Air Quality Modeling Given the Trump Administration’s Clean Air Act Actions
The Regulatory Backdrop
Air quality modeling often requires assumptions about future motor vehicle emissions standards. However, these assumptions have been complicated by (1) the 2025 revocation of California’s Clean Air Act waivers, which previously allowed California to adopt motor vehicle emissions regulations on its own, and (2) the 2026 federal rescission of the GHG endangerment finding under the Clean Air Act. More specifically, in May 2025, the U.S. Senate approved several bills under the Congressional Review Act which purported to rescind California’s Clean Air Act waivers. (H.J.Res. 87, H.J.Res. 88, H.J.Res. 89.) The U.S. Senate Parliamentarian and the Government Accountability Office both concluded that the bills were illegal. California and other states also filed a lawsuit challenging the waiver revocations. (See State of California et al v. U.S.A. et al. (2025) N.D. Cal Case No. 25-CV-04966.) Furthermore, the 2026 administration rescinded the GHG endangerment finding and removed federal GHG motor vehicle emissions standards from 40 CFR part 600. (Vol. 90 Federal Register, No. 146, 36288.) This decision has been challenged in American Public Health Association, et al. v. United States Environmental Protection Agency and Lee Zeldin, Administrator, Case No. 26-1037 (D.C. Cir., filed Feb. 18, 2026). Consequently, there is substantial legal uncertainty about future motor vehicle emissions standards. Given this uncertainty, public agencies should be able to “make reasonable assumptions based on substantial evidence about future conditions without guaranteeing that those assumptions will remain true.” (Cal. Pub. Resources Code, §21080(e); City of Del Mar v. City of San Diego (1982) 133 Cal.App.3d 401, 412.)
CARB’s Response
In response to this regulatory uncertainty, in January 2026 the California Air Resources Board (CARB) revised the emissions factors for air quality modeling programs and released revised greenhouse gas (GHG) adjustment factors for EMFAC2021 to remove the estimated emissions benefits underlying ACT, Zero-Emission Airport Shuttle, Omnibus and Warranty Phase I regulations. These adjustment factors were developed for the EMFAC2021 model rather than the more recent EMFAC2025 model version as the use of EMFAC2025 might result in underestimating future emissions if the rescission remains, depending in part upon the outcome of these cases.
Related Case Law
Similar modeling issues have arisen in other CEQA cases, which may provide useful guidance in navigating this uncertainty:
- Planning and Conservation League v. Castaic Lake Water Agency (2009) 180 Cal.App.4th 210, 249
- Bay Area Citizens v. Association of Bay Area Governments (2016) 248 Cal.App.4th 966, 1017.)