The Trump administration is threatening California with sanctions that include losing highway funding days after the state sued to block a federal rule revoking its authority to set its own greenhouse gas standards for vehicles.

In doing so, U.S. Environmental Protection Agency Administrator Andrew Wheeler accused the state in a Sept. 24 letter of failing to carry out its "most basic tasks" under the Clean Air Act and having "the worst air quality in the United States."

At issue is a backlog of more than 130 state implementation plans, known as SIPs, that are aimed at bringing areas within California into attainment with the National Ambient Air Quality Standards, or NAAQS, according to EPA officials. Most of those SIPs are inactive and "appear to have fundamental issues related to approvability, state-requested holds, missing information or resources," Wheeler said in his letter.

Wheeler therefore recommended that California withdraw any unapprovable plans and "work with the EPA to develop complete, approvable SIPs," warning that the EPA will begin the disapproval process if the state fails to do so. That could trigger sanctions that include a prohibition on federal transportation projects in certain parts of California, sanctions for large emissions sources, and the replacement of unapproved state plans with federal implementation plans, he noted.

Speaking to reporters on a Sept. 24 conference call, a senior EPA official declined to list other states that are significant contributors to the agency's SIP backlog. The same official said the agency's focus on California was unrelated to a joint rule the agency issued less than a week earlier with the National Highway Traffic Safety Administration. Originally included as part of an August 2018 proposal to freeze Obama-era vehicle efficiency standards, the separate regulation determined that California's long-standing authority to set its own tougher rules for vehicles under the Clean Air Act is preempted by federal law.

California and 22 other states asked a federal appeals court to vacate the rule a day after it was issued, arguing in part that it relies on its waiver authority to curb smog- and soot-forming emissions to meet the NAAQS. In particular, the state's zero-emission vehicles program and greenhouse gas tailpipe standards are necessary to meet the NAAQS in the extreme ozone nonattainment areas in the South Coast and San Joaquin Valley, where mobile source emissions have been the largest contributors to the formation of urban smog, California said in its legal brief.

Meanwhile, California Air Resources Board Executive Officer Richard Corey said Wheeler's letter contained multiple "inaccuracies, omissions and misstatements," contending that the EPA "sat on these documents for years and is now pounding the table about paperwork issues of its own creation."

"If the Trump administration is serious about air pollution it will reconsider revoking our waiver, and while they're at it, why not also fund the EPA to review submitted documents in less than a decade," Corey said in a statement.

Gay MacGregor, a former senior policy advisor in the EPA's Office of Transportation and Air Quality, said in a Sept. 24 interview that the EPA would likely need to regulate emissions sources such as locomotives and oceangoing vessels ⁠— two major drivers of air pollution that are exempt from state air regulations ⁠— if the agency planned to substitute federal implementation plans for California's state plans.

"If you look at the Central Valley and you look at South Coast, a lot of the emissions that are causing the air pollution problems currently are actually under the control of the federal government to regulate," MacGregor said.

NAAQS-related sanctions cannot be imposed until 18 months after the EPA administrator formally disapproves a SIP, and they may not be imposed if a deficiency has been corrected within that time period, MacGregor said. In his letter, Wheeler requested a response from California by Oct. 10 indicating whether it intends to withdraw the SIPs.