Glossary


Acid Deposition

– A comprehensive term for the various ways acidic compounds precipitate from the atmosphere and deposit onto surfaces. It can include: (1) wet deposition by means of acid rain, fog, and snow; and (2) dry deposition of acidic particles (aerosols).

Acute Exposure

– One or a series of short-term exposures generally lasting less than 24 hours.

Acute Health Effect

– A health effect that occurs over a relatively short period of time (e.g., minutes or hours). The term is used to describe brief exposures and effects that appear promptly after exposure.

Advanced Technology Partial Zero Emission Vehicle (AT-PZEV)

– A vehicle that meets the PZEV emissions and warranty standards and, additionally, makes use of ZEV-enabling clean technology such as alternative fuels, electric drive, or other advanced technology systems

Adverse Health Effect

– A health effect from exposure to air contaminants that may range from relatively mild temporary conditions, such as eye or throat irritation, shortness of breath, or headaches to permanent and serious conditions, such as birth defects, cancer or damage to lungs, nerves, liver, heart, or other organs.

After-Treatment Devices

– Devices which remove pollutants from exhaust gases after the gas leaves combustion chamber (e.g., catalytic converters or diesel particulate filters). The term “exhaust gas aftertreat-



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State and Federal Standards for Mobile-Source Emissions Glossary Acid Deposition – A comprehensive term for the various ways acidic compounds precipitate from the atmosphere and deposit onto surfaces. It can include: (1) wet deposition by means of acid rain, fog, and snow; and (2) dry deposition of acidic particles (aerosols). Acute Exposure – One or a series of short-term exposures generally lasting less than 24 hours. Acute Health Effect – A health effect that occurs over a relatively short period of time (e.g., minutes or hours). The term is used to describe brief exposures and effects that appear promptly after exposure. Advanced Technology Partial Zero Emission Vehicle (AT-PZEV) – A vehicle that meets the PZEV emissions and warranty standards and, additionally, makes use of ZEV-enabling clean technology such as alternative fuels, electric drive, or other advanced technology systems Adverse Health Effect – A health effect from exposure to air contaminants that may range from relatively mild temporary conditions, such as eye or throat irritation, shortness of breath, or headaches to permanent and serious conditions, such as birth defects, cancer or damage to lungs, nerves, liver, heart, or other organs. After-Treatment Devices – Devices which remove pollutants from exhaust gases after the gas leaves combustion chamber (e.g., catalytic converters or diesel particulate filters). The term “exhaust gas aftertreat-

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State and Federal Standards for Mobile-Source Emissions ment” is considered derogatory by some in the emission control industry, but there is no consensus on the use of such alternatives as “post-combustion treatment” or “exhaust emission control.” Air-to-Fuel Ratio – The ratio, by weight, of air to gasoline entering the intake in a gasoline engine. The ideal (stoichiometric) ratio for complete combustion is approximately 14.7 parts of air to 1 part of fuel, depending on the composition of the specific fuel. Air Quality Management Districts (AQMDs) – Administrative districts organized in California responsible for managing air quality on a regional or county basis. California is currently divided into 35 air districts. Air-Quality Model – A computer-based mathematical model used to predict air quality based upon emissions and the effects of the transport, dispersion, and transformation of compounds emitted into the air. Air Toxics – Air toxics refers to a host of carcinogens, respiratory toxicants, neurotoxicants, and other harmful atmospheric pollutants not included as criteria air pollutants. The Clean Air Act Amendments of 1990 listed 189 of these air toxics as hazardous air pollutants (HAPs) for future regulation. Also known as hazardous air pollutants Alternative Fuel Vehicle (AFV) – Any dedicated, flexible-fuel, or dual-fuel vehicle designed to operate on at least one alternative fuel such as compressed natural gas. Ambient Air – The air outside of structures. Often used interchangeably with “outdoor air.” California Air Resources Board (CARB) – The lead air quality management agency in California consisting of an eleven-member board appointed by the governor. CARB is responsible for attainment and maintenance of the state and federal air quality standards, and is fully responsible for motor vehicle pollution control in the state. It oversees county and regional air pollution management programs. Carbon Monoxide (CO) – A colorless, odorless, tasteless, and toxic gas that results from the incomplete combustion of fuels containing carbon.

