Under Sections 108-109 of the Clean Air Act (CAA), Congress mandated that the Environmental Protection Agency (EPA) set national ambient (outdoor) air quality standards (or NAAQS) for pollutants whose emissions (1) "may reasonably be anticipated to endanger public health or welfare"; and (2) "the presence of which in the ambient air results from numerous or diverse mobile or stationary sources." The statute further requires that EPA review the latest scientific studies and either reaffirm or modify previously established NAAQS every five years. EPA has identified and promulgated NAAQS for six principal pollutants commonly referred to as "criteria pollutants": particulate matter (PM), ozone (O 3 , a key measure of smog), nitrogen dioxide (NO 2 , or, inclusively, nitrogen oxides, or NOx), sulfur oxides (SOx, or, specifically, SO 2 ), carbon monoxide (CO), and lead (Pb). On October 17, 2006, EPA published its revisions to the NAAQS for particulate matter (PM) to provide protection against potential health effects associated with short- and long-term exposure to particulates (including chronic respiratory disease and premature mortality). The 2006 particulates NAAQS primarily tightened the pre-existing (1997) standard for "fine" particulate matter 2.5 micrometers or less in diameter (PM 2.5 ). The standard for slightly larger, but still inhalable, particles less than or equal to 10 micrometers (PM 10 ) established in 1987 was not similarly changed. Establishing NAAQS does not directly limit emissions; rather, it represents the EPA Administrator's formal judgment regarding the level of ambient pollution that will protect public health with an adequate margin of safety . Promulgation of NAAQS sets in motion a process under which the states and EPA first identify geographic nonattainment areas, those areas failing to meet the NAAQS based on monitoring and analysis of relevant air quality data. States then have three years from the date of EPA's final designations to submit nonattainment area State Implementation Plans (SIPs), which identify specific regulations and emission control requirements that will bring an area into compliance. EPA published the final designations for the 2006 PM 2.5 NAAQS on November 13, 2009. The final designation rule became effective December 14, 2009 (30 days from the date of publication). The EPA final designations reflected a change from those announced by the agency in December 22, 2008. The publication of final designations—and thus the effective date of the final designations—had been delayed since January 2009 pending review by the incoming Administration. EPA initiated review of several previous agency decisions, including those covered by the White House January 20, 2009, memorandum, and the Office of Management and Budget's subsequent January 21, 2009, memorandum , on regulatory review. The 2006 particulates NAAQS generated national interest and debate, and oversight in Congress. Additionally, several states and industry, agriculture, business, and public advocacy groups petitioned the U.S. Court of Appeals for the District of Columbia Circuit, challenging certain aspects of EPA's 2006 revisions. A February 24, 2009, decision by the D.C. Circuit granted the petitions in part, denying other challenges, and remanded the standards to EPA for further consideration. While the court did not specifically vacate the 2006 PM standards, and their implementation proceeded, the decision and EPA's ongoing actions have prompted renewed interest in PM NAAQS among Members of Congress. Simultaneously with the ongoing implementation of the 2006 PM NAAQS, EPA initiated its next round of statutorily required periodic review of the PM NAAQS not long after the October 2006 promulgation of the standards, in part in response to the D.C. Circuit Court's decision. On January 15, 2013, EPA published a final rule further revising the PM NAAQS. The January 2013 revisions changed the existing (2006) annual health-based ("primary") standard for "fine" particulate matter 2.5 micrometers or less in diameter (or PM 2.5 ), lowering the allowable average concentration of PM 2.5 in the air from the current level of 15 micrograms per cubic meter (µg/m 3 ) to a limit of 12 µg/m 3 . The existing primary daily (24-hour) standard for PM 2.5 that was reduced from 65 µg/m 3 to 35 µg/m 3 in 2006 was retained, as was the existing standard for larger, but still inhalable, "coarse" particles less than 10 micrometers in diameter, or PM 10 . "Secondary" standards that provide protection against "welfare" (non-health) effects, such as ecological effects and material deterioration, are identical to the primary standards and the same as in 2006. The proposed rule regarding the PM NAAQS published June 29, 2012, solicited comments on two options for a 24-hour PM 2.5 standard to improve visibility that were not adopted in the final rule. For an overview of the revisions to the PM NAAQS published January 15, 2013, see CRS Report R42934, Air Quality: EPA's 2013 Changes to the Particulate Matter (PM) Standard , by [author name scrubbed]. The following sections of this report focus primarily on the NAAQS implementation process for designating geographical nonattainment areas with respect to the revisions to the PM 2.5 standards under the 2006 particulates NAAQS, including comparisons with the final designations under the 1997 PM 2.5 NAAQS. EPA did not require new nonattainment designations for PM 10 based on the 2006 revisions to the standard. Also included is a brief overview of states' subsequent obligations for developing and submitting implementation plans (SIPs) for attaining or maintaining compliance with the NAAQS. Appendix A contains a comparative time line for implementing the 2006 and 1997 PM 2.5 NAAQS. Appendix B provides a state and county breakdown of EPA's November 13, 2009, final designated nonattainment areas and areas identified by EPA on December 22, 2008, areas proposed by the states for the 2006 24-hour PM 2.5 NAAQS, as well as the final EPA nonattainment designations for the 1997 PM 2.5 NAAQS. Appendix C identifies tribes included in EPA's final nonattainment designations 2006 24-hour PM 2.5 NAAQS. The Clean Air Act (CAA) provides for two types of NAAQS: primary standards, "the