	Introduction

Congressional actions on air quality issues have been dominated since 2011 by efforts—particularly in the House—to change the Environmental Protection Agency's (EPA's) authority to promulgate or implement new emission control requirements. Often under court order, the Obama Administration's EPA used authorities Congress gave EPA in the Clean Air Act amendments of 1970, 1977, and 1990 to address long-standing issues posed by emissions from various sources. 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 2022-2025 GHG standards for light-duty vehicles (cars and light trucks) in January 2017. Most of these rules are under review at EPA. The agency asked the D.C. On October 27, 2017, the D.C. The Clean Power Plan (CPP), which is the rule for existing units, would set state-specific goals for CO 2 emissions and emission rates from existing fossil-fueled power plants. President Obama vetoed both of the joint resolutions. 2042 , which would have delayed the compliance date of GHG emission standards for existing EGUs (including the date by which states must submit implementation plans) until after the completion of judicial review of any aspect of the rule, and would have allowed a state to opt out of compliance if the governor determined that the rule would have significant adverse effects on rate-payers or on the reliability of the state's electricity system. Legislation was also considered in the 113 th and 112 th Congresses: the House passed bills to restrict EPA's authority to implement GHG emission requirements for power plants on five separate occasions. Implementation of the CPP was stayed by the Supreme Court in February 2016, pending the completion of judicial review. Challenges to the rule were filed with the U.S. Court of Appeals for the D.C. Circuit by more than 100 parties, including 27 states. Circuit heard oral argument in the case in September 2016; as of this writing, the court has not issued a decision. 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. directed EPA to review the Clean Power Plan, the NSPS, and 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." The EPA and judicial processes could be short-circuited by Congress, through legislation overturning or modifying the CPP or NSPS. The threat of a filibuster, requiring 60 votes to proceed, might prevent Senate action, however. The Administration stated that the rule was a key component under President Obama's "Climate Action Plan," and that the plan's Strategy to Reduce Methane Emissions was needed to set the Administration on track to achieve its goal to cut methane emissions from the oil and gas sector by 40%-45% from 2012 levels by 2025, and to reduce all domestic greenhouse gas emissions by 26%-28% from 2005 levels by 2025. directed EPA to review the rule for consistency with policies that the E.O. EPA has begun a review of the rule and has proposed a two-year stay of the rule during the review process. 2015 Revision of the Ozone NAAQS

Since 2008, review of the NAAQS for ozone has sparked recurrent controversy. These are not the data EPA is using to designate nonattainment areas under the standard: EPA's final designation decisions, announced May 1 and July 17, 2018, are generally using data for 2014-2016, and have identified 201 counties or partial counties and 2 tribal areas in 22 states and the District of Columbia as nonattainment. In the 115 th Congress, H.R. 806 and S. 263 would, among other provisions, delay designation of nonattainment areas under the 2015 ozone standard until 2025 and submission of State Implementation Plans until 2026, require future NAAQS reviews every 10 years instead of every 5, and allow consideration of technological feasibility in the setting of some primary (health-based) standards. 806 on July 18, 2017. In addition, the House passed H.R. 1119 , the SENSE Act, which would ease emission limits for electric generating units powered by coal refuse; and H.R.
Review of regulations issued under the Obama Administration, with the possibility of their modification or repeal, has been the main focus of interest on Clean Air Act issues in the 115th Congress and in the executive and judicial branches in 2017 and 2018. Of particular interest are the ambient air quality standards for ozone promulgated by the Environmental Protection Agency (EPA) in October 2015; EPA rules to regulate greenhouse gas (GHG) emissions from power plants, cars and trucks, and the oil and gas sector; and emission standards for three groups of sources: brick kilns, wood stoves and heaters, and power plants that burn waste coal.
Reducing GHG emissions to address climate change was a major goal of President Obama, but, for a variety of reasons, many in Congress have been opposed to it. In the absence of congressional action, President Obama directed EPA to promulgate GHG emission standards using existing Clean Air Act authority. This authority has been upheld on three occasions by the Supreme Court, but the specifics remain controversial and subject to debate in Congress, the Trump Administration, and the courts.
The Clean Power Plan (CPP), which would limit GHG emissions from existing fossil-fueled power plants, has been a frequent subject of debate. Implementation of the CPP has been stayed by the Supreme Court since February 2016, pending the completion of judicial review. Prior to the stay, challenges to the rule had been filed with the U.S. Court of Appeals for the D.C. Circuit by more than 100 parties, including 27 states. The D.C. Circuit heard oral argument in the case in September 2016; as of this writing, the court has not issued a decision. The Trump Administration's EPA has proposed to repeal the CPP and has asked for public input on what should replace it. The D.C. Circuit has stayed the litigation during EPA's review of the CPP.
More broadly, 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. The E.O. directed EPA to review the Clean Power Plan 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 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.
Congress could, of course, short-circuit the EPA and judicial processes, through legislation overturning or modifying any of these regulations. The threat of a filibuster, requiring 60 votes to proceed, might prevent Senate action, however.
An EPA rule that has been the subject of action in the 115th Congress is the National Ambient Air Quality Standard (NAAQS) for ozone, which EPA revised in October 2015. Under the revised NAAQS, EPA was to have designated areas that have not attained the standard in October 2017, setting in motion tighter emission requirements for a multitude of industrial, commercial, and mobile sources of pollution. On May 1 and July 17, 2018, the agency took action, identifying 52 areas in 22 states and the District of Columbia as being in nonattainment of the revised standard.
On July 18, 2017, the House passed H.R. 806, which would have delayed designation of the nonattainment areas until 2025 and would require future reviews of the NAAQS every 10 years instead of every 5, among other provisions. The House has also passed legislation to delay emission standards for brick kilns and wood heaters (both in H.R. 1917) and to exempt coal-refuse-fired electric generating units (EGUs) from certain emission standards that apply to other coal-fired EGUs (H.R. 1119). All of the House-passed bills await action in the Senate.