United States Court of Appeals

FOR THE DISTRICT OF COLUMBIA CIRCUIT

Argued September 24, 2012 Decided January 22, 2013

No. 10-1413

SIERRA CLUB,

PETITIONER

v.

ENVIRONMENTAL PROTECTION AGENCY AND LISA PEREZ 

JACKSON, ADMINISTRATOR,

RESPONDENTS

UTILITY AIR REGULATORY GROUP,

INTERVENOR

On Petition for Review of Final Actions of 

the United States Environmental Protection Agency

David S. Baron argued the cause for petitioner. With him 

on the briefs were Seth L. Johnson and Emma C. Cheuse.

Jessica O'Donnell, Attorney, U.S. Department of Justice, 

argued the cause for respondents. With her on the brief were 

Brian L. Doster, Assistant General Counsel, U.S. 

Environmental Protection Agency, and Scott J. Jordan, 

Attorney.
2

Andrea Bear Field, Makram B. Jaber, Lucinda Minton 

Langworthy, and Elizabeth L. Horner were on the brief for 

intervenor Utility Air Regulatory Group in support of 

respondent.

Before: SENTELLE, Chief Judge, GRIFFITH, Circuit Judge, 

and EDWARDS, Senior Circuit Judge.

Opinion for the Court filed by Chief Judge SENTELLE.

SENTELLE, Chief Judge: In October 2010, the 

Environmental Protection Agency (“EPA”) issued a final rule 

establishing regulations for particulate matter less than 2.5 

micrometers (“PM2.5”) under § 166 of the Clean Air Act (“the 

Act”), 42 U.S.C. § 7476. See Prevention of Significant 

Deterioration (PSD) for Particulate Matter Less Than 2.5 

Micrometers (PM2.5)—Increments, Significant Impact Levels 

(SILs) and Significant Monitoring Concentration (SMC), 75 

Fed. Reg. 64,864 (Oct. 20, 2010). In this rule, the EPA 

established Significant Impact Levels (“SILs”) and a 

Significant Monitoring Concentration (“SMC”) for PM2.5, 

screening tools the EPA uses to determine whether a new 

source may be exempted from certain requirements under § 

165 of the Act, 42 U.S.C. § 7475. 75 Fed. Reg. at 64,890–

91, 64,895. Petitioner Sierra Club seeks review of this 

regulation. 

After the Sierra Club filed its petition, the EPA 

acknowledged that portions of the rule establishing SILs did 

not reflect its intent in promulgating the SILs, and now

requests that we vacate and remand some (but not all) parts of 

its PM2.5 SIL regulations. Notwithstanding the EPA’s 

concession, the Sierra Club maintains that the EPA lacks 

authority to establish SILs and requests that we rule 
3

accordingly. The Intervenor, Utility Air Regulatory Group 

(“UARG”), on the other hand, urges us to uphold the SIL 

provisions EPA established, or alternatively, to remand the 

SIL provisions without ordering that they be vacated.

Although the EPA conceded that it needs to revise some 

of the SIL provisions, it continues to assert that the portions of 

its rule establishing the SMC were valid. For the reasons 

stated below, we accept the EPA’s concession on the SILs, 

and vacate and remand some portions of the EPA’s rule 

establishing SILs. We further conclude that the EPA

exceeded its authority in establishing the SMC, and grant the 

Sierra Club’s petition as to those portions of the EPA’s rule.

I. BACKGROUND

A. The Clean Air Act

The Clean Air Act requires the EPA to set National 

Ambient Air Quality Standards (“NAAQS”) for various 

harmful air pollutants at levels necessary to protect the public 

health and welfare. 42 U.S.C. §§ 7401, 7409. Under the Act,

the EPA must designate areas as attainment, nonattainment, or 

unclassifiable for each NAAQS. Id. § 7407(d)(1)(A). States 

have primary responsibility for implementing the NAAQS, 

and must submit a state implementation plan (“SIP”) 

specifying how the State will achieve and maintain 

compliance with the NAAQS. Id. § 7407(a).

In 1977, Congress amended the Act to add the Prevention 

of Significant Deterioration (“PSD”) provisions “to protect 

the air quality in national parks and similar areas of special 

scenic or recreational value, and in areas where pollution was 

within the national ambient standards, while assuring 

economic growth consistent with such protection.” 
4

Environmental Defense Fund v. EPA, 898 F.2d 183, 184 

(D.C. Cir. 1990) (citing 42 U.S.C. § 7470). When Congress 

enacted the PSD provisions, it established maximum 

allowable increases over baseline concentrations — also 

known as “increments” — for certain pollutants in § 163 of 

the Act. See 42 U.S.C. § 7473; Environmental Defense Fund, 

898 F.2d at 184. For other pollutants, Congress delegated to 

the EPA the task of promulgating regulations to prevent the 

significant deterioration of air quality that would result from 

the emissions of these pollutants. 42 U.S.C. § 7476(a). For

pollutants that the EPA began regulating after Congress 

enacted the PSD provisions, which includes PM2.5, the EPA 

must promulgate PSD regulations within two years of 

establishing the NAAQS for that pollutant. Id. 