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State and Federal Standards for Mobile-Source Emissions Carboxyhemoglobin (COHb) – A molecule formed when CO reacts with hemoglobin, the intracellular protein that transports oxygen in the blood. The presence of carboxyhemoglobin increases hemoglobin’s affinity for oxygen, thereby reducing the transport of oxygen from the blood to the body’s tissues. Carcinogen – A cancer-causing substance. Carl Moyer Fund – A multimillion dollar incentive grant program in California designed to encourage reduction of emissions from heavy-duty engines. The grants cover the additional cost of cleaner technologies for on-road, off-road, marine, locomotive and agricultural pump engines, as well as forklifts and airport ground support equipment. Catalytic Converter – A mobile source emissions control device designed to reduce emissions of nitrogen oxides, hydrocarbons, and carbon monoxide. (See also “Two-Way Catalytic Converter” and “Three-Way Catalytic Converter”.) Chronic Exposure – Long-term exposure, usually lasting one year to a lifetime. Chronic Health Effect – A health effect that occurs over a relatively long period of time (e.g., months or years). (See also “Acute Health Effect”.) Clean Air Act (CAA) – The original Clean Air Act in the US was passed in 1963, but the national air pollution control program is based on the 1970 version of the law. The 1990 Clean Air Act Amendments (CAAA90) are the most recent revisions of the law. Closed Loop Fuel Control – A fuel metering system that uses real time feedback on combustion conditions for more effective emissions control. The closed loop fuel metering system of a contemporary vehicle uses sensors in the exhaust to evaluate the mixture exiting the engine and the catalyst to make adjustments to the air/fuel ratio through the use of an on-board computer that optimizes emissions performance. Cold Start Emissions – Tailpipe emissions that occur before a vehicle is fully warmed-up. Vehicle emissions are higher during initial operations

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State and Federal Standards for Mobile-Source Emissions because the engine and catalytic converter must come to operating temperature before the emissions control system become effective. The time to catalyst “light off,” when the catalyst is fully operational, is dependent on ambient temperature and vehicle technologies. Compressed Natural Gas (CNG) – Natural gas compressed to a volume and density that is practical as a portable fuel supply. Compression Ignition (CI) – A form of ignition that initiates fuel combustion in a diesel engine. The rapid compression of air within the cylinders generates the heat required to ignite the fuel as it is injected. Criteria Air Pollutants – An air pollutant for which acceptable levels of exposure can be determined and for which National Ambient Air Quality Standards have been set. There are six common air pollutants (carbon monoxide, lead, nitrogen dioxide, ozone, particulate matter, and sulfur dioxide) that have been designated as criteria pollutants. The Clean Air Act states that the presence of criteria pollutants “in the ambient air results from numerous or diverse mobile or stationary sources. The term “criteria air pollutants” derives from the requirement that EPA must describe the characteristics and potential health and welfare effects of these pollutants. EPA periodically reviews new scientific data and may propose revisions to the standards as a result. Diesel Oxidation Catalyst (DOC) – Catalyst promoting oxidation processes in diesel exhaust. Usually designed to reduce emissions of the organic fraction of diesel particulates, gas-phase hydrocarbons, and carbon monoxide. Diesel Particulate Filter (DPF) – A device which physically captures diesel particulates preventing their discharge from the tailpipe. Collected particulates need to be removed from the filter, usually by continuous or periodic oxidation in a process called “regeneration.” Diesel Particulate Matter (DPM) – Sub-micron size particles found in diesel exhaust. Most emission regulations specify DPM measurement methods in which particulates are sampled on filters from cooled exhaust gas. The cooling causes condensation of vapors in the gas sampling train. Thus, the DPM is composed of both solid and liquid particles and is generally classified into three fractions: (1) inorganic carbon (soot), (2) or-

