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30 CHAPTER 3 Evaluation of Current Methods The objective of this chapter is to evaluate the current the product of freight activity (e.g., fuel consumed, energy methods used to generate air emissions information from generated, or vehicle miles traveled) and emission factors (in freight transportation activities, including the following: grams of pollutant per measure of freight activity). Brief description of the state of the practice for the calcula- Emissions = Freight Activity Emission Factor (Equation 1) tion of emissions related to freight transportation, Depending on data availability and the complexity of ana- Analysis of the strengths and weaknesses of the main lytical methods, emissions might be calculated separately by methods and models, vehicle type or other factors that affect emission factors (e.g., Assessment of process uncertainty related to the main average speed, road level of service), and added up to a total methods and models, and by pollutant. With the exception of GHGs, which are summed Assessment of parameter uncertainty related to the inputs by multiplying their respective emissions by their global warm- required by the main methods and models. ing potential, the emissions of other pollutants are always reported separately. This chapter is organized by transportation mode, includ- Some emissions models incorporate both measures of ing heavy-duty trucks, rail, ocean-going vessels, harbor craft, freight activity and emission factors and output total emissions, cargo handling equipment, and aircraft. The following three while other emissions models are used to extract emission additional subsections, which are not dependent on mode, are factors only. also included: (1) a discussion of general methods for emis- sion calculations, (2) an evaluation of methods and models that estimate freight emissions at the national scale, and (3) an 3.1.1 Greenhouse Gases evaluation of how emissions estimates are used in air quality Transportation sources emit different gases that contribute dispersion models, health risk assessments, and other appli- to global warming, including CO2, CH4, N2O, and hydrofluo- cations. To the extent possible, each mode-specific subsection rocarbons (HFCs). Carbon dioxide is by far the most preva- is divided by geographic scale. lent GHG emitted by transportation sources. According to the This chapter is a combination of the results from Tasks 2 EPA GHG Inventory, nationally, more than 95% of trans- and 4. In Task 2, the project team examined the current state portation GHG emissions were in the form of CO2 in 2004, of practice for estimating freight transportation emissions of when measured in terms of global warming potential (i.e., CO2 criteria pollutants, air toxics, and GHGs. In Task 4, the proj- equivalent emissions). (1) The remainder of transportation ect team evaluated the current practices for estimating freight GHG emissions took the form of N2O, 2.2%; CH4, 0.1%; and emissions, including both freight activity estimates and freight HFCs, 2.3%. Note that GHG emissions typically are reported emission factors. in terms of CO2 equivalent to provide a common unit of mea- sure. Other GHGs are converted into CO2 equivalent on the 3.1 General Methods basis of their global warming potential, which is defined as the cumulative radiative forcing effects of a gas over a specified Although some methods and models are mode-specific, time horizon in comparison to CO2. Radiative forcing is the there are standard methods that can be applied to all modes. change in balance between radiation entering the Earth's at- As illustrated in Equation 1, freight emissions are generally mosphere and radiation being emitted back into space.

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31 Exhibit 3-1. Fuel types commonly used by different transportation modes. Heavy- Cargo Waterborne Fuel Duty Rail Handling Aircraft Vessels Trucks Equipment Motor Gasoline Diesel (Distillate) Jet Fuel Aviation Gasoline Residual Fuel Electricity Other Fuels* *Other fuels include compressed natural gas (CNG), liquefied petroleum gasoline (LPG), and other alternative fuels. Given the importance of CO2, it is usually appropriate and 100%; EPA recently recommended use of the 100% fraction acceptable for transportation GHG analyses to focus solely on for transportation for its international reporting.) The factor this gas, particularly if resources are limited and if the analysis 44/12 is the weight of CO2 in relation to the amount of car- is designed to provide a general indication of GHG impacts. bon in the fuel, assuming all carbon burned eventually oxi- A summary of the fuel types commonly used by various dizes to form CO2. Some carbon in fossil fuels is emitted in modes is provided in Exhibit 3-1. the form of carbon monoxide, which swiftly decays into CO2, and volatile organic compounds, which also decay into CO2. Carbon Dioxide Consequently, the key analysis that needs to be conducted to estimate CO2 is to determine the amount of fuel consumed The calculation procedures for estimating CO2 from on- by fuel type (e.g., motor gasoline, diesel, jet fuel, compressed road and nonroad sources are conceptually the same, since natural gas). CO2 is released in direct proportion to fuel consumption, Although conceptually simple, this calculation in practice with differences in the amount of emissions by fuel type. The is quite complex since transportation agencies do not typi- carbon content of a specific fuel (e.g., diesel) is the same re- cally collect data to track vehicle fuel consumption by fuel gardless of what mode consumes it (e.g., trucks, locomotives, type. In a limited number of cases, fuel data are available and ships). However, the tools available to analyze emissions from can be used directly in calculating CO2. For instance, for nonroad sources differ from those that can be used for exclu- GHG inventory development, state fuel tax records are often sively assessing on-road emissions. Moreover, state and local used to estimate motor vehicle fuel consumption and CO2. transportation agencies often have limited data on fuel con- The availability of direct measures of fuel consumption, how- sumption by nonroad modes. ever, is generally limited for transportation agencies, and fuel The amount of CO2 produced is a product of the amount consumption estimates may not be available at all for project- of fuel combusted, the carbon content of the fuel, and the level, corridor, or regional analysis. fraction of carbon that is oxidized when the fuel is com- Transportation modeling generally focuses on estimat- busted. A simple formula for the calculation of CO2 for each ing vehicle-miles traveled (VMT) for motor vehicles, or fuel is as shown in Equation 2. passenger-miles traveled (PMT) for transit and nonroad modes. Given the primary use of VMT as a metric for trans- CO2 emitted = Fuel Combusted Carbon Content Coefficient portation activity, the other key factor necessary to estimate Fraction Oxidized ( 44 12 ) (Equation 2) vehicle fuel consumption is vehicle fuel economy (miles per Fuel combustion (in gallons for liquid fuels or cubic feet gallon). Many factors influence vehicle fuel economy, includ- for natural gas) is converted into units of energy (Btus). The ing the following: carbon content of fuel varies by type of fuel and is usually ex- pressed in terms of units of carbon per Btu. The fraction of The mix of travel by vehicle type and model year; the carbon oxidized is a lesser consideration since it has tra- Vehicle operating characteristics, such as speeds and accel- ditionally been assumed to be 99% for all fossil fuel combus- erations, and amount of idling; and tion. (Recent analyses conducted for EPA suggest that the ox- Other factors, like vehicle maintenance, tire pressure, and idation fraction for light-duty gasoline vehicles is virtually air conditioner use.

