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.
A computer-based mathematical model used to predict air quality based on emissions and the effects of the transport, dispersion, and transformation of compounds emitted into the air.
Air toxics is a generic term referring to a host of carcinogens, neurotoxins, allergens, and other harmful atmospheric pollutants. The Clean Air Act Amendments of 1990 listed 189 of these air toxics as hazardous air pollutants (HAPs) for future regulation.
Alaska Department of Environmental Conservation (ADEC)—
The department that deals with clean air, land, and water issues in the state of Alaska.
The fraction of incoming sunlight that is reflected from the surface of the earth. Albedo is higher over whiter surfaces, such as snow, and lower over darker surfaces, such as oceans and forests.
The air outside of structures. Often used interchangeably with “outdoor air.”
A model that simulates how pollutant concentrations vary over time within a designated volume of air. The effects of emissions, exchange with the surrounding atmosphere, and chemical reactions are considered. Concentrations are assumed to be well-mixed within the box.
Carbon monoxide (CO)—
A colorless, odorless, tasteless, and toxic gas resulting from the incomplete combustion of fuels containing carbon.
Molecule formed when CO reacts with hemoglobin, an intracellular protein that transports oxygen in the blood. The presence of carboxyhemoglobin increases the hemoglobin’s affinity for oxygen, thereby reducing the transport of oxygen from the blood to the body’s tissues.
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.
Closed-loop fuel control—
A fuel-metering system that uses feedback for more effective emissions control. The air-to-fuel ratio of a contemporary vehicle is “closed-loop,” using a sensor in the exhaust to evaluate the mixture exiting the engine and adjusting the air-to-fuel ratio through the use of an onboard computer to optimize emissions performance.
Tailpipe emissions that occur before a vehicle is fully warmed up. Vehicle emissions are higher during the first few minutes of operation because the engine and the catalytic converter must come to operating temperature before they can become effective.
Criteria air pollutants—
A group of six common air pollutants (carbon monoxide, 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 the health and/or environmental effects of each pollutant.
For each pollutant, the emissions level above which a car is considered to have failed the emissions test for that pollutant.
Data link connector—
The connector where the scan tool interfaces with a vehicle’s onboard diagnostic system. Also know as the diagnostic link connector.
Diagnostic trouble codes—
Codes stored in the engine’s computer that identify emissions-control systems and components that are malfunctioning; they can be retrieved using a scan tool.
A treadmill-like machine that allows cars to be tested under the loads typical of onroad driving.
Allowable emissions levels identified as part of a state implementation plan (SIP) for pollutants emitted from mobile, industrial, stationary, and area sources. These emissions levels are used for meeting emission reduction milestones, attainment, or maintenance demonstrations.
The predicted ratio of the amount of pollution produced to the amount of raw material processed or burned or of the amount of pollution produced to the activity level. By using the emission factor of a pollutant and data regarding quantities of materials used by a given source or the activity level of a given source, it is possible to compute emissions for the source. In the case of mobile source emissions, estimated emissions are the product of an emissions factor in mass of pollutant per unit distance (e.g., grams per mile) and an activity estimate in distance (e.g., average miles traveled). In the case of stationary source emissions, estimated emissions are the product of an emissions factor in mass of pollutant per unit energy (e.g., pounds per million Btu) and the amount of energy consumed.
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.
Modeling which attempts to discern a statistically significant relationship between an outcome variable (e.g., ambient CO concentration) and various predictor variables (e.g., windspeed, temperature, and inversion strength).
See U.S. Environmental Protection Agency. Exceedance—An air pollution event in which the ambient concentration of a pollutant exceeds one of the National Ambient Air Quality Standards (NAAQS).
Hydrocarbon emissions that do not come from the tailpipe of a car. Evaporative emissions can come from evaporation, permeation, seepage, and leaks in a car’s fueling system. Often used interchangeably with nontailpipe emissions.
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.
Gaussian dispersion models—
A micro-scale model that simulates the dispersion of pollution from a source such as an intersection or a factory. The dispersion is assumed to be Gaussian in nature, and the ambient concentrations are assumed to be proportional to emissions.
