1 Refers to the primary National Ambient Air Quality Standard that protects the public health.
2 Based on 8-hour standard of 9 ppm.
3 Based on 1-hour standard of .12 ppm.
4 The particulates (PM-10) standard replaced the previous standard for total suspended particulates in 1987.
5 Based on 3-month standard of 1.5 @mg/m3.

Source: U.S. Environmental Protection Agency, National Air Quality and Emissions Trends Report, annual.

http://www.epa.gov/oar/oarpubs.html

*What is an ambient air quality standard?

An ambient air quality standard is a national target for an acceptable concentration of a specificpollutant in air. Under the Clean Air Act, EPA develops two standards for each pollutant ofconcern:

A primary standard to protect public health. The Clean Air Act mandates that primarystandards be based entirely on health-related information, without considering the costs ofattaining the standard.

A secondary standard to protect public welfare. Public welfare includes effects on soils,water, crops, vegetation, buildings, property, animals, wildlife, weather, visibility,transportation, and other economic values, as well as personal comfort and well-being.

*The Clean Air Act, which was last amended in 1990, requires EPA to set National Ambient Air Quality Standards forpollutants considered harmful to public health and the environment. The Clean Air Act established two types of national airquality standards. Primary standards set limits to protect public health, including the health of "sensitive" populations such asasthmatics, children, and the elderly. Secondary standards set limits to protect public welfare, including protection againstdecreased visibility, damage to animals, crops, vegetation, and buildings.

The EPA Office of Air Quality Planning and Standards (OAQPS) has set National Ambient Air Quality Standards for sixprincipal pollutants, which are called "criteria" pollutants. They are listed below. Units of measure for the standards are partsper million (ppm), milligrams per cubic meter of air (mg/m3), and micrograms per cubic meter of air (g/m3).

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Air Quality Data Base

The ambient air quality data are obtained from EPA's Aerometric Information Retrieval System (AIRS). These are direct measurements of pollutant concentrationsat monitoring stations operated bystate and local governments throughout thenation. The monitoring stations are generallylocated in larger urban areas. EPA and otherfederal agencies operate some air qualitymonitoring sites on a temporary basis as a partof air pollution research studies. The nationalmonitoring network conforms to uniformcriteria for monitor siting, instrumentation, andquality assurance.

The daily measurements are obtained from monitoringinstruments that produce one measurementper 24-hour period and typically operate ona systematic sampling schedule of once every sixdays, or 61 samples per year. Such instrumentsare used to measure PM-10 and lead. Morefrequent sampling of PM-10 (every other day orevery day) is also common. Only PM-10weighted (for each quarter to account for seasonality)annual arithmetic means that meet theAIRS annual summary criteria are selected asvalid means for trends purposes. Only lead siteswith at least six samples per quarter in three of thefour calendar quarters qualify as trends sites.Monthly composite lead data are used if at leasttwo monthly samples are available for at leastthree of the four calendar quarters.

Monitoring instruments that operate continuouslyproduce a measurement every hourfor a possible total of 8760 hourly measurementsin a year. For hourly data, only annualaverages based on at least 4380 hourly observationsare considered as trends statistics. TheSO2 standard-related daily statistics require 183daily values to be included in the analysis.Ozone sites meet the annual trends datacompleteness requirement if they have at least50 percent of the daily data available for theozone season, which varies by state, buttypically runs from May through September.

Emissions Estimates Methodology

Trends are presented for annual nationwideemissions of CO, lead, nitrogen oxides (NOx ),volatile organic compounds (VOCs), PM-10,and SO2 . These are estimates of the amountand kinds of pollution being emitted by automobiles,factories and other sources, basedupon best available engineering calculations.

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