The PSD provisions also establish requirements for 

preconstruction review and permitting of new or modified 

sources of air pollution. See id. § 7475. Subsection 165(a) of 

the Act lists the requirements an owner or operator proposing 

to construct a new source or modify an existing source must 

meet before starting construction, which include acquiring a 

PSD permit for the facility. Id. § 7475(a)(1)–(2). Of 

relevance to this petition, § 165(a)(3) requires that an owner

or operator proposing to construct a new major emitting 

facility or modify an existing facility demonstrate that 

emissions from construction or operation of the facility will 

not cause or contribute to any violations of the increment 

more than once per year, or to any violation of the NAAQS 

ever. Id. § 7475(a)(3).

 

Before a review of the § 165(a) requirements may be

undertaken, however, either a State or the owner or operator

of a facility applying for a PSD permit must conduct an 

analysis of the ambient air quality at the proposed site and in 

areas that may be affected by emissions from the facility for 
5

the relevant pollutants. Id. § 7475(e)(1). This analysis must 

include continuous air quality monitoring data gathered to 

determine whether the facility will exceed either the 

increments or the NAAQS. Id. § 7475(e)(2). The Act further 

mandates that this data be collected for a year before the date 

the applicant applies for a permit unless a State, in accordance 

with EPA regulations, “determines that a complete and 

adequate analysis for such purposes may be accomplished in a 

shorter time period.” Id. The results of the analysis must be 

made available to the public at the time of the public hearing 

on the application for a PSD permit. Id.

The Act requires States to address the PSD provisions in 

their SIPs. Id. § 7410(a)(2). The EPA has promulgated 

extensive regulations setting forth requirements and 

guidelines on how SIPs are to implement the PSD provisions. 

See 40 C.F.R. § 51.166. For States without an EPA-approved 

SIP, the EPA has promulgated separate regulations 

implementing the PSD provisions. See 40 C.F.R. § 52.21. 

B. Regulatory Background: Establishing the PM2.5

Increment, SILs, and SMC

In 1997, the EPA revised its NAAQS to include 

standards for PM2.5, see 62 Fed. Reg. 38,652 (July 18, 1997), 

and in 2006 revised the PM2.5 NAAQS, see 71 Fed. Reg. 

61,144 (Oct. 17, 2006). In 2007, the EPA proposed a rule 

establishing increments for PM2.5. See 72 Fed. Reg. 54,112 

(Sept. 21, 2007). In this rulemaking, the EPA also proposed 

two screening tools that would exempt a permit applicant 

from some of the air quality analysis and monitoring required 

under the Act and EPA regulations: significant impact levels

(“SILs”) and significant monitoring concentration (“SMC”). 

See id. at 54,138–42.
6

1. Significant Impact Levels

Under EPA regulations, the owner or operator of a 

proposed source or modification must undertake a source 

impact analysis to demonstrate “that allowable emission 

increases from the proposed source or modification, in 

conjunction with all other applicable emission increases or 

reductions (including secondary emissions), would not cause 

or contribute to” a violation of the increments or the NAAQS. 

40 C.F.R. § 51.166(k); id. § 52.21(k). In the proposed rule, 

the EPA discussed adopting SILs for PM2.5, which the EPA 

defines as “numeric values derived by EPA that may be used 

to evaluate the impact a proposed major source or 

modification may have on the NAAQS or PSD increment.”

72 Fed. Reg. at 54,138. This numerical value, measured in

micrograms per meter cubed (μg/m3), is the level of ambient 

impact below which the EPA considers a source to have an 

insignificant effect on ambient air quality. 72 Fed. Reg. at 

54,139. According to the EPA’s proposed rule, “a source that 

demonstrates its impact does not exceed a SIL at the relevant 

location is not required to conduct more extensive air quality 

analysis or modeling to demonstrate that its emissions, in 

combination with the emissions of other sources in the 

vicinity, will not cause or contribute to a violation of the 

NAAQS at that location,” an analysis the EPA terms the 

cumulative impact analysis, or the cumulative air quality 

analysis. 72 Fed. Reg. at 54,139. 

