further vehicle testing. Secondly, the models
inherently take into account dynamics. Thirdly,
emissions can be resolved spatially, and this
ought to lead to improvements in the prediction
of air pollution. However, detailed and precise
data on vehicle operation and location are
required, or any potential benefits may be lost.
Such data are difficult to obtain for many model
users. Moreover, it is extremely difficult to measure emissions on a continuous basis with a high
degree of precision, and then it is not straightforward to allocate those emission values to the correct operating conditions. During measurement in
the laboratory, an emission signal is dynamically
delayed and smoothed, and this makes it difficult
to align the emissions signal with the vehicle
operating conditions [77]. Such distortions have
not been fully taken into account in instantaneous
models until relatively recently.
Once the basic modeling approach has been
identified a number of other effects may need to
be considered, depending on the type of model
being used. These effects include potential future
improvements to fuels and technologies, the deterioration of vehicle exhaust emissions with age,
the presence and use of auxiliary devices (e.g., air
conditioning, windscreen wipers, radios), road
gradient, and vehicle load/occupancy. Such
effects may be taken into account via separate
models [78] or through adjustments to the basic
emission factors, with assumptions concerning
levels of uptake and effectiveness in future years.
Separate models also exist for the estimation of
cold-start emissions. In some cases, the hot emission factor is multiplied by a ratio of cold-start to
hot emissions for the fraction of kilometers driven
with a cold engine [33]. In others, a reference
cold-start excess emission value is adjusted to
take account of, for example, the ambient temperature, the average speed, the parking duration, and
the distance travelled (some trips are shorter than
the distance needed to fully warm the engine) [79,
80]. Specific models have also been developed to
determine the effects of different parking scenarios [81]. Cold-start emissions are usually included
in regional and national emission inventories, but
are often excluded from local air pollution studies.
One of the main reasons for this is the difficulty
associated with obtaining data on the temporal
and spatial allocation of cold-start operation.
Evaporation The earliest studies of evaporative
emissions were carried out in the United States in
the 1960s [82], but the first European studies were
only conducted 20 years later [36]. The models
that were developed from the resulting data were
not updated for many years. However, modelling
improvements have recently been possible following a number of new investigations [83–85].
Abrasion and Resuspension One non-exhaust
PM model in common use is that presented in
the European Environment Agency’s Air Pollutant Emissions Inventory Guidebook [33]. The
method uses reference emission factors (in g/km)
and incorporates corrections for both speed and,
in the case of HDVs, vehicle load and number of
axles. However, the use of average brake wear
emission factors (in g/km) means that differences
in the extent of braking for different traffic situations cannot be taken into account. Such emission
factors are likely to overestimate brake emissions
in areas of low brake use, and underestimate
emissions in areas of high brake use.
The calculation method presented in the US
Environmental Protection Agency’s AP-42
document [86] is one of the more widely used
for predicting resuspension on paved roads,
although some shortcomings have been identified [87]. There is no widely accepted European method for calculating emissions due to
resuspension.
Rail Transport Models
Emissions from electric trains occur at the power
stations which are responsible for generating the
electricity. The actual emissions will therefore be
a simple function of power consumption and
emission factors for power generation. However,
power losses between the power station and the
train, and within the train itself, also need to be
taken into account where possible.
Emissions from diesel locomotives are typically estimated using fuel consumption data and
fuel-specific emission factors. The types of train
which are typically included in models include
60
Air Quality, Surface Transportation Impacts on
inherently take into account dynamics. Thirdly,
emissions can be resolved spatially, and this
ought to lead to improvements in the prediction
of air pollution. However, detailed and precise
data on vehicle operation and location are
required, or any potential benefits may be lost.
Such data are difficult to obtain for many model
users. Moreover, it is extremely difficult to measure emissions on a continuous basis with a high
degree of precision, and then it is not straightforward to allocate those emission values to the correct operating conditions. During measurement in
the laboratory, an emission signal is dynamically
delayed and smoothed, and this makes it difficult
to align the emissions signal with the vehicle
operating conditions [77]. Such distortions have
not been fully taken into account in instantaneous
models until relatively recently.
Once the basic modeling approach has been
identified a number of other effects may need to
be considered, depending on the type of model
being used. These effects include potential future
improvements to fuels and technologies, the deterioration of vehicle exhaust emissions with age,
the presence and use of auxiliary devices (e.g., air
conditioning, windscreen wipers, radios), road
gradient, and vehicle load/occupancy. Such
effects may be taken into account via separate
models [78] or through adjustments to the basic
emission factors, with assumptions concerning
levels of uptake and effectiveness in future years.
Separate models also exist for the estimation of
cold-start emissions. In some cases, the hot emission factor is multiplied by a ratio of cold-start to
hot emissions for the fraction of kilometers driven
with a cold engine [33]. In others, a reference
cold-start excess emission value is adjusted to
take account of, for example, the ambient temperature, the average speed, the parking duration, and
the distance travelled (some trips are shorter than
the distance needed to fully warm the engine) [79,
80]. Specific models have also been developed to
determine the effects of different parking scenarios [81]. Cold-start emissions are usually included
in regional and national emission inventories, but
are often excluded from local air pollution studies.
One of the main reasons for this is the difficulty
associated with obtaining data on the temporal
and spatial allocation of cold-start operation.
Evaporation The earliest studies of evaporative
emissions were carried out in the United States in
the 1960s [82], but the first European studies were
only conducted 20 years later [36]. The models
that were developed from the resulting data were
not updated for many years. However, modelling
improvements have recently been possible following a number of new investigations [83–85].
Abrasion and Resuspension One non-exhaust
PM model in common use is that presented in
the European Environment Agency’s Air Pollutant Emissions Inventory Guidebook [33]. The
method uses reference emission factors (in g/km)
and incorporates corrections for both speed and,
in the case of HDVs, vehicle load and number of
axles. However, the use of average brake wear
emission factors (in g/km) means that differences
in the extent of braking for different traffic situations cannot be taken into account. Such emission
factors are likely to overestimate brake emissions
in areas of low brake use, and underestimate
emissions in areas of high brake use.
The calculation method presented in the US
Environmental Protection Agency’s AP-42
document [86] is one of the more widely used
for predicting resuspension on paved roads,
although some shortcomings have been identified [87]. There is no widely accepted European method for calculating emissions due to
resuspension.
Rail Transport Models
Emissions from electric trains occur at the power
stations which are responsible for generating the
electricity. The actual emissions will therefore be
a simple function of power consumption and
emission factors for power generation. However,
power losses between the power station and the
train, and within the train itself, also need to be
taken into account where possible.
Emissions from diesel locomotives are typically estimated using fuel consumption data and
fuel-specific emission factors. The types of train
which are typically included in models include
60
Air Quality, Surface Transportation Impacts on