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State and Federal Standards for Mobile-Source Emissions ganic fraction (often referred to as SOF or VOF), and (3) sulfate fraction (hydrated sulfuric acid). Direct Injection – With direct injection in diesel engines, the combustion chamber is not divided and fuel is injected directly to the cylinder. Design Standard – A technology-based standard that requires emitters to use a specifies technology to control emissions of a pollutant. These can also be called engineering standards. Design Value – The monitored reading used by EPA to determine an area’s air quality status; e.g., for the 1-hr ozone standard, the fourth highest reading measured over the most recent three years is the design value. Dose – The amount of a contaminant that is absorbed or deposited in the body of an exposed person for an interval of time--usually from a single medium. Total dose is the sum of doses received by interactions with all environmental media that contain the contaminant. Units (mass) of dose and total dose are often converted to units of mass or contaminant per volume of physiological fluid or mass of tissue. Dynamometer – A treadmill-like machine that allows cars to be tested under the loads typical of on-road driving. Electronic Control Module (ECM) – A microprocessor that determines the beginning and end of each injection cycle on every cylinder. The ECM determines both fuel metering and injection timing in response to such parameters as engine crankshaft position and rpm, engine coolant and intake air temperature, and absolute intake air boost pressure. Elemental Carbon (EC) – Inorganic carbon, as opposed to carbon in organic compounds, sometimes used as a surrogate measure for diesel particulate matter, especially in occupational health environments. Elemental carbon usually accounts for 40-60% of the total DPM mass. Emissions Budget – Allowable emissions levels identified as part of a state implementation plan for pollutants emitted from mobile, industrial, stationary, and area sources. These emissions levels are used for meeting emission reduction milestones, attainment, or maintenance demonstrations.

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State and Federal Standards for Mobile-Source Emissions Emissions Factor – For mobile sources, the emission factor is the relationship between the amount of pollution produced and the number of vehicle miles traveled. For stationary sources, the relationship between the amount of pollution produced and the amount of raw material processed or burned. By using the emission factor of a pollutant and specific data regarding activities (quantities of materials used by a given source or number of miles traveled), it is possible to compute emissions for the source. Emissions Inventory – An estimate of the amount of a pollutant emitted into the atmosphere from major mobile, stationary, area-wide, and natural sources over a specific period of time such as a day or a year. Emission Rate – The weight of a pollutant emitted per unit of time (e.g., tons / year). Environmental Protection Agency (EPA) – The federal governmental agency that establishes regulations and oversees the enforcement of laws related to the environment. Ethanol – Ethyl-alcohol, a volatile alcohol containing two carbon atoms (CH3CH2OH). For fuel use, ethanol is produced by fermentation of corn or other plant products. Evaporative emissions – Hydrocarbon emissions that do not come from the tailpipe of a car, but come from evaporation, permeation, seepage, and leaks in a car’s fueling system. The term is sometimes used interchangeably with non-tailpipe emissions. Ex Ante – Analysis of the effects of a policy based only on information available before the policy is undertaken. Also termed prospective analysis. Ex Post – Analysis of the effects of a policy based on information available after the policy has been implemented and its performance observed. Also termed retrospective analysis. Exceedance – An air pollution event in which the ambient concentration of a pollutant exceeds a National Ambient Air Quality Standard (NAAQS).

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State and Federal Standards for Mobile-Source Emissions Exposure – An event that occurs when there is contact at a boundary between a human and an environmental contaminant of a specific concentration for an interval of time; the units of exposure are concentration multiplied by time. Four-Stroke Engines – A type of internal combustion piston engines where the engine cycle is completed after four strokes (up or down) of the piston, which distinguishes it from the two-stroke engine. In a typical four stroke engine, the air/mixture intake occurs on the downward stroke of piston, followed by compression on upward stroke. The compressed mixture is ignited, which drives the piston down on its third stroke. Finally exhaust gases are exited on the upward fourth stroke. Federal Implementation Plan (FIP) – A plan prepared by the EPA in the absence of an approved state implementation plan that provides measures a nonattainment area 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 Dynamometer Driving Schedule, which attempts to simulate an urban driving cycle. Greenhouse Gas – Atmospheric gases such as carbon dioxide, methane, chlorofluorocarbons, nitrous oxide, ozone, and water vapor that slow the passage of re-radiated heat through the Earth’s atmosphere. 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). Hazardous Air Pollutants (HAPs) – Air toxics listed under section 112(b) of the CAAA90. Heavy-Duty Diesel Vehicle (HDDV) – An HDV using diesel fuel. Heavy-Duty Vehicle (HDV) – Any motor vehicle rated at more than 8,500 pounds GVWR or that has a vehicle curb weight of more than 6,000 pounds or a frontal area in excess of 45 square feet. This excludes