As the legal basis for adopting the SILs, the EPA cited 

Alabama Power Co. v. Costle, 636 F.2d 323 (D.C. Cir. 1979). 

72 Fed. Reg. at 54,139. In that case we discussed an 

administrative agency’s de minimis authority to establish 

exemptions from statutory commands, holding that 

“[c]ategorical exemptions may . . . be permissible as an 
7

exercise of agency power, inherent in most statutory schemes, 

to overlook circumstances that in context may fairly be 

considered de minimis.” 636 F.2d at 360. We further stated 

that “[u]nless Congress has been extraordinarily rigid, there is 

likely a basis for an implication of de minimis authority to 

provide exemption when the burdens of regulation yield a 

gain of trivial or no value.” Id. at 360–61. But that implied 

authority does not apply to situations “where the regulatory 

function does provide benefits, in the sense of furthering the 

regulatory objectives, but the agency concludes that the 

acknowledged benefits are exceeded by the costs.” Id. at 361. 

Applying this de minimis authority, the EPA explains that 

when a source’s ambient impact does not exceed the SIL —

i.e., is de minimis — the “EPA considers the conduct of a 

cumulative air quality analysis and modeling by such a source 

to yield information of trivial or no value with respect to the 

impact of the proposed source or modification.” 72 Fed. Reg. 

at 54,139. 

2. Significant Monitoring Concentration

In 1980, the EPA “adopted regulations that exempt 

sources from preconstruction monitoring requirements [i.e., 

§ 165(e)(2) of the Act] for a pollutant if the source can 

demonstrate that its ambient air impact is less than a value 

known as the [SMC].” Id. at 54,141. When the EPA first 

adopted SMCs for other pollutants in 1980,1 it described the 

SMCs as “air quality concentration de minimis levels for each 

 1 When the EPA established the preconstruction monitoring 

exemption in 1980, it did not label the emissions values below 

which the exemption applied as “SMCs,” instead terming them “de 

minimis emissions levels.” See 45 Fed. Reg. 52,676, 52,709 (Aug. 

7, 1980). But because the de minimis emissions levels promulgated 

in 1980 serve the same function as the PM2.5 SMC, we will refer to 

the 1980 de minimis levels as SMCs throughout this opinion. 
8

pollutant for the purpose of providing a possible exemption 

from monitoring requirements.” Id. (internal alterations and 

citations omitted). In its proposed rule establishing an SMC 

for PM2.5, the EPA explained that “[i]f a source can show 

through modeling of its emissions alone that its impacts are 

less than the corresponding SMC, there is little to be gained 

by requiring that source to collect additional monitoring data 

on PM2.5 emissions to establish background concentrations for 

further analysis.” Id. The EPA proposed different 

methodologies for establishing a value for the SMC and, as 

with the SILs, relied on the de minimis discussion from 

Alabama Power as the legal basis for establishing an SMC for 

PM2.5. 72 Fed. Reg. at 54,141.

C. Final Rule

In its final rule, the EPA adopted and set values for both 

the SILs and SMC for PM2.5. See 75 Fed. Reg. at 64,864. 

The EPA gave three purposes for the SILs in the final rule, 

which were to determine:

(1) When a proposed source’s ambient impacts 

warrant a comprehensive (cumulative) source impact 

analysis; (2) the size of the impact area within which 

the air quality analysis is completed, and (3) whether 

the emissions increase from a proposed new major 

stationary source or major modification is considered 

to cause or contribute to a violation of any NAAQS.

Id. at 64,890.

In adopting the SMC, the EPA emphasized that it 

retained discretion “to determine when it may be appropriate 

to exempt a proposed new major stationary source or major 

modification from the ambient monitoring data requirements 
9

under the PSD rules.” Id. at 64,895. The rule codified the 

SILs at 40 C.F.R. § 51.166(k)(2) and 40 C.F.R. § 52.21(k)(2), 

and the SMC at 40 C.F.R. § 51.166(i)(5)(i)(c) and 40 C.F.R. 

§ 52.21(i)(5)(i)(c). 75 Fed. Reg. at 64,902–07. 

The rule also codified the PM2.5 SILs in the EPA’s 

regulations on new source review and permitting 

requirements at 40 C.F.R. § 51.165(b)(2). Unlike the PSD 

regulations (40 C.F.R. §§ 51.166, 52.21), § 51.165(b)(2) does 

not use the SILs to exempt a source from conducting a 

cumulative air quality analysis. Instead, § 51.165(b)(2) states 

that a proposed source or modification will be considered to 

cause a violation of a NAAQS when that source or 

modification would, at a minimum, exceed the SIL in any 

area that does not or would not meet the applicable NAAQS.

