                       Remanded, April 2, 2007




                           PUBLISHED

UNITED STATES COURT OF APPEALS
                FOR THE FOURTH CIRCUIT


UNITED STATES OF AMERICA,               
                 Plaintiff-Appellant,
ENVIRONMENTAL DEFENSE; NORTH
CAROLINA SIERRA CLUB; NORTH
CAROLINA PUBLIC INTEREST RESEARCH
GROUP CITIZEN LOBBY/EDUCATION
FUND,
   Intervenors/Plaintiffs-Appellants,
                 v.
DUKE ENERGY CORPORATION,
              Defendant-Appellee.
                                                No. 04-1763


AMERICAN LUNG ASSOCIATION;
STATE OF NEW YORK; CONNECTICUT;
ILLINOIS; MARYLAND; NEW
HAMPSHIRE; PENNSYLVANIA;
WASHINGTON, DC; DELAWARE;
MAINE; MASSACHUSETTS; NEW
JERSEY; VERMONT,
         Amici Supporting Appellant,
                                        
2              UNITED STATES v. DUKE ENERGY CORP.


MANUFACTURERS ASSOCIATION WORK        
GROUP; JOE L. BARTON, U.S.
Representative; SOUTH DAKOTA;
STATE OF ALABAMA; STATE OF
KANSAS; STATE OF NEBRASKA;
STATE OF NORTH DAKOTA; TENNESSEE
VALLEY AUTHORITY; LAW
PROFESSORS, Jonathan Adler, Ronald
A. Cass, John C. Eastman, Ernest      
Gellhorn, James Huffman, Donald J.
Kochan, Gary Marchant, Roger
Meiners, Andrew Morriss,
         Amici Supporting Appellee,
               and
UTILITY AIR REGULATORY GROUP,
                         Movant.
                                      
           Appeal from the United States District Court
      for the Middle District of North Carolina, at Durham.
              Frank W. Bullock, Jr., District Judge.
                        (CA-00-1262-1)

                     Argued: February 3, 2005

                      Decided: June 15, 2005

          Before LUTTIG and MOTZ, Circuit Judges,
      and Samuel G. WILSON, United States District Judge
              for the Western District of Virginia,
                     sitting by designation.



Affirmed by published opinion. Judge Motz wrote the opinion, in
which Judge Luttig and Judge Wilson joined.
                UNITED STATES v. DUKE ENERGY CORP.                   3
                             COUNSEL

