General Conformity Training Module 3.2: Emissions Calculations

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

Federal agencies must prepare and evaluate the total direct and indirect emissions (i.e., TDI emissions or annual net emissions increases) caused by certain federal activities should their action to implement a federal project be approved. Such conformity evaluations are required for all activities applicable under § 93.153 and are not otherwise presumed to conform or exempt.

For the one or more activities that would be subject to the General Conformity provisions under 40 CFR part 93 subpart B, an agency must project the future emissions increase, by pollutant and by year, indicated under § 93.159(d), within the nonattainment or maintenance area affected by the activities’ emissions. The annual emissions would be compared against those annual emissions that would occur without implementation of the activities during the same future years. This is usually a comparison of what the National Environmental Policy Act (NEPA) procedures refer to as the "build alternative" vs “the no build alternative.”

The TDI emissions reflects the annual net emissions considering all emission increases and decreases caused by implementing the activities, where all the emissions considered must be reasonably foreseeable and quantifiable at the time the conformity evaluation is conducted, and includes only those sources over which the agency has operational oversight through its authority to approve, fund, permit or license, engage in, support, or otherwise authorize use of the sources, referred to as a continuing program responsibility, which can be enforced through the agency's decision document, such as a NEPA EIS Record of Decision (ROD), an EA Finding of No Significant Impact (FONSI), or other equally enforceable document that provides evidence of such a decision.

The term “reasonably foreseeable” means that the emissions can be estimated using standard EPA emission calculation techniques. The federal agency should consider its authority to impose physical, operational, or legal constraints to the use of the sources, consistent with the needs of the project (e.g., meets the purpose and need of the NEPA project scope of work), in order to reduce the emissions from the activities. The federal agency can allow for reasonable growth rates but that does not mean that a project must assume the maximum use of a source (e.g., potential to emit) unless the growth rates would justify such a level of activity within the timeframe of the analysis. The federal agency must not estimate projected emissions based on speculation.

“Practicable control” means the federal agency has the statutory authority to in some way control the emissions caused by the federal activities, if implemented. This may include having the authority to establish emissions reduction programs for a source, or having the authority to include conditions in permits, approvals, or contracts to limit or otherwise mitigate the emissions caused by certain activities. For example, a federal agency would have practicable control over the rate of vehicle emissions by adjusting the size of a proposed parking facility and developing requirements for employee trip reductions. The agency could also condition a permit, approval, or contract to require the use of low emitting equipment and vehicles used for construction or developing lower-emitting operational parameters.

The term “continuing program responsibility” means that the agency retains some oversight authority with regard to the activity, such as operational limitations or requirements, enforcement of permit conditions, and through contract requirements. The federal agency does not have to conduct these program responsibilities itself nor does the agency have the authority to set regulatory emissions standards for the engines that power the sources used for their project. What the agency does have is the authority to affect the emissions by controlling or adjusting how a source is used. For example, two federal agencies may be needed for approving the construction and operation of a pipeline across a wetland. One agency, the wetland permitting agency, would be responsible for authorizing construction within the wetland (e.g., excavating, removing, filling, or dredging, and installing the pipeline) and would be responsible to ensure that such construction emissions conform to the SIP/TIP. The other agency, the lead agency, would be responsible for approving operation of the pipeline once the wetland construction is complete and is also responsible for overseeing the safe operation of the pipeline and all other aspects of the project. In this case, the responsibilities of each agency to account for construction and operation of the pipeline do not overlap, are not duplicative, and each agency acts according to its own authority. The permitting agency has no authority to approve the pipeline project, and the lead agency has no authority to issue the permit to build. Each agency has a unique continuing program of responsibility. However, each agency must ensure the emissions it authorizes will conform to the purpose of the SIP/TIP.

For most federal actions, the lead federal agency must determine the TDI emissions caused by the proposed activities. The degree of detail in the emissions applicability analysis for determining if the annual emission increases, if any, caused by the activities, are below the annual de minimis rates depends upon how close the TDI emissions are to the associated rates in the tables at § 93.153(b)(1) and (b)(2). Each federal agency is responsible for how the projected TDI emissions are calculated and the degree of detail required to avoid objections to the results.

