Northeastern Illinois does not attain national ambient air quality standards (NAAQS) for ozone. Depending on the standard it is classified either as marginal or serious non-attainment area for the 8-hour ozone standard. The region must implement a transportation program which will help to reduce levels of these pollutants to national standards.
Nonattainment areas are designated by the U.S. Environmental Protection Agency (USEPA) based, in part, on recommendations from the Illinois Environmental Protection Agency (IEPA). IEPA's recommendation follows USEPA guidelines for identifying nonattainment areas. This includes not just monitor data, but also emissions data, urbanization patterns, meteorology, and so on. Technical information on this process can be found on the IEPA website.
The IL-IN-WI nonattainment area does not have the same geographic boundaries as MPO as they are established through a different process than then MPO boundaries are. With each standard a nonattainment area is established by the USEPA. The the nonattainment boundaries and the MPO boundaries are similar but not identical. The nonattainment area in northeast Illinois under the 1997, 2008, and 2015 ozone standards are the same and include: Cook, DuPage, Kane, Lake, McHenry, and Will counties and portions of Grundy (Aux Sable and Goose Lake Townships) and Kendall (Oswego Township) counties.
The nonattainment area under all three ozone standards also includes portions that are outside of Illinois. The 1997 ozone standard also included a portion of northwest Indiana and the 2008 and 2015 ozone standards also included portions of northwest Indiana and southeast Wisconsin. The Northwestern Indiana Regional Planning Commission and Southeastern Wisconsin Regional Planning Commission are responsible for fulfilling the transportation conformity requirements for those portions of the nonattainment area that are in there respective states while CMAP is responsible for transportation conformity in Northeast Illinois portion of the nonattainment area for each ozone standard.
The Chicago region (including the entire CMAP region) was redesignated as being in attainment of the 1997 PM2.5 NAAQS. As a result transportation conformity no longer applies on the effective date of the final PM2.5 SIP requirements rule, October 24, 2016. After that date, conformity determinations for the 1997 annual PM2.5 NAAQS for metropolitan transportation plans, metropolitan transportation improvement programs (TIPs), or transportation projects are no longer required in these areas for this NAAQS.
For more information regarding PM 2.5 and conformity see Transportation Conformity Guidance, specifically question 5 from the USEPA.
As part of the transportation planning and programming process, CMAP staff evaluates the impact of proposed transportation activities on the region's air quality. This evaluation, called a conformity analysis, must demonstrate that the a long-range regional transportation plan or Transportation Improvement Program (TIP) conforms before it is approved by the MPO or accepted by USDOT.
The conformity analysis must demonstrate that the emissions resulting from the long-range regional transportation plan or TIP meet the requirements of (i.e., "conform to") the regulations governing air quality.
Interagency consultation is required under the transportation conformity rule, as described in 40 CFR 51.402. In the northeastern Illinois region, these procedures are addressed through the Tier II Consultation process. Decisions made through this interagency consultation process guide the MPO in making conformity determinations.