ARGUED: Todd Sunhwae Kim, UNITED STATES DEPARTMENT
OF JUSTICE, Environment & Natural Resources Division, Washing-
ton, D.C., for the United States. James Blanding Holman, IV,
SOUTHERN ENVIRONMENTAL LAW CENTER, Chapel Hill,
North Carolina, for Appellants Environmental Defense, North Caro-
lina Sierra Club, and North Carolina Public Interest Research Group
Citizen Lobby/Education Fund. F. William Brownell, HUNTON &
WILLIAMS, Washington, D.C., for Appellee. ON BRIEF: Thomas
L. Sansonetti, Assistant Attorney General, Jason A. Dunn, John A.
Bryson, UNITED STATES DEPARTMENT OF JUSTICE, Environ-
ment & Natural Resources Division, Washington, D.C.; Alan Dion,
Monica Derbes Gibson, ENVIRONMENTAL PROTECTION
AGENCY, Washington, D.C., for the United States. Jeffrey M. Glea-
son, SOUTHERN ENVIRONMENTAL LAW CENTER, Charlottes-
ville, Virginia, for Appellants Environmental Defense, North Carolina
Sierra Club, and North Carolina Public Interest Research Group Citi-
zen Lobby/Education Fund. Garry S. Rice, DUKE ENERGY COR-
PORATION, Charlotte, North Carolina; Dean M. Moesser, DUKE
ENERGY CORPORATION, Houston, Texas; T. Thomas Cot-
tingham, III, Nash E. Long, III, HUNTON & WILLIAMS, L.L.P.,
Charlotte, North Carolina; Mark B. Bierbower, Henry V. Nickel,
Makram B. Jaber, HUNTON & WILLIAMS, L.L.P., Washington,
D.C., for Appellee. Alan Birnbaum, CLEAN AIR TASK FORCE,
Boston, Massachusetts, for Amicus Curiae, American Lung Associa-
tion, Supporting Appellant. Eliot Spitzer, Attorney General of New
York, Caitlin J. Halligan, Solicitor General, Daniel Smirlock, Deputy
Solicitor General, Peter Lehner, Assistant Attorney General, J. Jared
Snyder, Assistant Attorney General, Scott Bassinson, Assistant Attor-
ney General, Environmental Protection Bureau, Albany, New York,
for Amicus Curiae, State of New York, Supporting Appellant. Rich-
ard Blumenthal, Attorney General of Connecticut, Hartford, Connect-
icut, for Amicus Curiae, Connecticut, Supporting Appellant. Lisa
Madigan, Attorney General of Illinois, Chicago, Illinois, for Amicus
Curiae, Illinois, Supporting Appellant. J. Joseph Curran, Jr., Attorney
General of Maryland, Baltimore, Maryland, for Amicus Curiae,
Maryland, Supporting Appellant. Kelly A. Ayotte, Attorney General
of New Hampshire, Concord, New Hampshire, for Amicus Curiae,
New Hampshire, Supporting Appellant. Susan Shinkman, Chief
4               UNITED STATES v. DUKE ENERGY CORP.
Counsel, Pennsylvania, Department of Environmental Protection,
Harrisburg, Pennsylvania, for Amicus Curiae, Pennsylvania, Support-
ing Appellant. Robert J. Spagnoletti, Attorney General of District of
Columbia, Washington, D.C., for Amicus Curiae, Washington, D.C.,
Supporting Appellant. M. Jane Brady, Attorney General of Delaware,
Dover, Delaware, for Amicus Curiae, Delaware, Supporting Appel-
lant. G. Steven Rowe, Attorney General of Maine, Augusta, Maine,
for Amicus Curiae, Maine, Supporting Appellant. Thomas F. Reilly,
Attorney General of Massachusetts, Boston, Massachusetts, for
Amicus Curiae, Massachusetts, Supporting Appellant. Peter C. Har-
vey, Attorney General of New Jersey, Trenton, New Jersey, for
Amicus Curiae, New Jersey, Supporting Appellant. William H. Sor-
rell, Attorney General of Vermont, Montpelier, Vermont, for Amicus
Curiae, Vermont, Supporting Appellant. Charles H. Knauss, Robert
V. Zener, SWIDLER, BERLIN, SHEREFF, FRIEDMAN, L.L.P.,
Washington, D.C., for Amicus Curiae, Manufacturers Association
Work Group, Supporting Appellee. C. Boyden Gray, WILMER,
CUTLER, PICKERING, HALE AND DORR, L.L.P., Washington,
D.C., for Amicus Curiae, U.S. Representative Joe L. Barton, Support-
ing Appellee. Lawrence E. Long, Attorney General of the State of
South Dakota, Roxanne Giedd, Deputy Attorney General, Pierre,
South Dakota, for Amicus Curiae, South Dakota, Supporting Appel-
lee. Troy King, Attorney General of the State of Alabama, Montgom-
ery, Alabama, for Amicus Curiae, State of Alabama, Supporting
Appellee. Phill Kline, Attorney General of the State of Kansas, David
W. Davies, Deputy Attorney General, Topeka, Kansas, for Amicus
Curiae, State of Kansas, Supporting Appellee. Joe Bruning, Attorney
General of the State of Nebraska, Jodi M. Fenner, Assistant Attorney
General, Lincoln, Nebraska, for Amicus Curiae, State of Nebraska,
Supporting Appellee. Wayne Stenehjem, Attorney General of the
State of North Dakota, Charles M. Carvell, Assistant Attorney Gen-
eral, Lyle G. Witham, Assistant Attorney General, Dean J. Haas,
Assistant Attorney General, Bismarck, North Dakota, for Amicus
Curiae, State of North Dakota, Supporting Appellee. Maureen H.
Dunn, General Counsel, Harriet A. Cooper, Assistant General Coun-
sel, Gregory R. Signer, Senior Environmental Counsel, Frank H. Lan-
caster, TENNESSEE VALLEY AUTHORITY, Office of the General
Counsel, Knoxville, Tennessee, for Amicus Curiae, Tennessee Valley
Authority, Supporting Appellee. David B. Rivkin, Jr., Lee A. Casey,
                UNITED STATES v. DUKE ENERGY CORP.                    5
Darin R. Bartram, BAKER & HOSTETLER, L.L.P., Washington,
D.C., for Amici Curiae, Law Professors, Jonathan Adler, Ronald A.
Cass, John C. Eastman, Ernest Gellhorn, James Huffman, Donald J.
Kochan, Gary Marchant, Roger Meiners, Andrew Morriss, Support-
ing Appellee.


                              OPINION

DIANA GRIBBON MOTZ, Circuit Judge:

   The United States brought this enforcement action against Duke
Energy Corporation, which provides North Carolina and South Caro-
lina with electricity generated from eight plants located throughout
the two states. The United States maintains that Duke Energy on
numerous occasions modified these plants without first obtaining
appropriate permits in violation of the Clean Air Act. 42 U.S.C.
§§ 7401 et seq. (2000). The district court granted summary judgment
to Duke Energy. See United States v. Duke Energy Corp., 278 F.
Supp. 2d 619 (M.D.N.C. 2003). We affirm, albeit for somewhat dif-
ferent reasons than those relied on by the district court.

                                   I.

   The Clean Air Act is a complex statute supported by an elaborate
regulatory scheme; both have a complicated history. This case
involves two different, but complementary provisions of the Act: the
New Source Performance Standards ("NSPS") provisions, 42 U.S.C.
§ 7411, and the Prevention of Significant Deterioration ("PSD") pro-
visions, 42 U.S.C. §§ 7470-92.

