The CAA has established procedures and limitations for evaluating transportation projects in areas within the U.S. designated as nonattainment or maintenance 83 with respect to the NAAQS. The specific procedures, often referred to as Transportation Conformity regulations (see Section 8 , Conformity), require assessment of potential air quality impacts of all regionally significant planned and programmed transportation projects. The transportation conformity regulations for localized CO and PM hot-spot analyses are codified within 40 CFR Part 93, Subpart A 84 . 

In addition to the conformity regulation requirements, NEPA also requires environmental review of actions that have the potential to affect the environment (see Section 2 , Regulatory Framework ). Specifically, on-road transportation projects using federal -aid funds and/or requiring Federal Highway Administration (FHWA) or the Federal Transit Administration (FTA) approval actions must be evaluated for the potential impacts the actions will have on the natural and human environment. Air quality is one of several elements within the human environment to be considered. The NEPA requirements in regard to project -level FHWA/FTA air quality analyses are outlined in 23 CFR Part 771. 85 

The major difference between the project -level air quality requi rements under the CAA and those under the NEPA is that CAA hot -spot requirements apply to projects within specifically identified areas ( i.e., nonattainment/maintenance areas), whereas NEPA applies to federally - funded projects irrespective of location. 

When conducting a hot-spot analysis for the purposes of complying with NEPA or transportation conformity, the EPA -preferred/recommended dispersion and emission models 86 are the CAL3QHC/CAL3QHCR and AERMOD, and MOVES /EMFAC (see Section 5 , Air Quality Assessment Models ), respectively. 

## CO Project-level Analyses 

NEPA air quality analyses have typically focused on CO as the primary indicator for vehicular air quality impacts. A CO project -level hot-spot analysis is performed to ensure that new or worsened violations of the NAAQS will not occur as a result of a project/action. Presently, EPA in coordination with FHWA and FTA, has provided the following guidance documents for assessing CO hot-spot analysis for both project-level air quality requirements under the CAA and the NEPA: 