II. ANALYSIS

A. Significant Impact Levels

The Sierra Club argues that the EPA lacks de minimis

authority to promulgate the SILs. Specifically, the Sierra 

Club contends that the language of § 165 is so extraordinarily 

rigid that it bars de minimis exemptions, and that adoption of 

the SILs is contrary to the legislative design of the Act. Even 

if § 165 of the Act were not so extraordinarily rigid as to bar 

any de minimis exemption, the Sierra Club asserts that 

pollution increases below the SILs are not so trivial as to be

de minimis. 

To illustrate the latter point, the Sierra Club explains that 

if a proposed source or modification is in an area that is close 

to violating the NAAQS or an increment, that source could 

violate the NAAQS or increment even if its emissions would 

have an ambient impact below the SIL. For example, if a 
10

proposed source’s emission of PM2.5 would have a projected

air quality impact of 1 μg/m3 over a 24-hour average (below 

the SIL of 1.2 μg/m3 over a 24-hour average), and that source 

proposes to build in an area that already has an ambient PM2.5

concentration of 35 μg/m3 (the PM2.5 NAAQS over a 24-hour 

average), the construction of that source could cause a 

violation of the NAAQS. See id. §§ 50.13(c) (PM2.5

NAAQS), 51.166(k)(2) (PM2.5 SIL). The Sierra Club further 

notes that because the EPA’s regulation automatically 

exempts a source with a proposed impact below the SIL from 

demonstrating it will not cause or contribute to a violation of 

the NAAQS, unlimited numbers of sources whose impacts are 

less than the SILs could cumulatively cause a violation of the 

NAAQS or increments. Also, the Sierra Club points out that

sources whose impact is below the SILs that construct in an

upwind attainment area could worsen existing violations in a 

downwind nonattainment area. As the SIL regulations are 

currently written, sources in these scenarios would not be 

required to demonstrate that they would not cause or 

contribute to a violation of the NAAQS or increment, even 

though they likely would cause a violation (in an attainment 

area) or contribute to a violation (in a downwind 

nonattainment area), thus contravening the statutory 

command in § 165(a) of the Act.

In its brief, the EPA concedes that the SIL provisions, as 

codified, were flawed. When the EPA responded to

commenters in the final rule, it explained that 

“notwithstanding the existence of a SIL, permitting authorities 

should determine when it may be appropriate to conclude that 

even a de minimis impact will ‘cause or contribute’ to an air 

quality problem and to seek remedial action from the 

proposed new source or modification.” 75 Fed. Reg. at 

64,892. But as the EPA acknowledges in its brief, “the 

regulatory text it adopted does not allow permitting 
11

authorities the discretion to require a cumulative impact 

analysis, notwithstanding that the source’s impact is below 

the SIL, where there is information that shows the proposed 

source would lead to a violation of the NAAQS or 

increments.” Resp’t Br. at 34. Because the EPA asserts that 

it did not intend to automatically exempt a proposed source 

from the requirements of the Act without affording the 

permitting authorities discretion in applying the SILs, it 

requests that we vacate and remand the regulatory text 

promulgated in the rule at 40 C.F.R. §§ 51.166(k)(2) and 

52.21(k)(2). 

Despite the EPA’s concession, the Sierra Club asserts 

that vacatur and remand, while warranted, does not fully 

resolve its challenge, and asks that we determine whether the 

EPA has authority to promulgate SILs. We disagree with the 

Sierra Club that it is necessary to decide the EPA’s authority 

to promulgate SILs at this point. To do so would require that 

we answer a question not prudentially ripe for determination. 

On remand the EPA may promulgate regulations that do not 

include SILs or do include SILs that do not allow the 

construction or modification of a source to evade the 

requirements of the Act as do the SILs in the current rule. In 

such an event, we would not need to address the universal 

disallowance of all de minimis authority. If the EPA 

promulgates new SIL provisions for PM2.5 and those 

provisions are challenged, we can then consider the 

lawfulness of those SIL provisions. 

While the Sierra Club argues that simply vacating and 

remanding the SIL provisions does not go far enough, the 

UARG intervenes to argue that vacatur and remand go too far. 

The UARG asserts that remanding the SIL provisions for 

further rulemaking is unnecessary for two reasons. First,

intervenor asserts, the SIL provisions, as informed by the 
12

EPA’s statements during rulemaking, do allow permitting 

authorities discretion in how they apply the SILs. Second, it 

argues that if a source with an ambient impact below the SIL 

does cause a NAAQS or increment violation in an area, the 

permitting authority for that area is already obligated to revise 

its SIP to address the violation. See 40 C.F.R. § 51.166(a)(3).