   In order to "protect and enhance the quality of the Nation’s air
resources" and "promote the public health and welfare," 42 U.S.C.
§ 7401(b)(1), the Clean Air Amendments of 1970 directed the Envi-
ronmental Protection Agency ("EPA") to devise National Ambient
Air Quality Standards establishing the maximum concentrations of
certain air pollutants allowable in each region of the United States. 42
U.S.C. § 7409. The Act then directed each State to design a State
Implementation Plan to effect compliance with its air quality stan-
dards. 42 U.S.C. § 7410.
6               UNITED STATES v. DUKE ENERGY CORP.
   To help attain and thereafter maintain these air quality standards,
the 1970 amendments enacted the NSPS provisions, which required
the EPA to promulgate standards regulating emissions from both
newly constructed and modified sources of pollution at power plants.
42 U.S.C. § 7411. Congress defined "modification" in the NSPS pro-
visions as "any physical change in, or change in the method of opera-
tion of, a stationary source which increases the amount of any air
pollutant emitted by such source or which results in the emission of
any air pollutant not previously emitted." 42 U.S.C. § 7411(a)(4).

   Since 1971, the EPA has promulgated NSPS regulations that define
"modification" in virtually the same words as the statute. See, e.g., 36
Fed. Reg. 24,876, 24,877 (Dec. 23, 1971); 40 C.F.R. § 60.2 (1976);
40 C.F.R. § 60.2 (2004). In 1975, the EPA added a regulation elabo-
rating on this definition and further defining "modification" by refer-
ence to an increase in the hourly emission rate: a modification
includes "any physical or operational change to an existing facility
which results in an increase in the emission rate to the atmosphere of
any [regulated] pollutant," measured not in tons per year, but in kilo-
grams per hour. 40 Fed. Reg. 58,416, 58,419 (Dec. 16, 1975) (codi-
fied at 40 C.F.R. § 60.14(a) & (b)). Modified equipment becomes
subject to the NSPS’s "technology-based" standards, Alabama Power
Co. v. Costle, 636 F.2d 323, 346 (D.C. Cir. 1980), which mandate the
installation of the "best demonstrated pollution control technology."
Potomac Elec. Power Co. v. EPA, 650 F.2d 509, 518 (4th Cir. 1981)
[hereinafter PEPCo].

   The NSPS program was not entirely successful. See Wisconsin
Elec. Power Co. v. Reilly, 893 F.2d 901, 904 (7th Cir. 1990). In 1972,
the United States District Court for the District of Columbia issued
a preliminary injunction directing the EPA to promulgate regulations
to supplement the NSPS program and protect air quality from deterio-
ration in areas that had met or exceeded the relevant ambient stan-
dards. See Sierra Club v. Ruckleshaus, 344 F. Supp. 253 (D.D.C.
1972), aff’d 4 E.R.C. 1815 (D.C. Cir. 1972), aff’d by an equally
divided court sub nom. Fri v. Sierra Club, 412 U.S. 541 (1973). The
EPA duly disseminated the first PSD regulations in 1974. See 39 Fed.
Reg. 42,510 (Dec. 5, 1974). Congress thereafter enacted a PSD pro-
gram in the Clean Air Act Amendments of 1977. See 42 U.S.C.
§ 7470(1).
                 UNITED STATES v. DUKE ENERGY CORP.                    7
   As originally enacted, the PSD permit provisions in the Clean Air
Act applied only to the "construction" of major emitting facilities. See
Clean Air Act Amendments of 1977, Pub. L. No. 95-95, 91 Stat. 685,
735 (1977) ("No major emitting facility on which construction is
commenced after the date of the enactment of this part, may be con-
structed in any area to which this part applies unless . . . a permit has
been issued . . . ."). However, in November 1977, a few months after
the original enactment became effective, Congress passed the "Clean
Air Act Technical and Conforming Amendments." Pub. L. No. 95-
190, 91 Stat. 1393, 1399 (1977). These amendments added to the
"Definitions" section of the PSD provisions in 42 U.S.C. § 7479 a
subparagraph that provides: "The term ‘construction’ when used in
connection with any source or facility, includes the modification (as
defined in [section 7411(a)]) of any source or facility." Id. at 1402.
This amendment thus incorporated the NSPS statutory definition of
"modification," § 7411(a)(4), into § 7479 of the PSD statute.

   The PSD program imposes, inter alia, preconstruction review and
permit requirements on new or modified sources in areas that have
attained or exceeded their air quality standards. 42 U.S.C. § 7475.
Unlike the NSPS program, the PSD program does not focus primarily
on technology-based controls, but on the "net emissions from an
entire plant resulting from construction or modification of one or
more emitting sources within the plant." PEPCo, 650 F.2d at 518
(emphasis omitted). And so, while NSPS centers on technological
controls at an individual pollution-emitting apparatus, PSD fixes on
the actual emissions from a site. See N. Plains Res. Council v. EPA,
645 F.2d 1349, 1356 (9th Cir. 1981) ("The NSPS program is . . .
equipment oriented. On the other hand, the PSD program . . . is . . .
site oriented.").

