EPA's regulations on greenhouse gas emissions from both mobile and stationary sources and on emissions of all kinds from electric power plants have been of particular interest, as have the agency's efforts to revise ambient air quality standards for ozone and particulate matter. EPA's Greenhouse Gas Regulations

A continuing focus of congressional interest under the Clean Air Act (CAA) has been EPA regulatory actions to limit greenhouse gas (GHG) emissions using existing CAA authority. GHG Emission Standards for Motor Vehicles

Unlike its critics, EPA has concluded that the Clean Air Act requires action to control GHG emissions, without the need for additional congressional authorization: a 2007 Supreme Court decision interpreting EPA's existing CAA authority, Massachusetts v. EPA , found that the agency must weigh whether GHG emissions from new motor vehicles endanger public health and welfare (or present valid reasons why it is unable to make that determination) and, if it concludes that there is endangerment, proceed with regulation of such vehicles. In his 2014 State of the Union message and a subsequent directive to EPA and DOT, the President directed the agencies to develop a second round of these standards, to be proposed in 2015 and finalized a year later. EPA did not meet the deadlines of the two consent agreements, but it did finalize emission standards for new, existing, and modified power plants in August 2015. The rule for existing units, the Clean Power Plan (CPP), has received the most attention. Legislative and Judicial Actions

Following publication of the NSPS and the Clean Power Plan in the Federal Register , Congress considered and passed joint resolutions of disapproval of both rules ( S.J.Res. 24 ) under the Congressional Review Act (CRA). The President vetoed both of the joint resolutions that Congress passed on December 18, 2015. On June 24, 2015, the House passed H.R. 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 determines that the rule would have significant adverse effects on rate-payers or on the reliability of the state's electricity system. The Senate did not consider the bill. In the 113 th , the House passed H.R. 3826 , which would have prohibited EPA from promulgating or implementing GHG emission standards for fossil-fueled power plants until at least six power plants representative of the operating characteristics of electric generation units at different locations across the United States had demonstrated compliance with proposed emission limits for a continuous period of 12 months on a commercial basis. 3826 in H.R. Emissions of Other Pollutants from Power Plants

Issues related to emissions other than GHGs from electric power plants—principally sulfur dioxide (SO 2 ), nitrogen oxides (NOx), and mercury—have been another focus of interest in recent years. Two rules affecting these emissions—the Cross-State Air Pollution Rule (CSAPR, pronounced "Casper") and the Mercury and Air Toxics Standards (MATS) —have taken effect, in January and April 2015 respectively. As a result, under the Obama Administration, EPA developed new regulations to address the court's concerns:
CSAPR, promulgated on August 8, 2011, established a cap-and-trade system for SO 2 and NOx emissions; MATS, promulgated on February 16, 2012, set Maximum Achievable Control Technology standards for power plant emissions of mercury and other hazardous air pollutants. Both bills passed the House, but the Senate did not consider either. Air Quality Standards

The Obama Administration's EPA has reviewed several national ambient air quality standards (NAAQS), as it is required to do at five-year intervals by Section 109 of the Clean Air Act. In January 2010, EPA proposed a revision to the ozone NAAQS. The final standards were released on October 1, 2015, and appeared in the Federal Register , October 26, 2015. At least 13 bills were introduced in the 114 th Congress to modify EPA's authority or prohibit or delay the agency's proposed strengthening of the ozone NAAQS: H.R. 1044 ; H.R. 1327 / S. 640 ; H.R. 1388 / S. 751 ; H.R. 2111 ; H.R. 2822 (Section 438); H.R. 4000 ; H.R. 4265 ; H.R. 4775 / S. 2882 ; H.R. 5538 (Section 438); and S. 2072 . In addition, joint resolutions of disapproval of the ozone NAAQS revision were introduced in both the House and Senate, under the Congressional Review Act.
Oversight of Environmental Protection Agency (EPA) regulatory actions received significant attention in the 114th Congress. Of particular interest were two air quality issues: EPA's Clean Power Plan (CPP) and related rules to regulate greenhouse gas (GHG) emissions from new and existing power plants, promulgated on August 3, 2015; and a revision of the ambient air quality standard for ozone, promulgated on October 1, 2015.
Reducing GHG emissions to address climate change was a major goal of President Obama, but many in Congress have been less enthusiastic about it. In the absence of congressional action to reduce emissions, the President directed EPA to promulgate GHG standards using existing authority under the Clean Air Act. This authority has been upheld on three occasions by the Supreme Court, but it remains controversial in Congress.
In 2014, EPA proposed regulations to reduce GHG emissions from fossil-fueled (coal, oil, and natural gas) power plants, which EPA refers to as electric generating units (EGUs). The agency proposed standards for new EGUs in January 2014 and for existing and modified units five months later. It finalized these rules August 3, 2015. EGUs are the source of 30% of the nation's GHG emissions, so it is difficult to envision a regulatory scheme that reduces the nation's GHG emissions without addressing their contribution. At the same time, affordable and reliable electric power is central to the nation's economy and to the health and well-being of the population. Thus, the potential effects of the rules on the electric power system have been of considerable interest.
Even before proposal of the Clean Power Plan, the House had passed legislation (H.R. 3826 in the 113th Congress) that would effectively have prohibited EPA from promulgating or implementing power plant GHG emission standards. In September 2014, the House passed the same language a second time, in H.R. 2. The Senate did not consider either bill.
Following promulgation of the CPP, however, in December 2015 Congress passed and sent to the President S.J.Res. 24, a joint resolution disapproving the CPP under the Congressional Review Act. The President vetoed the resolution on December 18, 2015. Earlier, the House passed H.R. 2042, which would have delayed the compliance date of GHG emission standards for EGUs 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. The Senate did not act on the bill.
Besides addressing climate change, EPA took action on a number of other air pollution regulations affecting power plants and other sources, often in response to court actions remanding previous rules or setting deadlines for actions that are non-discretionary under the Clean Air Act. Remanded rules included the Clean Air Interstate Rule (CAIR) and Clean Air Mercury Rule—rules designed to control the long-range transport of sulfur dioxide, nitrogen oxides, and mercury from power plants through cap-and-trade programs. New rules—the Cross-State Air Pollution Rule (CSAPR) and the Mercury and Air Toxics Standards (MATS)—have replaced those the court remanded. Both CSAPR and MATS went into effect in 2015.
EPA also completed a review of the national ambient air quality standards (NAAQS) for ozone in 2015. NAAQS serve as EPA's definition of clean air for six widespread pollutants, and drive a range of regulatory controls. The ozone NAAQS review, completed October 1, 2015, resulted in tightening the ozone NAAQS from 75 ppb to 70 ppb. At least 13 bills were introduced to modify EPA's authority or prohibit or delay the agency's proposed strengthening of the ozone NAAQS: H.R. 1044, H.R. 1327/S. 640, H.R. 1388/S. 751, H.R. 2111, H.R. 2822 (Section 438), H.R. 4000, H.R. 4265, H.R. 4775/S. 2882, H.R. 5538 (Section 438), and S. 2072. In addition, joint resolutions of disapproval of the ozone NAAQS revision were introduced in both the House and Senate, under the Congressional Review Act, but not acted on.