Air Quality and Conformity Issue Brief________________

Author:  Claudio Ternieden, (703) 824-0504


What's at Issue?

For several decades the transportation industry in the United States has been subjected to regulations, emissions standards and mitigation measures designed to reduce air pollution. While pollution control measures have traditionally focused on automobiles, trucks and buses, more recently aircraft and airport-related sources have come under increased scrutiny. Because the EPA has increased its Clean Air Act (CAA) implementation and enforcement, airport sponsors are now realizing the importance of being involved in the air quality planning process and the importance of developing and managing airport-related programs to control air pollution.


Why it’s Important

Before a major improvement project is undertaken at an airport, the need for an air quality assessment and/or a conformity determination is investigated. Air quality assessments involve the identification of emissions sources that would be affected by the proposed project and the characterization of airborne pollutants from those sources. The increase or decrease in emissions of criteria pollutants resulting from the implementation of the project is compared to thresholds (de minimis) levels that have been established by the EPA. If pollutant emissions resulting from the proposed project are above specified de minimis levels or are regionally significant, a General Conformity determination is required.

Air quality assessments are typically required in nonattainment and maintenance areas. In such areas, a General Conformity analysis is required for a project that is not exempt, grandfathered, presumed to conform or de minimis.

Air quality assessments can be conducted using a variety of computer models. The EPA’s guideline model for air quality analyses at airports is the FAA/U.S. Air Force Emissions and Dispersion Modeling System (EDMS). The model has two primary components: (1) an emissions processor, which calculates total pollutant emissions from airport sources, individually and collectively; and (2) a dispersion model, which estimates ambient pollutant concentrations in the vicinity of an airport.


Major Provisions

The Clean Air Act of 1970, the Clean Air Act Amendments of 1990 and associated implementing federal regulations and guidance establish current air quality standards and define the responsibilities of local, state and federal governments in attaining these standards. The Clean Air Act establishes federal air quality standards for the following “criteria” pollutants: carbon monoxide (CO), nitrogen dioxide (NO2), ozone (O3), sulfur dioxide (SO2), lead (Pb), and particulate matter less than 10 microns in diameter (PM-10). These criteria pollutants and associated National Ambient Air Quality Standards (NAAQS) are periodically reviewed by the EPA.

Under the CAA, states are required to develop a State Implementation Plan (SIP) for attaining minimum air quality standards in areas that do not meet the NAAQS. Regions in which concentrations of EPA criteria pollutants exceed the NAAQS are designated “nonattainment” areas. Areas that the EPA has redesignated to attainment status for specific pollutants are known as “maintenance” areas. The SIP must also include measures to maintain air quality standards in maintenance areas.

State and regional transportation plans and transportation improvement programs (TIPs) must conform to the purpose of the SIP, eliminating or reducing air quality standard violations and achieving expeditious attainment and must result in emission levels that fall within the emission budget specified in the SIP for mobile sources. The CAA further requires that all federal action in those areas conform with the SIP. Federal actions that are required to conform with the SIP include approval of a revised Airport Layout Plan (ALP). Projects that are typically “categorically excluded” from the requirement for formal documentation under the National Environmental Policy Act of 1969 (NEPA) including construction of passenger handling facilities, roadways and parking facilities may also be required to conform to the SIP.

The indirect, or secondary, effects of airport development must also be considered. Indirect impacts result from the implementation or operation of a federal project/action. Under the provisions of 40 CFR Section 51.853 (the General Conformity Rule), indirect emissions are limited to those that the federal agency can “practicably control and will maintain control over due to a continuing program responsibility of the federal agency.” An example of indirect impacts might be the emissions from vehicular activity in a future airport industrial area identified on a revised ALP. Construction activity associated with a proposal must also be considered in determining conformity with the SIP.

Federal actions that, when compared with the “no-action alternative,” would emit less than specified de minimis levels of criteria pollutants are presumed to conform with the applicable SIP and are exempted from further analysis unless they are considered to be “regionally significant.” The term “regionally significant” means that total direct and indirect emissions resulting from the action would equal 10 percent or more of a region’s emissions for a given criteria pollutant. Generally airport projects are not likely to be regionally significant.

On September 25, 2002, the EPA and the Federal Aviation Administration issued General Conformity Guidance for Airports Questions and Answers which provides direction to airport managers on issues associated with their conformity requirements.  This document is a result of a broad stakeholder group and is a useful source of clarification for airports on particular regulatory provisions.


Related Links

Air Quality section, AAAE Environmental Resource Locator,


The General Conformity Guidance for Airports Questions and Answers (September 25, 2002):


TRANSOURCE, the Transportation Environmental Resource Center, visit