   The EPA promulgated regulations under the PSD provisions of the
statute in 1978, see 43 Fed. Reg. 26,380 (June 19, 1978), and
amended them in 1980, see 45 Fed. Reg. 52,676 (Aug. 7, 1980).
Under the 1980 PSD regulations, a plant cannot engage in a "major
modification" of equipment without first undergoing the EPA’s per-
mit process and acquiring a permit. 45 Fed. Reg. 52,676. The EPA’s
PSD regulations define a "major modification" as "any physical
change in or change in the method of operation of a major stationary
source that would result in a significant net emissions increase of any
8                UNITED STATES v. DUKE ENERGY CORP.
pollutant subject to regulation under the Act." 40 C.F.R.
§ 51.166(b)(2)(i) (1987).1 A "net emissions increase" is "[a]ny
increase in actual emissions from a particular physical change or
change in the method of operation" of a unit. 40 C.F.R.
§ 51.166(b)(3)(i). The PSD regulations measure emissions increases
relative to a baseline calculation of "actual emissions," i.e., "the aver-
age rate, in tons per year, at which the unit actually emitted" the regu-
lated pollutant for, usually, the two years prior to date of
measurement, "using the unit’s actual operating hours, production
rates, and types of materials processed, stored, or combusted during
the selected time period." 40 C.F.R. § 51.166(b)(21)(ii).

                                   II.

                                   A.

   Duke Energy’s eight plants in the Carolinas include thirty coal-
fired generating units that were placed in service between 1940 and
1975. Each unit contains, as one of its three major components, a
boiler, which is a large structure from six- to twenty-stories tall con-
taining thousands of steel tubes. The tubes are arranged into sets of
tube assemblies, including economizer tubes, in which water is ini-
tially heated; furnace waterwall tubes, in which water evaporates to
steam; superheater tubes, in which the temperature of the steam is
raised before being released into a turbine; and reheater tubes, in
which steam released from the turbine is reheated and returned to the
turbine.

   Between 1988 and 2000, as part of a plant modernization program,
Duke Energy engaged in twenty-nine projects on the coal-fired gener-
ating units, most of which consisted of replacing and/or redesigning
one or more of the boiler tube assemblies. These projects would both
extend the life of the generating units and allow the units to increase
their daily hours of operation. Duke Energy did not apply for or
    1
   The 1980 regulations, which the parties agree control the projects at
issue here, were recodified in the 1987 Code of Federal Regulations.
None of the relevant PSD or NSPS provisions were revised during the
period pertaining to this dispute. All subsequent references herein are to
the 1987 C.F.R. unless otherwise noted.
                UNITED STATES v. DUKE ENERGY CORP.                    9
acquire new permits from the EPA for these projects, some of which,
according to the Government, cost "more than seven times the origi-
nal cost of the unit." Brief of United States at 14.

   In December 2000, at the direction of the Administrator of the
EPA, the Attorney General brought this enforcement action against
Duke Energy, alleging that the life-extension projects violated, inter
alia, the Clean Air Act’s PSD provisions. In September 2001, the dis-
trict court granted Environmental Defense, the North Carolina Sierra
Club, and the North Carolina Public Interest Research Group Citizen
Lobby/Education Fund leave to intervene as plaintiffs, and these
groups filed a complaint against Duke Energy alleging similar viola-
tions.

   The EPA and the Intervenors maintain that these life-extension
projects constitute "major modifications" of Duke Energy’s furnaces
as defined in the PSD statutory and regulatory provisions — that is,
physical changes leading to a significant net emissions increase —
and thus Duke Energy was required to obtain permits for them. The
EPA does not contend that the post-project hourly rate of emissions
increased. Rather, it argues that the PSD requires measurement of the
net emissions increase by using an "actual-to-projected-actual" test,
comparing the actual pre-project emissions from a unit to the pro-
jected post-project emissions, which takes into account a unit’s ability
to operate for more hours. Because the Duke Energy projects enable
the units to operate for more hours each day, they will lead to an
increase in actual yearly emissions.

   Duke Energy counters that its projects do not constitute modifica-
tions subject to PSD because they did not increase the units’ levels
of emissions. The company maintains that, under the PSD program,
a net emissions increase will result only if there is an increase in the
hourly rate of emissions. Because none of its projects increased a
unit’s hourly capacity to emit pollution (but increased only the num-
ber of hours the unit could operate), the projects did not increase
emissions from pre-project levels, and so, according to Duke Energy,
it did not have to obtain permits.
10               UNITED STATES v. DUKE ENERGY CORP.
                                    B.

   The district court agreed with Duke Energy. It held that a modifica-
tion subject to PSD exists only if there is a post-project increase in
the hourly rate of emissions from a unit. Duke Energy Corp., 278 F.
Supp. 2d at 640.2 In reaching this conclusion, the district court relied
on the language of the PSD regulations, "contemporaneous interpreta-
tions" of the regulations by the EPA, and "the statutory language
incorporating the NSPS concept of modification into PSD." Id.

