Document ID: ./input/supremecourt_opinions/opinions/23pdf/23a349_0813.pdf
Page Number: 6

Cite as:  603 U. S. ____ (2024) 

3 

Opinion of the Court 

  Because the States bear “primary responsibility” for de-
veloping  compliance  plans,  §7401(a)(3),  EPA  has  “no  au-
thority to question the wisdom of a State’s choices of emis-
sion  limitations.”    Train  v.  Natural  Resources  Defense 
Council, Inc., 421 U. S. 60, 79 (1975).  So long as a SIP sat-
isfies  the  “applicable  requirements”  of  the  Act,  including 
the Good Neighbor Provision, EPA “shall approve” it within 
18  months  of 
  §7410(k)(3);  see 
§§7410(k)(1)(B), (k)(2).  If, however, a SIP falls short, EPA 
“shall” issue a Federal Implementation Plan, or FIP, for the 
noncompliant State—that is, “unless” the State corrects the 
deficiencies in its SIP first.  §7410(c)(1); EME Homer, 572 
U. S., at 498.  EPA must also ensure States meet the new 
air-quality standard by a statutory deadline.  See §7511. 

its  submission. 

B 
  A layer of ozone in the atmosphere shields the world from 
the sun’s radiation.  See National Resources Defense Coun-
cil v. EPA, 464 F. 3d 1, 3 (CADC 2006).  But closer to earth, 
ozone can hurt more than it helps.  Forming when sunlight 
interacts with a wide range of precursor pollutants, ground-
level ozone can trigger and exacerbate health problems and 
damage  vegetation.    80  Fed.  Reg.  65299,  65302,  65370 
(2015). 
  To  mitigate  those  and  other  problems,  in  2015  EPA  re-
vised its air-quality standards for ozone from 75 to 70 parts 
per billion.  Id., at 65293–65294.  That change triggered a 
requirement for States to submit new SIPs.  Id., at 65437.  
Along the way, EPA issued a guidance document advising 
States that they had “flexibility” in choosing how to address 
their Good Neighbor obligations.  See EPA, Memorandum, 
Information  on  the  Interstate  Transport  State  Implemen-
tation Plan Submissions for the 2015 Ozone National Am-
bient Air Quality Standards 3 (Mar. 27, 2018).  With that 
and other guidance in hand, many (though not all) States 
submitted SIPs.  See 84 Fed. Reg. 66612 (2019).  And many