Goal, Scope and Background. This study provides a life-cycle inventory of air emissions (CO2, NOx, PM10, and CO) associated with the transportation of goods by road, rail, and air in the U.S. It includes the manufacturing, use, maintenance, and end-of-life (EOL) of vehicles, the construction, operation, maintenance, and EOL of transportation infrastructure, as well as oil exploration, fuel refining, and fuel distribution. Methods. The comparison is performed using hybrid life-cycle assessment (LCA), a combination of process-based LCA and economic input-output analysis-based LCA (EIO-LCA). Results are summed by means of a common functional unit of grams of air pollutant per ton-mile of freight activity. Results and Discussion. Results show that the vehicle use phase is responsible for approximately 70% of total emissions of CO, for all three modes. This confirms that tailpipe emissions underestimate total emissions of freight transportation as infrastructure, pre-combustion, as well as vehicle manufacturing and EOL account for a sizeable share of total emissions. Depending on mode and pollutant, differences between tailpipe emissions and total systemwide emissions can range from only 4% for road transportation's CO emissions to an almost tenfold difference for air transportation's PM, emissions. Conclusion. Rail freight has the lowest associated air emissions, followed by road and air transportation. Depending on the pollutant, rail is 50-94% less polluting than road. Air transportation is rated the least efficient in terms of air emissions, partly due to the fact that it carries low weight cargo. It emits 35 times more CO2 than rail and 18 times more than road transportation on a ton-mile basis. It is important to consider infrastructure, vehicle manufacturing, and pre-combustion processes, whose life-cycle share is likely to increase as new tailpipe emission standards are enforced. Recommendation and Outlook. Emission factors, fuel efficiency, and equipment utilization contribute the most to uncertainty in the results. Further studies are necessary to address all variables that influence these parameters, such as road grade, vehicle speed, and vehicle weight. A focus on regional variation, EOL processes, fuel refining processes, terminals, as well as more accurate infrastructure allocation between freight and passenger transportation would strengthen the model.
