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The impending return of Annual SO2 NAAQS modeling

On April 3, 2024 the United States Environmental Protection Agency (EPA) proposed a new secondary annual National Ambient Air Quality Standard (NAAQS) for sulfur dioxide (SO2) of between 10 and 15 ppb. As part of the same action EPA proposed to maintain the existing secondary standards for nitrogen dioxide and particulate matter (which were identical to the primary NAAQS). This proposal was published in the Federal Register on April 15, 2024 (

In the world of dispersion modeling it has been quite some time since annual SO2 NAAQS modeling was required. The original secondary annual SO2 NAAQS was revoked in 1973 (recall that the secondary NAAQS are established to protect public welfare against adverse effects to things such as vegetation, whereas the primary NAAQS are established to protect public health), and the original primary annual SO2 NAAQS was revoked in 2010 (at the same time the primary 1-hr SO2 NAAQS was established). The previous primary annual SO2 NAAQS was 80 µg/m3, and what EPA is proposing here is much more stringent, between 26 and 39 µg/m3.

EPA states that it does not expect additional emissions reductions will be required to meet this new secondary annual SO2 NAAQS beyond those required to meet the primary 1-hr SO2 NAAQS. Of course, once this becomes final it will be another modeling analysis required when doing cumulative SO2 modeling.

EPA is currently taking comments on this proposal through June 14, 2024, and will hold a virtual public hearing on May 8, 2024.

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