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State and Federal Standards for Mobile-Source Emissions vehicles that will be classified as medium-duty passenger vehicles for the purposes of the Tier 2 emissions standards. Hybrid Electric Vehicle (HEV) –Various types of electric vehicles that use another power source to propel the vehicle or generate power for an electric drive train, or a combination of the two types. Hydrocarbons (HC) – Organic compounds containing hydrogen and carbon. (See also Appendix B.) Inspection and Maintenance (I/M) – State emissions testing programs that attempt to identify vehicles with higher than allowable emissions or that have malfunctioning emissions control equipment and ensure that such vehicles are repaired or removed from the fleet 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 also be included. Low-Emission Vehicle (LEV) – A vehicle that meets CARB’s LEV I or LEV II standards or EPA’s Clean Fuel Vehicle standards. Low-Emission-Vehicle (LEV) Standards – Vehicles that meet the CARB low-emission-vehicle standards adopted in 1990 and covering 1994 through 2003 model year vehicles. The LEV standards for light duty vehicles include four categories: transitional low emissions vehicles (TLEV), LEV I vehicles, ultra low emissions (ULEV) vehicles, and zero emissions (ZEV) vehicles. Low-Emission-Vehicle II (LEV II) Standards – Vehicles that meet the amended CARB low-emission-vehicle standards, known as LEV II standards, adopted in 1998 and covering 2004 and subsequent model years. The LEV II standards for light-duty vehicles include six categories: LEV II vehicles, ultra-low-emission (ULEV) vehicles, super ultra-low-emission (SULEV) vehicles, partial zero-emission (PZEV) vehicles, advanced-technology partial zero-emission (AT-PZEV) vehicles, and zero-emission (ZEV) vehicles.

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State and Federal Standards for Mobile-Source Emissions Malfunction Indicator Light (MIL) – The instrument panel light used by the onboard diagnostic (OBD) system to notify the vehicle operator of an emissions related fault. The MIL is also known as the “service engine soon” or “check engine” lamp. Marginal Benefit – The additional benefit gained from one more unit of output. In terms of reducing emissions, it represents the benefits from reducing emissions by one more unit. Marginal Cost – The additional cost associated with producing one more unit of output. In terms of reducing emissions, it represents the cost of reducing emissions by one more unit. Medium-Duty Passenger Vehicle (MDPV) – A new class of vehicles introduced with the federal Tier 2 emissions standards that includes sport utility vehicles and passenger vans rated at between 8,5000 and 10,000 GVWR. Model Year – Vehicles are certified for sale, marketed, and later registered as a certain “model year” indicating the year a vehicle was produced and offered for sale. Model years typically begin in September or October of the prior year, and run for roughly 12 months. In the last decade, certain vehicles have been introduced as a ‘pull-ahead’ vehicle, appearing as early as January of the preceding year. 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. There are four elements of a NAAQS: (1) the pollutant indicator (such as PM2.5), (2) the concentration of the indicator in the air, (3) the time over which measurements are made or averaged, and (4) the statistical form of the standard used to determine the allowable number of exceedances (such as the fourth highest value over a 3-year period). National Low Emission Vehicle (NLEV) – A vehicle that meets voluntary low emissions tailpipe standards that are more stringent than can be mandated by EPA prior to model-year 2004. The NLEV program introduced California low emissions cars and light-duty trucks into the Northeast beginning in model year 1999 vehicles and the rest of the country in the model-year 2001 vehicles.

