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While it is feasible to calculate project emissions directly, rather than as the difference between
two cases, this is recommended only for screening project options. Changes that affect
congestion or delay for aircraft or ground access vehicles have many secondary effects that are
difficult or impossible to calculate directly. Conversely, for a NAAQS assessment, emissions
must be estimated for each reasonable “build” and “no-build” alternative.
“Build” and “no-build” emission inventories should be prepared for the same year. It often will
be necessary to calculate emission impacts for future attainment years as well as for the year
during which the total direct and indirect emissions are greatest. In addition, an emissions
inventory will represent the net emissions, which is the sum of emission increases and decreases.
2 Analyses routinely do not consider the pollutant lead (Pb) since airports and air bases typically are not a
significant source of lead emissions. The chief source of lead at airports and air bases is the combustion
of leaded aviation gasoline in piston-engine aircraft. An analysis may need to consider lead if an airport
or air base has a significant amount of emissions from the combustion of leaded aviation gasoline in
piston-engine aircraft.
12
The type of emissions to include in each scenario also vary depending on the analysis being
performed. For example, under conformity regulations, emissions included in the inventory 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” (Reference 7).
Emission inventories can be very complex and difficult to calculate. For a more detailed
discussion on creating an emissions inventory, see Section Three.
2.1.3 Indirect Source Review
Some states require a review of emissions from indirect sources. In general, indirect sources are
stationary sources that attract or may attract sources of pollution (e.g., vehicles) and thus
indirectly cause the emission of air contaminants. Such indirect sources include airports as well
as highways and roads, parking facilities, sports and entertainment facilities, and office
buildings. The definition of indirect source may vary slightly by state. At this writing, indirect
source review requirements are required only for projects in certain parts of California,
Connecticut, Minnesota, New York, North Carolina, Oregon, Utah, Vermont, and Wisconsin.
The states that require indirect source review generally establish thresholds that serve as
guidelines for applying these rules. For example, a state may require indirect source review for
all projects that increase the total airport passengers by more than 100,000 passengers, add 1,000
new parking spaces, or increase aircraft operations by 1,000. Projects that exceed these
thresholds could be required to complete an indirect source analysis and obtain an indirect source
permit. See Appendix J for more information on state threshold criteria for indirect source
review.
2.1.4 Conformity
The FAA and USAF as Federal agencies are required to assure that an applicable proposed
action in a non-attainment or maintenance area “conforms” to any relevant State Implementation
Plan (SIP). This entails determining whether the emissions caused by the action at the airport or
air base are consistent with the state’s plan to meet the Federal air quality standards. Federal
actions subject to conformity are divided into two categories: transportation conformity and
general conformity.
A transportation conformity determination is required for any highway or transit project which is
proposed to receive funding assistance and approval through the Federal -Aid Highway program
or the Federal mass transit program, OR requires Federal Highway Administration (FHWA) or
Federal Transit Administration (FTA) approval for some aspect of the project, such as
connection to an interstate highway or deviation from applicable design standards on the
interstate system. 40 C.F.R. Part 51, Subpart T. Under such circumstances either the FHWA or
the FTA would be responsible for completing the transportation conformity determination not the
FAA or USAF. A transportation conformity determination is NOT required for INDIVIDUAL
PROJECTS which are NOT FHWA/FTA projects EXCEPT that Section 51.450 applies where
the highway or transit project is of REGIONAL SIGNIFICANCE.
Pursuant to Section 51.392, a project is of regional significance when it is on a facility which
serves regional transportational needs (such as access to and from the area outside the region,
major activity centers in the regions, major planned developments such as new retail malls, sport
　
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