Geoscience Reference
In-Depth Information
minutes in the city centre. It must be remembered,
however, that smog layers also impeded the re-radiation
of surface heat at night and that this blanketing effect
contributed to higher night-time city temperatures.
Occasionally, very stable atmospheric conditions
combine with excessive pollution production to give
dense smog of a lethal character. During the period
5 to 9 December 1952, a temperature inversion over
London caused a dense fog with visibility of less than
10 m for forty-eight consecutive hours. There were
12,000 more deaths (mainly from chest complaints)
during December 1952 to February 1953 compared with
the same period the previous year. The close association
of the incidence of fog with increasing industrialization
and urbanization was evident in Prague, where the mean
annual number of days with fog rose from seventy-nine
during 1860 to 1880 to 217 during 1900 to 1920.
The use of smokeless fuels and other controls cut
London's total smoke emission from 1.4
tions brought about a striking decline in sulphur dioxide
pollution in many European and North American cities
(Figure 12.21). Nevertheless, the effect of the regu-
lations was not always clear. The decrease in London's
atmospheric pollution was not apparent until eight
years after the introduction of the 1956 Clean Air
Act, whereas in New York City the observed decrease
began in the same year (1964) -
prior
to the air pollution
control regulations there.
Urban complexes in many parts of the world are
significantly affected by pollution resulting from the
combustion of gasoline and diesel fuel by vehicles
and aircraft, as well as from petrochemical industries.
Los Angeles, lying in a topographically constricted
basin and often subject to temperature inversions, is
the prime example of such pollution, although this
affects all modern cities. Even with controls, 7 per cent
of the gasoline from private cars is emitted in an
unburned or poorly oxidized form, another 3.5 per cent
as photochemical smog and 33 to 40 per cent as carbon
monoxide. Smog involves at least four main com-
ponents: carbon soot, particulate organic matter (POM),
sulphate (SO
4
2-
) and peracyl nitrates (Pans). Half of the
10
8
kg
(141,000 tons) in 1952 to 0.9
10
8
kg (89,000 tons) in
1960. Figure 12.20B shows the increase in average
monthly sunshine figures for 1958 to 1967 compared
with those of 1931 to 1960. Since the early 1960s annual
average concentrations of smoke and sulphur dioxide
in the UK have fallen from 160 ppm and 60 ppm,
respectively, to below 20 ppm and 10 ppm in the 1990s.
Visibility in the UK improved at many measuring
sites during the late twentieth century. In the 1950s
and 1960s, days with visibility at midday in the lowest
10th percentile were in the 4 to 5 km range, whereas in
the 1990s this had improved to 6 to 9 km. Annual
average 12 UTC visibility at Manchester airport was 10
km in 1950, but near 30 km in 1997. The improvements
are attributed to improved vehicle fuel efficiency and
catalytic converter installation in the 1970s.
b Gases
As well as particulate pollution produced by urban and
industrial activities involving coal and coke combustion,
there is the associated generation of pollutant gases.
Before the Clean Air Act in the UK, it was estimated
that domestic fires produced 80 to 90 per cent of
London's smoke. However, these were responsible for
only 30 per cent of the sulphur dioxide released into the
atmosphere - the remainder being contributed by
electricity power-stations (41 per cent) and factories (29
per cent). After the early 1960s, improved technology,
the phasing out of coal fires and anti-pollution regula-
Figure 12.21
Annual mean concentration of sulphur dioxide (mg
m
-3
) measured in New York and London during a twenty-five- to
thirty-year period. These show dramatic decreases of urban
pollution by SO
2
.
Source
: From Brimblecombe (1986).
