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 reviews the use of average vehicle speed (as used in the NAEI) to characterise exhaust emissions. Several alternative models for estimating emissions could be used in the NAEI. Some of these essentially use the same modelling approach as the NAEI, and could therefore be introduced with only minor changes to the model inputs. Others would require considerably more work, as the activity data would have to be reconfigured and transport statistics would have to be analysed differently. Various models were compared with the NAEI model. Generally, there was a very good agreement between the emission factors in the NAEI and those in the various models tested, but the results varied with vehicle category and pollutant. Four types of assessment were considered in an attempt to determine the accuracy of the predictions of different models. Model predictions were compared with: (i) on-board emission measurements; (ii) remote sensing measurements; (iii) the results from the inversion of an air pollution model; and (iv) measurements in road tunnels. The assessments included errors, assumptions and limitations which made it difficult to make general conclusions. Moreover, it is unlikely that such approaches could be conducted with enough regularity or consistency to enable changes in the accuracy of emission models to be checked with time. Nevertheless, the results indicate that the current UK emission factors probably provide a reasonably accurate characterisation of total emissions from road transport. However, the emission factors for specific vehicle types are associated with a high degree of uncertainty, not least due the difficulties associated with correctly identifying vehicle types and their operation. The Report concludes that there is little justification at present for replacing the current emission calculation method in the NAEI, but the emission factors for specific vehicle categories should be improved where possible. Further efforts are also required to categorise vehicles appropriately, and to properly characterise operational conditions (such as road gradient and load in the case of HDVs).

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