Large-scale winds in the atmosphere controlled by pressure differences. Geostrophic winds are not much influenced by the surface of the earth.
Heavy-duty vehicles (HDV)—
Any motor vehicle rated at more than 8,500 lb gross vehicle weight (GVWR) or that has a vehicle curb weight of more
than 6,000 lb or that has a basic vehicle frontal area in excess of 45 ft2. 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 that uses diesel fuel.
Organic compounds containing various combinations of hydrogen and carbon.
A meteorological phenomenon that occurs in Fairbanks, Alaska, on very cold days (colder than −20°F). Tiny ice particles are formed in the air when water vapor released from cars, humans, or power plants is frozen very quickly. If an elevated inversion is present, the ice fog can be trapped near the surface.
Inspection and maintenance (I/M) programs—
State emissions testings programs that attempt to identify vehicles with higher than allowable emissions and ensure that such vehicles are repaired or removed from the fleet.
See Temperature inversion.
The name for the emissions test used in some I/M programs, including those in Arizona and Colorado. The IM240 is a transient, loaded-mode emissions test. “Loaded-mode” refers to the fact that the test is run on a treadmill-like device called a dynamometer, which simulates driving with the engine in gear. “Transient” refers to the fact that the car drives under a load that varies from second to second during the test. The “240” in IM240 indicates that the test lasts for 240 seconds. The IM240 is intended by EPA to be a shortened version of part of the FTP and to correlate well with the FTP.
The rate at which temperature in the atmosphere changes with altitude. The average lapse rate is about −6.5°C/km. Under inversion conditions, the lapse rate can be positive.
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 lb GVWR are included (this limit previously was 6,000 lb). Small pickup trucks, vans, and sport utility vehicles may be included.
Loaded-mode emissions test—
An emissions test performed with the engine in gear.
Malfunction indicator light (MIL)—
The instrument panel light used by the onboard diagnostic 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.
Medium-duty passenger vehicle (MDPV)—
A new class of vehicles introduced with the Tier 2 emissions standards that includes sport utility vehi-
cles and passenger vans rated at between 8,500 and 10,000 lb GVWR.
A layer of air near the surface in which gaseous pollutants are well-mixed.
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 “pull-ahead” vehicles, 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.
A geographic area in which the concentration of a criteria air pollutant has exceeded the concentration allowed by the federal standards at some recent time. A single geographic area may have acceptable concentrations of one criteria air pollutant but unacceptable concentrations of one or more other pollutants; thus, an area can be both an attainment area and a nonattainment area at the same time.
Nitrogen oxides (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.
Numerical predictive model—
A meso-scale or large-scale model used to predict chemical concentrations in the atmosphere based on observed meteorological variables, emissions, and chemistry. Numerical predictive models represent the atmosphere as a three-dimensional grid of air parcels. Chemical transformations take place within the air parcels, and air is transported between them.
See Oxygen sensor.
See Onboard diagnostics generation I.
See Onboard diagnostics generation II.
Onboard diagnostic system—
A device incorporated into the computers of new motor vehicles to monitor the performance of the emission controls. The computer triggers a dashboard indicator light, referred to as a malfunction indicator light, when the controls malfunction, alerting the driver to seek maintenance for the vehicle. The system also communicates its findings to repair technicians by means of diagnostics trouble codes, which can be downloaded from the vehicle’s computer. Onboard diagnostic systems do not actually measure emissions.
Onboard diagnostics generation I (OBDI)—
An onboard automotive diagnostic system required by the California Air Resources Board since 1988. It uses the microprocessor and sensors to monitor and control various engine system functions.
Onboard diagnostics generation II (OBDII)—
OBDII expands upon OBDI to include emissions-system and sensor-deterioration monitoring.
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.)
Oxygen (O2) sensor—
A sensor placed in the exhaust that measures exhaust oxygen content. Typically, there are oxygen sensors before and after the catalytic converter.
Oxygenated fuel (oxyfuel)—
Gasoline containing oxygenates, 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 those areas, gasoline refiners must market oxygenated fuels, which typically contain 2–3% oxygen by weight.
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.
