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The amount and type of pollution generated depends greatly on
the mode of transport used. In general, however, one can say that
the amount of pollution is minimised if energy use is minimised. A separate article discusses the energy
efficiency of different modes. The word
"pollution", when mentioned with respect to
transportation, is often equated with "air pollution".
There are, however, other forms of pollution which should be
considered: noise pollution, land and water pollution. These
different types of pollution will be considered in order.
Air pollution
Combustion of fossil fuels
According to Environment Canada ([1]) the
combustion of gasoline results in the production of nitrogen
oxides (NOx), carbon monoxide (CO), carbon dioxide (CO2), and
volatile organic compounds (VOCs). All of these substances except
CO2 contribute to the formation of ground level ozone (O3).
Nitrogen dioxide reacts with water to result in "acid
rain". Carbon dioxide is a "greenhouse gas" which
contributes to global warming. Note that animals exhale carbon
dioxide and plants "breathe in" carbon dioxide, but the
excessive amounts produced by the combustion of fossil fuels is
what causes it to be classified as a pollutant.
The combustion of fossil fuels in cars and trucks in British
Columbia's Lower Mainland contributed 40% of the VOCs, 43% of the
nitrogen oxides, and 74% of the carbon monoxide ([2]).
As well, over half the airborne particulates is dust kicked up by
motor vehicles travelling on roadways. Overall, about 65% of
total air pollutants come from cars and light trucks.
The combustion of diesel fuel results in similar effects as
the combustion of gasoline except for the production of
particulates (the visible black "smoke").
Handling of fossil fuels
The transfer of gasoline between facilities and vehicles is
estimated to contribute 6% of all human-released volatile organic
compounds in Canada ([1]).
Combustion of natural gas
In theory the combustion of natural gas is extremely clean.
However, combustion is never perfect, and compressed natural gas
(CNG) engines produce the same pollutants that diesel engines do,
albeit at much lower levels. As well, carbon dioxide is still a
major by-product. Hence, treat the "Clean Air Bus"
paint scheme on the new Vancouver CNG buses with some scepticism;
they are certainly not as clean as trolleybuses powered by
hydroelectricity.
Generation of hydrogen for fuel cell bus
A fuel cell works without performing combustion and hence is
exceptionally clean. The generation of hydrogen for the fuel
cell, however, is not completely clean. (I will add more details
once I learn more, or once some kind reader emails me this
information!).
Different modes of electricity generation
Hydroelectric generating stations emit no air pollutants. The
vast majority of power produced in British Columbia is from
hydroelectric stations.
Generating stations which involve the combustion of a
non-renewable resource have basically the same problems as do
vehicles which burn the same resource. Note that there is a
difference, however - the vehicles use internal combustion
engines operating under widely varying loads, whereas a
generating station generally uses "external" combustion
(using the heat to produce steam to turn turbines) under a
relatively constant load. More complete combustion (and hence
fewer pollutants) is possible with external combustion. In
addition, the centralised nature of the power source makes it
feasible to add effective anti-pollution devices such as
"scrubbers". (For example, the natural gas fired
Burrard Thermal Generating Station in B.C. is being upgraded to
reduce its emissions of nitrogen oxides by 70% [3]).
As well, note that generating stations may be located away from
urbanised areas, thus not contributing to human health problems
to the same degree. Operation of the generating station can also
be curtailed when the air quality index indicates high level of
pollution (this is done in B.C. - the load is shifted to
hydroelectric stations). On the other hand, some efficiency is
lost in long distance electricity transmission.
Noise pollution
Anybody who has ever lived near a highway or a busy street
realises how noisy cars are. Even when stopped cars emit
substantial noise due to their internal combustion engines
idling. Buses powered by internal combustion engines have similar
characteristics, and are especially noisy when starting from a
stop and, often, when coming to a stop (squeaky brakes).
Unfortunately this is what diesel buses do all the time in
transit service!
By comparison, vehicles powered by electric motors are
extremely quiet (at least at low speed). The electric motor
itself is essentially noiseless, but the power electronics
usually emit a noticeable hum of various frequencies. Dynamic or
regenerative braking allows these vehicles to slow to nearly a
stop before having to use friction brakes which can emit the same
annoying noise alluded to above. Once stopped, however, the
vehicle is almost noiseless since the electric motor is stopped.
(I say "almost noiseless" because if I said
"noiseless" someone would point out the hum of the
fluorescent lights or some such thing). Note that some vehicles
(e.g. Vancouver trolleybuses) have a compressed air system (for
air brakes and operating the doors and wipers) and hence are
noisy when the air compressor operates occasionally.
