• a stationary source belonging to a defined group with the potential to emit 100 tons per year (tpy), or more, of any pollutant subject to regulation under the CAA; or
• any stationary source with the potential to emit 250 tpy, or more, of any pollutant subject to regulation under the CAA; or
• any physical change occurring at a stationary source that would constitute a major stationary source by itself. For example, modifying a source so that its emissions exceed 250 tpy of a regulated pollutant.

Any source subject to the PSD regulations must obtain a preconstruction permit. Prior to obtaining the permit, the source must undergo a New Source Review (NSR). The NSR will evaluate the affect the emissions from the source will have on the levels of criteria pollutants.

As part of the NSR, a source must demonstrate to the regulators it is using Best Available Control Technology (BACT) to control all criteria pollutants it emits. BACT is an emission limitation based on the maximum degree of reduction a permitting authority believes is achievable for the new source or major modification to existing source taking into consideration energy, environmental and economic impacts and other costs. The BACT is determined on a case-by-case basis.

The source must also undergo an air quality analysis for criteria pollutants to ensure it's emissions after installing a BACT will not cause an allowable increment to be exceeded. If it does, the permit will not be issued, and the source will either have to be modified or planned for another area where it will not cause the increment to be exceeded.

Nonattainment Areas - Nonattainment areas are areas where one or more NAAQS are not met. In spite of the efforts to reduce the level of criteria pollutants, by 1990 there were still many Air Quality Regions where one or more NAAQS had not been met. These areas, called attainment areas, are addressed by the CAA amendments of 1990. These amendments contain provisions to bring all areas of the country into attainment for three of the criteria pollutants, ozone, carbon monoxide and particulate matter in a specified number of years.

Nonattainment areas are classified based on the extent to which they fail to meet the standard: the worse the classification, the more stringent the controls. The classifications are:

Major new stationary sources and major modifications to existing sources in nonattainment areas must also obtain a preconstruction permit and undergo a NSR. However, a major source for a nonattainment area differs from that for PSD regulations and is defined as follows:

A major stationary source in an ozone nonattainment has the potential to emit the following quantities of volatile organic compounds (VOC) or oxides of nitrogen (NO, NO2):

1. 100 tpy or more in marginal or moderate nonattainment areas.
2. 50 tpy or more in serious nonattainment areas.
3. 25 tpy or more in severe nonattainment areas.
4. 10 tpy or more in extreme nonattainment areas.
5. 50 tpy of more in ozone transport regions* (VOCs only).

A major source in a particulate matter (PM-10**) nonattainment area, classified as serious, has the potential to emit 70 tpy or more of PM-10.

As part of the NSR, a major new source must demonstrate to the regulators it has achieved the lowest achievable emission rate (LAER). The LAER is the most stringent emission limitation within any state implementation plan or the most stringent level of pollution control achievable in the related industry group. Economics is not a factor in implementing LAER An area which is classified as nonattainment for one pollutant can be an attainment area for the others. Therefore, the NSR for a nonattainment area also includes an air quality analysis for other criteria pollutants to ensure allowable increments are not exceeded. The CAA amendments of 1990 require states with ozone nonattainment areas classified as moderate or worse, impose emission controls on existing sources of VOCs and nitrogen oxides (NOx). These limitations include reasonably available control technology (RACT). The EPA has not clearly defined RACT. However, the Agency has specified it will issue control technique guidelines (CTGs) for various types of pollutant sources which will qualify as RACT. The States can also determine RACT on a case-by-case basis.

Applicants for new major sources in ozone nonattainment areas must also demonstrate they produce a "net air quality benefit." Thus, if a new major source releases any emissions, it must offset them at another source by an amount greater than the amount being released.

In severe and extreme ozone nonattainment areas and serious carbon monoxide nonattainment areas, the EPA requires development of an Employee Commute Options (ECO) program. This program is designed to reduce solo-driving and promote alternative transportation modes to reduce air pollution. It is aimed at large employers (100 or more employees at a single work site). Employers can select appropriate strategies to reduce commuting trips and increase vehicle occupancy. Currently, the following nine areas in the country require an ECO program:

• Baltimore, MD;
• Houston/Galveston/Brazoria,TX
• Milwaukee WI
• Philadelphia, PA/Wilmington, DE/Trenton, NJ
• Ventura County, CA
• Chicago, IL
• Los Angeles, CA
• NY/NJ/CT metro area
• The Mohave Desert, CA.

