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4 requirements for data reporting, and (3) the role of agen- and/or long-term human health effects. The federal govern- cies overseeing the data collection. The data collection ment, via the EPA has established national ambient air qual- process has an effect on the complexity of methods because ity standards (NAAQS) for the following six pollutants: modeling might be required to compensate for a lack of available data (e.g., if vehicle activity is not collected by 1. Ground-level ozone (O3), vehicle type, alternate methods are necessary to estimate the 2. Carbon monoxide (CO), share activity by vehicle type). Additionally, if different data 3. Particulate matter (PM) less than 10 (PM10) and 2.5 (PM2.5) sources and models are based on different levels of data de- microns, tail, the integration of data types and the application of data 4. Nitrogen dioxide (NO2), by models could also become more complex; and 5. Sulfur dioxide (SO2), and System boundaries: the issue of system boundaries is espe- 6. Lead (Pb). cially critical for the modes that have an international seg- ment, such as marine and aviation. Allocation of emissions When specifically discussing diesel emissions, PM is often or fuel use to a specific system boundary may be difficult in referred to as diesel PM (DPM). Other emissions inventories cases where the fuel used in a region was not purchased in measure larger classes of nitrogen oxides (NOX) and sulfur that same region, as it is the case in the rail, marine, and oxides (SOX). NAAQS values typically are the maximum av- aviation sectors. erage level of ambient concentration acceptable under the law; in some cases, states may set more stringent standards or For most modes, the discussion is divided in three geo- include other pollutants than those listed here. graphic scales: national, regional, and project level. Because the Although not a criteria pollutant, organic species are often two main national methods to estimate emissions--Inventory considered along with criteria pollutants because they are of U.S. Greenhouse Gas Emissions and Sinks (hereafter referred chemical precursors for ground-level ozone. Depending on to as the EPA GHG Inventory) (1) and the National Emissions the report or methodology, these gases are referred to in var- Inventory (the NEI) (2)--include all modes, the discussion of ious forms as volatile organic compounds (VOCs), reactive national methods is done separately from the mode-specific organic gases (ROG), total organic gases (TOG), hydrocar- discussions. Regional and project-level methods are mode bons (HC), total hydrocarbons (THC), non-methane hydro- specific, so they are examined by transportation mode. carbons (NMHC), and diesel exhaust organic gases (DEOG). (Each has a specific definition depending on which species is included in the group but, in general, all are involved in reac- 1.4 Pollutants of Concern tions with NOx to form ozone. Strictly, total organic gases Pollutants of concern in this study include greenhouse gases, and total hydrocarbons contain species considered to be non- criteria pollutants, and toxic air pollutants. reactive, but may be grouped here for practicality.) Although each term defines specific subsets of VOCs, references to these terms in various methodologies all refer to the same class of 1.4.1 Greenhouse Gases VOC pollutants. Also, PM typically is expressed as primary Carbon dioxide (CO2), the primary greenhouse gas (GHG) PM (i.e., the amount emitted directly), as opposed to second- associated with the combustion of diesel (and other fossil ary PM, which is formed in the air from chemical reactions fuels), accounts for over 95% of the transportation sector's involving ammonia and other species. global warming potential-weighted GHG emissions. Methane (CH4) and nitrous oxide (N2O) together account for about 1.4.3 Toxic Air Pollutants 2% of the transportation total GHG emissions. Both gases are released during fuel consumption, although in much smaller Toxic air pollutants, also known as air toxics, hazardous air quantities than CO2, and are also affected by vehicle emis- pollutants (HAPs), toxic air contaminants (TACs), mobile sions control technologies. (3) More information on GHG source air toxics (MSATs), and non-criteria air pollutants pollutants, including sources, and methods to calculate emis- (NCAPs), are contaminants found in ambient air that are sions, is presented in Section 3.1.1. known or suspected to cause cancer, reproductive effects, birth defects, other health effects, or adverse environmental effects, but do not have established ambient air quality stan- 1.4.2 Criteria Pollutants dards. HAPs may have short-term and/or long-term expo- Criteria air pollutants (CAPs) are those for which either the sure effects. federal government and/or the California state government EPA currently has implemented programs to reduce emis- have established ambient air quality standards based on short- sions of 188 HAPs, (4) however 1,033 total HAPs are listed by