Recent measurements of mobile source emission factors in North American tunnels

Results of recent tunnel studies in the Cassiar, Tuscarora, Fort McHenry, and Caldecott tunnels are described, along with the methodology for calculating vehicle emissions in these studies. Results for Cassiar, Fort McHenry and Tuscarora generally agree within ±50% with the model predictions and the emissions ratios are also in good agreement with the models. These results contrast with the 1987 Van Nuys experiment, wherein the CO and HC were greatly underpredicted, as were the CONOx and HCNOx ratios by factors of two and more. The 1994 Caldecott CONOx results deviate significantly from model predictions and are similar to those seen in the 1987 Van Nuys study, although the HCNOx ratio is in agreement with the model prediction. Possible explanations for differences or lack of differences between the observed tunnel emissions and model predictions may include the evolution of the models and the increase in the basic emission factors incorporated in the models, differences in the tunnel fleet, manner of driving (lack of speed variability in some of the tunnels), and the impact of grade. None of these explanations, however, can adequately account for the range of deviations between on-road observations and model predictions.