts
Polluted atmospheres often display well-marked phys-
ical features around urban areas that are very dependent
upon environmental lapse rates, particularly the pres-
ence of temperature inversions, and on wind speed. A
pollution dome develops as pollution accumulates under
an inversion that forms the urban boundary layer (Figure
12.23A). A wind speed as low as 2 m s -1 is sufficient to
displace the Cincinnati pollution dome downwind, and
a wind speed of 3.5 m s -1 will disperse it into a plume.
Figure 12.23B shows a section of an urban plume with
the volume above the urban canopy of the building
tops filled by buoyant mixing circulations. When an
inversion lid prevents upward dispersion, but lapse
conditions due to morning heating of the surface air
allow convective plumes and associated downdrafts
to bring pollution back to the surface, this process is
termed fumigation . Downwind, lofting occurs above the
temperature inversion at the top of the rural boundary
layer, dispersing the pollution upwards. Figure 12.23C
illustrates some features of a pollution plume up to 160
km downwind of St Louis on 18 July 1975. In view of
the complexity of photochemical reactions, it is of note
that ozone increases downwind due to photochemical
reactions within the plume, but decreases over power
plants as the result of other reactions with the emissions.
This plume was observed to stretch for a total distance
of 240 km, but under conditions of an intense pollution
source, steady large-scale surface airflow and vertical
atmospheric stability, pollution plumes may extend
downwind for hundreds of kilometres. Plumes origi-
nating in the Chicago-Gary conurbation have been
Figure 12.22 The NO 2 photolytic cycle disrupted by hydro-
carbons to produce photochemical smog.
Sources : US DHEW (1970) and Oke (1978).
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