Geoscience Reference
In-Depth Information
In dealing with atmospheric pollution it must
be remembered, first, that the diffusion or
concentration of pollutants is a function both of
atmospheric stability (especially the presence of
inversions) and of the horizontal air motion. It is
also generally greater on weekdays than at the
weekends or on holidays. Second, aerosols are
removed from the atmosphere by settling out
and by washing out. Third, certain gases are
susceptible to complex chains of photochemical
changes, which may destroy some gases but
produce others.
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 due to natural processes. Pollution
trapped during the night beneath a stable layer a
few hundred meters 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, carbon aerosol accounts
for 40 percent of the total fine particle mass and
is the major cause of severe visibility decreases, yet
it is not routinely monitored. 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 25-55
percent of incoming solar radiation during the
period November to March. In 1945, it was
estimated that the city of Leicester lost 30 percent
of incoming radiation in winter, as against 6
percent 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
loses 15-21 percent of radiation when the sun's
altitude is 30
Aerosols
As discussed in Chapter 2A.2 and A.4, the global
energy budget is affected significantly by the
natural production of aerosols that are deflated
from deserts, erupted from volcanoes, produced
by fires and so on (see Chapter 13D.3). Over the
past century the average dust concentration has
increased, particularly in Eurasia, due only in
part to volcanic eruptions. The proportion of
atmospheric dust directly or indirectly attribut-
able to human activity has been estimated at 30
percent (see Chapter 2A.4). As an example of the
latter, the North African tank battles of World
War II 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 forest fires
of September 1997 and March 2000 were
transported across Southeast Asia.
The background concentration of fine particles
(PM
10
, radius <10μm) currently averages 20-30μg
m
-3
in the British countryside but daily average
values in industrial cities of Eastern Europe and in
many developing nations regularly exceed
50-100
, but the loss rises to 29-36 percent
with an altitude of 10°. The effect of smoke
pollution is dramatically illustrated by
Figure
12.20
, which compares conditions in London
before and after the enforcement of the UK Clean
Air Act of 1956 and the move to burning cleaner
fuels and the decline of heavy industry. Before
1950 there was a striking difference of sunshine
between the surrounding rural areas and the city
center (see
Figure 12.19A
), which could mean a
loss of mean daily sunshine of 16 minutes in the
outer suburbs, 25 minutes in the inner suburbs
and 44 minutes in the city center. 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,
°
g m
-3
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 character of
domestic heating and power demands causes city
smoke pollution to take on striking seasonal and
μ
