| This study provides a life-cycle inventory of air emissions (CO 2, NOx, PM10, CO, SO2, and Pb) associated with the transportation of goods by road, rail, and air in the U.S. It includes the manufacturing, use, maintenance, and EOL phases of vehicles, construction, operation, maintenance, and EOL of transportation infrastructure, as well as oil exploration, refining, and fuel distribution.; This is the first comprehensive study to compare road, rail, and air transportation of goods in the U.S. in terms of life-cycle emissions. It benefits those performing environmental research on transportation, including the internalization of social costs. Governments will receive input for the development of transportation policies that support more effective decision-making. As consumers become more environmentally aware, and governments start promoting regulations to internalize environmental externalities into transportation prices, environmental criteria will play a bigger role in transportation decisions by manufacturers and carriers. Additionally, more comprehensive information on life-cycle air emissions is useful to support the development of more accurate LCAs of diverse products.; Hybrid life-cycle assessment (LCA), a combination of process-based LCA and economic input-output analysis-based LCA (EIO-LCA), is the methodology used for the comparison. All the components are summed by a common functional unit of tons of air pollutant per ton-mile of freight activity. Even though results reflect a baseline scenario, the model also enables the differentiation of results based on alternative vehicle configurations (e.g., vehicle sizes), business practices (e.g., vehicle utilization, empty backhaulage), and geography (e.g., energy mix, road grade, rail track grade).; Depending on the pollutant, rail transportation rates 40-90% better than road transportation. Air transportation scored the lowest in terms of environmental performance, being from two to sixteen times worse than road transportation. However, the ranking can change by accounting for intra-modal variations (e.g., small trucks versus large trucks) and uncertainty. This is the case for CO and SO2 where road and rail overlap, as well as for PM10 and Pb where air and road overlap. The ranking of modes can also change for some pollutants depending whether the analysis accounts for total emissions or only fuel combustion emissions. This is the case of CO and SO2 emissions, where rail scores worse than road transportation if only fuel combustion is included in the analysis.; Results confirm that total emissions are significantly underestimated if only fuel combustion emissions are accounted for. Fuel combustion accounts for 70-90% of total emissions of CO2. NOx emissions tend to be more concentrated in the fuel combustion phase (70-98%). Infrastructure is the predominant phase for PM10 emissions due to construction processes, especially for road and air transportation. Due to regulations of CO emissions from road transportation, truck manufacturing dominates CO emissions from road transportation. Fuel combustion is still the predominant phase for CO emissions from rail transportation due to lack of emission standards. Airport operations rate high in CO emissions due to the operation of ground support equipment. In terms of SO2 emissions, road and air infrastructure are the predominant phases due to road construction processes, and ground support equipment, respectively. Fuel combustion is representative in rail transportation due to lack of fuel sulfur reduction standards. Due to the use of unleaded fuel, vehicle manufacturing is responsible for the majority of lead emissions for all three modes. Differences between tailpipe emissions and total systemwide emissions can range from 2% for rail transportation's NOx emissions to an almost twenty-threefold difference for air transportation's PM10 emissions. Therefore, it is of paramount importance to consider infrastructure, vehicle manuf... |
