AirToxics:
Chemicals in the air that are known or suspected to cause cancer or other
serious health effects, such as reproductive problems or birth defects.
Air toxics are also known as "hazardous air pollutants." Mobile
sources emit a number of air toxics associated with both long-term and
short-term health effects in people, including heart problems, asthma
symptoms, eye and lung irritation, cancer, and premature death.
Benzene:A cancer-causing hydrocarbon (C6H6)
derived from petroleum. Benzene is a component of gasoline. Benzene emissions
occur in exhaust as a byproduct of fuel combustion and also occur when
gasoline evaporates.
Carbon Monoxide (CO):A colorless,
odorless gas that forms when carbon in fuel is not burned completely.
Carbon monoxide is a component of exhaust from motor vehicles and engines.
Carbon monoxide emissions increase when conditions are poor for combustion;
thus, the highest carbon monoxide levels tend to occur when the weather
is very cold or at high elevations where there is less oxygen in the air
to burn the fuel.
Catalytic Converter:An anti-pollution
device located between a vehicle's engine and tailpipe. Catalytic converters
work by facilitating chemical reactions that convert exhaust pollutants
such as carbon monoxide and nitrogen oxides to normal atmospheric gases
such as nitrogen, carbon dioxide, and water.
Combustion:The process of burning. Motor
vehicles and equipment typically burn fuel in an engine to create power.
Gasoline and diesel fuels are mixtures of hydrocarbons,
which are compounds that contain hydrogen and carbon atoms. In "perfect"
combustion, oxygen in the air would combine with all the hydrogen in the
fuel to form water and with all the carbon in the fuel to form carbon
dioxide. Nitrogen in the air would remain unaffected. In reality, the
combustion process is not "perfect," and engines emit several
types of pollutants as combustion byproducts.
Emissions:Releases of pollutants into
the air from a source, such as a motor vehicle or a factory.
Emission Standards:Rules and
regulations that set limits on how much pollution can be emitted from
a given source. Vehicle and equipment manufacturers have responded to
many mobile source emission standards by redesigning vehicles and engines
to reduce pollution.
Evaporation:The process by which a substance
is converted from a liquid into a vapor. "Evaporative emissions"
occur when a liquid fuel evaporates and fuel molecules escape into the
atmosphere. A considerable amount of hydrocarbon pollution results from
evaporative emissions that occur when gasoline leaks or spills, or when
gasoline gets hot and evaporates from the fuel tank or engine.
Fossil Fuels:Fuelssuch as coal,
natural gas, and crude oil that come from the compressed remains
of ancient plants and animals. Gasoline and diesel are fossil fuels that
can be burned in internal combustion
engines to power everything from jet planes to automobiles to railroad
locomotives.
Haze:Atmospheric particulate
matter and gases that diminish visibility. Visibility is reduced when
light encounters tiny pollution particles, such as soot and dust, and
some gases (such as nitrogen dioxide) in the air. Some light is absorbed
by the particles and gases and other light is scattered away before it
reaches your eye. More pollutants mean more absorption and scattering
of light, resulting in more haze. Some haze-causing pollutants are directly
emitted to the atmosphere from vehicle emissions; others are formed indirectly
when pollutants from mobile sources react with other elements and materials
in the atmosphere.
Hydrocarbons (HC): Chemical compounds
that contain hydrogen and carbon. Most motor vehicles and engines are
powered by hydrocarbon-based fuels such as gasoline and diesel. Hydrocarbon
pollution results when unburned or partially burned fuel is emitted from
the engine as exhaust, and also when fuel evaporates directly into the
atmosphere. Hydrocarbons include many toxic compounds that cause cancer
and other adverse health effects. Hydrocarbons also react with nitrogen
oxides in the presence of sunlight to form ozone. Hydrocarbons, which
may take the form of gases, tiny particles, or droplets, come from a great
variety of industrial and natural processes. In typical urban areas, a
very significant fraction comes from cars, buses, trucks, and nonroad
mobile sources such as construction vehicles and boats.
