Real-World Measurements of Exhaust and Evaporative Emissions in the Cassiar Tunnel Predicted by Chemical Mass Balance Modeling

The chemical mass balance model has been used to separate non-methane hydrocarbon emission factors measured in the Cassiar tunnel study into exhaust and evaporative emission factors. The local gasoline composition has been used as a real-world surrogate profile for exhaust emissions and has been demonstrated to result in vastly improved model performance compared to the performance obtained with the use of an exhaust profile derived from dynamometer testing. Because of the approach used, the combustion and unburned gasoline components of exhaust emission gases could be estimated separately. Unburned gasoline was found to comprise 63.4 plus or minus 7.0% of exhaust gases for light-duty vehicles operating in steady-state driving conditions in this study. On-road benzene emissions were found to split 71%/27%/2% between the combustion, unburned gasoline, and evaporative sources. Evaporative non-methane hydrocarbons were found to represent 10.3 plus or minus 0.8% of the total on-road emission rate on average. The apportionment of total NMHC emission factors to exhaust and evaporative emission factors allowed a detailed comparison to exhaust and on-road evaporative emission factors predicted by the MOBILE4.1C and MOBILE5C models. (A)