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32 The relationships between these factors and fuel economy For nonroad modes, N2O and CH4 are generally assumed are not simple. For instance, the implications of vehicle oper- to be proportional to fuel consumption, making the calcula- ating speeds on fuel consumption are not linear and depend tion relatively simple. However, with the introduction of on vehicle type and size. Consequently, an approach that as- emission control technologies to nonroad sources, such as sumes an average fuel economy by vehicle category will not retrofits of diesel transportation construction equipment, more accurately account for the effects of transportation projects detailed analysis by control technology type may be needed that address vehicle speeds and traffic flow. The effects of ve- to accurately address the impacts of these technologies on hicle operating speeds on fuel economy also vary based on the N2O and CH4. model year and age of the vehicle; for instance, studies of vehicle fuel economy taken during the 1990s show less of a drop-off in vehicle fuel economy above 55 miles per hour HFCs and Other Gases than in similar studies of vehicles during the 1970s and 1980s, HFCs are synthetic chemicals that are used in vehicle air con- due to vehicle design changes affecting aerodynamics and ditioning and refrigeration systems as alternatives to ozone- engine operating efficiency, among other factors. (38) As a depleting substances being phased out under the Montreal result, an approach that assumes a standard formula for the Protocol. Leakage of HFCs during equipment operation, ser- level of fuel consumed per mile at a certain vehicle speed will vicing, and disposal also contributes to GHGs, so the level of not accurately account for the effects of changes in vehicle HFCs released depends on factors such as air conditioning designs over time. use and amount of refrigerated transport. Finally, the transportation sector also contributes to emis- Nitrous Oxide and Methane sions of several other compounds that are believed to have an indirect effect on global warming. These include ozone, car- Like CO2, N2O and CH4 are released during fossil fuel con- bon monoxide, and aerosols. Ozone traps heat in the atmos- sumption, but in much smaller quantities. CH4 emissions are phere and prevents a breakdown of CH4, but its lifetime in the greater from alternative-fuel vehicles such as LNG trucks that atmosphere varies from weeks to months, making it difficult store natural gas as a cryogenic liquid. To prevent build-up of to estimate net radiative forcing effects. CO indirectly affects pressure, gases are vented from the cryogenic tank, leading to global warming by reacting with atmospheric constituents fugitive emissions of CH4. However, since the market share that would otherwise destroy CH4 and ozone. Aerosols are of LNG vehicles is very small, these fugitive emissions do not small airborne particles or liquid droplets that have both di- impact the overall transportation GHG inventory. The emis- rect and indirect effects on global warming. The most promi- sions rates of N2O and CH4 are not directly proportional to nent aerosols are sulfates and black carbon, or soot. Sulfate fuel consumption. N2O and CH4 emissions rates per mile aerosols also have some cooling effect by reflecting light back are affected by vehicle emissions control technologies. The newest motor vehicle emission control technologies produce into space. Scientists have not yet been able to quantify the significantly less N2O and CH4 than do early emission control impact of ozone, carbon monoxide, or aerosols with reason- technologies--for instance, for a gasoline powered automo- able certainty; thus, these compounds are not included in re- bile, a vehicle with LEV technology produces only about porting GHG emissions. one-third the N2O emissions of a vehicle with Tier I emission controls. According to EPA, (1) N2O and CH4 from on-road 3.1.2 Criteria Air Pollutants and Air Toxics sources declined by over 20% between 2000 and 2003 while VMT rose. As a result, emission factors for on-road vehicles Emissions of criteria air pollutants and air toxics are not di- are usually presented in per mile units, and analyses of these rectly proportional to fuel consumption, with emissions rates pollutants require information on VMT and the distribu- per mile being affected by vehicle emissions control technolo- tion of miles by vehicle type (e.g., automobile, light-duty gies. Therefore, emission factors for on-road vehicles are usu- truck, heavy-duty truck), fuel type (e.g., gasoline, diesel), ally presented in per mile units, and analyses of these pollu- and technology type (e.g., Tier 0, Tier I, LEV). Knowing the tants require information on VMT and the distribution of emissions control technology used by vehicles is very im- miles by vehicle type (e.g., automobile, light-duty truck, heavy- portant for generating accurate results. A simple formula for duty truck), fuel type (e.g., gasoline, diesel), and technology the calculation of N2O or CH4 emissions for each individual type (e.g., Tier 0, Tier I, LEV). Knowing the emissions con- vehicle/fuel/technology type is as shown in Equation 3. trol technology used by vehicles is very important for gener- ating accurate results. Equation 3 shows a simple formula for Emissions = VMT( Vehicle, Fuel, Technology Type) (Equation 3) the calculation of criteria air pollutants and air toxics for each Emission Factor( Vehicle, Fuell, Technology Type) individual vehicle/fuel/technology type.