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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.
* What is an ambient air quality standard?
An ambient air quality standard is a national target for an acceptable concentration of a specific pollutant in air. Under the Clean Air Act, EPA develops two standards for each pollutant of concern:
A primary standard to protect public health. The Clean Air Act mandates that primary standards be based entirely on health-related information, without considering the costs of attaining 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 for pollutants considered harmful to public health and the environment. The Clean Air Act established two types of national air quality standards. Primary standards set limits to protect public health, including the health of "sensitive" populations such as asthmatics, children, and the elderly. Secondary standards set limits to protect public welfare, including protection against decreased 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 six principal pollutants, which are called "criteria" pollutants. They are listed below. Units of measure for the standards are parts per million (ppm), milligrams per cubic meter of air (mg/m3), and micrograms per cubic meter of air (g/m3).
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 concentrations at monitoring stations operated by state and local governments throughout the nation. The monitoring stations are generally located in larger urban areas. EPA and other federal agencies operate some air quality monitoring sites on a temporary basis as a part of air pollution research studies. The national monitoring network conforms to uniform criteria for monitor siting, instrumentation, and quality assurance.
The daily measurements are obtained from monitoring instruments that produce one measurement per 24-hour period and typically operate on a systematic sampling schedule of once every six days, or 61 samples per year. Such instruments are used to measure PM-10 and lead. More frequent sampling of PM-10 (every other day or every day) is also common. Only PM-10 weighted (for each quarter to account for seasonality) annual arithmetic means that meet the AIRS annual summary criteria are selected as valid means for trends purposes. Only lead sites with at least six samples per quarter in three of the four calendar quarters qualify as trends sites. Monthly composite lead data are used if at least two monthly samples are available for at least three of the four calendar quarters.
Monitoring instruments that operate continuously produce a measurement every hour for a possible total of 8760 hourly measurements in a year. For hourly data, only annual averages based on at least 4380 hourly observations are considered as trends statistics. The SO2 standard-related daily statistics require 183 daily values to be included in the analysis. Ozone sites meet the annual trends data completeness requirement if they have at least 50 percent of the daily data available for the ozone season, which varies by state, but typically runs from May through September.
Emissions Estimates Methodology
Trends are presented for annual nationwide emissions of CO, lead, nitrogen oxides (NOx ), volatile organic compounds (VOCs), PM-10, and SO2 . These are estimates of the amount and kinds of pollution being emitted by automobiles, factories and other sources, based upon best available engineering calculations.
These tables are based on figures supplied by the United States Census Bureau, U.S. Department of Commerce and are subject to revision by the Census Bureau.
Copyright © 2006 Photius Coutsoukis and Information Technology Associates, all rights reserved.