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How do Multi-modal
transport emissions work and how they are modelled?
When dealing with
transport emissions of pollutants at provincial and regional scale it is
necessary to include all the four existing transport “modes”: road, rail, water
and air. On this scale we normally encounter airports, ports, internal water
lines, major rail lines and of course motorways and road links as at urban
scale.
It must be recognised
here that, for the aim of correctly modelling urban air pollution, the
consideration of the four modes is sometimes necessary: it is not difficult to
identify urban areas close to a seaport and with an airport (or more) in the
vicinity of the residential areas. For these cases, it would be recommended
that both typical urban scale emission models are used (see Topic The role and prerequisites for Transport Emission Models in Urban Planning) and
models generally fitted for the wider regional scale.
Emission models from
the four transport modes present different difficulties connected with the
approach adopted for their modelling and the availability of proper
experimental data.
Emissions from Road transport vehicles are modelled in
a number of different ways. Emissions are calculated as the sum of at least two
main components: ‘hot emissions’ and ‘cold start emissions’. If the modeller is
interested in VOC emissions, then also the ‘evaporative term’ has to be added.
Hot emissions are the emissions emitted when engine and abatement devices have
reached a ‘regime’ temperature. They are influenced by a number of parameters:
vehicle kinematics, gradient of the road, altitude, maintenance level, vehicle
age, vehicle loading, electric loads (for car-lights and for Air conditioning).
Normally models refer to hot emissions as a function of kinematics (e.g.
average speed or instantaneous speed and acceleration) and then multiply the
‘ideal’ hot emission value for a number of corrective factors taking into
account the other parameters. Cold Start emissions are the emissions emitted
from the start up until the vehicle reaches an almost steady state in terms of
temperature of engine and abatements system. The cold start emissions affect
the first 3 – 4 km of trip and are particularly relevant for catalyst vehicles:
for CO and VOC the ‘cold start emission’ is roughly 10 times higher than the
hot emission. Since the EU fleets are getting more and more ‘catalyzed’ this
factor has a present and future relevance for emissions modellers. Evaporative
emissions are the emissions of unburned fuel from the ‘weak points’ of the
vehicle: tank and canister. Current modelling recognises three different
contributes to evaporative emissions: A) Running emissions, emitted when
vehicles are driven (tank level), B) Hot Soak emissions, emitted from the
canister at trip conclusion, C) Diurnal emissions, emitted at tank level by
cars parked (not included in usual traffic modelling). Evaporative emissions
are a relevant (30 – 40 %) of total transport related VOC emissions and then
have a major role in the planning of measures for reducing VOC related
pollution (e.g. policies for reducing benzene pollution).
Several correction
factors must be added to these primary terms:
·
Gradient correction,
particularly important for heavy vehicles;
·
Maintenance
correction, relevant for poorly maintained fleets;
·
Age correction,
important for older vehicles (‘aged fleets’);
·
Altitude correction,
relevant only in mountain areas crossed by vehicles ‘tuned’ at low altitude;
·
Load correction,
needed for heavy-duty vehicles;
·
Electric loading,
important especially for small and medium cars being equipped with A/C systems.
For road emission
models a variety of solutions have existed for several years at international
level, as for the other three modes (Water, Rail, Air) it is only with COST319
Action and EC DG Transport co-funded MEET and COMMUTE Projects (FP4, ended
1998-2000) that European scientists put together the available knowledge and
realised an innovative network orientated multi-modal model (the COMMUTE
‘tool’). The outcomes of these projects and the characteristics of the produced
tool are described also by the following notes on the Rail, Waterborne and Air
transport emissions.
Rail Transport emissions are modelled in a relatively simple way. For
these vehicles, emissions are derived from energy-fuel consumption figures and
strongly depend on the type of engine: electric or diesel.
The energy consumption
mostly depends on the maximum train speed, the average speed, the train mass
and front shape (Cx coefficient) and the number of
stops between origin and destination.
Air Transport emissions modelling presents unique characteristics linked
to the flight standard profile: this kind of ‘speed cycle’ describes the eight
phases of a flight mission: taxi out, take off, ascent, cruise, descent,
approach, landing and taxi in. Each of these phases is characterised by very
different consumption and emission rates and data exist for a rich set of
categories of planes (over 30).
Waterborne transport emissions are modelled in a rather similar way to rail
emissions. First fuel and energy consumption are calculated and from these the
emissions are estimated on the basis of average emissions units for used fuel
unit. Most of the engines here are diesel. A real complication of the
methodology is given by the emissions emitted within ports along the operation
of loading and unloading of the ship. These additional emissions are relevant
for urban centres near to the ports.
The emission models
available at international level (see reviews in COST319 and COST 346 web
sites, and FP4 DG VII COMMUTE Project results) include several different
‘categories’ of models that join some common characteristics. One of the most
used ‘categorisations’ split these models into two main ‘families’: Aggregated
Models and Disaggregated Models.
Aggregated Models deal normally with a whole Country and model traffic as a
global figure (total vehicles km driven in the area is the traffic amount
input). With reference to Road transport (e.g. COPERT tools) a country is split
into 3 entities: cities, motorways and rural roads. Vehicles kinematics is
represented only by average speed that is given as unique value for the three
‘contexts’ and differentiated through vehicle classes. Cold start is calculated
here from an average Trip Length without any spatial differentiation. Some
corrections factors are applicable (age, maintenance) while for other factors
the representation is basically impossible (e.g. gradient effect).
Disaggregated
(network based) Multi-modal Models
consider the transport network and normally receive input from transport
models. Emissions are calculated link by link. The network is in practice split
into four independent networks each of which describes one of the modes. Nodes
represent cities, ports, and airports. Links represent motorways and main roads
connecting cities, rail-lines, air routes, inland waterways and sea routes
(rather roughly). By means of these kinds of tools it is possible to evaluate
the impacts of transport policies on various scales, in particular those
involving new big infrastructures.
During the COMMUTE
Project the EC requested the running of a pilot study dedicated to the impacts
of the Trans European Network for Transport (TEN-T).
Critical Issues for
modelling Transport Emissions at Regional Scale
Vehicle kinematics do not play a relevant role at regional scale. Traffic
models on this scale provide the average speed as link attribute. Cold start
emissions also do not have an essential role. The extra urban average trip
length is estimated to be around 30 km depending on the Country. So the
fraction of cold vehicles is low on this scale.
The modelling of
network hilliness (distribution of gradients) can have a great importance in
regions with mountains and important commercial traffic (e.g. alpine regions).
As it regards rail
transport, a first difficulty is in the categorisation of trains: the available
model considers only 4 categories (high speed, intercity, freight and urban):
the result of this split can be a bit too coarse in some cases. Not simple at
all is the collection of data on trips, needed for assigning appropriate number
of trains passing in each link. Cooperation with national railways
organisations can solve the issue.
Air transport model use
is also affected by difficulties in getting airport traffic data. It is evident
that when the airport is close to the urban area its
modelling is crucial, due to the quantity of NOX, VOC and PM emitted
during flight phases and take off and ascent in particular. The relevance of
airport presence is connected with the direction and intensity of prevailing
winds compared with the position of the city.
Water transport has as
weak points the difficulty to know the size of the ships entering and leaving
ports. These parameters affect both the emissions during transit movements and
the operational emissions during goods loading-unloading.
This topic is
conceptually linked to the topics on urban emission models (The role and prerequisites for Transport Emission Models in
Urban Planning, ENEA) and the topic on How to develop urban Emission Inventories?.
Disaggregated regional scale
emission models are one of the fundamental ingredients for the building of
regional, provincial and urban emission inventories needed for modelling urban
air quality. |