Evaluation of Life-Cycle Air Emission Factors of Freight Transportation

Abstract

Life-cycle air emission factors associated with road, rail, and air transportation of freight in the United States are analyzed. All life-cycle phases of vehicles, infrastructure, and fuels are accounted for in a hybrid life-cycle assessment (LCA). It includes not only fuel combustion, but also emissions from vehicle manufacturing, maintenance, and end of life, infrastructure construction, operation, maintenance, and end of life, and petroleum exploration, refining, and fuel distribution. Results indicate that total life-cycle emissions of freight transportation modes are underestimated if only tailpipe emissions are accounted for. In the case of CO₂ and NO_x, tailpipe emissions underestimate total emissions by up to 38%, depending on the mode. Total life-cycle emissions of CO and SO₂ are up to seven times higher than tailpipe emissions. Sensitivity analysis considers the effects of vehicle type, geography, and mode efficiency on the final results. Policy implications of this analysis are also discussed. For example, while it is widely assumed that currently proposed regulations will result in substantial reductions in emissions, we find that this is true for NO_x emissions, because fuel combustion is the main cause, and to a lesser extent for SO₂, but not for PM₁₀ emissions, which are significantly affected by the other life-cycle phases.