Chemistry Reference
In-Depth Information
9
1
9
1
O
2
8
8
2
7
3
7
3
O
O
6
4
6
4
PCDD
PCDF
Scheme 3.3
glass manufacture, mercury from the chloralkali
industry and battery manufacture and trace metals
in coal, including arsenic, cadmium, chromium,
mercury, manganese, nickel, lead, antimony and
vanadium) also contribute to the hazards.
Lead emissions arise principally from the use of
lead tetraethyl additives in petrol. The lead is emitted
from the exhaust as a mixture of halides from reac-
tions with other additives. The halides react with
ambient ammonium sulfate in the atmosphere:
Secondary pollutants
Ozone.
Atmospheric reactions involving oxides of
nitrogen and hydrocarbons under solar ultraviolet
radiation yield a range of secondary products, the
most important of which is ozone:
UV
<310 nm
NO
2
O
•
+ NO
O
•
+ O
2
+ M Æ O
3
+ M
O
3
+ NO Æ O
2
+
NO
2
where M is a third body, e.g. N
2
, that carries away
excess energy.
In sunny urban areas such as Los Angeles, Cali-
fornia, ozone concentrations have reached 450 ppb.
In Britain's less intense sunlight, levels exceeding
100 ppb can occur during the fewer than 30 days per
year when anticyclonic conditions occur in summer,
and a peak of 250 ppb was measured at Harwell in
Berkshire in 1976.
Creation and destruction of O
3
are thus in equi-
librium. If, however, NO
2
is enhanced by contribu-
tions from anthropogenic sources, the equilibrium in
the last of the above three equation shifts, reducing
O
3
destruction. The NO
2
is formed in atmospheric
reactions involving OH
•
2PbBrCl + 2 NH
(
)
SO
Æ
PbSO
,
(
NH
)
SO
4
4
4
4
2
4
2
(
)
+
PbBrCl
,
NH
SO
4
2
4
Lead is a neurotoxin that induces intellectual dull-
ness, reduced consciousness and, in extreme cases,
coma and death.
The UK emissions of lead have declined sharply
following reductions in the lead content of petrol and
later the elimination of leaded petrol. Emissions
in 1997 were 1.31 kilotonne (kt), compared with
7.54 kt in 1970 [4]. Emissions of other metals also
are showing a strongly declining trend.
Other air toxics.
In addition to the elements and
compounds dealt with above, there are thousands of
commercial chemicals used in the UK. Many hun-
dreds of these may find their way into the atmos-
phere, where they may have adverse health or
environmental effects. Examples include asbestos
from construction, demolition, pipe and furnace
replacement and mining, hydrogen sulfide from
paper manufacture and oil refining and fluorides
from aluminium smelting and phosphate fertiliser
manufacture. A convenient list of hazardous air pol-
lutants is provided by the 1990 US Clean Air Act
Amendments (Table 3.2).
radicals, hydrocarbons, O
2
and NO:
OH
•
+ CH
4
Æ H
2
O
+ CH
3
•
CH
3
•
+ O
2
Æ CH
3
O
2
CH
3
O
2
+ NO Æ CH
3
O + NO
2
CH
3
O + O
2
Æ HCHO + HO
2
HO
2
+ NO Æ OH
•
+ NO
2
The OH
•
is regenerated at the end of the reaction
sequence and is recycled. A range of aldehydes,
depending upon which hydrocarbon begins the
sequence, also are formed as secondary products.