    One regulation promulgated by the EPA pursuant to the PSD stat-
ute, 40 C.F.R. § 51.166(b)(2)(i), characterizes a "major modification"
as "any physical change in or change in the method of operation
. . . that would result in a significant net emissions increase"; another,
40 C.F.R. § 51.166(b)(2)(iii)(f), excludes "[a]n increase in the hours
of operation or in the production rate" from the definition of "physical
change or change in the method of operation." Reading these two pro-
visions in conjunction, the district court determined that an emissions
increase traceable to increased hours of operation cannot trigger the
PSD provisions because an increase in hours is not a physical change.
Put another way, because increased hours are not a physical change
under the PSD regulations, calculation of post-project net emissions
cannot take into account increased hours of operation, but rather must
be based on pre-project hours of operation and rates of production.
Therefore, the court concluded, only if the hourly rate of emissions
increases can there be a net emissions increase under the PSD regula-
tions. Duke Energy, 278 F. Supp. 2d at 640-41.
  2
   Duke Energy asserted in the alternative that its projects constitute
maintenance, repair and replacement that is routine in the utility industry
and, for this reason, were exempt from the permit requirements. See 40
C.F.R. 51.166(b)(2)(iii)(a). The EPA and the Intervenors disagreed, con-
tending that this regulatory exemption only applies to repairs and
replacements routine within the life of a generating unit and, therefore,
Duke Energy’s life-enhancing projects did not qualify for this exemption.
The district court again agreed with Duke Energy’s interpretation, but
held that there was insufficient evidence to grant summary judgment to
Duke Energy on this claim. Duke Energy, 278 F. Supp. 2d at 638. Given
our resolution of this case, we need not reach this question.
                 UNITED STATES v. DUKE ENERGY CORP.                     11
   The district court recognized that the EPA interpreted its PSD regu-
lations differently, excluding a much smaller group of projects from
the definition of "major modification." The EPA would exclude only
those projects that increase hours of operation and involve no con-
struction. Id. at 641. According to the interpretation pressed by the
EPA, whenever there is an increase "in utilization coupled with a
physical change, any increase in hours of operation . . . may be con-
sidered in the emissions calculus." Id. The district court concluded
that it could not defer to the EPA’s interpretation because in addition
to being, in the court’s view, belied by the plain language of the PSD
regulations, the present EPA interpretation was "clearly contrary to
earlier [EPA] interpretations" of the regulations. Id. Specifically, the
court noted that "[i]mmediately after the promulgation of the PSD
regulations in 1980, the EPA’s Director of the Division of Stationary
Source Enforcement . . . , Edward E. Reich, confirmed in two separate
applicability determinations that the requirements of PSD would be
implicated only by an increase in the hourly rate of emissions." Id.

   Finally, the court determined that its interpretation of the PSD reg-
ulation was "also consistent with the NSPS [statutory] definition of
‘modification’" found in 42 U.S.C. § 7411(a), "which was incorpo-
rated by explicit reference into PSD" in 42 U.S.C. § 7479(2)(c)
(defining "construction" to include "the modification (as defined in
section 7411(a) . . . ) of any source or facility"). Id. at 642. The inter-
pretation of PSD regulations urged by the EPA in this suit would, the
district court concluded, be "inconsistent with the congressional
design of defining PSD construction in terms of NSPS modification
and should therefore be accorded little deference." Id. at 643.

   After resolution of this legal issue, the parties stipulated that the
Duke Energy projects would not result in an increase in the hourly
rate of emissions. The court then entered summary judgment for Duke
Energy.

                                   III.

   In cases in which an agency’s interpretation of its regulations are
at issue, a court engages in a modified Chevron analysis. See Chevron
U.S.A., Inc. v. Natural Res. Def. Council, 467 U.S. 837 (1984); United
States v. Deaton, 332 F.3d 698, 708-09 (4th Cir. 2003). First, as in
12               UNITED STATES v. DUKE ENERGY CORP.
the usual Chevron analysis, a court must determine "whether Con-
gress has directly spoken to the precise question at issue." Chevron,
467 U.S. at 842. "The judiciary is the final authority on issues of stat-
utory construction" and "[i]f a court, employing traditional tools of
statutory construction, ascertains that Congress had an intention on
the precise question at issue, that intention is the law and must be
given effect." Id. at 843 n.9. Only if the statute is silent or ambiguous
on the point is Congress deemed to have delegated authority to the
agency to clarify the point in its regulations. Id. at 843-45. Thus, only
in such cases does a court examine the regulation itself, determining
its legitimate meaning, asking whether the regulation is based on a
permissible construction of the statute, and, if so, deferring to it. See
id. at 843; Deaton, 332 F.3d at 708-09.

   The EPA and the Intervenors expressly acknowledge that these
principles govern our review in the case at hand. See Brief of United
States at 14-15; Brief of Intervenors at 7-9. They fail to understand,
however, that straightforward application of these principles can lead
to only one conclusion: affirmance of the judgment of the district
court.

