EPA Proposes SO₂ NAAQS Remain Same, Looks for Alternatives to Implementation

On May 25, 2018, Environmental Protection Agency (EPA) Administrator Scott Pruitt signed a proposed rule titled “Review of the Primary National Ambient Air Quality Standards (NAAQS) for Sulfur Oxides.” 

The review of the primary sulfur dioxide (SO₂) standard was conducted pursuant to a requirement of the Clean Air Act that it be reviewed every five years. In this notice, EPA proposes to keep the existing standard for SO₂ in place. According to the EPA’s notice, emissions of SO₂ and associated concentrations have declined nationally by 82% from 2000 to 2016. The EPA’s Clean Air Scientific Advisory Committee also recommended retaining the existing limit. The deadline for EPA to finalize its decision is January 28, 2019.

While EPA proposes to retain its existing primary SO₂ standard, it is also considering changes to the implementation of the standard. The standard is currently 75 parts per billion (ppb) over one hour, and attainment of the standard is determined by statistical “form”—the 99^th percentile of daily maximum 1-hour SO₂ concentrations averaged over 3 years. Under the Obama Administration in 2010, EPA established the current standard of 75 ppb using a one-hour averaging time to protect against short-term but intense spikes in pollution. Under Administrator Pruitt, EPA is considering altering this formula to make it easier for states to comply with the NAAQS and to help the states avoid a “nonattainment” designation for certain areas. Changing the formula could also make it easier for industry applicants for air permits to demonstrate compliance with the current standard.

In the May 2018 notice, EPA is soliciting comments on the four basic elements of the current NAAQS: indicator, averaging time, level, and statistical form. The “indicator” is the pollutant, which in this case is sulfur dioxide. The “averaging time” represents the length of time over which the pollutant is to be measured for the standard. The “level” is the amount of the pollutant measured—commonly in parts per billion (ppb) or parts per million (ppm). The “form” provides the guide for measuring the standard such as “not to be exceeded more than once per year” or “annual mean, averaged over 3 years.” The EPA is asking that comments to its notice include alternative approaches to the current formula, especially regarding the “averaging time” or “statistical form” that could provide a comparable level of public health protection. Alternative approaches to how the current primary standard is expressed could potentially ease implementation of the NAAQS for states and applicants for air permits in the future.