Historical analysis of previous activities can be adapted to the current proposed activities in cases where the proposed activities are similar in size and scope to activities approved for previous projects. More complex projects may require more detailed activity analyses to determine whether emissions equal or exceed the annual de minimis rates. The NEPA analyses, if one is associated with the action, may provide the necessary emissions studies.

3.2.1 Construction emissions

Federal actions often involve construction activities that are essential to successful implementation of a project, program, or plan proposed by a federal agency and such emissions from construction must be evaluated under General Conformity. If the project alternative that will ultimately be implemented depends on completion of construction activities, it is likely the agency is preparing an environmental assessment under the National Environmental Policy Act (NEPA), where the potential adverse impact caused by the construction emissions on the project area is included in the air quality discussion within the EIS, EA, or Categorical Exclusion. When the agency makes its decision to implement a project, all the activities subject to General Conformity are also approved. Therefore, the agency is applying its authority and can impose conditions relating to how the construction is phased, e.g., perhaps the agency wants all construction to occur outside the months where ozone concentrations may be highest within the project area, or the agency can require that only Tier 3 or Tier 4 nonroad equipment can be used on the job site. Thus, the federal agency has practicable control over the emissions from any sources of emissions proposed, including construction sources.

The annual rate of construction emissions must be included in the TDI emissions (i.e., the annual net emissions increases) and must be projected for, at a minimum, each future year and scenario required under § 93.159(d), where the EPA is particularly interested in the rate of emissions during the year of greatest annual net emissions. This applies not only to emissions from construction sources, but to all sources of emissions for which General Conformity applies for a particular project.

The federal agency would identify each type of equipment needed for construction, including the horsepower (hp) of each engine (e.g. a large 350 hp excavator), the type of fuel used (e.g., diesel or gasoline), how many annual hours the unit will be operated (e.g., 23 days @ 6 hours a day, is 138 hours/year). Identify whether each unit is Tier 2, Tier 3, Tier 4, etc. informing the agency of the maximum allowable emissions rate. The earth-moving phase of construction tends to produce the most emissions for a project.

The EPA does not allow a federal agency to estimate the annual average emissions caused by construction equipment. The future projected emissions rates must be calculated on an annual basis, a calendar year. The EPA recommends the agency keep on file all the calculations and tabulations on which the agency relied for their analysis. The appendix to this training manual provides examples of the calculations of construction emissions.

3.2.2 Emission factors

An emission factor is a coefficient that describe the rate at which a given source releases a pollutant into the atmosphere, e.g., 3.157 grams per horsepower-hour. These factors usually consider the pollutant as a mass (e.g., gram or lb) per unit of distance (e.g., mile), and duration (e.g. hours per year). 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 projecting estimated emissions is:

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

where:

E = emissions of a specific pollutant;
A = activity rate, e.g., miles or hours per year;
EF = emission factor, e.g., grams of NOx per gallon of fuel burned by a vehicle engine per mile traveled; and
ER =overall emission reduction efficiency, if known as a %

The EPA’s Compilation of Air Pollutant Emissions Factors, known as AP-42 and WebFIRE provide emission factors for a wide range of sources, e.g., engine types and fuel types. The emission factors for stationary and area sources 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 those listed above, the federal agency should coordinate their use of such data and supplemental material with the EPA, and state, Tribal, 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: the MOBILE program series for on-road mobile sources, the NONROAD program series for non-road mobile sources, and the FAA AEDT program series that must be used for 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, that is source-specific and is collected by some federal agencies or non-federal entities 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.

3.2.3 Precursors of secondary pollutants

Secondary pollutants are criteria pollutants that are not emitted directly into the atmosphere. Instead, their precursor pollutants are emitted and, through complex atmospheric chemical reactions, are transformed into the criteria pollutants. For example, volatile organic compounds (VOC) combine with oxides of nitrogen (NOx) to form ozone in the presence of sunlight or heat. The SIPs/TIPs recognize the role of precursors and establish their budget of emissions to limit the formation of the criteria pollutants for which a NAAQS exists. The General Conformity requirements recognize the importance of precursors and establishes annual de minimis emission rates for precursors as well as the criteria pollutants. Precursors such as VOC, NOx, and ammonia (NH₃) are not regulated by a NAAQS. In determining whether the emissions of criteria or precursor pollutants equal or exceed their annual de minimis rates, the annual rate of emission increases of each criteria and precursor pollutant is analyzed separately for each year analyzed, including those years as required under § 93.159(d). Federal agencies do not need to sum the total of the precursor emissions. For example, if the annual de minimis emission rate for NOx is 100 tons per year and the annual de minimis emission rates 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 annual de minimis emissions rates for the area.

