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Transportation Air Quality Selected Facts and Figures



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.


CAAAThe Clean Air Act Amendments of 1990.
COCarbon monoxide - criteria pollutant - product of incomplete combustion.
CO2Carbon dioxide - not a criteria pollutant - greenhouse gas.
CondensablesPM2.5 formed in the atmosphere from 'precursor' gases, such as SO2 and NOx.
Fugitive dustLargely windblown dust from paved and unpaved roads.
HCHydrocarbons - gaseous compounds made of carbon and hydrogen (used interchangeably with VOC).
MSATMobile source air toxics.
NAAQSNational Ambient Air Quality Standards - federally established standards for pollutant concentrations that states, cities, and towns must meet by specified deadlines.
NOxOxides of nitrogen - collective term for all compounds of nitrogen and oxygen (includes nitrogen monoxide, nitrogen dioxide, etc.).
Nonattainment areasAreas that have failed to meet the NAAQS.
Non-road enginesAircraft, trains, boats, off-road recreational vehicles, farm and construction equipment, and yard tools.
On-road vehiclesCars, vans, buses, light-duty and heavy-duty trucks, and motorcycles.
O3 Ozone - criteria pollutant - oxygen compound that can develop when NOx, VOC, and sunlight interact in the lower atmosphere; the primary constituent of smog.
PM10Particulate matter with a diameter less than 10 micrometers.
PM2.5Particulate matter with a diameter less than 2.5 micrometers.
Point and area sourcesStationary sources of emissions, including electric utilities, factories, petroleum refineries, dry cleaners, and others.
PrecursorsPollutants that contribute to the formation of other pollutants; HC and NOx are precursors of ozone and PM.
SO2Sulfur dioxide.
VMTVehicle Miles Traveled.
VOCVolatile organic compounds - gaseous compounds made of carbon and hydrogen (used interchangeably with HC).
Updated: 3/17/2015
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