attainment and maintenance of which in the judgment of the [EPA] Administrator ... are requisite to protect the public health," with "an adequate margin of safety"; and secondary standards, necessary to protect public welfare, a broad term that includes visibility impairment as well as damage to crops, vegetation, property, building materials, etc. The primary NAAQS include a daily (24-hour) limit for both PM 2.5 and PM 10 , and an annual limit for PM 2.5 (the previous annual limit for PM 10 was revoked). To attain the annual standard, the three-year average of the weighted annual arithmetic mean PM concentration at each monitor within an area must not exceed the maximum limit set by the agency. The 24-hour standards are a concentration-based percentile form, indicating the percentage of the time that a monitoring station can exceed the standard. For example, a 98 th percentile 24-hour standard indicates that a monitoring station can exceed the standard 2% of the days during the year. For PM 2.5 and PM 10 , the secondary (welfare) NAAQS are the same as the primary standards. As modified and published in the October 17, 2006, Federal Register Notice, the primary PM 2.5 and PM 10 standards are as follows: PM 2.5 : tightened the daily (24-hour) standard, which had allowed no more than 65 micrograms per cubic meter (µg/m 3 ) under the 1997 PM 2.5 NAAQS, by setting a new limit of 35 µg/m 3 , based on the three-year average of the 98 th percentile of 24-hour PM 2.5 concentrations; retained the annual standard at 15 µg/m 3 . PM 10 : retained the daily standard at 150 µg/m 3 set in 1987 but changed from the 99 th percentile to no more than one exceedance per year on average over three years; eliminated the annual maximum concentration (50 µg/m 3 ) standard for PM 10 . As discussed in more detail in the following section, strengthening the daily (24-hour) standard for PM 2.5 had implications for those counties and partial counties designated nonattainment for the 1997 PM 2.5 NAAQS. With only a few exceptions, most of the designated nonattainment areas for the 1997 PM 2.5 NAAQS were not meeting the annual standard , but were meeting the daily (24-hour) standard . EPA is not requiring new nonattainment designations for PM 10 since the standards were not strengthened. To the contrary, a number of counties previously designated nonattainment for PM 10 have been determined by EPA to be in attainment since the 2006 revisions of the particulates NAAQS were promulgated. Designating geographical areas not achieving the established NAAQS based on monitoring and analysis of relevant air quality data is a critical step in NAAQS implementation. Section 107(d) of the CAA (42 U.S.C. 7407) establishes the process for designating "attainment" and "nonattainment" areas and setting their boundaries, but allows the EPA Administrator some discretion in determining what the final boundaries of the areas will be. Areas are identified as nonattainment when they violate or contribute to the violation of NAAQS. The NAAQS designation process is intended as a cooperative federal-state-tribal process in which states and tribes provide initial designation recommendations to EPA for consideration. In Section 107(d)(1)(A) (42 U.S.C. 7407), the statute states that the governor of each state shall submit a list to EPA of all areas in the state, "designating as ... nonattainment, any area that does not meet ( or that contributes to ambient air quality in a nearby area that does not meet ) an air quality standard" (emphasis added). Areas are identified as "attainment/unclassified" when they meet the standard or when the data are insufficient for determining compliance with the NAAQS. Following state and tribal designation submissions, the EPA Administrator has discretion to make modifications, including to the area boundaries. As required by statute (§107(d)1(B)(ii)), the agency must notify the states and tribes regarding any modifications, allowing them sufficient opportunity to demonstrate why a proposed modification is inappropriate, but the final determination rests with EPA. Measuring and analyzing air quality to determine where NAAQS are not being met is a key step in determining an area's designation. Attainment or nonattainment designations are made primarily on the basis of three years of federally referenced monitoring data. EPA began developing methods for monitoring fine particles at the time the PM 2.5 NAAQS were being finalized in 1997, and operation of the network of monitors for PM 2.5 was phased in from 1999 through 2000. The network of monitors and their locations have been modified over time. In a separate action in conjunction with the October 2006 publication of the revised particulates NAAQS, EPA amended its national air quality monitoring requirements, including those for monitoring particle pollution. The amended monitoring requirements were intended to help federal, state, and local air quality agencies by adopting improvements in monitoring technology. EPA's final designations for the 2006 24-hour PM 2.5 NAAQS published November 13, 2009, were based on air quality monitoring data for calendar years 2006 through 2008. In addition to air emission and air quality data, EPA considers a number of other relevant factors, and recommends that states apply these factors in their determinations in conjunction with other technical guidance. Examples of these factors include population density and degree of urbanization (including commercial development), growth rates, traffic and commuting patterns, weather and transport patterns, and geography/topography. States and tribes may submit additional information on factors they believe are relevant for EPA to consider. Nonattainment areas include those counties where pollutant concentrations exceed the standard as well as those that contribute to exceedance of the standard in adjoining counties. Entire metropolitan areas tend to be designated nonattainment, even if only one county in the area has readings worse than the standard. In addition to identifying whether monitored violations are occurring, states' or tribes' boundary recommendations for an area are to also show that violations are not occurring in those portions of the recommended