	Introduction

Review of Clean Air Act regulations issued under the Obama Administration, with the possibility of their modification or repeal, has been a major focus of the Trump Administration since it took office in 2017. The U.S. Environmental Protection Agency (EPA) has conducted these reviews as part of the Trump Administration's "regulatory reform" initiative under which the Administration has directed federal agencies to evaluate existing regulations and identify those that should be considered for replacement, repeal, or modification. In addition, Executive Order (E.O.) 13783 has directed EPA and other federal agencies to review existing regulations and policies that "potentially burden the development or use of domestically produced energy resources" for consistency with policies that the E.O. enumerates, and as soon as practicable, to "suspend, revise, or rescind the guidance, or publish for notice and comment proposed rules suspending, revising, or rescinding those rules." EPA rules to regulate greenhouse gas (GHG) emissions from power plants, cars and trucks, and the oil and gas sector have been of particular interest. EPA's Greenhouse Gas Regulations

EPAs regulatory actions to limit GHG emissions have relied on authority that Congress granted the agency in the Clean Air Act (CAA) Amendments of 1970. EPA finalized GHG standards for power plants in August 2015; set GHG emission standards for oil and gas industry sources in June 2016; finalized a second round of GHG standards for trucks in August 2016; and completed a Mid-Term Evaluation (MTE) of the already promulgated GHG standards for model years 2022-2025 light-duty vehicles (cars and light trucks) in January 2017. Most of these rules are under review at EPA; the agency has proposed repeal or modification in several cases. The Clean Power Plan (CPP), which is the rule for existing units, would set state-specific goals for CO 2 emissions or emission rates from existing fossil-fueled power plants. The agency therefore proposed repeal of the CPP on October 16, 2017, and a rule to replace it (the Affordable Clean Energy (ACE) rule) on August 21, 2018. The ACE rule would apply a narrower interpretation than the CPP of the best system of emission reduction (BSER), defining it as on-site heat rate improvements for existing coal-fired units. The NSPS revision and repeal and replacement of the CPP are still at the proposal stage. The EPA and judicial processes could be short-circuited by Congress, through legislation overturning, modifying, or affirming the CPP or NSPS. Congressional action is considered unlikely, however, as the threat of a filibuster, requiring 60 votes to proceed, could prevent Senate action. The new House majority has expressed a strong interest in addressing climate change. 13783, signed by President Trump on March 28, 2017. The second round of GHG standards for cars and light trucks is being phased in over model years (MY) 2017-2025. The proposed standards would lead to an estimated average fuel economy of 37 mpg for MY2020-2026 vehicles, causing a projected increase in fuel consumption of about 0.5 million barrels per day (equivalent to about 186,000 metric tons of carbon dioxide per day), according to EPA and DOT. Further, under the proposal, EPA aims to withdraw California's CAA preemption waiver for its vehicle GHG standards applicable to MYs 2021-2025. The CAA has minimal requirements for how the agency is to conduct NAAQS reviews, leaving the details to the EPA Administrator. Congress may undertake oversight, as EPA moves forward with these reviews. In October 2018, EPA made an unprecedented change and eliminated the pollutant-specific scientific review panels, which have historically helped agency staff conduct the five-year reviews. The standards have been challenged in court; the D.C. EPA faces a statutory deadline of October 2020 to complete a review of the ozone NAAQS and decide whether to modify or retain it. The agency is reviewing additional regulations, among them air toxics rules applicable to power plants, brick and ceramic kilns, and industrial sources of ethylene oxide as well as NSPS rules applicable to particulate matter from wood heaters. Mercury from Power Plants

EPA is reviewing the benefit-cost analysis it prepared in 2011 for the Mercury and Air Toxics (MATS) rule, raising questions about whether the agency will take additional action on the rulemaking in 2019. Other legislation was introduced in the 115 th Congress and may be reintroduced in the 116 th Congress.
Review and rollback of Clean Air Act rules to regulate greenhouse gas (GHG) emissions from power plants, cars and trucks, and the oil and gas sector has been a major focus of the Trump Administration since it took office in 2017. On March 28, 2017, President Trump signed Executive Order 13783, to require the review of regulations and policies that "burden the development or use of domestically produced energy resources." The E.O. directed the U.S. Environmental Protection Agency (EPA) to review the Clean Power Plan (CPP), which set limits on GHG emissions from existing power plants, and several other regulations for consistency with policies that the E.O. enumerates, and as soon as practicable, to "suspend, revise, or rescind the guidance, or publish for notice and comment proposed rules suspending, revising, or rescinding those rules." GHG rules for new power plants, for cars and trucks, and for methane emissions from the oil and gas industry, in addition to the CPP, are subject to the executive order and are under review at EPA, as well as being challenged in the courts.
The CPP, which was promulgated by the Obama Administration's EPA in 2015 and would limit GHG emissions from existing fossil-fueled power plants, has been one focus of debate. The Trump Administration's EPA has proposed to repeal the CPP and replace it with the Affordable Clean Energy rule (ACE), a rule that defines the "best system of emission reduction" for coal-fired power plant GHGs as efficiency improvement technologies. As proposed, the CPP repeal and ACE rules would remove federal numerical carbon dioxide (CO2) emission limits for existing coal- and natural gas-fired power plants, eliminating one backstop on power plant GHG emissions. Final agency action on ACE is expected later this year. Some Members of Congress have submitted comments to EPA on the ACE proposal. Congress may be interested in conducting oversight of the ACE rule.
Clean Air Act GHG standards for cars and light trucks are the subject of another EPA review. An August 2018 proposal would freeze EPA's GHG standards for new cars and light trucks at the level required in model year (MY) 2020. Current regulations, promulgated in 2012 and reaffirmed in January 2017, set increasingly stringent emission standards through MY2025. The EPA proposal would cause a projected increase in vehicle fuel consumption of about a half million barrels of gasoline per day (equivalent to about 186,000 metric tons of carbon dioxide per day) when fully implemented, according to EPA and the Department of Transportation. The proposal would also withdraw California's Clean Air Act waiver for new vehicle GHG standards applicable to MY2021-MY2025. The California standards have been adopted by 12 other states and cover about 35% of the new vehicle market.
Following promulgation of these or other Clean Air Act regulations, Congress could address the issues through legislation affirming, modifying, or overturning them. The threat of a filibuster, requiring 60 votes to proceed, however, has generally prevented Senate action. In the 116th Congress, the new majority in the House has indicated a greater interest in addressing climate change issues rather than rolling back regulations. One result may be a new focus on oversight of agency actions to address climate change and its impacts.
The 116th Congress may also be interested in issues related to EPA air quality standards for what are called "conventional" or "criteria" pollutants. EPA faces statutory deadlines to complete reviews of the National Ambient Air Quality Standards (NAAQS) for the two most widespread of this group: ozone and particulate matter (PM). The agency has proposed to speed up the review process, while simultaneously eliminating the scientific review panels that have historically assisted agency staff in conducting the reviews. The Clean Air Act has minimal requirements for how the agency is to conduct NAAQS reviews, leaving the details to the EPA Administrator. Nevertheless, congressional oversight is considered possible as EPA moves forward with the ozone and PM reviews.
Other issues Congress might consider include air toxics regulations (e.g., the Mercury and Air Toxics rule for power plants), standards for new residential wood heaters, and the Renewable Fuel Standard.