A Fuel-Based Inventory for Heavy-Duty Diesel Truck Emissions

A fuel-based method for estimating heavy-duty diesel truck emissions is described. In this method, emission factors are normalized to fuel consumption; vehicle activity is measured by the amount of diesel fuel consumed. For the San Francisco Bay Area during summer 1996, on-road heavy-duty diesel trucks were estimated at the upper bound to emit 110 × 103 kg/day of nitrogen oxides (NOx) and 3.7 × 103 kg/day of fine black carbon (BC) particles. These upper bound values were 2.3 and 4.5 times, respectively, the corresponding predictions of California’s motor vehicle emission inventory model, MVEI 7G. Significant decreases in diesel truck activity and emissions, 70– 80% below typical weekday levels, were observed in the Bay Area on weekends. Reductions in diesel NOx and BC particle emissions on weekends may contribute to higher ambient ozone concentrations and higher organic carbon (OC) to BC ratios observed on weekends. Heavy-duty truck traffic peaks on weekdays during the middle of the day and falls off before the afternoon rush hour. Therefore, the diurnal pattern of heavy-duty truck travel may contribute to increases in ambient OC/BC ratios observed during late afternoon hours. INTRODUCTION Heavy-duty diesel trucks (i.e., diesel-powered trucks with gross vehicle weight exceeding 3,860 kg, or ~4 tons) are an important source of fine particle and nitrogen oxide (NOx) emissions. 1 Cass and Gray2 estimate that during the IMPLICATIONS The emissions of nitrogen oxides and fine black carbon particles from heavy-duty diesel trucks may be understated in current emission inventories. The daily and weekly cycles of diesel truck activity differ from corresponding light-duty vehicle patterns. Weekend decreases in diesel truck activity and emissions may contribute to the observed “weekend effect” of higher ambient ozone concentrations in some urban areas on weekends. The contribution of secondary organic aerosol to total fine particle mass may have been overestimated in some cases. 1980s, heavy-duty diesel engines accounted for 70% of total fine black carbon (BC) particle emissions from onroad vehicles in the Los Angeles area. Current estimates from California’s MVEI 7G motor vehicle emission inventory model for the San Francisco Bay Area indicate that in 1996, heavy-duty diesel trucks contributed 74% of exhaust PM emissions and 18% of total NOx emissions from on-road vehicles.3 Motor vehicle emissions are currently estimated using the travel-based MOBILE4 and EMFAC5 emission factor models in the U.S. and California, respectively. In this approach, estimates of vehicle travel are combined with emission factors expressed on a mass per unit distance traveled basis to obtain a motor vehicle emission inventory. Traditionally, vehicle activity has been estimated using travel demand models.6,7 Spatiallyand temporally-resolved vehicle activity is predicted using socioeconomic data such as population, employment, automobile ownership, and income, combined with knowledge of travel times between points, available modes of transportation, and a description of the roadway network. Heavy-duty truck travel represents only a small fraction of total vehicle travel, so little effort has been made to describe truck travel explicitly within travel demand models.7 In current modeling practice, it is common to estimate heavy-duty truck travel as a fixed percentage of predicted traffic volumes.7 However, as noted by Schlappi et al.,8 heavyduty truck travel does not follow the same spatial and temporal patterns as light-duty vehicle travel. Consequently, heavyduty truck activity estimates should not be based upon lightduty vehicle travel patterns. Alternatively, measurements of vehicle kilometers of travel (VKT) for trucks may be used to estimate truck activity. In California, truck VKT is measured only on the state highway system, so reported truck VKT does not include all truck activity. However, truck VKT may be used in conjunction with statewide fuel sales to estimate total heavy-duty truck activity, using the amount of fuel consumed as a measure of activity. Accurate diesel fuel sales data are available at the state level, and truck VKT is reported at the county level.9 Light-duty vehicle emissions are regulated per unit distance of travel. Likewise, current emission inventory models