Guensler, Randall L., Daniel Sperling, Paul P. Jovanis (1991) Uncertainty in the Emission Inventory for Heavy-Duty Diesel-Powered Trucks. Institute of Transportation Studies, University of California, Davis, Presentation Series UCD-ITS-RP-91-33
Emissions from motor vehicles are a function of how, and under what conditions, the vehicles are operated. In general, mobile source emission inventories are developed by defining vehicle activities (through vehicle activity models), and coupling the activities with activity-specific emission rates (appropriately corrected for environmental conditions and trip characteristics). The emissions from individual vehicle activities are summed to determine the total emission inventory.
We find that the estimated emissions for heavy-duty diesel trucks are highly uncertain due principally to the following: 1) test methods used to determine emission rates are somewhat imprecise and inaccurate, 2) the activity-specific emission rates employed are not representative of actual vehicle activity, 3) highly questionable activity-specific emission rate correction factors are employed, 4) the application of "typical" or "average" emission factors to specific vehicle activity does not represent the diversity of vehicle activity, 5) emission rates are based upon non-statistically-representative numbers of vehicles, 6) vehicle activity parameters are highly aggregated, 7) the actual vehicle activities being monitored are not representative of emission producing activities, 8) questionable surrogate indicators are used to estimate actual activity parameters (e.g. traffic counts used to estimate VMT are suspect), and 9) a number of critical vehicle activities are omitted from the models.
Policy analysts are faced with evaluating emission control measures for which the existing models are the only analytical tools available. However, the modeling results are highly uncertain because the models were only designed to roughly estimate a "bulk" emission inventory, and were never designed to evaluate policy issues in the manner that they are often employed. Significant modeling and data collection improvements are needed for analyzing changes in vehicle activity and activity-specific emission rates.