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General Conformity

General Conformity Training Module 3.2: Emissions Calculations

Words that are shown in bold and italics are defined in the Glossary.

Federal agencies must evaluate the total direct and indirect emissions caused by their actions. In determining the total direct and indirect emissions caused by the action, agencies must project the future emissions in the area with the action versus the future emissions without the action, what the National Environmental Policy Act (NEPA) entitles “the no build option.” The total direct and indirect emissions is the net emissions considering all emission increases and decreases and must be reasonably foreseeable at the time that the conformity evaluation is conducted and are possibly controllable through agency's continuing program responsibility to affect emissions. 

The term “reasonably foreseeable” means that the emissions can be estimated using standard emission techniques, assuming activity levels are consistent with the projected use. The projected use should consider any limits on the operation of the facility imposed by physical or legal constraints. This includes use of emission control equipment and operational limitations. The term includes reasonable growth rates, but does not mean that the projects must assume the maximum use of a facility unless the growth rates would justify that level of activity within the timeframe of the analysis.

Practicably controllable” means the ability of the federal agency to regulate, in some way, the emissions caused by the federal action. The ability to regulate may include having the authority to establish emissions reduction programs, or having the authority to include conditions in permits, approvals or contracts to limit the emissions from certain activities. For example, a federal agency could practicably control the level of vehicle emissions by controlling the size of the parking facility and setting requirements for employee trip reductions. The agency could also condition a permit, approval or contract to require the use of low emitting vehicles during construction or for operations. The term “continuing program responsibility” means that the agency retains some oversight authority over the activity such as operational control, enforcement of permit conditions, contract review, or leasing authority. The federal agency does not have to conduct these activities itself; the agency just has the authority to affect them. For example, two federal agencies may be approving the construction of a pipeline across a wetland. One agency may be responsible for approving the permit to fill the wetland. That agency would be responsible to ensure that the construction emissions conform to the SIP/TIP. Once the wetland construction is complete, the agency would not have responsibility for the operational emissions, since the agency would not have a continuing program responsibility. The other agency with responsibility for overseeing the safe operation of the pipeline must ensure that both the construction and operational emissions conform to the SIP/TIP.

The degree of detail in the emissions analysis for determining if the emissions are below the de minimis levels depends upon how close the total emissions are to the de minimis levels. If the emissions are significantly below the de minimis levels, only the rough but conservative estimates are needed. However, if the total emissions are close to the de minimis levels, then a more detailed study is called for. Controversial actions may require more detailed studies to avoid challenges to the determination. Historical analysis of similar actions could be used in cases where the proposed projects are similar in size and scope to previous projects. More complex projects may require more detailed activity analyses to determine whether emissions exceed de minimis levels. The NEPA analyses may provide the necessary emissions studies.

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3.2.1 Baseline emissions

The baseline emissions for the future year are the projected emissions for that year considering the historic activity levels, expected growth without the federal action and the appropriate emission factors for the future year. The historic level is determined by the level of activity used in the SIP/TIP, or where EPA has not approved a SIP/TIP for the area, the most recent calendar year with a complete emission inventory available before the area was designated, unless EPA sets another year for determining the baseline.

If a federal agency is approving an action in an area with an approved SIP, then baseline emissions would be the emissions included in the SIP for the years being evaluated. However, if the area does not have an approved SIP or the SIP does not cover the years being evaluated, then the baseline emissions would need to be calculated. To calculate the emissions one must determine the appropriate activity level based upon historic activity levels, growth rates, and the appropriate emission factors for the type of equipment to be used. 

For example, we will assume that a federal agency is approving a 5-year expansion of an airport in a nonattainment area without an approved SIP and the airport presently handles 100 flights per day but can safely handle 125 fights per day. The flights are a mixture of 80% regional jets and 20% medium size jets. For the past five years, the airport has seen a 5% growth in the number of passengers using its facilities. We will assume that the airport will reach capacity before the CAA mandated attainment. Since there is no SIP for the area, the baseline must be calculated upon historic activity levels growth rates and appropriate emission factors. Without the expansion the airport would reach its capacity of 125 flights per day and the activity level would be set at that number. However, to service the growth in the number of passengers, airlines would have to use more of the mid size aircraft. Therefore, the appropriate emission factor would be based upon a different mixture of mid size and regional jets.

3.2.2 Out of season emissions

Conformity evaluations are required for all actions covered by the regulations. The de minimis emission levels are established on a tons-per-year basis without regard for the time of year that the pollutant is emitted. Thus, annual emission rates per calendar year are used. However, if a conformity determination is required, then the season of the emissions may be relevant to determining if the emissions conform with the SIP/TIP. If the emissions from a federal action occur outside of the pollutant season, you should discuss this issue with the appropriate state, tribe or local air quality agency.

