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 presents a review of methodologies and models used for estimating evaporative emissions of volatile organic compounds. The Report deals with three mechanisms by which petrol fuel evaporates from vehicles: “diurnal” emissions, “hot-soak” emissions and “running” emissions. Evaporative emissions from diesel vehicles are negligible. The NAEI approach for evaporative emissions is based largely on the COPERT II and COPERT III models. Comparisons were made between the NAEI, ARTEMIS and draft COPERT 4 models. The total fleet-weighted annual emission factors in the three models were rather similar. This is in spite of the fact that the relative contributions of diurnal, hot-soak and running emissions in the three models were very different. The Report concludes that, given that considerable effort has been put into developing the new method in COPERT 4 and the flexibility it offers, there would be sufficient justification for changing the NAEI to include this method. However, further work is required to assess its applicability and reliability. Allowance should also be made in the NAEI for the failure of evaporative control systems.

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