Chemistry Reference
In-Depth Information
Table 3.2
Continued
Dichloroethyl ether
Methylene diphenyl diisocyanate
Glycol ethers
1,3-Dichloropropene
4,4
¢
-Methylenedianiline
Lead compounds
Dichlorvos
Naphthalene
Manganese compounds
Diethanolamine
Nitrobenzene
Mercury compounds
N
,
N
-Diethyl aniline
4-Nitrobiphenyl
Fine mineral fibres
Diethyl sulfate
4-Nitrophenol
Nickel compounds
3,3-Dimethoxybenzidine
2-Nitropropane
Polycyclic organic matter
Dimethyl aminoazobenzene
N
-Nitroso-
N
-methylurea
Radionuclides (including radon)
Selenium compounds
2,4-D
=
2,4-dichlorophenoxyacetic acid; DDE
=
p
,
p
¢
-dichlorophenyl dichloroethene.
The O
3
and NO
2
concentrations depend on sun-
light intensity and emission volume and therefore
vary with the time of day as well as with the time of
year. The morning rush produces an NO and hydro-
carbon peak. Oxidation of the NO to NO
2
then leads
to a rise in O
3
during the middle of the day, which
decreases as the sun goes down and further NO from
the evening rush contributes to its removal:
Benzaldehyde from aromatics in the fuel is con-
verted to peroxybenzoyl nitrate (PBzN). Both per-
oxides are eye irritants and phytotoxicants.
Enhanced greenhouse effect
The sun radiates with a mean surface temperature
of 5700°C and the Earth with a mean surface tem-
perature of -23°C at the cloud tops (Fig. 3.2). The
Earth therefore receives and radiates heat in differ-
ent parts of the spectrum. Gases in the atmosphere
absorb both incoming and outgoing radiation. Ozone
absorbs most of the incoming short-wavelength UV
and H
2
O, CH
4
, CO
2
, O
3
and N
2
O absorb in the out-
going infrared. The net effect is that the Earth's
surface is maintained some 30°C warmer than it
would be otherwise. This global warming is termed
the greenhouse effect. However, the greenhouse
gases, especially CO
2
, CH
4
and N
2
O, are increased by
human activities and other gases such as chloro-
fluorocarbons (CFC) also are produced, which
absorb in the 8-13 mm window through which the
Earth loses most of its heat.
The CFCs are a family of short-chain halocarbon
compounds used as refrigerants, foam inflators and
propellants and include CFCl
3
, CF
2
Cl
2
and C
2
F
3
Cl
3
.
They were considered initially to be inert but now
are known to have an impact not only on global
warming but also on stratospheric ozone depletion.
The contribution of a gas to global warming is a
function of its electromagnetic radiation absorption
efficiency (its radiative forcing potential) and its con-
centration. Thus, CH
4
is about 10 times and CF
2
Cl
2
about 7000 times as effective as CO
2
at absorbing
energy, but CO
2
makes the major contribution
because of its higher concentration. Estimates of
the relative contributions to global warming put CO
2
NO + O
3
Æ NO
2
+ O
2
Hydrocarbons differ in their potential for photo-
chemical ozone creation, depending upon how
quickly they react with OH
•
. Methane has a low
potential but a high concentration and so still makes
the major contribution. Substituted aromatics such
as toluene and the xylenes and unsaturated hydro-
carbons such as ethene and propene have high
potentials but lower concentrations and so con-
tribute less.
Ozone is a powerful oxidant and causes detriment
to human lung function in the form of inflammation
and scarring at levels above about 50 ppm. The O
3
levels in London are declining at an average rate of
0.9 ppb per year due to emission legislation but levels
in other cities, e.g. Athens, are continuing to rise [9].
Peroxyacetyl nitrate (PAN).
As we have seen,
atmospheric photochemistry generates O
3
, aldehydes
and free radicals such as HO
2
•
and OH
•
. There is no
effective sink for these radicals and they recycle
during daylight hours. In the presence of NO
2
this
can lead to the formation of peroxides such as PAN:
CH
3
CHO + OH
•
Æ CH
3
CO
•
+ H
2
O
CH
3
CO
•
+ O
2
Æ CH
3
CO O
2
CH
3
CO O
2
+ NO
2
Æ CH
3
CO O
2
NO
2
(PAN)