The UARG bases the first of these arguments on the 

premises that an agency’s interpretation of its own regulations 

is given deference, and that the EPA has interpreted the SIL 

provisions so that permitting authorities retain discretion in 

applying the SILs. See Auer v. Robbins, 519 U.S. 452, 461 

(1997) (explaining that an agency’s interpretation of its own 

regulations is “controlling unless plainly erroneous or 

inconsistent with the regulation.”) (internal citations and 

quotation marks omitted). Although the first premise is true, 

the latter premise is contradicted by the EPA’s statements in 

its brief that the regulatory text it adopted does not give 

permitting authorities sufficient discretion to require a 

cumulative air quality analysis. That the EPA itself requests 

that we remand these provisions strongly argues that the 

current SIL provisions do not give permitting authorities 

sufficient discretion in applying the SILs.

The text of the SIL regulations as codified in the Code of 

Federal Regulations supports the EPA’s interpretation that the 

SILs do not allow a permitting authority sufficient discretion. 

Cf. Auer, 519 U.S. at 461 (opining that a critical phrase in the 

contested regulation “comfortably bears the meaning the 

[agency] assigns.”). Although 40 C.F.R. § 51.166(k)(2), 

which applies to SIPs, states that a plan “may provide” for the 

use of SILs to exempt a proposed source or modification from 

undertaking a cumulative air quality analysis, it does not give 

permitting authorities that implement the SILs discretion to 

require a cumulative air quality analysis for sources that are 
13

below the SIL, but could nevertheless cause a violation of the 

NAAQS or increment. And 40 C.F.R. § 52.21(k)(2), which 

applies to states without an approved SIP, goes even further 

and simply states that the demonstration required under 

§ 165(a)(3) is deemed to have been made if a proposed source 

or modification’s air quality impact is below the SIL.

The UARG’s second argument, that remand is 

unnecessary because the EPA requires permitting authorities 

to address violations by revising their SIPs, also does not 

persuade us that we should deny the EPA’s request to remand 

its regulations on the PM2.5 SILs. The PSD provisions 

Congress enacted may not have specified how the owner or 

operator of a proposed source or modification must 

demonstrate compliance, but they do require demonstration 

that the source will not cause or contribute to a violation of 

the NAAQS or increment as a precondition to construction.

See 42 U.S.C. § 7475(a)(3). As the Sierra Club notes, relying 

on permitting authorities to address violations, rather than to 

prevent violations by requiring demonstration that a proposed 

source or modification will not cause a violation, conflicts 

with this statutory command. 

The UARG finally argues that if we remand the SIL 

regulations, we should not vacate the regulations, based on 

our holding in Fertilizer Institution v. EPA, 935 F.2d 1303 

(D.C. Cir. 1991), where we stated that “when equity demands, 

an unlawfully promulgated regulation can be left in place 

while the agency provides the proper procedural remedy.” Id.

at 1312. According to the UARG, leaving the SIL provisions 

in place during the new rulemaking would cause no harm to 

air quality, while vacating the SIL provisions would have 

“disruptive consequences” for economic growth — i.e., by 

adding additional burdens to sources with de minimis impacts. 
14

Therefore, the UARG asserts that equity requires we do not 

vacate the SIL provisions. 

The UARG’s equitable argument does not persuade us. 

In Fertilizer Institution we left in place administrative 

exemptions the EPA adopted without providing adequate 

notice and comment, a procedural defect, while in this case 

the EPA has requested we vacate and remand the SILs 

because it did not have authority to promulgate such a broad 

exemption. See id. Because this is a substantive defect, and 

because the EPA explicitly requested we vacate and remand 

some of its SIL provisions, we will grant its request 

notwithstanding the UARG’s opposition.

