TRL was commissioned by the Department for Transport to review the approach used in the National Atmospheric Emissions Inventory (NAEI) for estimating emissions from road vehicles, and to propose new methodologies. This Report addresses the emission deterioration functions and fuel/technology scaling factors currently used in the NAEI. Scaling factors are applied to the basic emission factors to enable the modelling of emissions in different years. These scaling factors cover to the changes in emissions associated with vehicle age (“degradation” or “deterioration”), and the effects of the penetration of improved fuels and vehicle technologies. The current assumptions concerning vehicle age are rather simplistic, and do not take into account the characteristics of the vehicle samples used to derive emission factors. Similarly, the fuel and technology scaling factors were devised several years ago, and many were assumed to stabilise after 2005. The Report provides a brief review of the mileage, fuel and technology effects given in the literature, and describes how new scaling factors (to be applied to the new emission factors) were derived. Scaling factors for different years were developed to account for the following: (i) mileage effects relating to vehicle samples; (ii) fuel composition effects; (iii) increased market penetration of biofuels; and (iv) the effects of future technologies.

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