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 shied the earth from excessive ultraviolet rays, as well as in the lower atmosphere near 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 solid or liquid states 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 reactions in the atmosphere.
A term referring to a chemical reaction brought about by the light of the sun. The formation of ozone from nitrogen oxides and hydrocarbons in the presence of sunlight involves photochemical reactions.
An electric device used to heat the engine in order to facilitate en-
gine starting and reduce the time for emissions-control devices to be activated.
A subset of particulate matter that includes those tiny particles with an aerodynamic diameter less than or equal to a nominal 2.5 micrometers. This fraction of particulate matter penetrates most deeply into the lungs and causes the majority of visibility reduction.
A major air pollutant consisting of small particles with an aerodynamic diameter less than or equal to a nominal ten micrometers (about one-seventh 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.
A NAAQS for criteria air pollutants based on health effects.
Process numerical model—
A meso-scale or large-scale model used to analyze atmospheric processes and their effect on air quality, typically for research purposes. Like numerical predictive models, process numerical models represent the atmosphere as a three-dimensional grid of air parcels. Process numerical models better resolve the coupling between meteorology and chemistry than do numerical predictive models and are not necessarily constrained by observations.
Reformulated gasoline (RFG)—
Specifically formulated fuel blended such that, on average, the exhaust and evaporative emissions of volatile organic carbons (VOCs) and hazardous air pollutants (chiefly benzene, 1,3-butadiene, polycyclic aromatic hydrocarbons, formaldehyde, and acetaldehyde) resulting from RFG use in motor vehicles might be significantly and consistently lower than such emissions resulting from use of conventional gasolines. The 1990 Clean Air Act Amendments require sale of RFG in the nine areas with the most severe ozone pollution problems. RFG contains, on average, a minimum of 2.0 weight percent oxygen.
A method for measuring pollution levels in a vehicle’s exhaust while the vehicle is traveling down the road. Remote-sensing systems employ infrared absorption to measure VOC and carbon monoxide emissions relative to carbon dioxide. These systems typically operate by continuously projecting a beam of infrared radiation across a roadway and making measurements on the exhaust plume after a vehicle passes through the beam.
A hand-held computer that is plugged into a vehicle’s data link connector allowing a technician to read diagnostic trouble codes, readi-
ness status, and other information collected by the onboard diagnostic system.
Participate 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. Secondary particles are distinguished from primary particles, which are emitted directly into the atmosphere.
A NAAQS for criteria air pollutants based on environmental effects such as damage to property, plants, or visibility.
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 air conditioner operation (SC03 cycle).
State implementation plan (SIP)—
A detailed description of the programs 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 the review and approval of SIPs.
Statistical roll-back model—
A model that estimates the emissions reductions needed for a desired improvement in air quality. The emissions reductions are assumed to be linearly related to ambient concentrations of pollutants.
Steady-state emissions test—
An emissions test performed under one stable operating condition, such as when a vehicle is tested at idle or under a constant engine load.
Slow descent of air due to radiative cooling.
Used to describe meteorological processes that occur over regional spatial extents and over several days.
The malfunctioning of one or more emissions-control devices due to either deliberate disablement or mechanical failure.
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 oxidize CO
and VOCs and reduce NOx emissions from gasoline-fueled vehicles.
Transient emissions test—
An emissions test performed under a load that varies from moment to moment during the test.
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 education 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.
Transportation-supply improvement (TSI) strategies—
TSI strategies attempt to reduce emissions by changing the physical infrastructure of the road system to improve traffic flow and reduce stop-and-go movements.
Two-way catalytic converter—
A first generation catalytic converter designed to oxidize CO and VOC emissions from gasoline-fueled vehicles.
Urban airshed model (UAM)—
A three-dimensional photochemical air quality grid model for calculating the concentrations of both inert and chemically reactive pollutants in the atmosphere. It simulates the physical and chemical processes that affect concentrations of pollutants.
U.S. Environmental Protection Agency (EPA)—
The federal government agency that establishes regulations and oversees the enforcement of laws related to the environment.
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.
Sources: California Air Resources Board 2002; Davis 1997; EPA 2002; FHWA 2000; Harvey and Deakin 1993; IMRC 2000.