Characteristics and emissions of heavy-duty vehicles in Tennessee under the MOBILE6 model.

Heavy-duty vehicle (HDV) classifications used for modeling emissions in the MOBILE6 model have been expanded from 2 classifications in MOBILE5 to 16 classifications in MOBILE6. The new classifications are based on vehicle weight and fuel used (i.e., gasoline or diesel). The heavier vehicles have higher emissions, so it is important to use correct vehicle weight distributions. Tennessee's HDV registration data show a distribution very similar to the national defaults, but with more vehicles in the heaviest weight category (HDV8B). More than 50% of Tennessee's HDVs fall in the lightest vehicle category (HDV2B). The biggest difference in truck characteristics in Tennessee versus national defaults in MOBILE6 is the higher HDV fraction on Tennessee rural Interstates. Also, the ratio of single-unit trucks to trailer trucks varies considerably by facility type. The emissions of volatile organic compounds and carbon monoxide per mile of travel of gasoline-fueled single-unit trucks can be 2.5 to 5 times higher than those of heavy-duty diesel trailer trucks. The emissions of nitrogen oxides per mile of travel of diesel-fueled tractor-trailer trucks can be five times higher than those of gasoline-fueled single-unit trucks. For these reasons it is important to accurately characterize the HDV fleet. The characteristics of the Tennessee HDV fleet are compared with national defaults used in MOBILE6, and a new scheme for classifying vehicles by road type is presented.