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Glossary Air-quality model A computer-based mathematical model used to pre- dict air quality based upon emissions and the effects of the transport, dis- persion, and transformation of compounds emitted into the air. Arterial A roadway that serves major traffic movements, and second- arily provides access to abutting land. Arterials generally carry higher traffic volumes at higher speeds than collectors and local streets, but carry lower volumes at lower speeds than expressways, freeways, and other lim- ited access and grade separated facilities. Ambient airThe air outside of structures. Often used interchangeably with "outdoor air." California Air Resources Board (CARB) A part of the California Environmental Protection Agency whose mission it is to promote and pro- tect public health, welfare and ecological resources through the effective and efficient reduction of air pollutants while recognizing and considering the effects on the economy of the state. Carbon monoxide (CO) A colorless, odorless gas resulting from the incomplete combustion of hydrocarbon fuels. Clean Air Act (CAA) The original Clean Air Act was passed in 1963, but our national air pollution control program is actually based on the 1970 version of the law. The 1990 Clean Air Act Amendments (CAAA90) are the most recent and far-reaching revisions of the 1970 law. 226

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GtossAaY 227 Clean screening The use of methods such as remote sensing measure- ments or vehicle profiling by states to excuse cars from a scheduled inspec- tion and maintenance (I/M) emissions test. Collector A street that provides access within neighborhoods as well as commercial and industrial districts, and which channels traffic from local streets to arterials. Conformity (transportation conformity) A process to demonstrate whether a federally supported activity is consistent with the air quality goals in State Implementation Plans (SIPs). Transportation conformity demonstrates that plans, programs, and projects approved or funded by the Federal Highway Administration, or the Federal Transit Administra- tion for regionally-significant projects do not create new violations, in- crease the frequency or severity of existing violations, or delay timely at- tainment of NAAQS. General conformity refers to projects approved or funded by other federal agencies. Criteria air pollutants A group of common air pollutants (carbon mon- oxide, lead, nitrogen dioxide, ozone, particulate matter, and sulfur dioxide) regulated by the Federal Government since the passage of the Clean Air Act in 1970 on the basis of information on health and/or environmental effects of each pollutant. Dose The amount of a contaminant that is absorbed or deposited in the body of an exposed organism for an increment of time-usually from a sin- gle medium. Total dose is the sum of doses received by a person from a contaminant in a given interval resulting from interaction with ad envi- ronmental media that contain the contaminant. Units of dose and total dose (mass) are often converted to units of mass per volume of physiologi- cal fluid or mass of tissue. Dose-response The relationship between the dose of a pollutant and the response or effect it produces on a biological system. Emissions budgetAllowable emissions levels identified as part of a state implementation plan for pollutants emitted *om mobile, industrial, stationary, and area sources. These emissions levels are used for meeting emission reduction milestones, attainment, or maintenance demonstra- tions. Emissions factor The relationship between the amount of pollution produced and the amount of raw material processed or burned, or between

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228 MODELING MOBI[E-SOURCE EMISSIONS the amount of pollution produced and the activity level. By using the emis- sion factor of a pollutant and specific data regarding quantities of materi- als used by a given source or the activity of a given source, it is possible to compute emissions for the source. In the case of mobile source emissions, the product of an emission factor estimated in mass of pollutant per unit distance (e.g., grams per mile) and an activity estimate in distance (e.g., average miles traveled) produces an estimate of emissions. In the case of stationary source emissions, the product of an emission factor in mass of pollutant per unit energy (e.g., pounds per million Btu) and the amount of energy consumed produces an estimate of emissions. Emissions inventory Estimates of the amount of pollutants emitted into the atmosphere from major mobile, stationary, area-wide, and natural source categories over a specific period of time such as a day or a year. Exceedance Air pollution event in which the ambient concentration of a pollutant exceeds a National Ambient Air Quality Standard (NAAQS). Ethanol EthyI-alcohol, a volatile alcohol containing two carbon atoms (CH3CH2OH). For fuel use, ethanol is produced by fermentation of corn or other plant products. ExposureAn event that occurs when there is contact at a boundary be- tween a human and the environment with a contaminant of a specific con- centration for an interval of time; the units of exposure are concentration multiplied by time. Fast pass Fast pass is a process that recognizes very clean cars early in the IM240 test cycle and passes them without the need to complete the fuB test. Federal Implementation Plan (FIP) In the absence of an approved state implementation plan, a plan prepared by the U.S. Environmental Protection Agency (EPA) that provides measures that nonattainment ar- eas must take to meet the requirements of the Federal Clean Air Act. Federal Test Procedure (FTP)A certification test for measuring the tailpipe and evaporative emissions from new vehicles over the Urban Dy- namometer Driving Schedule, which attempts to simulate an urban driv- ing cycle. Gasoline volatilityThe evaporative properties of gasoline. Gasoline vapor contains volatile organic compounds.