Mobile Sources: Motor vehicles, engines,
and equipment that move, or can be moved, from place to place. Mobile
sources include vehicles that operate on roads and highways ("on-road"
or "highway" vehicles), as well as nonroad vehicles, engines,
and equipment. Examples of mobile sources are
cars, trucks, buses, earth-moving equipment, lawn and garden power tools,
ships, railroad locomotives, and airplanes.
Nitrogen Oxides (NOx):A
group of highly reactive gases that contain nitrogen and oxygen in varying
amounts. Many of the nitrogen oxides are colorless and odorless. The common
pollutant nitrogen dioxide (NO2)
can often be seen combined with particles in the air as a reddish-brown
layer over many urban areas. Nitrogen oxides are formed when the oxygen
and nitrogen in the air react with each other during combustion. The formation
of nitrogen oxides is favored by high temperatures and excess oxygen (more
than is needed to burn the fuel). The primary sources of nitrogen oxides
are motor vehicles, electric utilities, and other industrial, commercial,
and residential sources that burn fuels.
Ozone:A gaseous molecule that contains three
oxygen atoms (O3). Ozone can
exist either high in the atmosphere, where it shields the Earth against
harmful ultraviolet rays from the sun, or close to the ground, where it
is the main component of smog. Ground-level
ozone is a product of reactions involving hydrocarbons
and nitrogen oxides in the presence
of sunlight. Ozone is a potent irritant that causes lung damage and a
variety of respiratory problems.
Particulate Matter (PM):Tiny particles or liquid
droplets suspended in the air that can contain a variety of chemical components.
Larger particles are visible as smoke or dust and settle out relatively
rapidly. The tiniest particles can be suspended in the air for long periods
of time and are the most harmful to human health because they can penetrate
deep into the lungs. Some particles are directly emitted into the air.
They come from a variety of sources such as cars, trucks, buses, factories,
construction sites, tilled fields, unpaved roads, stone crushing, and
wood burning. Other particles are formed in the atmosphere by chemical
reactions.
Particulate Matter 2.5 (PM2.5):Particles
that are less than 2.5 microns in diameter. These particles are often
referred to as "PM fine." PM fine particles are so small that
they are not typically visible to the naked eye. In the atmosphere, however,
they are significant contributors to haze.
Smaller particles are generally more harmful to human health because they
can penetrate more deeply into the lungs than larger particles. Virtually
all particulate matter from mobile sources is PM2.5.
Particulate Filter: An anti-pollution
device designed to trap particles in diesel exhaust before they can escape
into the atmosphere.
Pollutants (Pollution): Unwanted
chemicals or other materials found in the environment. Pollutants can
harm human health, the environment, and property. Air pollutants occur
as gases, liquid droplets, and solids. Once released into the environment,
many pollutants can persist, travel long distances, and move from one
environmental medium (e.g., air, water, land) to another.
Smog: A commonly used term for pollution
caused by complex chemical reactions involving nitrogen
oxides and hydrocarbons in the
presence of sunlight. Ozone is a key
component of smog. Smog-forming chemicals come from a wide variety of
combustion sources and are also found in products such as paints and solvents.
Smog can harm human health, damage the environment, and cause poor visibility.
Major smog occurrences are often linked to heavy motor vehicle traffic.
Source:Any place or object from which pollutants
are released, such as a power plant, factory, tractor, car, or other machines.
Mobile sources move (e.g., cars and buses), while stationary sources do
not (e.g., factories).
Vapor Recovery System: An anti-pollution
system designed to capture gasoline vapors that would otherwise escape
into the atmosphere from hot vehicle engines and fuel tanks.
Vehicle Miles Traveled (VMT):The total number
of miles traveled in a given period of time (e.g., day, year) by a given
vehicle or fleet of vehicles. VMT, combined with pollution rates per mile
traveled, provide an estimate of the total amount of vehicle pollution
in a given period of time.
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