area that have been excluded, and that they do not contain emission sources that contribute to the observed violations. On November 13, 2009, EPA published its final designations of 31 areas in 18 states, comprising 120 counties (89 counties and portions of 31 additional counties) for nonattainment of the revised 2006 24-hour PM 2.5 standard. (See Table B -1 in Appendix B for state-by-state county/area nonattainment designations.) The counties (and partial counties) that EPA designated as nonattainment for the 2006 24-hour PM 2.5 NAAQS are indicated in the map in Figure 1 . The map distinguishes those counties not previously designated nonattainment for the PM 2.5 NAAQS and those areas that are being designated for the 2006 24-hour PM 2.5 24-hour standard, that were previously designated nonattainment for the 24-hour 1997 PM 2.5 NAAQS. In the February 3, 2011, Federal Registe r, EPA published its final designations of three areas as "nonattainment" or "unclassifiable/attainment" for the 2006 24-hour PM 2.5 NAAQS that were deferred at the time of the agency's November 13, 2009, promulgated designations. Designations were deferred so that EPA could evaluate measurements of high fine particle concentrations by violating monitors. EPA designated a portion of Pinal County, AZ, as nonattainment. This designation is not reflected in the map in Figure 1 . Eight surrounding counties were designated unclassifiable/attainment, and Gila River Indian Community and the Ak-Chin Indian community designations were deferred pending completion of the formal consultation process. Plumas County, CA, and Shasta County, CA, as well as surrounding counties, were designated unclassifiable/attainment. EPA's November 2009 final designations, which were based on air quality monitoring data for calendar years 2006 through 2008, do not include counties violating the annual standard, as the level is unchanged from the 1997 PM 2.5 NAAQS. As with the designations for the 1997 PM 2.5 and other criteria pollutant NAAQS standards, several areas include counties from multiple states. EPA also included portions of tribal lands of 22 tribes in areas designated nonattainment (see Appendix C for list of tribes included in the final 2006 24-hour PM 2.5 NAAQS nonattainment areas). EPA's final designations for the 2006 24-hour PM 2.5 standard differed from those identified by the agency on December 22, 2008, based on air quality monitoring data for calendar years 2005 through 2007. At that time, EPA had identified 58 areas in 25 states, comprising 211 counties (154 counties and portions of 57 additional counties) for designation as nonattainment of the revised 2006 24-hour PM 2.5 standard (see Table B -1 in Appendix B for a comparison of the state-by-state county/area nonattainment designations; counties (and partial counties) designated as nonattainment for the 2006 24-hour PM 2.5 NAAQS as of December 22, 2008, are indicated on the map in Figure D -1 of Appendix D ). Publication of a final designation rule for the 2006 24-hour PM 2.5 NAAQS was delayed pending review by the agency under the current Administration. The review of the final designation rule, along with several other agency proposed and final actions introduced toward the end of the previous Administration, was initiated, in part, in response to a White House January 20, 2009, memorandum, and the Office of Management and Budget's subsequent January 21, 2009, memorandum, regarding regulatory review. During this review, EPA revised its designations based on more current monitoring data (calendar years 2006-2008). Delaying publication of the final designation rule resulted in the delay of the expected effective date, which had been scheduled for April 2009. The effective date for the final designation rule for the 2006 PM NAAQS is December 14, 2009 (30 days from the date of publication). The majority of U.S. counties (including partial counties and tribal lands) were designated attainment/unclassifiable. While the final number of nonattainment areas and associated counties can seem small compared with the approximately 3,000 counties throughout the United States, nonattainment counties tend to have larger populations than those in attainment: for example, more than 70 million people (more than 20% of the U.S. population) live in the 120 counties designated nonattainment for the 2006 24-hour PM 2.5 NAAQS. The CAA does not specifically require combining neighboring counties within the same nonattainment area, but it does require the use of metropolitan statistical area boundaries in the more severely polluted areas (§107(d)(4)(A)(iv)). However, unlike the 1997 PM 2.5 standards, Metropolitan Statistical Areas or Consolidated Metropolitan Statistical Areas did not generally serve as the "presumptive boundary" for nonattainment areas under the 2006 24-hour PM 2.5 standards. Rather than establish a presumption for the minimum size of an area, in its June 2007 guidance EPA instructed states and tribes to evaluate each area on a case-by-case basis. EPA expected that nonattainment areas for the 2006 PM 2.5 would include counties with monitors violating the 24-hour standard and nearby counties that contribute to that violation. EPA also recommended that states and tribes consider using common boundaries for areas to be designated as nonattainment for both the annual and 24-hour PM 2.5 standards. This information in conjunction with air emission and air quality data, as well other relevant factors as recommended in EPA's guidance, such as population density, growth rates, traffic and commuting patterns, weather and transport patterns, and geography/topography, were used by states in determining the boundaries for the designated areas. By December 2007, 20 states provided EPA with recommended nonattainment boundaries for the 2006 revised particulates NAAQS based on 2004 to 2006 monitoring data. The states identified 46 areas comprising 116 counties, including 31 partial counties. The state-recommended designations were primarily based on air quality monitoring data for calendar years 2004 through 2006, criteria and technical guidance from EPA and assistance from its regional offices, and states' own relevant information and criteria. As required by statute, EPA