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GLOSSARY 229 Gross Vehicle Weight Rating (GVWR) The value specified by the manufacturer as the maximum design loaded weight of a single vehicle (i.e., vehicle weight plus rated cargo capacity). Heavy-duty vehicles (HDV) Any motor vehicle rated at more than 8,500 pounds gross vehicle weight GVWR or that has a vehicle curb weight of more than 6,000 pounds or that has a basic vehicle frontal area in ex- cess of 45 square feet. This excludes vehicles that will be classified as medium-duty passenger vehicles for the purposes of the Tier 2 emissions standards. Heavy-duty diesel vehicles (HDDV) An HDV using diesel fuel. Inspection anti maintenance (~/M) program A periodic emissions testing and inspection program, usually done once a year or once every two years, to ensure that the catalytic or other emissions control devices on in- use vehicles are operating. The test can be an idle or dynamometer loaded mode emission test. Light-duty vehicle (LDV) A passenger car or passenger car derivative capable of seating 12 or fewer passengers. All vehicles and trucks under 8,500 GVWR are included (this limit previously was 6,000 pounds). Small pick-up trucks, vans, and sport utility vehicles may be included. Meclium-cluty passenger vehicle (MD PV) A new class of vehicles in- troduced with the Tier 2 emissions standards that includes sport utility vehicles and passenger vans rated at between 8,5000 and 10,000 GVVVR. Metropolitan Planning Organization (MPO) The organized entity designated by law with lead responsibilities for developing transportation plans and programs for urbanized areas with population of 50,000 or more people. MPOs are established by agreement of the Governor and units of general purpose local government which together represent 75 percent of the affected population of an urbanized area. Moile choiceA process by which an individual selects a transportation mode (e.g., automobile, transit, bicycle) for use on a trip, given the trip's purpose, origin, and destination. National Low Emission Vehicle (NLEV)Vehicles that meet volun- tary low emissions tailpipe standards that are more stringent than can be mandated by EPA prior to model year 2004. The NLEV program intro- duces California low emissions cars and light-duty trucks into the North-

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230 MODEL/NO MOB/NE-SOURCE EMISSIONS east beginning in model year 1999 vehicles and the rest of the country in the model year 2001 vehicles. National Ambient Air Quality Standards (NAAQS) Standards set by EPA for the maximum levels of criteria air pollutants that can exist in the outdoor air without unacceptable effects on human health or the public welfare. Nonattainment area- A geographic area in which the level of a criteria air pollutant is higher than the level allowed by the federal standards. A single geographic area may have acceptable levels of one criteria air pol- lutant but unacceptable levels of one or more other criteria air pollutants; thus, an area can be both attainment and nonattainment at the same time. Nitrogen oxides (Oxides of Nitrogen, NOX) A general term pertain- ing to nitric oxide (NO), nitrogen dioxide (NO2), and other oxides of nitro- gen. Nitrogen oxides are typically created during combustion processes, and are major contributors to smog formation and acid deposition. On-Board Diagnostic (OBD) systems Devices that are incorporated into the computers of new motor vehicles to monitor fuel delivery and emission controls. The computer triggers a dashboard indicator light when the controls malfunction, alerting the driver to seek maintenance for the vehicle. Diagnostic systems are required on vehicles beginning with 1994 models. Oxygenated gasoline (oxyfuel) Gasoline containing oxygenates, typi- cally methyl tertiary-butyl ether (MTBE) or ethanol, that burns more completely than regular gasoline and reduces production of carbon monox- ide, a criteria air pollutant. In some parts of the country, carbon monoxide emissions from cars make a major contribution to pollution. In these ar- eas, gasoline refiners must market oxygenated fuels, which contain a high- er oxygen content than regular gasoline. For oxygenated gasoline pro- grams to reduce carbon monoxide (CO) pollution, the minimum oxygen content is typically 2.7 weigh percent OxygenatesCompounds containing oxygen (alcohols and ethers) that are added to fuels to increase its oxygen content. Methyl tertiary-butyl ether (MTBE) and ethanol are the most common oxygenates currently used, although there are a number of other oxygenates. Ozone A reactive gas consisting of three oxygen atoms. It is a product of

