How many cars are used to measure the emissions factors?
The emission factors for cars used in the UK national inventory are based on measurements made on around 2,400 cars from test programmes carried out in the UK and across Europe. The emission factors are from analysis of the test data results carried out by Transport Research Laboratory (TRL) who compile and hold the database of emissions data from cars.
Where can I find an emissions factor for a particular activity?
A comprehensive set of emissions factors applicable to the UK can be found on the UK Emissions Factors Database (UK EFD). Available from here
What is an Emission Factor?
An emission factor is the relationship between the amount of pollution produced and the amount of raw material processed or burned. For road traffic it is the relationship between the amount of pollution produced and the number of vehicle miles travelled. By using the emission factor of a pollutant and specific data regarding quantities of materials used by a given source, it is possible to compute emissions for the source. This approach is often used in preparing an emissions inventory.
How are the emission factors for vehicles calculated?
The emission factors for cars used in the UK national inventory are based on measurements made on around 2,400 cars from test programmes carried out in the UK and across Europe. The emission factors are from analysis of the test data results carried out by Transport Research Laboratory (TRL) who compile and hold the database of emissions data from cars. The road transport emission factors are from analysis of the test data results carried out by Transport Research Laboratory (TRL) who compile and hold the database of emissions data from cars. For further information on the methodology please refer to the TRL website www.trl.co.uk.
What are emission inventories?
An emission inventory is a comprehensive listing by source of air pollutant emissions. They cover a specific geographical area and a specific time period. The UK NAEI estimate national total emissions of a wide range of air pollutants for each year since 1970. For further details please go to the report section of the NAEI website and download the most recent UK Emissions of Air Pollutants report.
The units for road transport emission factors in the NAEI relate to distance travelled (i.e. g/km). Can you provide any assistance for determining EFs for stationary vehicles which have their engines idling?
Unfortunately there are no emission factors available for stationary vehicles. I suggest that you use a low speed such as 5 km/hr and use this to give you an estimate of what the emissions might be for idling vehicles.
Do emission factors for road transport just relate to emissions from the exhaust or do they also include other factors such as brake / clutch dust, tyre and road wear?
In the data warehouse section of the NAEI website under 3. UK Emission Factors, you have the choice of selecting either General UK Emission Factor's or Road transport Emission Factors. If you select General UK Emission factors you will be able to select the type of road transport that you are interested in, you will then be provided with Emission Factors for both exhaust and brake and tyre wear. The spreadsheet provided in the Road Transport Emission Factors section (from which speed dependent Emission Factors can be calculated) only shows emissions from the exhaust.
In your 'General UK emissions factors' database what kind of car is the emission factor for 'all cars' based upon?
The emission factors for the category 'all cars' are fleet weighted emission factors. Therefore the 2000 data for example will take into account the fact that some cars on the road will be pre Euro I (i.e. without a catalytic converter) but that the majority will Euro I and better. For detailed emission factors by Euro standard please go to the Data Warehouse section of the NAEI website and then to 'UK Emission Factors' and then to 'Road Transport Emission Factors'.
Are any emission factors available for vehicles driving up and down hills?
Unfortunately no road transport emission factors are available specifically for driving vehicles up and down hills. I would suggest that you vary the speed to take into account changes in gradient.
Do you have emission factors for cold starts in terms of g/km?
Cold start emission factors are provided in the data warehouse section of the NAEI website. The units are grams per trip. The EF have been derived for cars and LGVs from tests performed with the engine starting cold and warmed up. The difference between the two measurements can be regarded as an additional cold start penalty paid on each trip a vehicle is started with the engine (and catalyst) cold. Hence the cold start EFs are provided in grams per trip rather than g/km. | 