   This is so because Congress has indeed "directly spoken to the pre-
cise question at issue." Chevron, 467 U.S. at 842. As the EPA itself
concedes, the critical first "question at issue" here is whether the EPA
"can interpret the statutory term ‘modification’ under PSD differently
from how EPA interpreted that term" in the NSPS. Brief of United
States at 1. As the EPA also concedes, see id. at 4, Congress expressly
defined "modification" in the NSPS provisions of the Clean Air Act,
42 U.S.C. § 7411(a), and then expressly directed that the PSD provi-
sions of the Act employ this same definition. See 42 U.S.C.
7479(2)(c) (providing that "construction" in the PSD includes "modi-
fication . . . as defined in section 7411(a)"). When Congress mandates
that two provisions of a single statutory scheme define a term identi-
cally, the agency charged with administering the statutory scheme
cannot interpret these identical definitions differently. Thus, because
Congress mandated that the PSD definition of "modification" be iden-
tical to the NSPS definition of "modification," the EPA cannot inter-
                 UNITED STATES v. DUKE ENERGY CORP.                      13
pret "modification" under the PSD inconsistently with the way it
interprets that term under the NSPS.3

   Common sense would seem to dictate this result. Supreme Court
precedent certainly does. See Rowan Cos. v. United States, 452 U.S.
247 (1981). In Rowan, the Court faced a situation strikingly similar
to the one at hand, and held that when Congress itself provided "sub-
stantially identical" statutory definitions of a term in different statutes,
the agency charged with enforcing the statutes could not interpret the
statutory definitions "differently." Id. at 257.4

   The question presented in Rowan was whether the Commissioner
of the Internal Revenue Service could interpret the statutory term
"wages" differently for, on the one hand, the Federal Insurance Con-
tributions Act ("FICA") and the Federal Unemployment Tax Act
("FUTA"), and, on the other, the statute governing income-tax with-
holding. Id. at 250. Congress had defined the term "wages" in these
statutes in substantially the same language: for both FICA and FUTA,
"wages" were defined as "all remuneration for employment, including
the cash value of all remuneration paid in any medium other than
cash"; for income tax withholding, "wages" were defined as "all
remuneration (other than fees paid to a public official) for services
performed by an employee for his employer, including the cash value
of all remuneration paid in any medium other than cash." Rowan, 452
U.S. at 249 n.4.5 The Commissioner, however, issued regulations
  3
     Since Congress has "directly spoken to the precise question at issue
. . . , that is the end of the matter." Chevron, 467 U.S. at 842. A court
need not, indeed cannot, go further. Thus, the language and various inter-
pretations of the PSD regulations, on which the district court partially
based its holding and which the parties exhaustively discuss, are largely
irrelevant to the proper analysis of this case.
   4
     Because none of the parties or the thirty amici cited Rowan in their
original briefs or reply briefs, we requested supplemental briefing
addressing the case. Predictably (but inexplicably in view of its original
failure to cite Rowan), Duke Energy asserts that Rowan strongly supports
its position. More significantly, although the United States and Interve-
nors attempt to counter Rowan in various ways (which we address
within), they do not contend that the Rowan analysis is no longer good
law.
   5
     In 1983, Congress amended two of the statutes at issue in Rowan. See
Pub. L. No. 98-21 § 327(b)(1), 97 Stat. 65, 127 (1983) (adding to 26
14               UNITED STATES v. DUKE ENERGY CORP.
interpreting "wages" under FICA and FUTA to include the value of
meals and lodging provided to employees for the convenience of the
employer, and "wages" under the income-tax withholding statute to
exclude this value. See Rowan, 452 U.S. at 250.

   In holding the Commissioner’s interpretation impermissible, the
Court relied on the plain language of the statutes and their legislative
history. First, the Court noted that when Congress enacted the precur-
sors to FICA and FUTA as part of the Social Security Act of 1935,
it chose "wages" as the basis for employer taxation and then statu-
torily defined the term. Id. at 255. Similarly, seven years later, when
Congress enacted the original income-tax withholding statute, it chose
"wages" as the basis for taxation and statutorily defined the term "in
substantially the same language that it used in FICA and FUTA." Id.
The Rowan Court held that, "[i]n view of this sequence of consis-
tency, the plain language of the statute is strong evidence that Con-
gress intended ‘wages’ to mean the same thing under FICA, FUTA,
and income-tax withholding." Id.6 The Court then examined the stat-
utes’ history, finding indications that Congress intended to "coordi-

U.S.C. §§ 3121(a)(21) and 3306(b)(17) a paragraph stating, "Nothing in
the regulations prescribed for purposes of . . . income tax withholding
. . . which provides an exclusion from ‘wages’ as used in such chapter
shall be construed to require a similar exclusion from ‘wages’ in the reg-
ulations prescribed for purposes of this chapter").
   6
     We note that the standards of judicial review followed in Rowan dif-
fer somewhat from those established three years later in Chevron. The
most significant change for the case at hand is that Chevron abolished
judicial deference to agency interpretation of a statute in step one. See
John F. Coverdale, Chevron’s Reduced Domain: Judicial Review of
Treasury Regulations and Revenue Rulings After Mead, 55 Admin. L.
Rev. 39, 77 (2003); cf. United States v. Mead Corp., 533 U.S. 218, 229
(2001); Christensen v. Harris County, 529 U.S. 576, 582-83 (2000). This
difference does not in any way lessen the precedential value of Rowan
here. Indeed, if anything, it strengthens Rowan’s support for our holding
because even though in Rowan the Court gave some deference to the
agency’s interpretation of the statutes (rather than examining the statutes
de novo as Chevron step one directs), the Court nevertheless rejected the
agency’s position that it could interpret very similar statutory language
differently.
                 UNITED STATES v. DUKE ENERGY CORP.                    15
nate the income-tax withholding system with FICA and FUTA . . . to
promote simplicity and ease of administration," and concluding that
"[c]ontradictory interpretations of substantially identical definitions
do not serve that interest." Id. at 257.