Hazardous Air Pollutants Approach
The second approach is to identify pollutants which are a significant public health threat and are not regulated under NAAQS. In 1970 the CAA authorized the EPA to establish special emission standards for new and existing sources of these pollutants. Called National Emission Standards for Hazardous Air Pollutants (NESHAPS), the EPA, since 1970, issued standards for only seven substances: arsenic, asbestos, benzene, beryllium, mercury, radionuclides, and vinyl chloride.

The CAA amendments of 1990 significantly increased the number of pollutants considered to be a serious public health threat. In fact, the amendments require emission control for existing and new sources of 189 listed pollutants and categories with a new designation of hazardous air pollutants (HAPs). The EPA must develop technology-based emission standards for HAPs based on Maximum Achievable Control Technology (MACT). The standards, which are source-specific and are based on the best demonstrated contr ol technologies and practices are called National Emission Standards for Hazardous Air Pollutants for Source Categories (NESHAPSC).

MACT emission standards are slightly different for new and existing sources, but basically strive to achieve the lowest emission possible for a specific source category or subcategory. They take into consideration the cost of achieving such emission reduction, and health and environmental effects resulting from impacts on media other than air.

The HAP emission standards will apply only to major sources. For HAPs, a major source is defined as any stationary source or group of stationary sources located within a contiguous area that emits 10 tpy or more of a single HAP or 25 tpy or more of any combination of HAPs. However, the EPA administrator can establish a lesser quantity, or, for radionuclides, different criteria.

Air Pollutants Creating Significant Environmental Impacts
The third approach in the CAA is to address those air pollutants causing regional or global environmental damage such as acid air pollutants and stratospheric ozone depleting air pollutants.

Acid Air Pollutants - Acid rain which is part of a larger category of air pollutants known as acid pollutants has received a great deal of publicity in the northeast. A majority of the acid pollutants (acid rain, acid snow, acid fog, acid gas and acid dust) found in the northeast portion of the United States result from the release of sulfur dioxide (SOx) and NOx into the atmosphere by large coal burning power plants primarily in the midwest. In moist air, these compounds can eventually form sulfuric acid and nitric acid. When transported eastward, they eventually return to earth in the form of rain, snow, mist or solid particles possibly acidifying lakes and streams, damaging plant life and property, and causing health problems, especially among children and those with asthma.

To control sulfur dioxide emissions from the midwest power plants, the EPA has set up a market-based system of emission allowances. Every existing power plant covered by the Acid Air Pollutants program is issued a certain number of SO2 allowances, each worth one ton of sulfur dioxide. The system is market-based because the allowances can be bought and sold. However, new units coming on line after 12 December 1995 will not be given new allowances. They must obtain them from existing sources.

The NOx emission reduction program is not allowance-based. The EPA will control NOx emissions by establishing emission limits for existing units and new source performance standards (NSPSs) for new units.

Stratospheric Ozone Depletion - The other major air pollutants causing environmental concern are the chemicals possibly causing the hole in the stratospheric ozone layer. The ozone layer depletion is thought to be due mainly to man-made chemicals released into the atmosphere. These chemicals are divided into two classes depending on their ozone-depleting potential. Class I ozone depleting substances (ODSs) have a high potential, while the ODS depletion potential for class II substances is lower.

The EPA has banned the production of all but one of the class I substances effective 1 January 1996. The banned substances include all chlorofluorocarbons (CFCs), halons, carbon tetrachloride, methyl chloroform, and hydrobromofluorocarbons (HBFCs). The only class I substance that will continue to be produced is methyl bromide. It will be banned effective 1 January 2001.

Class II substances consist of hydrochlorofluoro-carbons (HCFCs). Currently they are used as substitutes for class I substances. However, they too have a phase-out schedule. HCFC-141b will be banned in 2003, HCFC-142b and HCFC-22 in 2020 and all other HCFCs in 2030.

The EPA has also established certification requirements for technicians who service, maintain, repair, and dispose of refrigeration and air conditioning systems containing CFCs and HCFCs. For all equipment, except motor vehicle air conditioners (MVACs), the technicians must pass a certification examination, but are not required to take a training course. Technicians working on MVACS must take both a training course and pass an examination.

Another program developed in accordance with the CAA to combat ozone depletion is the Significant New Alternatives Policy (SNAP) program established by the EPA. The EPA intends to publish quarterly updates to the SNAP list as new substitutes are developed and approved. The objectives of SNAP are:

• To identify substitutes for ozone-depleting substances;
• To evaluate the acceptability of these substitutes;
• To promote the use of those substitutes believed to present lower overall risks to human health and the environment, relative to the class I and class II substances being replaced, as well as to other substitutes for the same end use; and
• To prohibit the use of those substitutes found to increase overall risk.