responded to the states with its modifications to the area designation recommendations for the 2006 PM 2.5 NAAQS for the 24-hour standard in letters dated August 19, 2008. The agency identified 57 areas in 26 states, comprising 213 counties (167 counties and portions of 46 additional counties) for designation as nonattainment for the revised 2006 24-hour PM 2.5 standard. EPA solicited the states' comments and additional information for consideration in determining the final designations. As it did in implementing the 1997 PM 2.5 NAAQS and has done with other NAAQS, EPA used its discretion to expand the size of nonattainment areas (added more counties or portions of counties) or to combine areas that a state listed as separate areas into a single larger unit, EPA also combined nonattainment counties across state lines into the same nonattainment area, if the counties are part of the same metropolitan area. States (and tribal groups) had 120 days to respond to EPA's recommendations, and the agency also issued a notice for a 30-day public comment period. As occurred with EPA's proposed nonattainment designations for the 1997 PM 2.5 NAAQS, several states challenged the agency and maintained support for their original recommendations in response to the agency's proposed nonattainment areas for the 2006 24-hour PM 2.5 NAAQS. EPA designations for the 2006 24-hour PM 2.5 NAAQS as of December 22, 2008, based on 2005 through 2007 air monitoring data reflected minor modifications to those proposed; primarily, two counties were removed from the list of nonattainment areas, and other counties were redefined by designating only specified locations ("partial") within the county as nonattainment. As discussed previously in this report, the November 13, 2009, final designations for the 2006 24-hour PM 2.5 NAAQS based on 2006 through 2008 monitoring data resulted in fewer nonattainment areas/counties than EPA had proposed in August 2008 and identified as of December 2008. ( Table B -1 in Appendix B includes a state-by-state comparison of EPA's final county/area nonattainment designations with the agency's designations as of December 22, 2008.) EPA's final designations for the 2006 24-hour PM 2.5 NAAQS published in November 2009 included 27 fewer nonattainment counties than identified as of December 2008, 26 fewer areas than proposed in August 2008, and 17 fewer areas than recommended by the states. EPA's final nonattainment designation areas included 7 fewer states and 91 fewer counties than identified as of December 2008, 7 fewer states and 95 fewer counties than proposed, and 3 fewer states and 27 fewer counties than recommended by the states. EPA's final designations for nonattainment of the 1997 PM 2.5 NAAQS (those areas with or contributing to air quality levels exceeding the annual and 24-hour standards or both) included all or part of 204 counties in 20 states and the District of Columbia. As indicated in the map in Figure 2 , below, the designated nonattainment areas for the 1997 24-hour and annual PM 2.5 NAAQS are primarily concentrated in the central, mid-Atlantic, and southeastern states east of the Mississippi River, as well as in California. More than 2,900 counties in 30 states were designated attainment/unclassifiable for the 1997 PM 2.5 NAAQS. Table 1 below illustrates the comparative geographic distribution of counties designated nonattainment for the 2006 24-hour PM 2.5 NAAQS (based on EPA's final designations as published in the November 13, 2009, Federal Register ), and those counties in EPA's final area designations for the annual and 24-hour 1997 PM 2.5 NAAQS. Based on the final designations for the 2006 24-hour PM 2.5 NAAQS, 38 counties would be designated nonattainment for PM 2.5 for the first time but the majority of the counties identified overlap with EPA's final nonattainment designations for the 1997 PM 2.5 NAAQS. Most of the 1997 PM 2.5 nonattainment areas were only exceeding the annual standard; only 12 counties were exceeding both the 24-hour and the annual standards. Thus, tightening the 24-hour standard resulted in an increased number of areas (82 counties) being designated nonattainment based on exceedances of both the 24-hour and the annual standard. The map in Figure 3 , below, shows the overlap of the final nonattainment designations for the 24-hour standard, as modified under the 2006 24-hour PM 2.5 NAAQS, with the final designated nonattainment areas for the annual standard under the 1997 PM 2.5 NAAQS. It is difficult to assess what effect the overlap of nonattainment designations may have had on current control measures in these areas. In some areas, current measures that focused on achieving attainment for the annual standard may have been sufficient to attain the 24-hour standard as well. Other areas may have required supplementing current measures or significant modifications to ensure compliance over a shorter averaging period. The impacts could vary substantially from area to area within a state and from state to state depending on many factors, including the type and locations of primary emission sources, current control measures, the extent to which the area is exceeding the standard, topography, weather, etc. Once designations take effect, they become an important component of state, local, and tribal governments' efforts to reduce fine particle pollution. The designations govern what subsequent regulatory actions states, tribes, and EPA must take in order to improve or preserve air quality in each area. These often involve promulgation of new regulations by states, leading to the issuance of revised air permits, which affect pollution control requirements at various facilities. Under the CAA, EPA sets the nationwide standard for criteria pollutants, and EPA and states are responsible for placing limits on emissions that contribute to criteria pollution and for regulating entities emitting criteria pollutants. Within three years of issuance of a NAAQS, all states are required to submit "infrastructure" plans demonstrating that they have the basic air quality management components necessary to implement the NAAQS. Areas designated attainment/unclassifiable will not have to take steps to improve air quality but under the statute they