   The plain language of the Clean Air Act provides even stronger
evidence that Congress intended the statutory definitions of "modifi-
cation" in the PSD and NSPS provisions to be interpreted identically.
While Congress used only "substantially the same language" in the
statutory definitions at issue in Rowan, id. at 255, here Congress man-
dated that the definition of "modification" in the PSD provisions pre-
cisely mirror the definition of "modification" in the NSPS provision.
Congress did this by directly incorporating the NSPS definition,
which it had enacted in 1970, into the PSD provisions, which it
enacted seven years later. See Pub. L. No. 95-190, 91 Stat. 1393, 1402
(1977) ("The term ‘construction’ when used in connection with any
source or facility, includes the modification (as defined in [section
7411(a)]) of any source or facility."); 42 U.S.C. § 7479(2)(C).

   Moreover, as in Rowan, the legislative history of the statutes at
issue here does not in any way suggest that Congress intended these
identical statutory definitions to receive different interpretations. Not-
withstanding the EPA’s contentions to the contrary, the fact that the
PSD definition of modification became part of the statute through
"Technical and Conforming Amendments" does not change the fact
that the definition is a statutory enactment, entitled to be treated as
such. See United States v. R.L.C., 503 U.S. 291, 305 n.5 (Opinion of
Souter, J.) ("[A] statute is a statute, whatever its label," and must be
interpreted using "the usual tools of statutory construction."). Indeed,
the expressed intent in the congressional summary of the legislative
amendments to "conform" the definition of modification in the PSD
provisions "to usage in other parts of the Act," 123 Cong. Rec. 36,253
(Nov. 1, 1977), indicates congressional concern with the same sort of
simplicity and consistency that the Rowan Court discerned from the
legislative history examined there. As the Court explained in Rowan,
"[i]t would be extraordinary for a Congress pursuing this interest to
intend, without ever saying so, for identical definitions to be inter-
preted differently." 452 U.S. at 257.7
  7
   Confronted with FICA and FUTA regulations that directly contra-
dicted established rules developed under income taxation, see Rowan,
16               UNITED STATES v. DUKE ENERGY CORP.
   The EPA points to Senator Muskie’s statement that it was "not the
purpose of these amendments to re-open substantive issues" in the
Act, 123 Cong. Rec. 36,252, as evidence that Congress was merely
using an expedient method to correct the inadvertent omission of the
word "modification" from the PSD provisions. To the extent that Sen-
ator Muskie’s remarks demonstrate Congress’ intent, see Runnebaum
v. NationsBank of Maryland, N.A., 123 F.3d 156, 169 n.7 (4th Cir.
1997) (en banc), they do not support the EPA’s position. The asser-
tion that the Technical and Conforming Amendments were not "de-
signed to resolve issues that were not resolved" in the debate and
passage of the Act, see 123 Cong. Rec. 36,252 (statement of Sen.
Muskie), says nothing about whether Congress had previously
resolved the issue of whether the interpretation of "modification" was
to be congruent under the PSD and NSPS statutory provisions.

  The EPA and Intervenors also emphasize the "vital differences"
between PSD and NSPS. Brief of United States at 22; see also id. at