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State and Federal Standards for Mobile-Source Emissions Nonattainment Area – A geographic area designated by the EPA to have concentrations of a criteria pollutant in excess of a NAAQS at some recent time. A single geographic area may have acceptable levels of some criteria air pollutants but unacceptable levels of others; thus, an area can be both an attainment area for one pollutant and a nonattainment area for another. Nitrogen Oxides (Oxides of Nitrogen, NOx) – A general term referring to nitric oxide (NO) and nitrogen dioxide (NO2). Nitrogen oxides are formed when air is raised to high temperatures, such as during combustion or lightning, and are major contributors to smog formation and acid deposition. Onboard Diagnostic (OBD) Systems – A system incorporated into new motor vehicles to monitor the performance of emissions control devices. When the system detects a problem, an onboard computer triggers a dashboard indicator light, referred to as a malfunction indicator light (MIL), alerting the driver to seek maintenance for the vehicle. The current OBDII system also communicates its findings to repair technicians by means of diagnostics trouble codes, which can be downloaded from the computer. OBD systems do not directly measure emissions. Onboard Diagnostics Generation I (OBDI) – An onboard automotive diagnostic system required by the California Air Resources Board since 1988, which uses a microprocessor and sensors to monitor and control various engine system functions. A MIL illuminates when a malfunction is noted, but engine technicians cannot connect to the system and download trouble codes (MIL flash patterns communicate the problem). Onboard Diagnostics Generation II (OBDII) – OBDII expands upon OBDI to include monitoring of both the emissions system and sensor deterioration and to provide diagnostic information to repair technician. Open Loop Fuel Control – A system in which the air/fuel mixture is preset by design and contains no feedback correction signal to optimize fuel metering for emissions control. (See also “Closed Loop Fuel Control.”) Original Equipment Manufacturer (OEM) – Manufacturers of equipment (such as engines and vehicles) that provide the original product de-

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State and Federal Standards for Mobile-Source Emissions sign and materials for its assembly and manufacture. OEMs are directly responsible for manufacturing and modifying the products, making them commercially available, and providing the warranty. Oxygen Sensor – A sensor placed in the exhaust manifold to measure oxygen content. On some vehicles, oxygen sensors are located both before and after the catalytic converter. Oxygenated Gasoline – Gasoline containing an oxygenate, typically methyl tertiary-butyl ether (MTBE) or ethanol, intended to reduce production of carbon monoxide, a criteria air pollutant. In some parts of the country, carbon monoxide emissions from cars make a major contribution to pollution. In some of these areas, gasoline refiners must market oxygenated fuels, which typically contain 2-3% oxygen by weight. Oxygenates – Compounds containing oxygen (alcohols and ethers) that are added to gasoline 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 others. Ozone (O3) – A reactive gas whose molecules contain three oxygen atoms. It is a product of photochemical processes involving sunlight and ozone precursors, such as hydrocarbons and oxides of nitrogen. Ozone exists in the upper atmosphere (stratospheric ozone), where it helps shield the earth from excessive ultraviolet rays, as well as in the lower atmosphere (tropospheric ozone) near the earth’s surface. Tropospheric ozone causes plant damage and adverse health effects, and is a criteria air pollutant; it is a major component of smog. Partial Zero Emission Vehicle (PZEV) – Vehicles that have achieved the CARB’s cleanest tailpipe emission standard under the LEV II program, the super ultra-low-emission vehicle (SULEV) standard, and have nearly zero evaporative emissions and their emission control equipment warranted for 15 years/150,000 miles. Particulate Matter (PM) – Any material, except uncombined water, that exists in the solid or liquid droplet states in the atmosphere. Particulate matter includes wind-blown dust particles, particles directly emitted as combustion products, and particles formed through secondary reactions in the atmosphere.