For example, a federal agency might fund or approve construction activities in a northern carbon dioxide nonattainment or maintenance area. The violations of the carbon monoxide (CO) standard only occur in the winter months and the SIP requirements and emission budgets only apply to those months. Because of the climate, most if not all of the construction activities cannot take place in the winter months. If the total direct and indirect emissions of CO exceeded the 100 tons per year de minimis level, then a full conformity determination would be required. However, a federal agency could legally limit the construction activities to non-winter months and get the state or tribe agree that the emissions are included in the SIP because they are only non-winter emissions.

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3.2.3 Offsite emissions

In some cases, federal actions at a facility can cause increased emissions off the facility’s property. These emissions would be considered indirect emissions if the agency could practicably control them through a program with continuing responsibility. For example, if a federal agency builds a new facility, it would include the emissions from personnel commuting to a facility since it could institute programs to limit the single vehicle commuting. The agency could then estimate the future emissions by assuming an average commute distance and future vehicle emission factors. However, the additional emissions from other off-facility activities of the employees such as shopping and recreational driving would not be included, since the agency would not have a responsibility for these activities. Emissions from vehicles servicing the facility are treated in the same manner as those from employee commuters.

3.2.4 Emission factors

An emissions factor is a representative value that relates the quantity of a pollutant released to the atmosphere with an activity associated with the release of that pollutant. These factors are usually expressed as the weight of the pollutant divided by a unit weight, volume, distance, or duration of the activity emitting the pollutant (e.g., kilograms of particulate emitted per megagram of coal burned). Such factors facilitate estimation of emissions from various sources of air pollution. In most cases, these factors are simply averages of all available data of acceptable quality, and are generally assumed to be representative of long-term averages for all facilities in the source category (i.e., a population average).

The general equation for emissions estimation is:

E = A x EF x (1-ER/100)


  • E = emissions;
  • A = activity rate;
  • EF = emission factor, and
  • ER =overall emission reduction efficiency, %

The EPA’s Compilation of Air Pollutant Emissions Factors, known as AP-42, provides a wide range of emission factors. The emission factors for stationary and area sources and can be accessed through EPA’s CHIEF website. Besides providing the emission factors, AP-42 also provides information on the quality of the factors.

In cases where emission factors are unavailable or inappropriate, the federal agency should use the best available information. When using data on emission factors from sources other than AP-42, the federal agency should coordinate their use of such data and supplemental material with the EPA, and state, tribe and local air quality agencies.

In addition to the manual approach, EPA and other agencies have developed emission-estimating software that incorporates EPA emission factors. Some of the available software includes: MOBILE6 for on-road mobile sources, NONROAD2002a for off-road mobile sources, and EDMS airports and air bases. See mobile source emissions models for the latest information.

Emission estimates for existing sources can also be obtained from emission test data. Source test information may have been collected by facilities as part of their efforts to demonstrate compliance with existing regulations. If such data are to be used, the federal agency should ensure that the conditions under which the data were collected represent the conditions being evaluated and the agency should coordinate the use of such data with the state or tribal air quality agency.

Appendix A to this training manual provides additional information regarding emission factors and includes sample calculations from various source types.

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3.2.5 Precursors of secondary pollutants

Secondary pollutants are criteria pollutants that are not emitted directly into the atmosphere. Instead, the precursors of those pollutants are emitted and, through atmospheric chemistry, are transformed into the criteria pollutants. For example, volatile organic compounds (VOC) combine with oxides of nitrogen (NOx) to form ozone. The SIPs and TIPs recognize the role of precursors and limit their emissions. The General Conformity Regulations also recognize their importance and establishes de minimis emission levels for precursors. In determining if the emissions exceed the de minimis levels, each precursor is analyzed separately. Federal agencies do not need to sum the total of the precursor. For example, if the de minimis emission levels for NOx is 100 tons per year and the de minimis emission levels for VOC is 100 tons per year and the total direct and indirect emissions from the action is 75 tons per year of VOC and 75 tons per year of NOx, the action would be below the de minimis emissions levels for the area. De minimis emission levels are available for precursors of ozone and fine particles – those are the only precursors for which federal agencies are required to evaluate in the conformity analysis. Although the precursors are analyzed separately for the de minimis emission level evaluation, if permitted by the SIP/TIP, the federal agency, with the concurrence of state or tribe, can reduce the emission of one precursor to offset or mitigate the increase in emission of another precursor of the same pollutant.

3.2.6 Construction emissions

Major federal actions often involve construction activities. The emissions from those activities can be either direct or indirect emissions. If the federal agency is controlling the construction activities or is approving the construction, then the emissions should be considered as direct emissions. Otherwise, the emissions would most likely be considered as indirect emissions. To do an analysis of the construction emissions the federal agency would identify the type of equipment needed for the activity, the duration it is needed and when during the construction phase it would be used. The earth-moving phase of construction tends to produce the most emissions. The appendix to this training manual provides examples of the calculations of construction emissions.

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