Although the EPA asks us to vacate and remand the parts 

of its rule codifying SILs at §§ 51.166(k)(2) and 52.21(k)(2), 

it requests that we let the promulgation of SILs in 

§ 51.165(b)(2) remain operative, emphasizing that the Sierra 

Club’s challenge of the EPA’s authority to promulgate SILs 

was directed only at the first two regulations. We agree that 

the parts of the EPA’s rule codifying SILs in § 51.165(b)(2) 

should remain. We are remanding the other regulations 

because they allow permitting authorities to automatically 

exempt sources with projected impacts below the SILs from 

having to make the demonstration required under 42 U.S.C. § 

7475(a)(3), even in situations where the demonstration may 

require a more comprehensive air quality analysis. These 

concerns, which are based on whether the EPA has authority 

to exempt those requirements, are not present in § 

51.165(b)(2), which simply states that a source may be 

deemed to violate the NAAQS if it exceeds the SILs in certain 

situations. Apparently, for that reason, the Sierra Club only 

addresses § 51.165(b)(2) in the section of its brief challenging 

the EPA’s methodology in setting SILs, and not in the section 

challenging the EPA’s authority to promulgate SILs. See 
15

Pet’r Br. at 32 n.12, 37 n.17. We are not now ruling on the 

methodology the EPA used to determine the SILs. Instead, 

we are vacating and remanding §§ 51.166(k)(2) and 

52.21(k)(2) based on the EPA’s lack of authority to exempt 

sources from the requirements of the Act. Therefore, vacatur 

and remand of § 51.165(b)(2) is not necessary at this point.

Accordingly, we vacate and remand the portions of the 

EPA’s rule regarding SILs, with the exception of those 

portions codified in 40 C.F.R. § 51.165(b)(2). 

B. Significant Monitoring Concentrations

As with the SILs, the Sierra Club argues that the EPA 

does not have de minimis authority to promulgate an SMC for 

PM2.5 that can be used to exempt an owner of a proposed 

source or modification from undertaking the year-long preconstruction air quality monitoring requirement under 

§ 165(e)(2) of the Act. As a threshold issue, however, the 

EPA argues that the Sierra Club’s challenge is time-barred

under § 307(b)(1) of the Act because the EPA has used SMCs 

as a screening tool since 1980. See 42 U.S.C. § 7607(b)(1); 

45 Fed. Reg. 52,676, 52,710 (Aug. 7, 1980) (explaining that a 

source owner may be exempt from preconstruction 

monitoring if the source’s projected impact is de minimis).

We disagree with the EPA that the Sierra Club’s petition is 

time-barred, and we agree with the Sierra Club that the EPA 

did not have de minimis authority to promulgate the SMC 

because we hold Congress was “extraordinarily rigid” in 

mandating preconstruction air quality monitoring.

Section 307(b)(1) of the Act requires a petitioner seeking 

review of an EPA regulation to file its petition within sixty 

days from the date the challenged regulation was published in 

the Federal Register. 42 U.S.C. § 7607(b)(1). The EPA relies 
16

on our decision in Medical Waste Institute and Energy 

Recovery Council v. EPA, 645 F.3d 420 (D.C. Cir. 2011), to 

argue that § 307(b)(1) bars the Sierra Club’s challenge to the 

PM2.5 SMC. In that case, the petitioner challenged the EPA’s 

approach to setting the level of emissions control for 

pollutants emitted by medical waste incinerators. Id. at 422. 

The EPA had initially set these levels in 1997, but we 

remanded its regulations after granting an environmental 

organization’s petition for review. Id. at 423. The EPA 

issued a new rule in 2009 setting even more stringent levels

for emissions control than it had in 1997, prompting another 

petition for review, this time by an industry organization. Id.

at 424. In remaking the rule, the EPA used the same approach 

to setting the levels of emissions control for the same set of 

pollutants as it did in 1997, but used a different data set. Id. 

at 426–27. We held that because no one challenged the 

approach to setting levels of emissions control in 1997 — the 

same approach the EPA used in its 2009 regulation — the 

petitioners had failed to file a timely petition, and their 

challenge was thus barred by § 307(b)(1). Id. at 427. 

Our holding in Medical Waste Institute, however, does 

not apply in this case. The EPA has promulgated new 

regulations for a pollutant it did not regulate in 1980. See 45 

Fed. Reg. at 52,733–34 (listing SMCs for various pollutants 

that does not include PM2.5). By establishing a new 

monitoring exemption for a new pollutant, the EPA exposes 

its PM2.5 regulations, including whether it has authority to 

adopt the SMC exemption for PM2.5 and whether it used an 

appropriate method to determine the level of the SMC, to 

challenge by a timely filed petition. In Ohio v. EPA, 838 F.2d 

1325, 1328 (D.C. Cir. 1988), we opined that “the period for 

seeking judicial review may be made to run anew when the 

agency in question by some new promulgation creates the 

opportunity for renewed comment and objection.” Although 
17

not a parallel to this case in that the Ohio case concerned a 

reopening, we consider its reasoning instructive. 

