Ambient Air Standards

Assessing Project Impacts - Ambient Air Standards

A. National Ambient Air Quality Standards

The 1970 Clean Air Act and the 1990 Clean Air Act Amendments were enacted by Congress to protect the health and welfare of the public from the adverse effects of air pollution.  Subsequently, the EPA established National Ambient Air Quality Standards for six criteria pollutants: sulfur dioxide or SO2, particulate matter (PM10 with an aerodynamic diameter less than 10 microns and PM2.5 with an aerodynamic diameter less than 2.5 microns), nitrogen dioxide or NO2, carbon monoxide or CO, ozone or O3, and lead or Pb.  The NAAQS specify maximum concentrations for various averaging times below which the air quality is considered acceptable with an adequate margin of safety and they include both primary and secondary standards.

Pollutant Primary Standards Secondary Standards
Level Averaging Time Level Averaging Time
Carbon Monoxide(CO) 9 ppm(10 mg/m3) 8-hour None
35 ppm(40 mg/m3) 1-hour
Lead(Pb) 0.15 µg/m3 Rolling 3-Month Average Same as Primary
1.5 µg/m3 Quarterly Average Same as Primary
Nitrogen Dioxide(NO2) 53 ppb Annual(Arithmetic Average) Same as Primary
100 ppb 1-hour None
Particulate Matter(PM10) 150 µg/m3 24-hour Same as Primary
Particulate Matter(PM2.5) 12.0 µg/m3 Annual(Arithmetic Average) Same as Primary
35 µg/m3 24-hour Same as Primary
Ozone 0.075 ppm(2008 standard) 8-hour Same as Primary
0.08 ppm(1997 standard) 8-hour Same as Primary
0.12 ppm 1-hour Same as Primary
Sulfur Dioxide(SO2) 0.03 ppm Annual(Arithmetic Average) 0.5 ppm 3-hour
0.14 ppm 24-hour
75 ppb 1-hour None

The primary standards are intended to protect human health; whereas, the secondary standards are intended to protect public welfare (e.g., damage to vegetation) from any known or anticipated adverse effects associated with the presence of air pollutants.  The more stringent of the primary or secondary standards is applicable to the modeling evaluation.  The NAAQS have been developed for various averaging periods.

B.    PSD Increments

PSD increment is the amount of pollution an area is allowed to increase.  PSD increments prevent the air quality in clean areas from deteriorating to the level set by the NAAQS.  The NAAQS is a maximum allowable concentration "ceiling."  A PSD increment, on the other hand, is the maximum allowable increase in concentration that is allowed to occur above a baseline concentration for a pollutant.  The baseline concentration is defined for each pollutant and, in general, is the ambient concentration that exists at the time that the first complete PSD permit application affecting the area is submitted.  Significant deterioration is said to occur when the amount of new pollution would exceed the applicable PSD increment.  It is important to note, however, that the air quality cannot deteriorate beyond the concentration allowed by the applicable NAAQS, even if not all of the PSD increment is consumed.  PSD increments have been established for three pollutants: SO2, particulate matter (PM10 and PM2.5), and NO2.  Both short term and annual increments exist for these pollutants.

Pollutant Period Class I Increment
Class II Increment
Class III Increment
Nitrogen Dioxide (NO2) Annual 2.5 25 50
Sulfur Dioxide (SO2) 3-hr 25 512 700
24-hr 5 91 182
Annual 2 20 40
Particulate Matter ≤ 10mm (PM10) 24-hr 8 30 60
Annual 4 17 34
Particulate Matter ≤ 2.5mm (PM2.5) 24-hr 2 9 18
Annual 1 4 8

C.   Significant Ambient Air Concentration

In addition to the six criteria pollutants, the DEQ regulates specific toxic pollutants.  Toxic pollutants are designated in the Air Toxics Program Priority Implementation Policy and the toxicity values of these are found in the 1991-1992 ACGIH Handbook (not including the six criteria pollutants).  Each pollutant has an hourly or a combination of an hourly and an annual Significant Ambient Air Concentration.  For specifics on toxic pollutants, refer to the Virginia regulations for control and abatement of air pollution, 9 VAC 5 Chapter 60, Part II, Article 5 for New and Modified Stationary Sources.


An air quality modeling analysis for a PSD proposed facility, such as a base load power plant, is required to demonstrate that its emissions of specific pollutants will not cause or significantly contribute to a violation of any ambient air quality standard or PSD increment.  The modeling analysis must be done in conformance with the EPA and DEQ guidance and procedures.  In the case of a Class I (defined in the following section on assessments) analysis, direct consultation with the specific federal land manager is required to adequately determine the air quality impacts of this proposed facility.

The basic goals of an air quality analysis are:

  1. To ensure that economic growth will occur in harmony with the preservation of existing natural resources.
  2. To protect the public health and welfare from any adverse effect that might occur even at air pollution levels less than the NAAQS.
  3. To preserve, protect, and enhance the air quality in areas of special, natural, recreational, scenic, or historic value such as national parks and wilderness areas.
  4. To require that major new stationary sources and major modifications be carefully reviewed prior to construction to ensure compliance with the NAAQS and applicable air quality increments.

Protecting Exceptional Resources

The third purpose of the PSD program is more explicitly defined to preserve, protect, and enhance the air quality in national parks, national wilderness areas, national monuments, national seashores, and other areas of special national or regional natural, recreational, scenic, or historic value. These are mandatory Class I areas include the following areas that were in existence as of August 7, 1977:

  1. International parks
  2. National wilderness areas and national memorial parks in excess of 5,000 acres
  3. National parks in excess of 6,000 acres

These Class I areas are managed by the USDA Forest Service, National Park Service, and the Fish and Wildlife Service.  These federal land managers have the affirmative responsibility to protect these areas unique attributes and air quality.

The first level of protection is estimating the proposed facility's impact on the Class I PSD increments.  These were small amounts of additional pollution that Congress dictated.

In addition, the Clean Air Act referenced air quality-related values which could not be adversely impacted by the proposed facility.  These air quality-related values include visibility, odor, flora, fauna, and geological resources: archeological, historical, and other cultural resources; and soil and water resources.  The values for various Class I areas differ depending on the purpose and characteristics of a particular area and the assessment by an area's federal land manager.  The thresholds of each Air Quality Related Values for compliance assessment are determined by the specific federal land manager. The two Class I areas in Virginia are the Shenandoah National Park (managed by the National Park Service) and James River Face Wilderness Area (managed by the USDA Forest Service).

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Virginia Department of
Environmental Quality
P.O. Box 1105
Richmond, VA 23218
(804) 698-4000

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