EPA Acts on Clean Air Act "Interstate Transport" Plans
Time 6 Minute Read
EPA Acts on Clean Air Act "Interstate Transport" Plans

A previous post, EPA Makes Room for State Flexibility in Addressing “Interstate Transport” Under the Clean Air Act, discussed the evolving policy of the US Environmental Protection Agency (EPA) regarding approval of state plans—required under the “Good Neighbor Provision” of the federal Clean Air Act—addressing “interstate transport” of air pollution. That article reviewed a series of guidance documents EPA issued in 2018 to allow states flexibility in addressing wind-borne emissions that can contribute to ground-level ozone pollution in other states located downwind. At stake are not only downwind states’ air quality objectives but the prospect of expensive additional emission controls on upwind states’ manufacturing facilities and power plants.

One of EPA’s 2018 guidance documents addresses the seemingly technical question of what “contribution threshold” to apply. That term refers to the quantity—measured in parts per billion (ppb) of ozone in the air at ground level—below which an upwind state’s impact on a downwind state’s ozone concentrations is small enough that any contribution would be considered essentially de minimis. Generally, a state will want its emission contributions to be deemed low enough that it would be clear that its emission sources would not need new control requirements.

In regulations EPA issued in 2011 and 2016—the “Cross-State Air Pollution Rule” (CSAPR) and the CSAPR Update Rule, respectively—EPA used a contribution threshold of one percent of the applicable national ambient air quality standard (NAAQS). For the ozone NAAQS established by EPA in 2015 at a level of 70 ppb (which is the NAAQS that states are currently addressing through implementation plans), that one-percent threshold would be 0.70 ppb, a level some states and others have criticized as unjustifiably low. An August 2018 EPA guidance document included an analysis supporting the conclusion that using a threshold of 1 ppb—still low, but nearly 50 percent higher than a 0.70-ppb, one-percent-of-the-NAAQS threshold—“may be reasonable and appropriate for states to use” in developing interstate transport plans for the 2015 ozone NAAQS. Using a 1-ppb, rather than a 0.70-ppb, threshold could mean fewer upwind states will have to consider imposing additional controls on emitting facilities within their borders.

EPA has reviewed and proposed to approve several state plans to address interstate transport for the 2015 ozone NAAQS and, in a few cases, has given final approval. (So far, at least, EPA has not disapproved, or proposed to disapprove, any of these state plans.) But until recently, EPA had not proposed action on a plan submitted by an upwind state for which EPA’s air quality computer modeling had projected a contribution to any downwind state’s ozone problems in excess of 0.70 ppb. On March 2, 2020, however, EPA published a proposal to approve the plan submitted by Iowa, which presents exactly that situation. (Any public comments on EPA’s proposal on Iowa’s plan are due to EPA by April 1.) In its proposal, EPA notes that its modeling projects Iowa’s emissions will not contribute ozone to any other state in an amount above 1 ppb but will make ozone contributions to two states in amounts that, in each case, fall between the 0.70-ppb and 1-ppb levels—i.e., a 0.79-ppb contribution to a Wisconsin air quality monitor and a 0.77-ppb contribution to a Michigan monitor.

EPA proposes to agree with Iowa that it is reasonable for the state to deem these contributions as not constituting state-to-state “linkages” that would otherwise require assessment of whether (and to what extent) Iowa’s contributions are “significant.” If a state’s contributions are found to be significant, the state must conduct an assessment of whether more emission reductions from in-state sources are needed to address impacts on downwind states’ problems with attaining the NAAQS.

In its proposed approval for Iowa, EPA explains that it “first assessed whether the general observation in [EPA’s] August 2018 memorandum that a 1 ppb threshold captures a comparable amount of upwind collective contribution as a 1 percent [i.e., 0.70-ppb] threshold holds true for the specific [ozone air quality monitor] receptors at issue here.” According to EPA’s proposal, for the Wisconsin and Michigan monitors at issue, EPA calculates that a 1-ppb threshold captures 83 percent and 94 percent, respectively, of the total contribution that would be captured if the lower, 1-percent threshold were used instead. Based on its calculations, EPA proposes that Iowa meets this “comparable amount” test.

Next, referring to quantitative information in EPA’s August 2018 memorandum, the proposed approval asks three additional questions: (1) “How does the impact of in-state emissions [in this case, emissions from Iowa] on ozone levels at this [downwind air quality] receptor compare to collective upwind impacts?”; (2) “What are the impacts of individual upwind states linked at 1 ppb or higher to the [downwind] receptor?”; and (3) “Are individual upwind states impacting this receptor [in amounts] between 1 percent and 1 ppb linked above 1 ppb to other receptors?” (emphasis in original). Assessing information relevant to these questions, EPA proposes to find it reasonable to treat Iowa’s contributions to the Wisconsin and Michigan receptors as falling below the amounts that potentially could reflect significant contribution. Thus, if EPA makes final this proposed finding, it would conclude that Iowa need not impose additional controls on its emission sources to address interstate transport for the 2015 ozone NAAQS.

The proposed rule suggests that other upwind states in a position similar to that of Iowa—that is, those with projected ozone contributions that exceed 0.70 ppb but none that exceed 1 ppb—might have their plans reviewed by EPA in a similar way, assuming they rely on a 1-ppb contribution threshold. Besides Iowa, states whose largest contributions to downwind states fall within the band between 0.70 and 1 ppb include Connecticut, Kansas, Mississippi and New Mexico.

The proposed rule also can be read to indicate that, at least as a general matter, EPA may expect the much larger number of upwind states with projected contributions that do exceed 1 ppb to include in their plans at least an assessment of possible additional in-state emission controls. Based on EPA’s modeling, it appears there are 20 upwind states with contributions to individual downwind states that exceed 1 ppb. These upwind states are Arkansas, California, Illinois, Indiana, Kentucky, Louisiana, Maryland, Michigan, Missouri, New Jersey, New York, Ohio, Oklahoma, Pennsylvania, Texas, Utah, Virginia, West Virginia, Wisconsin and Wyoming. These are the states that may receive the greatest scrutiny as EPA continues its review of plans to address interstate transport for ozone.


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