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GLOSSARY 23 7 photochemical process involving the sunlight and ozone precursors, such as hydrocarbons and oxides of nitrogen. Ozone exists in the upper atmo- sphere ozone layer (stratospheric ozone) as well as at the earth's surface (tropospheric ozone). Tropospheric ozone causes plant damage and adverse health effects and is a criteria air pollutant. Tropospheric ozone is a major component of smog. Particulate matter (PM) Any material, except uncombined water, that exists in the solid or liquid state in the atmosphere. The size of particulate matter can vary from coarse, wind-blown dust particles to fine particles directly emitted as combustion products or formed through secondary reac- tions in the atmosphere. Photochemical reaction A term referring to chemical reactions brought about by the light of the sun. The reaction of nitrogen oxides with hydrocarbons in the presence of sunlight to form ozone is an example of a photochemical reaction. PM-2.5 A subset of particulate matter that includes those tiny particles with an aerodynamic diameter less than or equal to a nominal 2.5 microm- eters. This fraction of particulate matter penetrates most deeply into the lungs, and causes the majority of visibility reduction. PM-10 A major air pollutant consisting of small particles with an aero- dynamic diameter less than or equal to a nominal 10 micrometers (about 1/7 the diameter of a single human hair). Their small size allows them to make their way to the air sacs deep within the lungs where they may be deposited and result in adverse health effects. PM10 also causes visibility reduction. Preconditioning A term used in inspection and maintenance programs that use the IM240 test cycle. The cut points, which determine passing or failing for such a vehicle, are based on testing a fully warmed-up vehicle in which the emissions control equipment, including the catalytic converter, are hot and fully functional. If an owner drives a short distance to the test station or if the vehicle has to wait in the test station for a long time, the vehicle may not be fully warmed up. This may result in a false reading; a car which would have passed if fully warmed (i.e., fully preconditioned) would fail. Thus, a preconditioned vehicle is a vehicle that is fully warmed up so that it can give a valid result from an IM240 inspection test. Primary standardA NAAQS for criteria air pollutants based on health effects.

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232 MODEl/NG MOB/[E-SOURCE EMISSIONS Reformulated gasoline (RFG)Specifically formulated fuels blended such that, on average, the exhaust and evaporative emissions of VOCs and hazardous air pollutants (chiefly benzene, 1,3-butadiene, polycycTic organic matter, formaldehyde, and acetaidehyde) resulting from RFG use in motor vehicles are significantly and consistently lower than such emissions re- sulting from use of conventional gasolines. The 1990 Clean Air Act Amend- ments requires sale of reformulated gasoline in the nine areas with the most severe ozone pollution problems. RFG contains, on average, a mini- mum of 2.0 weight percent oxygen. Remote sensing A method for measuring pollution levels in a vehicle's exhaust while the vehicle is traveling down the road. Remote sensing sys- tems employ an infrared absorption principle to measure VOC and carbon monoxide emissions relative to carbon dioxide emissions. These systems typically operate by continuously projecting a beam of infrared radiation across a roadway. As a vehicle passes through the beam, the device mea- sures the ratios of CO and VOC to carbon dioxide in the vehicle exhaust plume. Reid vapor pressure (RVP) fuel vapor pressure (often expressed in units of pounds per square inch) at 100 degrees F. Secondary particle Particulate matter that is formed in the atmo- sphere. Secondary particles are generally composed of species such as am- monia and the products of atmospheric chemical reactions including ni- trates, sulfates and organic material. Secondary particles are distin- guished from primary particles, which are emitted directly into the atmo- sphere. Secondary standard A NAAQS for criteria air pollutants based on en- vironmental effects such as damage to property, plants, visibility, etc. Speed-correction factor (SCF)- Factors used in the MOBILE model to adjust emissions factors from the average speed used in the Federal Test Procedure (which is used to obtain emissions data) to other average speeds as driven by vehicles in the geographical area being modeled. Supplemental Federal Test Procedure (SFTP) The SFTP is a certif- ication test for measuring the tailpipe and evaporative emissions from new vehicles that includes two driving cycles not represented in the FTP. The SFTP includes a test cycle simulating high speed and high acceleration driving (US06 cycle) and a test cycle simulating air conditioner operation (SC03 cycle).

