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On October 1, 2015, the U.S. EPA strengthened the National Ambient Air Quality Standard (NAAQS) for ground-level ozone by lowering the standard from a value of 75 parts per billion (ppb) to 70 ppb. The revised standard was established based on extensive scientific evidence obtained by EPA about ozone’s effects on public health and welfare.

Based on current monitoring networks, EPA projects that the majority of the U.S. will meet the revised standard by 2025. To do so, however, states will need to work closely with EPA to finalize area designations and implement control measures to comply with the lowered standard through rulemaking.


For each criteria pollutant, Sections 108 and 109 of the Clean Air Act (CAA) require EPA to set a primary standard that protects public health with “an adequate margin of safety” and a secondary standard that protects public welfare from “any known or anticipated adverse effects.” The CAA also requires periodic review of the science upon which the standards are based and review of the standards themselves. Ozone, the primary component of smog, is one of the regulated “criteria pollutants” under the NAAQS program.

The previous ozone NAAQS of 75 ppb was established in 2008. In 2010, EPA proposed to strengthen the NAAQS for ground-level ozone to a level within the range of 60-70 ppb. In September 2011, President Obama issued a statement requesting EPA withdraw the proposed ozone NAAQS to reduce regulatory burdens and regulatory uncertainty during the period of economic recovery. In April 2014, the U.S. District Court (Northern District of California) ordered EPA to review and issue a final ozone standard by October 1, 2015, a court-mandated deadline.

The Revised Ozone NAAQS

The 2015 final rule sets both primary and secondary 8-hour ozone NAAQS at 70 ppb. In setting the primary ozone standard, EPA examined the body of scientific evidence on ozone and health, including new clinical studies. These studies provided evidence of health effects in adults, determining that ozone can be harmful to healthy adults at 72 ppb. EPA also reviewed results of analyses of exposure to ozone and looked at how different levels of the standard would reduce risk. Advice from the Clean Air Scientific Advisory Committee (CASAC) and public comments on the November 2014 proposal were also considered. For the establishment of the secondary (welfare) ozone standard, EPA took into consideration new studies which add to evidence that repeated exposure to ozone reduces the growth of plants and trees and has other harmful vegetative effects. Areas will meet the standards if the fourth-highest daily maximum 8-hour ozone concentration per year, averaged over three years, does not exceed 70 ppb. The new standard will cause parts of the country that are currently in attainment with the 2008 ozone NAAQS to be reclassified as non-attainment with the revised ozone NAAQS. Figures 1 and 2 demonstrate that revising the ozone NAAQS from 75 ppb down to 70 ppb increases the total number of counties violating the new standard from 28 to 241 as of October 2015.

EQ Winter 2016 Ground-Level Ozone Fig 1

Figure 2 _Ozone Map 3

As part of the revised ozone standard, EPA also updated the current Air Quality Index (AQI), a number used by government agencies to communicate to the public how polluted the air currently is Figure 3_ozone chartor how polluted it is forecasted to be. A comparison of AQI under the 2008 and revised ozone NAAQS is provided in Figure 3, where the revised primary NAAQS now equals 100 on the index.

Other changes made by EPA as part of the revised ozone rulemaking include the following:

  • Addition of an Appendix U to 40 CFR Part 50 detailing data selection, handling, and reporting requirements for ozone NAAQS
  • Revision of ambient monitoring requirements for ozone monitoring
  • Addition of a grandfathering provision to the Prevention of Significant Deterioration (PSD) permitting program, exempting pending permits from the revised ozone NAAQS when they are fully promulgated

As part of the final rule, EPA has outlined initial steps the agency will take to help states implement the revised standards, including the anticipated area designation schedule targeted for late 2017. All states with nonattainment areas (NAAs) must develop emission inventories and implement a preconstruction permitting program designed to provide additional air quality safeguards for those areas. States with nonattainment areas classified as “Moderate” or higher must develop state implementation plans (SIPs) to ensure reasonable further progress (RFP) toward attaining the standard. The SIPs must detail the state’s intended efforts to meet the NAAQS by describing the targets, plans, and control strategies for each area designated as nonattainment. In developing SIPs and the control strategies proposed for nonattainment areas, states will need to consider various parameters such as monitoring data, emissions inventories, and photochemical modeling in order to determine the degree of emission reduction necessary for NAAQS attainment. Emissions inventories must also be developed for various types of sources in the affected areas to facilitate the investigation of control strategies. Upon completion, the SIPs must be submitted to EPA for review and approval. Upon final SIP approval, the states will commence rulemaking to promulgate and adopt the proposed rules intended to achieve the required reductions of NOX and/or VOCs (ozone precursors).

States also must adopt reasonably available control technology (RACT) standards for certain types of emission sources in the nonattainment area to implement the proposed control strategies and achieve the required reductions in NOX and/or VOC as determined by the SIPs. Each state may choose to develop its own RACT requirements on a case-by-case basis. For certain source categories, states may also decide to use Maximum Achievable Control Technology (MACT) standards to establish the RACT level of control.