must take steps to prevent air quality from deteriorating to unhealthy levels. For those areas designated nonattainment, state, local, and tribal governments must outline detailed control requirements in plans demonstrating how they will meet the 2006 PM 2.5 NAAQS. These plans, defined as state implementation plans and referred to as SIPs (TIPs for tribal implementation plans), were to be submitted to EPA by December 2012, three years after the effective date of the agency's final designations of December 14, 2009. If states fail to develop an adequate implementation plan, EPA can impose one. Under CAA, states are required to meet the 2006 PM 2.5 standard "as expeditiously as practicable," but no later than five years from the effective date of designation, which would be December 2014, unless an extension allowed under the CAA is granted. Within three years of promulgation of a new or revised NAAQS (or shorter period as EPA may prescribe), Section 110(a)(1) of the CAA requires states to submit revised SIPs that provide for implementation, maintenance, and enforcement of the new or revised NAAQS. All states are required to submit SIPs that include the basic program requirements for managing air quality required in Section 110(a)(2) of the CAA, showing that they have the capacity to attain, maintain, and enforce the revisions associated with the 2006 PM 2.5 NAAQS. These "infrastructure SIP" submissions must address a number of basic elements, including ambient air quality monitoring and data systems; programs for enforcement of control measures; adequate authority and resources to implement the plan; and prohibition of interstate pollution transport. Section 110(a)(2)(D)(i) of the act contains four elements that revised SIPs must address. The first two elements of this section require each state in its SIP to demonstrate adequate provisions for the ability to prohibit air emissions within the state that (1) contribute significantly to another state's nonattainment of the NAAQS, or (2) interfere with another state's maintenance of the NAAQS. Three years from the effective date of the final designations, states and local governments are required to develop and implement new or revised plans (SIPs) for addressing emissions in those "nonattainment" areas that do not meet the new or revised NAAQS. Nonattainment area SIPS for the 2006 PM 2.5 NAAQS were required to be submitted to EPA for review and approval by December 2012. As previously indicated, the effective date of the final designation rule is December 14, 2009 (30 days from the date of publication). Nonattainment area SIPs include pollution control measures that will be implemented by federal, state, and local governments, and rely on models of the impact on air quality of projected emission reductions to demonstrate attainment. SIPs must identify, among other items, specific regulations, emissions limitations, and monitoring provisions that will bring an area into compliance. Several counties designated as nonattainment for the PM 2.5 NAAQS were designated for the first time. As noted earlier, a large portion of the nonattainment areas for the 2006 PM 2.5 NAAQS designated by EPA November 2009, overlap with those areas designated nonattainment for the 1997 PM 2.5 NAAQS. However, as discussed in the previous section, these areas were unable to meet the 2006 revised 24-hour standard, whereas previously many of them were designated nonattainment based on their inability to meet the annual standard. Exceeding both an annual and 24-hour standard may have implications with respect to existing SIPs, the extent of which could vary significantly from area to area based on many factors. In some cases SIPs may require substantial modifications, while in other cases the current SIP may be sufficient to achieve compliance with both standards. In a February 2006 advance notice of proposed rulemaking (ANPR) outlining an implementation plan for the transition to the 2006 particulates standards, EPA indicated that it would be beneficial for states to consider control strategies that may be useful in attaining the 2006 revised PM 2.5 NAAQS when developing their strategies for the 1997 PM 2.5 standards. The basic framework of the implementation rule for the 1997 PM2.5 NAAQS published in April of 2007 provides guidance for implementation of the 2006 standards. The implementation rule describes the requirements that states and tribes must meet in their implementation plans to achieve and maintain attainment. The rule also provides guidance and procedures for establishing controls to achieve and maintain attainment. The implementation rule also includes guidance for submitting a SIP when reaching attainment within the five-year requirement is impractical. The implementation rule takes into account existing (oftentimes pending) federal regulations that contribute to controlling criteria pollutants and their precursors. In addition to the 2007 implementation rule for the 1997 PM 2.5 NAAQS, EPA outlined implementation guidance regarding the development of SIPs to demonstrate attainment with the 2006 PM 2.5 NAAQS in a March 12, 2012, memorandum to EPA Regional Air Directors. On June 9, 2010, EPA published a finding that 23 states, the District of Columbia, and 5 territories had failed to submit "infrastructure" SIPs to address the requirements of CAA Section 110(a)(2)(D)(i)(I) with respect to the 2006 PM 2.5 NAAQS. On July 20, 2011, EPA published a notice finding that one additional state had failed to submit a SIP to address those requirements. Both of EPA's findings notices addressed the first two of four elements under Section 110(a)(2)(D)(i) to satisfy interstate transport requirements. In an August 8, 2011, Federal Register notice, EPA promulgated a series of FIPs through the Cross-State Air Pollution Rule (CSAPR) which were to address the emissions from 28 states that significantly affect downwind states' ability to attain and maintain compliance with the 2006 (and 1997) PM 2.5 NAAQS (as well as the 1997 ozone NAAQS). , These FIPs limited emissions through the regulation of electric generating units. Eleven states identified in the findings notices were included in EPA's CSAPR. On August 21, 2012, the U.S. Court of Appeals for the