A typical life cycle analysis includes analysis of the inventory of environmental releases, evaluation of potential environmental impacts, and interpretation of the inventory analysis and impact evaluation. Environmental releases include emissions to air, water, and solid waste emissions; these are defined in document 14040 of the International Standards Organization (ISO), Environmental Management, Life-Cycle Assessment—Principles and Framework. Prior fuel cycle analyses of vehicle/fuel systems have focused primarily on air emissions and energy use. For air emissions, studies often include the following criteria:
1. Greenhouse gas emissions: carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and several other gases, including hydrofluorocarbons, perfluorocarbons, and sulfur hexafluorides.
2. Criteria pollutant emissions: volatile organic compounds (VOCs), carbon monoxide (CO), nitrogen oxides (NOx), particulate matter [usually smaller than 10 mm (PM10)], and sulfur oxides (SO*).
3. Emissions of air toxics: acetaldehyde, benzene, 1,3-butadiene, and formaldehyde.
4. Energy use parameters: total energy use, fossil energy use, and petroleum use.
To estimate CO2-equivalent GHG emissions, the GHGs are often combined with their global warming potentials (GWPs) as described by the Intergovernmental Panel on Climate Change (IPCC) in its 2001 report, Climate Change 2001: The Scientific Basis, Technical Summary of the Group I Report. The three major GHGs for evaluating vehicle/fuel systems are CO2, CH4, and N2O. The GWPs of the greenhouse gases vary with the time horizon used to calculate them. With a time horizon of 100 years, GWPs are 1, 23, and 296 for CO2, CH4, and N2O, respectively.
Emissions of criteria pollutants are regulated in the United States by the EPA (the term ‘‘criteria pollutants’’ comes from the EPA’s establishment of criteria for ambient concentrations and emissions of these pollutants). These pollutants primarily cause adverse health effects. Thus, the locations in which they are emitted as well as their amounts are important. Total emissions are emissions that occur everywhere, including unpopulated or sparsely populated regions, and urban emissions are those that occur in urban areas with large populations.
Air toxics are hydrocarbons identified by EPA as potential cancer-causing species. Acetaldehyde, benzene, 1,3-butadiene, and formaldehyde are the major air toxics emitted by motor vehicles. Air toxic emissions of various vehicle/fuel combinations have been assessed in a study at Argonne National Laboratory by Winebrake and colleagues.
All complete fuel cycle analyses of energy impacts include total energy use; which means that all energy sources, such as renewable energy and nonrenewable energy (i. e., fossil energy), are taken into account. Some analyses further analyze fossil energy impacts; in these studies, assessments take into account fossil energy, including petroleum, natural gas, and coal. Because fossil energy resources are finite, analyses of fossil energy use reveal the impacts of particular vehicle/fuel systems on energy resource depletion. This aspect of life cycle analysis is especially important for biofuels such as ethanol and biodiesel, for which analysis of total energy impacts may not provide adequate information because the energy in ethanol and biodiesel comes from solar energy during crop growth, and use of solar energy per se is not a concern. Some studies also include analysis of petroleum displacement effects by vehicle/fuel systems. This is an important issue for countries that rely on imported oil and seek to reduce their oil consumption.