Geoscience Reference
In-Depth Information
forest fires of 1999 September 1997 and March 2000
were transported across the region.
The background concentration of fine particles
(PM
10
, radius <10 µm) currently averages 20 to 30 µg
m
-3
in the British countryside but daily average values
regularly exceed 50 µg m
-3
, and occasionally exceed
100 µg m
-3
in industrial cities near ground level. The
greatest concentrations of smoke generally occur with
low wind speed, low vertical turbulence, temperature
inversions, high relative humidity and air moving
from the pollution sources of factory districts or areas
of high-density housing. The temporal character of
domestic heating and power demands causes city smoke
pollution to take on striking seasonal and diurnal cycles,
with the greatest concentrations occurring at about
08:00 hours in early winter (Figure 12.19). The sudden
morning increase is also partly a result of natural
processes. Pollution trapped during the night beneath
a stable layer a few hundred metres above the surface
may be brought back to ground level (a process termed
fumigation
) when thermal convection sets off vertical
mixing.
The most direct effect of particulate pollution is
to reduce visibility, incoming radiation and sunshine.
In Los Angeles, aerosol carbon accounts for 40 per cent
of the total fine particle mass and is the major cause of
severe visibility decreases, yet it is not routinely moni-
tored. Half of this total is from vehicle exhausts and
the remainder from industrial and other stationary fuel
burning. Pollution, and the associated fogs (termed
smog
), used to cause some British cities to lose 25 to 55
per cent of incoming solar radiation during the period
November to March. In 1945, it was estimated that the
city of Leicester, England, lost 30 per cent of incoming
radiation in winter, as against 6 per cent in summer.
These losses are naturally greatest when the sun's
rays strike the smog layer at a low angle. Compared with
the radiation received in the surrounding countryside,
Vienna lost 15 to 21 per cent of radiation when the
sun's altitude is 30°, but the loss rises to 29 to 36 per
cent with an altitude of 10°. The effect of smoke
pollution is dramatically illustrated in Figure 12.20,
which compares conditions in London before and
after enforcement of the UK Clean Air Act of 1956.
Before 1950, there was a striking difference of sunshine
between the surrounding rural areas and the city centre
(see Figure 12.20A), which could mean a loss of mean
daily sunshine of sixteen minutes in the outer suburbs,
twenty-five minutes in the inner suburbs and forty-four
Figure 12.20
Sunshine in and around London. (A) Mean
monthly bright sunshine recorded in the city and suburbs for the
years 1921 to 1950, expressed as a percentage of that in adjacent
rural areas. This shows clearly the effects of winter atmospheric
pollution in the city. (B) Mean monthly bright sunshine recorded
in the city, suburbs and surrounding rural areas during the period
1958 to 1967, expressed as a percentage of the averages for the
period 1931 to 1960. This shows the effect of the 1956 Clean Air
Act in increasing the receipt of winter sunshine, in particular in
central London.
Sources
: (A) After Chandler (1965); (B) After Jenkins (1969), reprinted
from
Weather
, by permission of the Royal Meteorological Society.
Crown copyright ©.
African tank battles of the Second World War disturbed
the desert surface to such an extent that the material
subsequently deflated was visible in clouds over the
Caribbean. Soot aerosols generated by the Indonesian