Annual de minimis emission rates are available for precursors of ozone and fine particle emissions, which are the only criteria precursors for which federal agencies are required to include in their applicability analysis. Although the precursors are evaluated separately against their annual de minimis emission rate, 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 emissions of another precursor of the same pollutant.

3.2.4 Multiple affected areas

In some cases, a proposed project might include activities that, if implemented, would cause emissions to originate at a distance from the largest part or main focus of the proposed project but are still within the same nonattainment or maintenance affected area. Other times, an activity might cause emissions to originate within an altogether different nonattainment or maintenance area than the largest part or main focus of the project. Further, the main focus of a project may be located within an attainment area (but not a maintenance area) while one or more of its activities would be implemented within a nonattainment or maintenance area. Any of these emissions that originate within a nonattainment or maintenance area must all be included in the General Conformity applicability analysis.

A federal agency must not assume that because, for example, construction activities, which are part of the scope of work, but occur at some arbitrary distance from the main focus of the project or occur within an entirely different area that is a nonattainment or maintenance area, that the emissions are somehow exempt or otherwise not subject to General Conformity. Neither can a federal agency claim it has no jurisdiction to control emissions from any source that is otherwise applicable under General Conformity but located outside the attainment, nonattainment, or maintenance area where the main focus of the project is located.

The federal agency is required to include in its applicability analysis emissions from all applicable activities included in the scope of work, particularly where the activities are assessed in the NEPA document, if one exists. If the project includes the use of a lengthened local roadway or access road (not a federal transit project), the agency should assume there will be vehicles using the roadway and include those emissions in the applicability analysis. Also, for construction, add the emissions from an average commute distance for vehicles driven to/from the site by employees. Emissions from vehicles servicing the facility are treated in the same manner as those from employee commuters.

However, the additional emissions from other activities of the employees such as shopping and recreational driving within the affected area would not be included in the applicability analysis, since the agency would not have a continuing program of responsibility for these activities.

3.2.5 Out of season emissions

The annual de minimis rates are established on a tons-per-year basis without regard for the time of year that a pollutant is emitted. Thus, annual emission rates per calendar year are used, and not on a “rolling 12 months” basis. If a conformity demonstration analysis is required because implementation of one or more activities would equal or exceed the associated annual de minimis rates, then the season within which the emissions would occur may be relevant to determining if the emissions conform with the SIP/TIP. If the emissions from the activities occur outside of the pollutant season, the agency should discuss this issue with the appropriate state, Tribal, or local air quality agency.

For example, a federal agency might propose to fund or approve construction activities within a carbon dioxide nonattainment or maintenance area. Violations of the carbon monoxide (CO) standard would only occur in the winter months and the SIP requirements and emission budgets only apply within those months. If the implementation of construction activities are not possible within the winter months because of the climate, meaning the SIP/TIP has no budget for construction emissions for those months, and the TDI emissions of CO in those months equal or exceed the 100 tons per year de minimis rate for CO, then the federal agency must prepare a demonstration of conformity and based upon that demonstration, make a General Conformity determination. However, by acting according to its statutory continuing program authority, a federal agency could legally limit the construction activities to occur during non-winter months when the SIP/TIP includes an emission budget for construction. The state or Tribe would need to concur that the SIP/TIP budget of emissions accounts for the construction emissions.

3.2.6 SIP/TIP Baseline emissions

The future years’ baseline emissions estimated for the nonattainment or maintenance area affected by the proposed activities are usually adjusted by the state or Tribal air agency to consider the historic activity levels, expected growth, and the appropriate emission factors for future years, but do not consider implementation of the proposed activities subject to General Conformity. Historic levels of activities is determined by the rate of activity increase or decrease used in the SIP/TIP. Where the EPA has not approved a SIP/TIP for the affected area, the most recent calendar year for which complete SIP/TIP emission projections were available before the area was designated nonattainment should be used, unless EPA sets another year for determining the baseline.

If a federal agency is approving its proposed activities within an area with an approved SIP/TIP, the baseline emissions would be defined as those emissions included in the SIP/TIP for the years being evaluated according to § 93.159.