This, of course, does not mean that a petitioner’s 

challenge to the EPA’s authority will always survive, as the 

EPA’s authority to promulgate certain regulations could be 

well-settled. The solution, however, is not to bar any

challenges to that authority under § 307(b)(1) of the Act, but 

instead to consider the timely challenge and any relevant

precedent demonstrating that the EPA has the authority in 

dispute. Because we have not yet decided whether the EPA’s 

de minimis authority allows it to establish SMCs as a 

screening tool to determine when to exempt sources from the 

Act’s preconstruction monitoring requirement, we will

consider whether the EPA had authority to adopt an SMC for 

PM2.5. 

Subsection (e) of § 165 of the Act requires that before a 

PSD permit application can be reviewed, either the State or 

the permit applicant must conduct an analysis of the ambient 

air quality at the proposed site and in areas which the 

applicant’s facility may affect. 42 U.S.C. § 7475(e)(1). 

Under subsection (e)(2), this analysis 

shall include continuous air quality monitoring data 

gathered for purposes of determining whether 

emissions from such facility will exceed the 

[increment] or the maximum allowable concentration 

permitted under [the NAAQS]. Such data shall be 

gathered over a period of one calendar year preceding 

the date of application for a permit under this part 

unless the State, in accordance with regulations 

promulgated by the [EPA], determines that a complete 

and adequate analysis for such purposes may be 

accomplished in a shorter period. The results of such 
18

analysis shall be available at the time of the public 

hearing on the application for such permit.

Id. § 7475(e)(2).

We read § 165(e)(2) of the Act as an “extraordinarily 

rigid” mandate that a PSD permit applicant undertake 

preconstruction monitoring. Indeed, we recognized the 

rigidity of this subsection in Alabama Power when we held 

that “[t]his is a plain requirement for inclusion of monitoring 

data.” Alabama Power, 636 F.2d at 372 (holding that the 

EPA did not have authority to dispense with monitoring 

where Congress mandated the use of that technique, even 

though monitoring technology at the time was limited). 

Congress’s use of the word “shall” in each sentence of the Act 

evidences a clear legislative mandate that the preconstruction 

monitoring requirement applies to PSD permit applicants.

That Congress provided only one exception to this monitoring 

requirement — a shorter monitoring period — suggests that 

Congress did not intend any other exceptions. See Sierra 

Club v. EPA, 294 F.3d 155, 160 (D.C. Cir. 2002). If Congress 

sought to give the EPA discretion to eliminate the monitoring 

requirement it could have used less rigid language to achieve 

that result, as it has in other subsections of § 165. For 

example, in 42 U.S.C. § 7475(e)(2), Congress provided that

“[air quality] data shall be gathered over a period of one 

calendar year preceding the date of application for a permit 

under this part unless the State, in accordance with regulations 

promulgated by the [EPA], determines that a complete and 

adequate analysis for such purposes may be accomplished in a 

shorter period.” (emphasis added). In contrast, § 7475(a)(7) 

requires as a condition for obtaining a PSD permit, that an 

owner or operator of a proposed source or modification agree 

to post-construction monitoring as “may be necessary to 

determine the effect which emissions” from the facility may 
19

have on air quality. (emphasis added). Moreover, the 

exception for a shorter monitoring period only applies when 

the permitting authority determines that a complete and 

adequate analysis may be accomplished in a shorter period. 

EPA has not explained how a “complete and adequate” 

analysis may be accomplished without any of the monitoring 

data required by § 165(e)(2).

Given how extraordinarily rigidly Congress stated its 

monitoring mandate in § 165(e)(2), we are not persuaded by 

the EPA’s arguments that it has de minimis authority to 

exempt the preconstruction monitoring requirement. The 

EPA argues that the Sierra Club fails to show that the statute 

is so rigid that it precludes the exercise of the EPA’s de 

minimis authority. The EPA, however, does not explain how 

the statute is ambiguous, but instead asserts that there is a 

“virtual presumption” of inherent agency authority. Resp’t 

Br. at 46; see Public Citizen v. Young, 831 F.2d 1108, 1113 

(D.C. Cir. 1987). This argument is circular. Even if a

“virtual presumption” exists, that presumption can be rebutted 

by an “extraordinarily rigid” statutory mandate. See Public 

Citizen, 831 F.2d at 1113. Whether we call preconstruction 

monitoring a “plain requirement” or a requirement mandated 

by an “extraordinarily rigid” statute, the result is the same: the 

EPA has no de minimis authority to exempt the requirement. 

