Purpose and Terms

Purpose

This brochure provides an overview of facts and figures regarding the linkages between transportation and air quality. The focus of this brochure is primarily on transportation-related emissions trends, policies, technologies, and standards that affect on-road mobile sources, including automobiles, light-duty trucks, and heavy-duty trucks.

Terms

CAAA	The Clean Air Act Amendments of 1990.
CO	Carbon monoxide - criteria pollutant - product of incomplete combustion.
CO₂	Carbon dioxide - not a criteria pollutant - greenhouse gas.
Condensables	PM_2.5 formed in the atmosphere from 'precursor' gases, such as SO₂ and NO_x.
Fugitive dust	Largely windblown dust from paved and unpaved roads.
HC	Hydrocarbons - gaseous compounds made of carbon and hydrogen (used interchangeably with VOC).
MSAT	Mobile source air toxics.
NAAQS	National Ambient Air Quality Standards - federally established standards for pollutant concentrations that states, cities, and towns must meet by specified deadlines.
NO_x	Oxides of nitrogen - collective term for all compounds of nitrogen and oxygen (includes nitrogen monoxide, nitrogen dioxide, etc.).
Nonattainment areas	Areas that have failed to meet the NAAQS.
Non-road engines	Aircraft, trains, boats, off-road recreational vehicles, farm and construction equipment, and yard tools.
On-road vehicles	Cars, vans, buses, light-duty and heavy-duty trucks, and motorcycles.
O₃	Ozone - criteria pollutant - oxygen compound that can develop when NO_x, VOC, and sunlight interact in the lower atmosphere; the primary constituent of smog.
PM₁₀	Particulate matter with a diameter less than 10 micrometers.
PM_2.5	Particulate matter with a diameter less than 2.5 micrometers.
Point and area sources	Stationary sources of emissions, including electric utilities, factories, petroleum refineries, dry cleaners, and others.
Precursors	Pollutants that contribute to the formation of other pollutants; HC and NO_x are precursors of ozone and PM.
SO₂	Sulfur dioxide.
VMT	Vehicle Miles Traveled.
VOC	Volatile organic compounds - gaseous compounds made of carbon and hydrogen (used interchangeably with HC).

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