Opinion ID: 187215
Heading Depth: 3
Heading Rank: 6

Heading: PM2.5 Contribution Threshold

Text: North Carolina argues that EPA acted arbitrarily by proposing an air quality threshold for PM2.5 at 0.15 µg/m3 but finally settling on an air quality threshold of 0.2 µg/m3. The air quality threshold for PM2.5 is the amount of PM2.5 that sources in a state must contribute to a downwind nonattainment area to be regulated as an upwind state in CAIR's PM2.5 program. North Carolina also challenges EPA's decision to truncate, rather than round, the numbers it compared to the threshold. As a result, states that contributed 0.19 µg/m3 or less to a downwind nonattainment area were not linked with North Carolina by CAIR. EPA contests North Carolina's standing to raise this issue. It notes that only two states would be affected if EPA were to use the 0.15 µg/m3 threshold. Illinois, which is already subject to CAIR's requirements for PM2.5 contributions, would be subject to the exact same requirements for an additional reasonits contributions to Catawba County, North Carolina. Technical Support Document, at Appendix H. This additional upwind-downwind link would not change any of Illinois's duties under CAIR; therefore it would not change any effects felt by Catawba County, North Carolina. The lower threshold would also subject Arkansas to CAIR's PM2.5 controls. CAIR, 70 Fed.Reg. at 25,191; Technical Support Document, at 42 tbl. VII-1. EPA states that Arkansas does not contribute at threshold levels to nonattainment in North Carolina, but it cites no record support for this assertion. North Carolina has standing to raise this issue for three reasons. First, if in repromulgating CAIR to comply with section 110(a)(2)(D)(i)(I), EPA removes or modifies its interstate trading options, Illinois would be barred outright from contributing significantly to North Carolina's nonattainment areas. Second, EPA does not provide support for its assertion that Arkansas does not contribute to nonattainment areas in North Carolina because it never modeled the State. North Carolina claims that models for sources in Louisiana, Missouri, and Texas, which are further from North Carolina than those in Arkansas, show that Arkansas contributes at the 0.15 µg/m3 threshold to nonattainment areas in North Carolina. Third, because EPA designed CAIR to be a complete statutory remedy, whether North Carolina is linked with Illinois by CAIR under section 110(a)(2)(D)(i)(I) is likely to affect related remedies that North Carolina may have against Illinois, for example, pursuant to section 126, 42 U.S.C. § 7426. Although we cannot anticipate what a new rule will look like, there is a substantial probability that a favorable decision by this court would redress the injury North Carolina asserts. Because North Carolina has demonstrated an injury-in-fact caused by the rule it is challenging which a favorable decision by this Court could likely remedy, we can turn to the merits of North Carolina's petition. North Carolina notes that EPA first considered a threshold of 0.1 µg/m3. NPR, 69 Fed.Reg. at 4584. In the Notice of Proposed Rulemaking, EPA stated that a 0.1 µg/m3 threshold is the smallest one that can make the difference between compliance and violation of the NAAQS for an area very near the NAAQS.... Id. EPA then decided that it is on balance, more appropriate to adopt a small percentage value of the standard level and chose the percentage of the NAAQS standard of 15.0 µg/m3 that is closest to 0.1 µg/m3, which was one percent. Id. One percent of 15.0 µg/m3 is 0.15 µg/m3, so EPA initially chose that number as the threshold. Id. However, EPA then request[ed] comments on the use of higher or lower thresholds for this purpose. Id. In CAIR, EPA finally settled on a threshold value of 0.2 µg/m3. It did so because EPA was persuaded by commenters['] arguments on monitoring and modeling that the precision of the threshold should not exceed that of the NAAQS, which only measure PM2.5 concentration to the tenths column. CAIR, 70 Fed.Reg. at 25,191; see id. at 25,190 (commenters). North Carolina believes it was arbitrary for EPA to round 0.15 µg/m3 up to 0.2 µg/m3 instead of reverting to the earlier 0.1 µg/m3 number that is the smallest one that can make the difference between compliance and violation of the NAAQS. See NPR, 69 Fed.Reg. at 4584. EPA did not explain why it chose the larger number instead of the smaller number in the final rule; it only explained why it chose a number that ended at the tenths column. CAIR, 70 Fed.Reg. at 25,191. Based on EPA's reasoning in the Notice of Proposed Rulemaking, it may have made more sense to return to the 0.1 µg/m3 threshold instead of [r]ounding the proposal value of 0.15, which is what it did. See id. But EPA was concerned that the 0.15 µg/m3 threshold it originally proposed was too low, requesting comments on the use of higher or lower thresholds. NPR, 60 Fed.Reg. at 4584. And in raising the threshold number, EPA was responding to comments citing concerns about the measurement precision of existing PM2.5 monitors. CAIR, 70 Fed.Reg. at 25,190. We cannot say in this circumstance that EPA's decision to round the 0.15 µg/m3 threshold to 0.2 µg/m3 instead of reverting to the original threshold considered of 0.1 µg/m3 was wholly unsupported by the record. Likewise, we cannot say that EPA's decision to truncate rather than round the PM2.5 contribution levels it compared to the 0.2 µg/m3 threshold was arbitrary. The parties dispute which C.F.R. provision applies to the number it compares to the threshold  one mandating rounding, 40 C.F.R. pt. 50, App. N, § 4.3(a) (preferred by petitioner), or another mandating truncating, 40 C.F.R. pt. 50, App. N § 3.0(b) (preferred by EPA). The number EPA compares to the threshold, which is measured as the average of annual means [of PM2.5 contribution] from three successive years, is the contribution of PM2.5 from one upwind state to a nonattainment area. CAIR, 70 Fed.Reg. at 25,190. Section 4.3(a) applies to annual PM2.5 standard design values. Design values are the metrics (i.e., statistics) that are compared to the NAAQS levels to determine compliance. 40 C.F.R. pt. 50 App. N § 1.0(c). Design values are composed of the average of annual means of PM2.5 for three consecutive years, 40 C.F.R. pt. 50 App. N § 4.1(b), but design values are measurements of PM2.5 levels in a stationary areanot levels of PM2.5 moving from one area to another. Because the contribution level is not a design value, section 4.3(a)'s rounding mandate does not apply. Similarly, section 3.0(b)'s truncation mandate applies to PM2.5 hourly and daily measurement data and says nothing about the contribution level EPA is assessing in CAIR. Without a rule mandating any particular method, EPA is free to round or truncate the numbers it is comparing to the 0.2 µg/m3 threshold as long as its choice is reasonable. EPA chose to truncate numbers because the truncation convention for PM2.5 is similar to that used in evaluating modeling results in applying the ozone significance screening criterion of 2 ppb in the NOx SIP call and the CAIR proposal, as well as today's final action. CAIR, 70 Fed.Reg. at 25,191 n. 42 (internal citation omitted). EPA's choice to truncate the numbers is reasonable. As a result, we deny North Carolina's petition challenging the 0.2 µg/m3 threshold and EPA's choice to truncate the numbers compared to it.