Chemistry Reference
In-Depth Information
about 5.2 ¥ 10
9
million tonnes (Mt) and is the most
vulnerable reservoir to pollution. It did not always
have its present composition. The primitive atmos-
phere probably comprised CO
2
, N
2
, H
2
, H
2
O and H
2
S
with traces of CH
4
, HCl, HBr and HI and the inert
gases, similar to volcanic gases today. There was no
oxygen. We know this because 2.5-billion-year-old
pebbles of the oxidisable minerals pitchblende and
pyrite, which clearly in the past have been rounded
by water on the planet's surface but are now locked
under old sediments, show no signs of oxidation.
The origins of life are conjectural but we do know
that photosynthetically active organisms evolved
around 2 billion years ago. They used solar radiation
to separate hydrogen from water to build their
bodies and released the unwanted oxygen. This first
pollutant changed the world forever, allowing
all modern life to evolve by taking advantage of
the energetically favourable oxidative metabolic
processes.
Air pollutants may be in gaseous, aerosol, droplet
or particulate form. Pollutants emitted directly into
the atmosphere are termed primary. Pollutants that
are formed in the atmosphere from primary pollu-
tants by chemical or photochemical reaction are
termed secondary.
by weight in coal and 2-3% in heavy fuel oils, with
decreasing amounts in lighter oils and natural gas.
More than 97% of the sulfur emitted in com-
bustion is as SO
2
. The remainder is largely as sulfur
trioxide, which in the presence of water vapour is
transformed rapidly into H
2
SO
4
. Sulfur dioxide per-
sists for an average of ten days in the atmosphere
before oxidation and transformation to the acid.
SO
2
+ OH
•
Æ HSO
3
•
HSO
3
•
Æ SO
3
+ HO
2
•
+ O
2
SO
3
+ H
2
O Æ H
2
SO
4
SO
2
+ O
3
Æ SO
3
+ O
2
Both SO
2
and H
2
SO
4
at sufficient concentrations will
produce harmful effects. For the average person SO
2
at above 5 ppm (13 mg m
-3
) irritates the mucous
membrane, inducing coughing. Continuous expo-
sure diminishes the sense of smell and taste and, in
more serious cases, causes oedema (swelling and
fluid in the lungs). The frequency with which the
cilia in the lungs oscillate and hence the efficiency
of the lung protection mechanism is reduced. The
bronchii are constricted and the risk of bronchitis is
increased.
The formation of H
2
SO
4
combined with natural fog
and man-made smoke in weather inversion condi-
tions led to the notorious London smogs of the first
half of the twentieth century, culminating in 1952
when a surplus of 3500-4000 deaths over the
expected was recorded [3].
The Clean Air Act (1956) legislation that followed
the London smog incidents initiated substantial
reductions in emissions. The change from coal-fired
domestic heating to electricity and the increased use
of nuclear energy and natural gas made the major
contributions to achieving this. The UK emissions,
which had totalled 6.34 Mt of sulfur in 1970,
declined to 1.66 Mt in 1997.
Foliage is damaged by contact with SO
2
, and
inorganic materials also suffer corrosion. Carbonates
in sandstones, limestones and mortars are
attacked:
2.2 Tropospheric pollution
Pre-1970, tropospheric pollution was considered
principally to be smoke, soot and odours. Today
the definition must embrace many other substances
that produce objectionable or deleterious effects on
humans, animals or vegetation or otherwise alter the
natural balance of an ecosystem.
Primary pollutants
Sulfur species.
The most abundant natural sulfur
species is sulfate in sea spray. This, however, rapidly
returns to the ocean and is not considered further
here. Other natural sources, including sulfur dioxide
(SO
2
) and hydrogen sulfide from volcanos and
dimethyl sulfide, carbonyl sulfide and carbon disul-
fide from biological processes, are estimated to intro-
duce some 20-30 Mt of sulfur into the global
atmosphere annually. The anthropogenic total is
70-100 Mt, principally from combustion but also
from sulfide ore roasting and sulfuric acid (H
2
SO
4
)
manufacture. Sulfur is present in most fuels: 1-2%
CaCO
3
+ H
2
SO
4
+ H
2
O Æ CaSO
4
·2H
2
O + CO
2
gypsum
The gypsum produced is soluble and may leach out
in rainwater. However, if it remains in situ it occu-
pies a larger volume than the original carbonate,
causing internal pressure and mechanical stress
to the structure. Old glass, with its high alkali and
