Abstract:This report focuses on the sources of uncertainty in emission factors and activity estimates used by regulatory agencies to estimate the emission contribution of heavy-duty trucks. This investigation into the areas of uncertainty in the heavy-duty diesel truck emission inventory evaluates the methods and data used by regulatory agencies.

For the most part, heavy-duty diesel and gasoline truck emission inventories are more uncertain than light-duty vehicle emission inventories, due to limited heavy-duty laboratory testing of vehicles and scant knowledge about heavy-duty vehicle activity patterns. Additional concentrated research efforts are called for on all fronts of the heavy-duty emission inventory preparation methodology.

Sources of modeling uncertainty are described and discussed in this report, and a research framework designed to reduce uncertainty in the emission inventory for heavy-duty trucks is provided. The CARB is strongly encouraged to initiate its proposed research program for testing and specifying mode-specific heavy-duty truck emission rates, and emission rate correction factors. A complementary research effort to provide accurate estimates of applicable heavy-duty vehicle activities is also necessary.