452 U.S. at 258, the Court invalidated the FICA and FUTA regulations,
id. at 263. In contrast, notwithstanding the contentions of the United
States and Intervenors in their supplemental briefs, no question as to the
validity of the PSD regulations is (or could be, see 42 U.S.C. § 7607(b))
presented here. Unlike the FICA and FUTA regulations at issue in
Rowan, the PSD regulations can be interpreted consistently with pre-
existing principles — the NSPS regulations — as the district court dem-
onstrated and as the EPA’s Director of the Division of Stationary Source
Enforcement twice opined shortly after promulgation of the PSD regula-
tions. Our choice of this interpretation of the PSD regulations — as
required under the statute — over the EPA’s interpretation is not an
invalidation of those regulations. The PSD regulations remain fully intact
and enforceable and, indeed, could even be enforced as the EPA urges
provided that, as long as the PSD and NSPS statutes define "modifica-
tion" identically, the NSPS regulations are similarly interpreted and
enforced. Thus, the only question that we need resolve is one properly
before us, indeed one that EPA itself poses, i.e., whether it "can interpret
the statutory term modification under PSD differently from how" it has
interpreted that term under NSPS. See Brief of United States at 1, 22-30;
Reply Brief of United States at 2. Given that the EPA expressly asks that
we resolve this question, we have difficulty understanding its suggestion
in its supplemental brief that Duke Energy "waived" resolution of this
question.
                UNITED STATES v. DUKE ENERGY CORP.                   17
4-6; Brief of Intervenors at 13-21. We do not ignore or minimize
those differences. Although both statutes are part of the Clean Air Act
and designed to serve its purpose "to protect and enhance the quality
of the Nation’s air resources so as to promote the public health and
welfare and the productive capacity of its population," 42 U.S.C.
§ 7401(b)(1), they address somewhat different problems. PSD exists
primarily to prevent significant deterioration of ambient air quality in
areas meeting clean air standards, see 42 U.S.C. § 7470(1), while
NSPS requires new sources to implement particular technologies to
limit their own emissions. Id. § 7411. These differences have led us
and other courts to approve different regulatory definitions for an
identical statutory term in the two statutes. In PEPCo, for example,
we held that "significant difference[s] between the PSD and NSPS
programs" justified a different interpretation of the statutory term
"stationary source." 650 F.2d at 518. But in PEPCo, although Con-
gress had defined the term "stationary source" in the NSPS provi-
sions, see 42 U.S.C. § 7411(a)(3), it had not defined that term in the
PSD provisions. Thus, while in PEPCo both statutes contained the
same term, the statutes did not define that term in the same manner;
nor was the use of the term in the PSD provisions linked to the statu-
tory definition of the term in the NSPS provisions. Similarly, in
Northern Plains Resource Council, the Ninth Circuit allowed the EPA
to interpret the statutory term "commenced" differently in the NSPS
and PSD regulations. 645 F.2d at 1357. But again, although one stat-
ute — there the PSD provisions — defined the term, see 42 U.S.C.
§ 7479(2)(A), the other — the NSPS provisions — did not. And, Con-
gress had not linked the PSD definition of the term to its use in the
NSPS provisions of the statute.

   PEPCo and Northern Plains Resource Council illustrate the princi-
ple that the same word or phrase will generally be presumed to have
the same meaning when used in different parts of the statute, but this
"presumption of the uniform usage . . . relents" when there is "a varia-
tion in the connection in which the words are used as reasonably to
warrant the conclusion that they were employed in different parts of
the act with different intent." Gen. Dynamics Land Sys., Inc. v. Cline,
124 S. Ct. 1236, 1245 (2004) (internal quotation marks and citation
omitted). Thus, in PEPCo and Northern Plains Resource Council, the
difference in purpose between the NSPS and PSD programs justified
the conclusion that the same words had different meanings in the two
18               UNITED STATES v. DUKE ENERGY CORP.
sections of the statute. See N. Plains Res. Council, 645 F.2d at 1355-
56.

   In the case before us, however, the presumption of uniform usage
has become effectively irrebutable because Congress’ decision to
create identical statutory definitions of the term "modification" has
affirmatively mandated that this term be interpreted identically in the
two programs. The different purposes of the NSPS and PSD programs
cannot override that mandate. Neither the United States nor the Inter-
venors have cited a single case in which any court has held that iden-
tical statutory definitions can be interpreted differently by the agency
charged with enforcement of the statute. Moreover, in Rowan the
Supreme Court expressly rejected the argument, which was successful
in the Fifth Circuit, that the different purposes of FICA/FUTA and
income-tax withholding justified the different regulatory interpreta-
tions of the same statutory definition. See Rowan, 452 U.S. at 250,
257-58. The Rowan Court concluded that to permit the Commissioner
to interpret the same statutory terms differently would "fail to imple-
ment the congressional mandate in a consistent and reasonable man-
ner." Id. at 253.

   So it is here. Congress mandated that the PSD statute incorporate
the NSPS statutory definition of "modification." No one disputes that
prior to enactment of the PSD statute, the EPA promulgated NSPS
regulations that define the term "modification" so that only a project
that increases a plant’s hourly rate of emissions constitutes a "modifi-
cation." The EPA must, therefore, interpret its PSD regulations defin-
ing "modification" congruently. Of course, this does not mean that
this regulatory interpretation must be retained indefinitely. The EPA
retains its authority to amend and revise this and other regulations
"through exercise of appropriate rulemaking powers." Helvering v.
Wilshire Oil Co., 308 U.S. 90, 100-01 (1939) (noting that "[t]he con-
trary conclusion would . . . drastically curtail the scope and materially
impair the flexibility of administrative action"); McCoy v. United
States, 802 F.2d 762, 766 (4th Cir. 1986). Indeed, the parties point out
that the EPA has already amended some of the regulations at issue
here. See Brief of Duke Energy at 62-63; Reply Brief of United States
at 17-18. As long as Congress mandates that "modification" be
defined identically in the NSPS and PSD statutes, however, EPA must
               UNITED STATES v. DUKE ENERGY CORP.                    19
interpret that term in a consistent manner in the NSPS and PSD regu-
lations.

                                IV.

  For the foregoing reasons, the judgment of the district court is

                                                        AFFIRMED.