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G1OSSARY 233 State implementation plan (SIP)A detailed description of the pro- grams a state will use to carry out its responsibilities under the Clean Air Act for complying with the NAAQS. SIPs are a collection of the programs used by a state to reduce air pollution. The Clean Air Act requires that EPA approve each SIP. The public is given opportunities to participate in review and approval of SIPs. Two-way catalytic converter- First generation catalytic converters designed to reduce CO and VOC emissions from gasoline-fueled vehicles. Three-way catalytic converter Catalytic converters designed to re- duce CO. VOC and NOX emissions from gasoline-fueled vehicles Tier O vehicles- Vehicles that meet Tier O tailpipe standards. For light- duty vehicles, these tailpipe standards began with model year 1981 vehi- cles and were phased out in model year 1995 for passenger cars and most light-duty trucks. Tier 1 vehiclesVehicles that meet Tier 1 tailpipe standards. For light- duty vehicles, these tailpipe standards began with model year 1994 vehi- cles. Tier 2 vehicles Vehicles that will meet Tier 2 tailpipe standards. For light-duty vehicles, these standards would not begin until model year 2004 vehicles. Travel-demand modelan analysis procedure using heuristics or formal systems of equations to estimate the number, distribution, mode choice, and/or route choice of trips made by a household or individual that can be aggregated to estimate the number of trips starting and/or ending in a spe- cific geographic area. The model determines the amount of transportation activity occurring in a region based on an understanding of the daily activ- ities of individuals and employers as well as the resources and transporta- tion infrastructure available to households and individuals when making their daily activity and travel decisions. Transportation Control Measure (T CM) Any control measure to re- duce vehicle trips, vehicle use, vehicle miles traveled, vehicle idling, or traffic congestion for the purpose of reducing motor vehicle emissions. TCMs can include encouraging the use of carp ools and mass transit. Transportation Improvement Program (TIP) Also known as a transportation program, a TIP is a short-term prioritized list of projects

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234 MODELING MOBI[E-SOURCE EMISSIONS (covering 3 years out at a minimum and updated at least every 2 years) proposed to be funded or approved by the Federal Highways Administra- tion or Federal Transit Authority. The TIP-listed projects are drawn from or consistent with the long-range transportation plan. Transportation PlanA long-range plan that identifies facilities that should function as an integrated transportation system. Under the Intermodal Surface Transportation Efficiency Act of 1991, MPOs must have transportation plans in place that present a 20-year perspective on transportation investments for their region. The transportation plan gives emphasis to those facilities that serve important national and regional transportation functions, and includes a financial plan that demonstrates how the long-range plan can be implemented. Urban Airshed Model (UAM)An air quality model that is a three-di- mensional photochemical grid model calculating the concentrations of both inert and chemically reactive pollutants in the atmosphere. It simulates the physical and chemical processes that affect pollution concentrations. Vehicle miles traveler] (VMT)- The number of miles driven by a single vehicle, or by a fleet of vehicles over a set period of time, such as a day, month, or year. Volatile organic compounds (VOCs) An organic compound is a com- pound containing carbon combined with atoms of other elements, com- monly hydrogen, oxygen, and nitrogen. Simple carbon-containing com- pounds such as carbon monoxide and carbon dioxide are usually classified as inorganic compounds. A volatile organic compound is a compound that can exist as a gas under typical atmospheric conditions. Many volatile organic chemicals are hazardous air pollutants; for example, benzene causes cancer. See Appendix B for details of how the term VOC is used in this report. Zero Emission Vehicle (ZEV) Vehicles which produce no emissions from the on-board source of power, e.g., an electric vehicle. Sources: California Air Resources Board at www.arb.ca.gov/htmI/gloss. him; U.S. EPA at www.epa.gov/oar/oagps/peg_caa/pegcaalO.htmI; U.S. EPA at www.epa.gov/oms/stds-ld.htm; Harvey and Deakin 1993; Davis 1997; FHWA 1997.