District of Columbia issued a 2-1 decision to vacate and remand the CSAPR. On October 5, 2012, the U.S. Department of Justice filed a petition seeking en banc rehearing of the D.C. Circuit's August 21, 2012, decision regarding the CSAPR. The D.C. Circuit denied requests for both a panel and for the en banc rehearing on January 24, 2013. In a separate Notice published September 8, 2011, EPA notified six states, the District of Columbia, and the Commonwealth of Puerto Rico that they failed to submit complete infrastructure SIPs that include required elements to ensure the implementation of the 2006 24-hour PM 2.5 NAAQS. In some cases the submission was missing only one or two elements, while others were missing multiple elements. At the time this report was updated, EPA had approved, or approved portions of, submissions from a number of states. In most cases the elements not approved were with regard to interstate requirements. EPA anticipated that in many cases emission reductions from implementing national regulations and strategies would provide a framework for helping states achieve attainment with the PM 2.5 NAAQS. These national actions include the Cross-State Air Pollution Rule (CSAPR); Mercury and Air Toxics Standards (MATS); Light-Duty Vehicle Tier 2 Rule; Heavy Duty Diesel Rule; Clean Air Nonroad Diesel Rule; Regional Haze Regulations and Guidelines for Best Available Retrofit Technology Determinations; NOx Emission Standard for New Commercial Aircraft Engines; Emissions Standards for Locomotives and Marine Compression-Ignition Engines; Emission Standards Ignition Engines, Control of Emissions for Nonroad Spark Ignition Engines and Equipment; Category 3 Oceangoing Vessels; Reciprocating Internal Combustion Engines (RICE) National Emissions Standards for Hazardous Air Pollutants (NESHAPS); and New Source Performance Standards and Emissions Guidelines for Hospital/Medical/Infectious Waste Incinerators Final Rule Amendments. Stakeholders and some Members of Congress have been skeptical about EPA's expectations with respect to the corollary benefits associated with some of these regulations, and have raised concerns about pending efforts to delay some of the more recent programs and historical delays of others. For example, one of the key federal regulations that was designed to control emissions of air pollution that causes air quality problems in downwind states, EPA's May 2005 Clean Air Interstate Rule (CAIR), was vacated in a July 11, 2008, decision ( North Carolina v. EPA ), by the U.S. Court of Appeals for the D.C. Circuit. The D.C. Circuit subsequently modified its decision on December 23, 2008, in response to an EPA motion, reversing its decision to vacate CAIR while EPA developed a replacement rule. As discussed in the previous section of this report, the subsequent replacement rule, the Cross-State Air Pollution Rule ("Cross-State Rule" or CSAPR), was to have gone into effect in 2012 but was stayed in December 2011, then vacated on August 21, 2012, by the D.C. Circuit Court of Appeals. The U.S. Department of Justice filed a petition on October 5, 2012, seeking en banc rehearing of the D.C. Circuit's August 21, 2012, decision regarding the CSAPR, which was subsequently denied by the D.C. Circuit on January 24, 2013. Additionally, EPA itself has stayed the Mercury and Air Toxics Standards (MATS), pending reconsideration. Other remanded rules include the hazardous air pollutant ("MACT") standards for boilers and cement kilns. EPA has delayed implementation of the boiler MACT rules for more than a year and a half while considering changes to the requirements. The agency has also extended the compliance deadline for the cement kiln MACT by two years. These actions have had a disruptive effect on certain aspects of the implementation of the 2006 revised PM 2.5 NAAQS. Designated nonattainment areas also are subject to new source review (NSR) requirements. Enacted as part of the 1977 CAA Amendments and modified in the 1990 CAA Amendments, NSR is designed to ensure that newly constructed facilities, or substantially modified existing facilities, do not result in violation of applicable air quality standards. NSR provisions outline permitting requirements both for construction of new major pollution sources and for modifications to existing major pollution sources. The specific NSR requirements for affected sources depend on whether the sources are subject to "Prevention of Significant Deterioration" (PSD) or nonattainment provisions. EPA promulgated a rule in May 2008 that contained several NSR program requirements for sources that emit PM 2.5 and the pollutants that contribute to its formation (precursors), including SIPS modifications to state NSR programs to account for emissions of PM 2.5 . On October 20, 2010, EPA published a final rule amending requirements under the Clean Air Act PSD program for PM 2.5 . This final rule establishes several components addressing air quality modeling and monitoring provisions when making PSD permitting determinations for fine particle pollution. The final rule adds maximum allowable increases in ambient pollutant concentrations or "increments," which is the mechanism used to estimate significant deterioration of ambient air quality for a pollutant. EPA also added two screening tools, referred to as Significant Impact Levels (SILs) and Significant Monitoring Concentration (SMC) for PM 2.5 . SILs are used to determine whether a proposed source's emissions will have a "significant" impact on air quality in the area; the SMC may be used to determine if a source must submit to the permitting authority one year of pre-construction air quality monitoring data prior to constructing or modifying a facility. On May 10, 2011, EPA issued a final rule to repeal a provision in the rule implementing the NSR permitting program for PM 2.5 , known as "the grandfathering provision," in part in response to petitions to reconsider the May 2008 final NSR rule. The provision allowed applicants for federal PSD permits to rely on requirements for coarse particles (PM 10 ) as a surrogate for meeting the permit requirements for PM 2.5 if they applied for, and were awaiting approval for, a permit before July 15, 2008. The EPA decided not to take any