APPORTIONMENT OF NMHC TAILPIPE VS NON-TAILPIPE EMISSIONS IN THE FORT MCHENRY AND TUSCARORA MOUNTAIN TUNNELS

Measurements of on-road emissions of non-methane hydrocarbons (NMHCs) were made in the Fort McHenry Tunnel (Baltimore) and Tuscarora Mountai n Tunnel (Pennsylvania) during the summer of 1992. Measurements were m ade during 11 one-hour periods in the Fort McHenry Tunnel and during 1 1 one-hour periods in the Tuscarora Mountain Tunnel. The observed ligh t-duty fleets were quite new, with a median model year of approximatel y 1989. Speciated NMHC values were obtained from analyses of canister and Tenax samples, and light-duty speciated emission factors were calc ulated for the two tunnels. Fuel samples were collected in the area ar ound the tunnels for use in constructing headspace and liquid Fuel pro files for the chemical mass balance (CMB) receptor model. Profiles of tailpipe emissions were obtained from the literature, The CMB was used to apportion tailpipe from non-tailpipe emissions. Non-tailpipe sourc es were found to constitute approximately 15% of the light-duty NMHC e missions. The Federal automotive emission-rate models, MOBILE4.1 and M OBILES, underpredicted non-tailpipe emissions, assigning approximately 9% and 6.5%, respectively, to non-tailpipe sources. In terms of total absolute emissions, MOBILES predictions were approximately a factor o f 2 greater than MOBILE4.1 predictions. Both MOBILE4.1 and MOBILES und erestimated the NMHC emissions in the Fort McHenry Tunnel and overpred icted the NMHC emissions in the Tuscarora Mountain Tunnel. In all case s, the MOBILE models underestimated the absolute value of the non-tail pipe emissions. The ability of the MOBILE models to account for observ ed emissions when conditions are more variable than those studies in t he Fort McHenry and Tuscarora Mountain tunnels is still an open questi on. Copyright (C) 1996 Elsevier Science Ltd