The American Public Power Association on March 23 filed comments supporting the U.S. Environmental Protection Agency’s proposed Phase 1 reconsideration of the Interstate Transport Plan Review for the 2015 Ozone National Ambient Air Quality Standards.
In light of recent court decisions, EPA is revisiting earlier proposed state implementation plan (SIP) disapprovals and related error corrections.
EPA now concludes the affected states do not significantly contribute to downwind nonattainment or interfere with maintenance of the ozone standard.
EPA is proposing to approve eight SIPs and to withdraw prior proposed disapprovals and error-correction actions for two additional states. The state SIPs cover Alabama, Arizona, Kentucky, Minnesota, Mississippi, Nevada, New Mexico, and Tennessee. The two states with error corrections are Iowa and Kansas.
APPA’s comments supported EPA’s reliance on the states’ original modeling and a 1 part per billion (ppb) significance threshold, emphasizing consistency with prior guidance, court rulings, and use of the 2023 analytic year to promote a fair and predictable review process.
“APPA strongly supports EPA’s decision to rely primarily on the modeling states used when developing their SIPs,” it said in the comments.
“This approach appropriately respects the CAA’s cooperative federalism structure, under which states have primary responsibility and broad discretion to determine how to meet the NAAQS, while EPA plays a secondary review role. Meaningful state discretion depends on a stable analytical baseline; states cannot reasonably exercise that discretion if the standards used to judge their SIPs change after submission,” it said.
By evaluating SIPs against the modeling available to states at the time of development, “EPA provides the predictability necessary for compliance and avoids the procedural chaos that would result from outcome determinative reliance on post submission modeling,” APPA said.
Courts found that EPA had improperly relied on newer modeling (2016v3) in a way that was “outcome determinative,” contrary to reasonable state reliance on EPA’s March 2018 modeling (2011 base-year, 2023 analytic year), APPA noted.
“The Proposed Rule evaluates each state’s chosen modeling. EPA consults newer modeling only to confirm that any potential linkages do not persist. EPA finds that, even when checked using EPA’s updated models, states' linkages do not exceed 1 ppb.”
Although EPA is not statutorily required to provide states with guidance on meeting their interstate transport obligations, many states lack the resources to develop independent modeling.
As a result, EPA’s modeling and guidance were essential to supporting those states’ analyses, APPA said.
“Once EPA provides guidance, it is reasonable -- and expected -- for states to rely on it in preparing their SIPs,” APPA said.
In supporting the 1 ppb significance threshold, APPA said the agency could strengthen the Proposed Rule by more clearly explaining the scientific and technical basis for this threshold.
“EPA could clarify why contributions below 1 ppb do not meaningfully affect air quality downwind, why the threshold is appropriate for all downwind receptors, and how it satisfies the CAA’s requirement to address emissions that ‘contribute significantly’ to nonattainment.”
This explanation should address modeling uncertainty, including whether contributions at this level can be reliably distinguished from inherent variability in the model, APPA said.
EPA should also clarify that the 1 ppb level is only a screening threshold, and that SIPs may still be approvable when -- based on a weight-of-evidence analysis -- they reasonably determine that contributions above 1 ppb are insignificant, APPA added.
EPA intends to engage in Phase II rulemaking to address the remaining states.
