lubricant temperature in the engine and transmission. These “cold-start” emissions can constitute a
significant proportion of total road transport emissions, particularly in urban areas. Cold-start emissions and fuel consumption are higher for lower
engine starting temperatures which will, in turn,
depend upon factors such as the length of time the
vehicle has been parked and the prevailing ambient temperature.
Rail locomotives are mainly powered by either
diesel or electricity. The nature of the pollution
emitted from rail transport is dependent upon the
type of traction. In the case of diesel locomotives,
pollutants are emitted directly into the local environment as result of combustion, whereas for
electric locomotives the emission is effectively
transposed to the power stations which are responsible for producing the electricity. Emissions from
electric trains are therefore normally allocated to
the power generation sector. Compared with road
vehicles, trains generally have low emissions per
passenger-kilometer although this depends, of
course, on the passenger load factors.
Shipping is responsible for a significant fraction of NO x and primary PM emissions in some
countries. Many ships have old engines or old
technology, and compared with road transport
few exhaust aftertreatment devices are in use.
The quality of the fuel is also poor relative to
that used for road vehicles. In particular, the residual fuel oil used in ship engines contains very high
levels of sulfur (up to 4.5% by mass), which leads
to high PM emissions [34]. To put this in perspective, the PM emission factor for ship engines is
typically around 1.25 g/kWh [35], compared with
the legislative limit value of 0.02 g/kWh for a
modern Euro V truck. There is little information
on the size distribution of PM emissions from
ships, although some recent data indicate that a
large fraction of the emitted particles can be classified as PM 2.5 [34]. NO x and sulfur dioxide (SO 2 )
emissions from shipping also contribute significantly to secondary particle formation.
Evaporation
Volatile organic compounds (VOCs) are emitted
from the fuel systems of petrol vehicles as a result
of evaporation. The compounds that are emitted
are mainly of light hydrocarbons (C 4 –C 6 )
[36]. Evaporative emissions from diesel-fuelled
vehicles are considered to be negligible due to
the low volatility of diesel fuel.
There are several different mechanisms of
evaporation. “Diurnal losses” are due to the thermal expansion and emission of vapor, mainly in
the fuel tank, as a result of changes in ambient
temperature during the day. “Hot-soak losses”
occur when a warm engine is turned off and heat
is dissipated into the fuel system. Whilst a vehicle
is being driven, the engine provides a continuous
input of heat to the fuel system, resulting in “running losses.” In addition, “refueling losses” occur
while the tank is being filled and the saturated
vapors are displaced and vented into the atmosphere, but these are usually attributed to the fuel
handling chain.
Evaporative emissions are dependent upon
four major factors: the vehicle design, the ambient
temperature, the volatility of the petrol, and the
driving conditions. Emissions have decreased significantly in recent years as a result of new cars
being equipped with sealed fuel injection systems
and activated carbon canisters in fuel tank
vents [5].
Abrasion
As noted earlier, particles are also generated as a
result of various unregulated abrasion processes,
including tire wear, brake wear, and road surface
wear. These sources are becoming increasingly
important (Fig. 4), but the understanding of emissions of abrasion particles is relatively poor and
their health effects are not well documented.
There is little information on the composition
of tire wear particles. Components that have been
suggested as environmental tracers of tire wear
include styrene butadiene rubber [38],
benzothiazole [39], and organic zinc [40]. Studies
conducted in the 1970s revealed that wear particles had a bimodal distribution, with mass in the
both the fine (<2.5 mm) and coarse (2.5–10 mm)
modes [38, 41–43]. Several laboratory and field
investigations have demonstrated that most particles are present in the coarse fraction [38, 44, 45],
or are larger still [46], whereas others have indicated substantial mass in both the PM 2.5 and PM 1
Air Quality, Surface Transportation Impacts on
55
significant proportion of total road transport emissions, particularly in urban areas. Cold-start emissions and fuel consumption are higher for lower
engine starting temperatures which will, in turn,
depend upon factors such as the length of time the
vehicle has been parked and the prevailing ambient temperature.
Rail locomotives are mainly powered by either
diesel or electricity. The nature of the pollution
emitted from rail transport is dependent upon the
type of traction. In the case of diesel locomotives,
pollutants are emitted directly into the local environment as result of combustion, whereas for
electric locomotives the emission is effectively
transposed to the power stations which are responsible for producing the electricity. Emissions from
electric trains are therefore normally allocated to
the power generation sector. Compared with road
vehicles, trains generally have low emissions per
passenger-kilometer although this depends, of
course, on the passenger load factors.
Shipping is responsible for a significant fraction of NO x and primary PM emissions in some
countries. Many ships have old engines or old
technology, and compared with road transport
few exhaust aftertreatment devices are in use.
The quality of the fuel is also poor relative to
that used for road vehicles. In particular, the residual fuel oil used in ship engines contains very high
levels of sulfur (up to 4.5% by mass), which leads
to high PM emissions [34]. To put this in perspective, the PM emission factor for ship engines is
typically around 1.25 g/kWh [35], compared with
the legislative limit value of 0.02 g/kWh for a
modern Euro V truck. There is little information
on the size distribution of PM emissions from
ships, although some recent data indicate that a
large fraction of the emitted particles can be classified as PM 2.5 [34]. NO x and sulfur dioxide (SO 2 )
emissions from shipping also contribute significantly to secondary particle formation.
Evaporation
Volatile organic compounds (VOCs) are emitted
from the fuel systems of petrol vehicles as a result
of evaporation. The compounds that are emitted
are mainly of light hydrocarbons (C 4 –C 6 )
[36]. Evaporative emissions from diesel-fuelled
vehicles are considered to be negligible due to
the low volatility of diesel fuel.
There are several different mechanisms of
evaporation. “Diurnal losses” are due to the thermal expansion and emission of vapor, mainly in
the fuel tank, as a result of changes in ambient
temperature during the day. “Hot-soak losses”
occur when a warm engine is turned off and heat
is dissipated into the fuel system. Whilst a vehicle
is being driven, the engine provides a continuous
input of heat to the fuel system, resulting in “running losses.” In addition, “refueling losses” occur
while the tank is being filled and the saturated
vapors are displaced and vented into the atmosphere, but these are usually attributed to the fuel
handling chain.
Evaporative emissions are dependent upon
four major factors: the vehicle design, the ambient
temperature, the volatility of the petrol, and the
driving conditions. Emissions have decreased significantly in recent years as a result of new cars
being equipped with sealed fuel injection systems
and activated carbon canisters in fuel tank
vents [5].
Abrasion
As noted earlier, particles are also generated as a
result of various unregulated abrasion processes,
including tire wear, brake wear, and road surface
wear. These sources are becoming increasingly
important (Fig. 4), but the understanding of emissions of abrasion particles is relatively poor and
their health effects are not well documented.
There is little information on the composition
of tire wear particles. Components that have been
suggested as environmental tracers of tire wear
include styrene butadiene rubber [38],
benzothiazole [39], and organic zinc [40]. Studies
conducted in the 1970s revealed that wear particles had a bimodal distribution, with mass in the
both the fine (<2.5 mm) and coarse (2.5–10 mm)
modes [38, 41–43]. Several laboratory and field
investigations have demonstrated that most particles are present in the coarse fraction [38, 44, 45],
or are larger still [46], whereas others have indicated substantial mass in both the PM 2.5 and PM 1
Air Quality, Surface Transportation Impacts on
55