A substantial part of the noise of road vehicles at high speed
is from the tires. Rail vehicles produce noise of a different
type - sometimes a high pitched squeal is emitted when the flange
of the wheel rubs against the rail on a corner. Newer systems
have steerable axles which mostly avoid this problem. Rail
vehicles can also emit a low frequency noise (thump thump) when
passing over switches. Overall, however, new rail systems with
electrically propelled trains are by far the quietest mode of
transport.
Land and water pollution
Landfill
The construction of all vehicles using non-recyclable items
results in eventual landfill material. Road vehicles have one
problem which rail vehicles do not in this regard: tires. An
estimated 19.5 million tires are discarded every year in Canada,
about 13 million from passenger vehicles ([1]).
62% of these tires are landfilled, 18% recycled or retreaded, 6%
burned as a fuel source, and 14% stockpiled. 11.5 million of
these stockpiled tires caught fire at Hagersville, Ontario in
1990, causing large amounts of air, land and water pollution.
Transit vehicles fare better than private cars in terms of
amount of landfill waste because of their more intensive use (see
the article on efficiency).
Pollution generated by production and delivery of liquid
fuels
From [1]:
Between 1985 and 1990, an average of 7.9 million litres of
gasoline and 16.2 million litres of crude oil per year were
reported to have been spilled in Canada during extraction,
transportation, refining, storage and delivery. It is
suspected that unreported events, such as the dumping of
contaminated ballast from tankers and runoff from roads and
sewers, may release even greater amounts. Furthermore,
leaking gasoline from underground storage tanks has recently
begun to emerge as a significant contributor to the
contamination of soil and water. A single litre of gasoline
can make up to 1 million litres of water unfit for human
consumption.
In 1987, crude oil refineries collectively discharged, on
a daily basis, 1080 kg of oil and grease, 4039 kg of
suspended solids, 77 kg of phenols, 21 kg of sulphide, and
726 kg of ammonia nitrogen.
Road runoff
Road salt, leaked motor oil and particulate emissions wash off
road surfaces and concentrate in ditches and sewers. The overall
effect of these problems is not yet known.
Destruction of farmland
This may be stretching the definition of land pollution a bit,
but one of the most disturbing facets of automobile dependency is
the conversion of prime agricultural land to car oriented sprawl
development. Between 1981 and 1986, 55 200 hectares of rural land
near 70 Canadian cities was urbanised. Of this, 59% was prime
agricultural land ([1]).
"Free" energy in fossil fuels?
Some people may argue that the energy contained in fossil
fuels is "free", in the sense that it should not count
in the overall energy use of a transport mode which burns fossil
fuels, and hence such modes are really "cleaner" than
they appear. Even if one were to accept this notion, it is clear
that depleting non-renewable resources can only be done for so
long, and in fact has environmental consequences of its own. As
well, of course, the extraction, shipping, refining, distribution
and consumption of fossil fuels is most certainly not free, and
has large environmental problems of its own (as discussed above).
Flooding of land by the construction of hydroelectric
stations
The construction of a hydroelectric station involves damming a
river and creating a water reservoir (a.k.a. lake) where there
wasn't one before. This can have environmental consequences, but
they are more of a one-time nature and vary considerably from
project to project. In any event, the environmental damage done
cannot be attributed to any significant degree to the use of
electric propulsion in British Columbia. The trolleybus system
consumes only 0.038% of all electricity produced in British
Columbia, or 0.22% of the output of the Revelstoke dam. (This is
not to say that the energy all comes from the Revelstoke dam,
just that I happen to know the power output of the Revelstoke
dam!).
In addition, hydroelectric dams may cause silt to accumulate
behind the dam. The magnitude or significance of this effect in
British Columbia is not known to the author.
Conclusions
The "hidden" pollution of the fossil fuel
infrastructure is considerable, and adds to the huge strain
imposed by the burning of those fuels - e.g. cars contribute two
thirds of air pollution in B.C.'s Lower Mainland.
The closest to benign type of transportation is the directly
powered trolleybus or electric train, supplied by hydroelectric
generating stations. These transport modes, in operation, do not
produce any air pollution at all, and produce little noise.
References:
[1] Environment Canada, "Environmental implications of the
automobile"
[2] Go Green (Greater Vancouver Regional District, British
Columbia Ministry of Transportation and Highways, British
Columbia Environment Ministry, Environment Canada, BC Transit),
"Facts about motor vehicles and air pollution"
[3] BC Hydro, "Making the Connection"
James Strickland
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