carbon monoxide, lead, ozone, nitrogen dioxide, particulate matter with a diameter of 10 µm or less (known as PM10) and sulfur oxides (measured as sulfur dioxide).

The act establishes an array of programs intended to ensure that the standards are attained and maintained and that air quality superior to the standards is protected.

Each state must submit to EPA a state implementation plan (SIP) that demonstrates that primary NAAQS will be attained by the statutory deadlines and that secondary NAAQS will be attained as expeditiously as practicable. If a state does not do so, then EPA must prepare a federal implementation plan (FIP) for the state.

SIPs usually concentrate on existing sources in or near nonattainment areas—areas that violate one or more NAAQS. For many types of new sources, SIPs are supplemented by federal regulations that apply to clean air as well as nonattainment areas. For instance, EPA may promulgate new source performance standards for categories of stationary sources that contribute to air pollution. These standards require the use of the best demonstrated technology by new and modified sources in the category. Similarly, the act establishes emissions standards for new mobile sources and standards for fuel content; the act also authorizes EPA to establish additional regulations on those subjects.

Two programs under the act are concerned specifically with visibility in national parks and wilderness areas. One of these, the prevention of significant deterioration (PSD) program (§160–169), is directed primarily at new sources; the other, the visibility program (§169A–B), is aimed primarily at existing sources.

The PSD program requires that each applicant for a new or modified major emitting facility seeking to locate in a clean-air area (an area in which the NAAQS are met for one or more air pollutants) show that the facility will use the best available control technology (BACT) to minimize additional air pollution. (Every area meets the NAAQS for at least one pollutant; therefore, the PSD program applies nationwide.) BACT is defined as the maximum achievable degree of emission reduction, taking into account energy, environmental, and economic effects and other costs. It is determined on a case-by-case basis, but must be at least as stringent as any new source performance standard that applies to the facility's category.

The applicant must also demonstrate that the proposed new or expanded source will comply with air-quality "increments" that limit increases



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