Without pointing out any ambiguity in Congress’s 

mandate, the EPA asserts that the purpose of the statute’s 

preconstruction monitoring requirement “is to provide data 

for purposes of performing an air quality analysis,” and that it 

can reasonably conclude “the statute permits an exemption for 

collection of data that is not useful to carrying out the 

purposes of the statute.” Resp’t Br. at 49. The EPA confuses 

the purpose of § 165(e)(2)’s monitoring requirement. The 

statute explicitly states that one purpose of the monitoring 
20

requirement is to determine whether emissions from a 

proposed source or modification will exceed the increments or 

NAAQS. 42 U.S.C. § 7475(e)(2). We logically infer from 

this statement that Congress intended the monitoring 

requirement to establish the baseline air quality in an area 

before the owner of a proposed source or modification even 

applies for a PSD permit. If an area’s pre-existing ambient 

PM2.5 concentration is so high that a violation of the NAAQS 

or increment is imminent, a source below the SMC may 

nevertheless cause a violation if built or modified. This is 

true even if the source’s projected ambient impact on PM2.5 is 

so low that the difference in air quality before and after 

construction would be impossible to measure with accuracy. 

But a permitting authority cannot know how close an area is 

to violating the NAAQS or increment unless it knows the 

existing ambient concentrations of PM2.5 before a source is 

constructed or modified. 

The EPA’s argument also fails to address Congress’s 

mandate that the results of the air quality analysis required by 

§ 165(e) be made available to the public at the time of a 

hearing for a PSD permit. Id. § 7475(e)(2). Indeed, one of 

Congress’s stated purposes in enacting the PSD provisions 

was “to assure that any decision to permit increased air 

pollution in any area to which” the PSD provisions apply be 

made only after careful evaluation by the permitting authority 

and “after adequate procedural opportunities for informed

public participation in the decisionmaking process.” 42 

U.S.C. § 7470(5) (emphasis added). Congress’s express 

statement that the public shall have air quality data to allow 

for informed participation in PSD application hearings 

bolsters our conclusion that the EPA has no authority to 

exempt the monitoring requirement.
21

In addition to arguing that § 165(e)(2) was 

extraordinarily rigid, the Sierra Club contends that the EPA

has no de minimis authority because the PM2.5 SMC thwarts 

the legislative design of the Act. The EPA addresses this

argument by making two arguments. First, the EPA states 

that it has advised permitting authorities not to apply the 

monitoring exemption when an area’s ambient concentration 

is close to the NAAQS or the consumption of the increment. 

Second, the EPA asserts that exempting preconstruction 

monitoring in areas where the ambient concentration itself is 

below the SMC (and thus not capable of accurate 

measurement, regardless of a proposed source’s projected 

impact) furthers legislative design by avoiding pointless 

expenditures of effort. 

Both these points ignore the rigidity of the statute. 

Because the statute leaves no room for exemptions, such as 

those at issue, granting the permitting authorities discretion to 

apply the exemption is beyond the EPA’s statutory authority. 

As to the EPA’s second point, we agree with the Sierra Club 

that the estimation that an area is below the SMC does not 

render monitoring superfluous because monitoring could 

reveal that the estimate was incorrect. More importantly, 

Congress provided a clear mandate that the EPA does not 

have authority to disregard, even if the mandated 

requirements appear to it to be superfluous.

To authorize the EPA to exempt the plain requirement of 

preconstruction monitoring and to retain (and delegate) 

discretion on when such an exemption should apply would 

allow the EPA to engage in an impermissible cost-benefit 

analysis. As we explained in Alabama Power, “implied 

authority is not available for a situation where the 

regulatory function does provide benefits, in the sense of 

furthering the regulatory objectives, but the agency concludes 
22

that the acknowledged benefits are exceeded by the costs.” 

Alabama Power, 636 F.2d at 361. To engage in cost-benefit 

decisions, the EPA’s implied authority “must be based not on 

a general doctrine but on a fair reading of the specific statute, 

its aims and legislative history.” Id. The monitoring 

requirement is a regulatory function that provides benefits, 

and the statute precludes the EPA from exempting that 

requirement. Although the year-long preconstruction 

monitoring requirement may be onerous and, in some cases,

EPA deems it more costly than beneficial, the EPA may not

substitute its policy for that of Congress. 

III. CONCLUSION

For the foregoing reasons, we vacate and remand to the 

agency for further consideration the portions of the EPA’s 

rule addressing SILs, except for the parts of its rule codifying 

PM2.5 SILs in 40 C.F.R. § 51.165(b)(2). We grant the Sierra 

Club’s petition as to the parts of the EPA’s rule establishing a 

PM2.5 SMC, and vacate them because these parts of the rule

exceed the EPA’s statutory authority. See 42 U.S.C. 

§ 7607(d)(9)(3).

So ordered.