EPA’s projections indicate that the vast majority of U.S. counties will meet the new ozone standards by 2025, with the deadline for all nonattainment areas to meet the primary (health) standard being 2037. Figure 4 illustrates the expected designation and compliance schedule for the revised ozone NAAQS.

ozone timeline_new

Implications of the Revised Ozone NAAQS

Impact on Major Source Thresholds
The lowering of the ozone NAAQS will affect the definition of a “major stationary source.” Table 1 Table 1_ozonesummarizes the new major source thresholds and major modification thresholds for ozone precursors (NOX and VOC), based on the classification of the nonattainment area. Note that EPA has not yet released the thresholds for the classification categories for the revised ozone NAAQS, and has indicated the thresholds will be established as part of future implementation guidance/rulemaking.

NNSR Permitting Implications
For areas that are reclassified as non-attainment for ozone, the process of acquiring proper authorization to construct new major sources of air pollutants or make significant modifications to existing major sources will become more difficult. This is mainly because the pertinent permitting program for new non-attainment areas changes from PSD or minor New Source Review (NSR) to Non-Attainment New Source Review (NNSR). Under the more stringent NNSR program, new or modified sources must offset emission increases of ozone precursors (often at a ratio greater than 1:1) by identifying and acquiring emission reductions from other existing sources in the non-attainment area. Finding available offsets in newly designated non-attainment areas could pose a challenge. The cost for emission credits in existing non-attainment areas that are reclassified to a more stringent nonattainment designation is also expected to increase. New or modified sources must also install more expensive control technologies as part of the project to satisfy regulatory requirements, i.e., Lowest Achievable Emission Rate (LAER) will replace Best Available Control Technology (BACT), which can significantly increase the capital cost of the project as LAER does not consider the cost effectiveness of the control requirement. In addition, some states may require the applicant to operate Continuous Emission Monitors (CEMS) and/or monitor certain parameters of the control devices to guarantee that the devices are operating optimally and achieving the required emission reductions. An extended permitting timeline would also be expected and should be taken into consideration for planning purposes.

A lower major source threshold could also trigger Title V permitting requirements for current minor NSR sites. The significance threshold for NNSR permitting may also be reduced, and a higher emission offset ratio may be required for projects that do trigger NNSR review.

PSD Grandfathering Provision
The final ozone NAAQS rule includes a grandfathering provision to ensure that compliance with the updated ozone standards will not delay final processing of certain pending preconstruction permit applications. This is important because, without a grandfathering provision, any facility that did not have a final PSD permit issued prior to the effective date of the new or revised NAAQS would have been required, per established EPA policy, to evaluate the project’s impacts against the NAAQS effective at the time of permit issuance. The grandfathering provision will apply to a PSD permit application if either of the following conditions is met: 

  • The permitting agency has formally determined the application to be complete as of Oct. 1, 2015; or
  • The public notice for a draft permit or preliminary determination has been published prior to the date the revised standards become effective (60 days after publication in the Federal Register – December 28, 2015).

Modeling for Ozone Impacts
While ozone has typically not been the focus of PSD permitting in the past, with only general qualitative assessments of ozone impacts completed as part of the PSD permitting analysis, EPA has recently been developing additional tools and guidance for advanced assessment of ozone impacts, potentially including air dispersion modeling. EPA has provided, as part of a separate proposed rulemaking (40 CFR Part 51, Appendix W, the Guideline on Air Quality Models), new guidance for ozone impact assessments. It is strongly recommended that owners/operators with a PSD permit application currently under review, or who will be submitting one soon to a permitting regulatory authority, ensure clear understanding of the expectations of the permitting authority in relation to any ozone impact assessment. The proposed rule was published in the Federal Register on July 29, 2015, and the public comment period closed on October 27, 2015. The rule is expected to be finalized in the spring of 2016.


With the implementation of the revised ground-level ozone NAAQS, areas which previously demonstrated attainment for the 2008 standard may be re-designated as non-attainment for the new standard. Due to the changes in the ozone attainment designations for counties within a state, each state must develop plans to address the achievement of attainment in these areas. Industries located in these areas must be aware of the implications of these re-designations with regard to RACT, NSR permitting, and Title V permitting, and begin to plan accordingly. Failure to consider these re-designations may result in significant costs to a site, which may have been otherwise avoided. Any permitting project involving ozone precursors should be executed with extra care before the initial state attainment designations are made, and permit applicants will benefit from maintaining a close dialogue with the agency of concern throughout the permitting project.

1 Ozone map from USEPA website. Note that nonattainment area projections in Figure 2 are based on 2012-2014 data. Many of the areas shown may not be nonattainment based on more recent data (2013-2015, 2014-2016), which are the data on which the initial determinations will be based.