action that would end the PM 10 Surrogate Policy. The policy enables sources to make a demonstration of compliance with PSD requirements for PM 10 as a surrogate for requirements for PM 2.5 in light of various technical issues associated with undertaking a PM 2.5 analysis, which EPA has now determined have generally been resolved. States with EPA-approved PSD programs could use the surrogate policy until May 2011, or until EPA approved the revised State Implementation Plan (SIP) for PM 2.5 . The agency had proposed to end the policy which had been in place since 1987, sooner than currently scheduled. If new or revised SIPs for attainment establish or revise a transportation-related emissions budget, or add or delete transportation control measures (TCMs), they will trigger "conformity" determinations. Transportation conformity is required by the CAA, Section 176(c), to prohibit federal funding and approval for highway and transit projects unless they are consistent with ("conform to") the air quality goals established by a SIP, and will not cause new air quality violations, worsen existing violations, or delay timely attainment of the national ambient air quality standards. On March 24, 2010, EPA published a final rule amending the transportation conformity regulation primarily to incorporate the October 17, 2006, strengthening of the 24-hour PM 2.5 air quality standard and revocation of the annual PM 10 standard. The final rule, which affects implementation of conformity in PM 2.5 and PM 10 nonattainment and maintenance areas, also addresses a court remand concerning hot-spot analyses as they apply to PM 2.5 and PM 10 , as well as to carbon monoxide and nonattainment and maintenance areas. On March 14, 2012, EPA published a final rule restructuring sections of the conformity rule (40 C.F.R. 93.109 and 93.119) so that existing requirements apply to new or revised NAAQS, and released associated implementation guidance in July 2012. The rule is intended to remove the need to amend the transportation conformity rule merely to reference specific new NAAQS. The designation of geographical areas unable to meet the NAAQS is a critical step in NAAQS implementation, and historically has been an issue of concern and debate among EPA, states and tribes, various stakeholders, and some Members of Congress. EPA's 2006 revisions of the PM 2.5 standards increased the number of areas (typically defined by counties or portions of counties) in nonattainment, and subsequently will potentially result in an encumbrance on states to achieve compliance. A February 24, 2009, decision by the U.S. Court of Appeals for the District of Columbia Circuit did not vacate the 2006 standards but granted petitions in part, denying other challenges, and remanded the standards to EPA for further consideration. Although EPA and states continue the implementation of the 2006 revised PM 2.5 standards, the EPA recently completed the next round of periodic review of the particulates NAAQS, and on January 15, 2013, EPA published a final rule further revising the PM 2.5 standards. On November 13, 2009, EPA published its final designations of areas as nonattainment and unclassifiable/attainment for the 2006 24-hour PM 2.5 NAAQS. Publication of a final designation rule for the 2006 24-hour PM 2.5 NAAQS was delayed pending review by the agency as part of the Administration's review of several proposed and final actions introduced toward the end of the previous Administration. Delaying publication of the final designation rule resulted in the delay of the expected effective date of April 2009 to December 14, 2009 (30 days from the date of publication). The November 2009 final designations, based on more current monitoring data (calendar years 2006-2008), differed from those areas EPA had included in its December 22, 2008, notifications to governors and tribal leaders and from EPA's August 2008 proposed modifications to nonattainment designation recommendations submitted by states. Historically, there have been disagreements between EPA and states, and other stakeholders, with regard to final NAAQS nonattainment designations, and questions have been raised with regard to the final designations for the 2006 24-hour PM 2.5 NAAQS. Within three years of promulgation of the 2006 PM 2.5 NAAQS, all states were required to demonstrate that they have the basic infrastructure and capacity necessary to implement the 2006 PM 2.5 NAAQS. By December 19, 2012, three years following the publication of the final designations, state, local, and tribal governments were to submit detailed control requirements in plans (or nonattainment SIPs) demonstrating how areas designated nonattainment will meet the 2006 standards. Although a large portion of the nonattainment areas for the 2006 PM 2.5 NAAQS designated by EPA in November 2009 overlap with those areas previously designated nonattainment for 1997 PM 2.5 NAAQS, these new designations are based on the 24-hour standard, whereas the majority of the previous designations were based on the annual standard. EPA anticipated that in many cases emission reductions from implementing national regulations and strategies would provide a framework for helping states achieve attainment with the PM 2.5 NAAQS. However, decisions by the D.C. Circuit to vacate two key federal regulations designed to control emissions of air pollution that causes air quality problems in downwind states, EPA's May 2005 CAIR and its replacement CSAPR published in 2011, have elicited concerns with regard to the implications this may have on the implementation of the 2006 PM 2.5 NAAQS. Nevertheless, recent air quality trends suggest that several factors, including national pollution control regulations, appear to be contributing to improved air quality. In a February 2012 report, "Our Nation's Air—Status and Trends through 2010," EPA reported that nationally, annual and 24-hour PM 2.5 concentrations were 24% and 28% lower, respectively, in 2010 compared to 2001. In its February 2012 report, EPA indicated that nationally, annual mean concentrations of SO 2 and NOx (both PM 2.5 precursors) decreased 33% and 50%, respectively, between 2001 and 2010. EPA expects air quality to continue to improve with implementation of more