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Abstract-As part of the 1993 Lower Fraser Valley Oxidants Study, measurements of mobile source emission factors were performed in the Cassiar Tunnel on the Trans-Canada Highway to measure the on-road contribution to the ozone-forming precursors (NO, and speciated hydrocarbons) along with CO. ObSe~ed emission factors were compared to the Canadian versions of the U.S. Environmental Protection Agency' s MOBILE models, MOBILE4.1C and MOBILESC, to assess uncertainty in the predicted mobile source contributions to the Vancouver emissions inventory.A total of 16 l-h runs were made. The timing of the individual runs was designed to encompass different traffic volumes, driving conditions, and times of day. A total of 24,513 vehicles traversed the tunnel during the study, with approximately 91% light-duty vehicles, 4% heavy-duty spark ignition vehicles, and 5% heavy-duty dim1 vehicles. MOBILESC overpredicted the observed value of CO by -2%, NMHC by 24%, and NO, by 13%, while MOBILE4.1C underpredicted the observed values by 36,29, and 23% for CO, NMHC, and NO, respectively. @ 1997 Elsevier Science Ltd.

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