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State and Federal Standards for Mobile-Source Emissions Photochemical Reaction – A term referring to a chemical reaction brought about by sunlight, such as the formation of ozone from the interaction of nitrogen oxides and hydrocarbons in the presence of sunlight. Plug-In – An electrical device used to heat the engine under extreme cold conditions in order to facilitate engine starting and reduce the time for emissions control devices to be fully operational. PM2.5 – PM2.5 refers to a subset of particulate matter collected by a sampling device with a size-selective inlet that has a 50% collection efficiency for particles with an aerodynamic diameter of 2.5 micrometers (μm). This fraction of PM penetrates most deeply into the lungs, and causes the majority of visibility reduction. PM10 – PM10 refers to a subset of particulate matter collected by a sampling device with a size-selective inlet that has a 50% collection efficiency for particles with an aerodynamic diameter of 10 μm. Primary Standard – A NAAQS for criteria air pollutants based on health effects. Purge Test – A test used to determine if fuel vapors are properly drawn from the evaporative canister and the fuel tank into the engine for combustion. If the purge system is not working properly, the evaporative canister can become saturated and vent hydrocarbons into the atmosphere. Regulatory Impact Analysis (RIA) – An analysis document produced by EPA for each major rulemaking listing the expected impacts of the rule including environmental impacts, health impacts, cost-benefit analyses, economic impacts, small business impacts. Reformulated Gasoline (RFG) – Specifically formulated fuels blended such that, on average, the exhaust and evaporative emissions of HCs and related hazardous air pollutants (chiefly benzene, 1,3-butadiene, polycyclic aromatic hydrocarbons, formaldehyde, and acetaldehyde) are significantly and consistently lower than such emissions resulting from use of conventional gasolines. The 1990 Clean Air Act amendments required sale of reformulated gasoline in the nine areas with the most severe ozone pollution problems.

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State and Federal Standards for Mobile-Source Emissions Regional Haze – The haze produced by a multitude of sources and activities which emit fine particles and their precursors across a broad geographic area. Federal regulations require states to develop plans to reduce the regional haze that impairs visibility in national parks and wilderness areas. Remote Sensing – A method for measuring pollutant concentrations from a vehicle’s exhaust with the use of a roadside monitoring device (known as remote sensing devices or RSDs). Infrared (IR) and Ultraviolet (UV) light is directed across the road and passively reflected back to detectors that monitor light intensity at characteristic wavelengths. The amount of characteristic infrared or ultraviolet light absorbed is translated into the exhaust concentration of the three regulated pollutants of interest, CO, HCs and NOx. Secondary Particulate Matter – Particulate matter that is formed in the atmosphere, and is generally composed of species such as ammonia or the products of atmospheric chemical reactions, such as nitrates, sulfates and organic material, in addition to some water. Secondary particles are distinguished from primary particles, which are emitted directly into the atmosphere. Secondary Standard – A NAAQS for criteria air pollutants based on environmental effects such as damage to property, plants, visibility, etc. Selective Catalytic Reduction (SCR) – Term frequently used as a synonym for catalytic reduction of NOx in diesel exhaust or flue gases by nitrogen containing compounds, such as ammonia or urea. Such SCR systems are commercially available for stationary applications and are being developed for mobile diesel engines. Since “selective catalytic reduction” is a generic term which also applies to other reactions, its use may lead to confusion in some situations. Spark Ignition (SI) – The form of ignition that initiates combustion in a gasoline, natural gas, or liquefied petroleum gas fueled engine or any other type of engine with a spark plug (or other sparking device). State Implementation Plan (SIP) – A detailed description of the scientific methods and emissions reduction programs a state will use to carry out its responsibilities under the Clean Air Act for complying with the

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State and Federal Standards for Mobile-Source Emissions NAAQS. The Clean Air Act requires that EPA approve each SIP after the public has had an opportunity to participate in its review and approval. Super Ultra-Low-Emission Vehicle – Vehicles meeting CARB’s super ultra-low-emission vehicle (SULEV) standard, the cleanest emission standard that a gasoline vehicle can meet under the LEV II program. Supplemental Federal Test Procedure (SFTP) – The SFTP is a certification 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 that evaluates the effects of simulating air conditioner operation (SC03 cycle). Technology-Based Standards – A type of standard that dictates polluters use specific techniques (e.g., a particular type of pollution abatement equipment) or follow a specific set of operating procedures and practices. Technology Forcing – The establishment by a regulatory agency of a requirement to achieve an emissions limit, within a specified time frame, that can be reached through use of unspecified technology or technologies that have not yet been developed for widespread commercial applications and have been shown to be feasible on an experimental or pilot-demonstration basis. Temperature Inversion – An atmospheric condition in which temperature in the lower part of the atmosphere increases with altitude, rather than decreasing with altitude, as is more typical. Inversion conditions can trap pollution near the surface because warmer, less dense air is resting above colder, more dense air. Three-way Catalytic Converter – A catalytic converter designed to both oxidize CO and HCs and reduce NOx emitted from gasoline-fueled vehicles. Tier 0 Vehicles – Vehicles that meet federal Tier 0 tailpipe standards. For light-duty vehicles, these standards began with model-year 1981 vehicles and were phased out in model-year 1995 for passenger cars and most light-duty trucks.

