Future Air Quality in Danish cities

3. Vehicle Emission Factors

3.1 Background Data for Vehicle Emissions
3.2 Calculated Emission Factors for the Scenario Years

To provide the present study with emission data a simplified model has been made to forecast the emission factors for vehicles in the years 2000, 2005, 2010, 2015 and 2020 using 1995 as a reference year. The model covers the hot, cold and evaporative (running loss) emission types respectively for the emission species: CO, NO_x, VOC (NMVOC and CH₄), particulates, CO₂ and N₂O. CO and NO_x emission factors are used for air quality modelling of urban background and street levels.

The following vehicle types are included in the model:

	Conventional gasoline passenger cars
	Closed loop catalyst gasoline passenger cars
	Conventional gasoline light duty vehicles (vans)
	Closed loop catalyst gasoline light duty vehicles (vans)
	Diesel passenger cars
	Diesel light duty vehicles (vans)
	Lorries (3.5-7.5 tonnes)
	Lorries (7.5-16 tonnes)
	Lorries (16-32 tonnes)
	Lorries (> 32 tonnes)
	Urban buses.

3.1 Background Data for Vehicle Emissions

The travel speed dependent hot emission factors from the European road traffic emission model COPERT III are used as background emission data for all of today’s and future vehicle types (Ntziachristos, 1999). An overview of the different emission legislation levels for present and future vehicle layers are given in Table 3.1 and Table 3.2. A vehicle layer consists of the vehicles with comparable data for emissions and fuel consumption. In the present study the emission factors are picked out from COPERT III at a travel speed of 50 km/h. This choice of travel speed facilitates the subsequent use of data in models for air quality, where the single set of emission factors are scaled to represent the emission factors at travel speeds found in the selected case study streets.

An exception to the use of original emission data from COPERT III is made for conventional gasoline light duty vehicles. In this situation no classification is made in COPERT III to take into account vehicle age and technology levels. Instead the emission factors for these vehicles are represented by the emission factors from conventional gasoline light duty vehicles corresponding to first registration years, and emissions are multiplied with a factor of 1.5.

COPERT III does not give emission factors for benzene. Benzene emission factors have been established by so-called invert calculations by the OSPM model assuming that the OSPM gives a perfect description of the dispersion whereby emission factors for light and heavy vehicles can be estimated. In the reduction of benzene emission factors in the different scenario years, it is taken into account that the benzene content in petrol has decreased from 3% to 1% between 1995 and 2000.

The emissions from catalyst equipped vehicles increase with increasing mileage due to wear of the catalytic converter. Emissions continue to increase until a certain cut-off mileage is reached. At this time the emissions stabilise at a constant level due to On Board Diagnostics (OBD) in future catalyst cars and the implementation of the Danish inspection and maintenance programme. This is true when the emissions from the entire fleet is considered on average. For the individual vehicles the emission curves may be serrated. The deterioration factors and cut-off mileage from COPERT III are used in the present project to simulate the influence on emissions and fuel consumption due to catalyst ageing, OBD and the Danish inspection and maintenance programme.

In general the cold start emission factors are calculated as the hot emission factors times the hot/cold emission ratio, the latter ratio, see Appendix 1, is given in COPERT III. For catalyst gasoline vehicles the ratios exist for three engine sizes of passenger cars and one ratio for vans. The ratios are equivalent for gasoline fuelled conventional passenger cars and vans, and for diesel passenger cars and vans, respectively. Even though, the hot/cold ratios are constant for each of the individual vehicle classes, the cold start emission factors will decrease in the future. This is due to the gradually decrease in the emissions from the hot engines as stricter emission standards come into force. Additionally, the average length of each trip driven with a cold engine will gradually become shorter in the future due to stricter emission legislation for cold starts. The cold driven part of the trip length for trips starting with cold engines are expressed by the so-called beta-factors.

Table 3.1
Vehicle Layers for Passenger Cars According to EU Emission Legislation

Table 3.2
Vehicle Layers for Vans, Lorries and Buses According to EU emission Legislation

3.2 Calculated Emission Factors for the Scenario Years

The core in the simulation of emission factors for future years is the hot emission factors from COPERT III for the different vehicle types. The hot emission factors are further processed taking into account lower emissions for future new registered vehicles, and for each layer the number of vehicles and their corresponding annual mileage. Catalyst vehicle emissions are also simulated taking into account the decline in catalyst efficiency (deterioration factors). The lowering of emissions for vehicles in compliance with future EU emission legislation levels compared to EURO I levels are given in Table 3.3 and Table 3.4 for passenger cars and vans, and lorries and buses, respectively.

No forecast of the vehicle stock and annual mileage in layers for the future scenario years could be made available for the present study. The absence of fleet and mileage projections is partly compensated for by the use of the baseline year (1997) distributions of vehicle stock and annual mileage per first registration year (see Appendix 2). The use of 1997 distributions assume constant percentage shares for all future scenario years of the number of one-year old, two-years old etc. vehicles and correspondent mileage.

For all scenario years the emission factors for this study’s vehicle categories are subsequently calculated taking into account the implementation dates for new vehicle technologies and the number of km’s driven by vehicles of a certain age; this approach keeps track on the number of vehicles in each layer and the degree of catalyst wear, which in turn affects the aggregated results.

The calculated hot and cold emission factors for the present study’s vehicle categories are listed in Appendix 3 together with the weighted beta-factors (which represent the cold driven part of the trip length for trips starting with cold engines). Table 3.5 and Table 3.6 shows the development in the hot and cold emission factor levels and the beta-factors for the future scenario years with 1995 as base.

Table 3.3
Future Emissions of Passenger Cars and Vans Compared to EURO I

Table 3.4
Future Emissions of Lorries and Buses Compared to EURO I

Table 3.5
Levels of Hot and Cold Emission Factors and Beta-factors (Index) for Passenger Cars and Vans

* Exhaust

Table 3.6
Levels of Hot and Cold Emission Factors and Beta-factors (Index) for Lorries and Buses

* Exhaust