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EPA sets and implements primary and secondary National Ambient Air Quality Standards (NAAQS) for six common air pollutants. Primary NAAQS protect the public health, while secondary NAAQS protect the public welfare. Secondary NAAQS have traditionally not been more stringent than primary ones, yet EPA’s staff and science advisors are developing recommendations that EPA promulgate such standards. Any new, more stringent secondary NAAQS would raise significant implementation questions.

EPA is in the process of reviewing its secondary NAAQS for nitrogen oxides, sulfur oxides, and particulate matter. The agency has entered into a consent decree requiring it to sign a notice of proposed rulemaking concerning these NAAQS by February 9, 2024, and to sign a notice of final rulemaking on these standards no later than December 10, 2024. In preparation for that rulemaking proposal, EPA’s air office staff recently released draft recommendations for revisions to the secondary NAAQS for each of these pollutants, including recommendations for standards that would be more stringent than the related primary NAAQS. Specifically, EPA staff released a draft recommendation to lower the level of annual secondary NAAQS for nitrogen dioxide (NO2) to “as low as 40 ppb.” Such a standard would clearly be more stringent than the current 50 ppb annual primary NO2 NAAQS. Furthermore, the agency staff‘s draft recommendations included an annual secondary NAAQS for sulfur dioxide (SO2) in the range of 10 to 22 ppb. EPA currently has no primary or secondary annual NAAQS for SO2, but, until 2010, it had a primary annual SO2 NAAQS of 30 ppb. An annual secondary NAAQS within the range recommended by EPA staff would be more stringent than this revoked primary one.

In reviewing those staff recommendations at a public meeting, EPA’s Clean Air Scientific Advisory Committee (CASAC) went even further. Although CASAC’s recommendations have not yet been formalized in writing, that committee agreed to recommend an annual secondary NO2 NAAQS of less than 20 ppb, twice as stringent as the one the agency’s air office is recommending. CASAC’s members also endorsed an annual secondary NAAQS for SO2 towards the bottom of the range recommended by EPA staff, one of 10 to 15 ppb. Furthermore, CASAC members agreed that the level of the 24-hour secondary NAAQS for fine particulate matter (PM2.5) should be reduced to 25 µg/m3, significantly more stringent than the primary 24-hour PM2.5 NAAQS of 35 µg/m3.

Whatever the scientific merits of such more stringent NAAQS, what will they mean for the regulated community if they are adopted? As a statutory matter, implementation of secondary NAAQS differs in some respects from implementation of primary NAAQS. For example, the Act specifies deadlines by which primary NAAQS must be met but, as a general matter, provides only that secondary NAAQS must be met “as expeditiously as practicable.”

In other ways, though, the Act requires secondary NAAQS be implemented in a manner that parallels implementation of primary NAAQS. Designations of areas as nonattainment, attainment, or unclassifiable are explicitly required for both primary and secondary NAAQS. States adopt and submit to EPA a State Implementation Plan (SIP) within three years after promulgations of either a primary or secondary NAAQS. This plan, which EPA refers to as an Infrastructure SIP, must provide for “implementation, maintenance, and enforcement” of the NAAQS, whether primary or secondary. The Infrastructure SIP must also prohibit emissions activity within the state that contributes significantly to nonattainment or interferes with maintenance of any primary or secondary NAAQS in another state. (EPA has previously relied on this “good neighbor” provision to impose federal implementation plans reducing emissions across multi-state areas).

The applicability to secondary NAAQS of many other CAA provisions is less clear, however. EPA’s interpretation of these provisions will significantly affect the impact of a more stringent secondary NAAQS. For example, what is meant by attainment “as expeditiously as practicable”? How will that be reflected in sources’ control obligations?

For many industries, a key question will be what new source permitting requirements apply to secondary NAAQS? The Act provides an exemption from some permitting requirements for certain sources of particulate matter or sulfur oxide emissions that “will not cause or contribute to ambient air quality levels in excess of the national secondary ambient air quality standard for either of those pollutants.” Will EPA infer that new source permitting requirements otherwise apply to secondary NAAQS?

Which, if any, of the Act’s requirements for controls on sources in nonattainment areas apply to secondary NAAQS as well as primary ones? Although there may be superficial appeal to saying that unless applicability of a requirement solely to primary NAAQS is specified, it applies to both primary and secondary NAAQS, a thoughtful analysis calls such a conclusion into question. For example, how can a SIP for a nonattainment area be required to include emissions controls on a schedule that provides Reasonable Further Progress (RFP) towards attainment by the attainment date when the Act does not specify any attainment date? How can a SIP be required to include contingency measures that are automatically triggered if an area fails to attain by the applicable attainment date when the Act does not specify any such date? The relevance of these requirements only to SIPs for primary NAAQS is supported by a provision added to the Act in 1990 concerning their applicability only for new nonattainment areas for primary NAAQS for sulfur oxides, NO2, or lead.

This post does not seek to resolve the questions of what is required for implementing a secondary NAAQS more stringent than a primary one. It merely points to questions that arise in such a case, and that the resolution of these questions may substantially affect the impact on regulated entities of such a NAAQS. EPA must answer these questions no later than the promulgation of any more stringent secondary NAAQS.