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State and Federal Standards for Mobile-Source Emissions Tier 1 Vehicles – Vehicles that meet federal Tier 1 tailpipe standards. For light-duty vehicles, these standards began with model-year 1994 vehicles. Tier 2 Vehicles – Vehicles that will meet Tier 2 tailpipe standards. For light-duty vehicles, these standards begin with model-year 2004 vehicles. Total Carbon – The sum of the elemental carbon and organic carbon. For diesel particulates, this is typically 80-85% of the total DPM mass. Transitional-Low-Emission Vehicle (TLEV) – A vehicle meeting either EPA’s clean fuel vehicle TLEV standards or CARB’s California low-emission vehicle (LEV I) TLEV standards. TLEVs produce lower emissions than federal Tier 1 vehicles. Transportation Control Measure (TCM) – Any control measure to reduce 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 carpools and mass transit. Transportation Demand Management (TDM) Strategies – Strategies which use regulatory mandates, economic incentives, or educational campaigns to change driver behavior. TDM strategies attempt to reduce the frequency or length of automobile trips or to shift the timing of automobile trips. Travel-demand Model – An 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 specific geographical area. The model determines the amount of transportation activity occurring in a region based on an understanding of the daily activities of individuals and employers, as well as the resources and transportation infrastructure available to households and individuals when making their daily activity and travel decisions. Two-Stroke Engines – A type of internal combustion piston engines where the engine cycle is completed after just two strokes (up or down) of the piston, which distinguishes it from the four-stroke engine. In a typical two-stroke engine, the air/fuel mixture is drawn into the crank-

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State and Federal Standards for Mobile-Source Emissions case as the piston moves up on its first stroke to compress the mixture above it. Then the compressed mixture is ignited, and hot gases are produced, which drive the piston down on its second stroke. As it moves down, it uncovers an opening (port) that allows the fresh fuel mixture in the crankcase to flow into the combustion space above the piston. At the same time, the exhaust gases leave through another port. Two-way Catalytic Converter – A first generation catalytic converter designed to oxidize CO and HC emissions from gasoline-fueled vehicles. Ultra-Low Emission Vehicle (ULEV) – A vehicle meeting CARB’s California low-emission vehicle ULEV standards or EPA’s clean fuel vehicle ULEV standards. ULEVs produce fewer emissions than LEV I or LEV II vehicles. Vehicle Miles Traveled (VMT) – The number of miles driven by a fleet of vehicles over a set period of time, such as a day, month, or year. One vehicle traveling one mile is one vehicle mile. Volatile Organic Compounds (VOCs) – Organic compounds that can include oxygen-, nitrogen-, and sulfur-containing compounds. Alkanes, alkenes and aromatic hydrocarbons are all VOCs (as well as being HCs). The simple carbon-containing compounds carbon monoxide and carbon dioxide are usually classified as inorganic compounds. A volatile organic compound is one that can exist as a gas at ambient temperatures. Many volatile organic chemicals are hazardous air pollutants; for example, benzene causes cancer. (See also Appendix B.) Zero Emission Vehicle (ZEV) – A vehicle that emits no tailpipe exhaust emissions. Sources: CARB at www.arb.ca.gov/html/gloss.htm; EPA at www.epa.gov/oar/oaqps/peg_caa/pegcaa10.html, and www.epa.gov/oms/stds-ld.htm.