recently adopted regulations and states' work to meet current and recently revised national air quality standards. Key regulations include the Locomotive Engines and Marine Compression-Ignition Engines Rule, the Tier 2 Vehicle and Gasoline Sulfur Rule, the Heavy-Duty Highway Diesel Rule, the Clean Air Non-Road Diesel Rule, and the Mobile Source Air Toxics Rule. Ultimately, the continued implementation of these national regulations and associated air quality improvements should help nonattainment areas in achieving compliance with the tighter 2006 PM 2.5 NAAQS. The D.C. Circuit's February 2009 decision regarding challenges to the 2006 PM NAAQS, delays in designating nonattainment areas and implementing the 2006 PM 2.5 NAAQS, impacts of the D.C. Circuit's decisions regarding the 2005 CAIR and 2011 CSAPR transport regulations, and potential implications of EPA's January 2013 revisions to the PM 2.5 NAAQS have prompted increased interest in the 2006 PM 2.5 NAAQS revisions. Implications of the various issues could vary significantly from area to area based on numerous factors. The associated impacts on specific geographical nonattainment areas would be speculative at best. Implementation milestones and attainment deadlines for the 2006 PM 2.5 NAAQS remain an area of concern for many stakeholders and interest groups, as well as Congress. Appendix A. Comparative Time Line for Implementing the 1997 and 2006 PM 2.5 NAAQS The time line presented in Table A -1 reflects the most recent key milestone dates for implementing the 1997 and the 2006 PM 2.5 NAAQS, including actual completions. These milestones are driven primarily by statutory requirements. The table follows an EPA milestone schedule outlined in an April 1, 2003, memorandum to EPA regional administrators that also provided the nonbinding guidance for implementation of the 1997 PM 2.5 area designations, and the agency's previous and current projected time lines for the 2006 PM 2.5 NAAQS. Appendix B. Comparison of EPA's Final and Interim Nonattainment Designation Areas for the 2006 24-Hour PM 2.5 NAAQS and the Final Nonattainment Designation Areas for the 1997 PM 2.5 NAAQS Appendix C. Tribal Lands: U.S. EPA Designations for the 2006 PM 2.5 NAAQS 24-Hour Standard Appendix D. Map Depicting Counties in Nonattainment for the 2006 24-Hour PM 2.5 NAAQS: EPA's Designation as of December 22, 2008 On December 22, 2008, EPA had identified 58 areas in 25 states, comprising 211 counties (154 counties and portions of 57 additional counties) for designation as nonattainment for the revised 2006 24-hour PM 2.5 standard. These designations, based on air quality monitoring data for calendar years 2005 through 2007, are indicated in the map below for purposes of comparison with the November 2009 final designations.

Summary: The Environmental Protection Agency (EPA) published revisions to the Clean Air Act (CAA) National Ambient Air Quality Standards (NAAQS) for particulate matter (particulates, or PM) on October 17, 2006. EPA's actions leading up to and following promulgation of the 2006 standard have been the subject of considerable congressional oversight. EPA and states' ongoing implementation of the standard, beginning with the designation of those geographical areas not in compliance, likewise has been an area of concern and debate among some Members of Congress, states, and other stakeholders for some time. EPA's most recent round of periodic review of the particulates NAAQS and final rule revising the PM NAAQS, published January 15, 2013, prompted further scrutiny of the ongoing implementation of the standards. Promulgation of NAAQS sets in motion a process under which the states and EPA identify areas that exceed the national standard ("nonattainment areas") using multi-year air quality monitoring data and other criteria, requiring states to take steps to reduce pollutant concentrations in order to achieve it. The publication of the final designations for the 2006 NAAQS—and thus the effective date of the final designations—had initially been delayed pending review by the current Administration. On November 13, 2009, EPA published its final designations for the 2006 PM NAAQS that included 120 counties and portions of counties in 18 states as nonattainment areas based on 2006 through 2008 air quality monitoring data. The final designations, which include tribal land of 22 tribes, were effective as of December 14, 2009. States have three years from the effective date to submit nonattainment area State Implementation Plans (SIPs), which identify specific regulations and emission control requirements that would bring a nonattainment area into compliance. The 2006 NAAQS strengthened the pre-existing (1997) standard for "fine" particulate matter 2.5 micrometers or less in diameter (PM2.5) by lowering the allowable daily concentration of PM2.5 in the air. The daily standard averaged over 24-hour periods was reduced from 65 micrograms per cubic meter (µg/m3) to 35 µg/m3. However, the annual PM2.5 standard, which addresses human health effects from chronic exposures to the pollutants, was unchanged from the 1997 standard of 15 µg/m3. The 2006 NAAQS did not substantially modify the daily standard for slightly larger, but still inhalable, particles less than or equal to 10 micrometers (PM10), retaining the 24-hour standard but revoking the annual standard for PM10. EPA's final nonattainment designations are only for the revised 2006 24-hour PM2.5 standard. EPA did not require new nonattainment designations for the PM2.5 annual standard and for PM10. The final designations for the 2006 PM2.5 NAAQS included a few areas designated nonattainment for PM2.5 for the first time, but, as expected, the majority of the counties identified overlapped with EPA's final nonattainment designations for the 1997 PM2.5 NAAQS. EPA's designations for the 1997 PM2.5 NAAQS included all or part of 204 counties in 20 states and the District of Columbia. Most of them were only exceeding the annual standard; only 12 counties were exceeding both the 24-hour and the annual standards. Thus, the 2006 tightening of the 24-hour standard resulted in an increased number of areas being designated nonattainment based on exceedances of both the 24-hour and the annual standards.