Car emission factors understate particle pollution

Official estimates could underestimate total road transport emissions of fine particles by 7.5% or more, according to tests for the Department for Transport.

The UK's current emissions inventory and projections are based on emissions factors for a range of vehicles. Since 2001, all new cars have had to meet so-called Euro III emission standards but, until now, no test data have been available to confirm in-service emissions from these vehicles at varying speeds.

However, work by Ricardo suggests that emissions factors for some pollutants need to be revised considerably. The engineering firm conducted tests on 37 Euro III cars, as well as a handful of Euro IV vehicles and light commercial vehicles.

Ricardo concludes that particle emissions from diesel cars, which are expected to account for 42% of new car sales by 2010, are higher than currently assumed. It proposes a revised emissions factor that would increase forecast road transport emissions by about 3.6% in 2010 and 7.5% in 2020.

Importantly, Ricardo says the increase in emission factors could be higher at urban traffic speeds. Any revision to emission factors could force more local authorities to declare air quality management areas to meet air quality targets for particle pollution.

The picture is brighter for other pollutants. Transport-related emissions of oxides of nitrogen are responsible for most declarations of UK air quality management areas.

However, Ricardo says its proposed emission factors for petrol cars would lead to an overall reduction of about 7% in estimates of road transport NOx emissions in 2010. Likewise, the report says tailpipe emissions of hydrocarbons may be overstated by 13% or more.

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