150 SOUTH NINTH STREET 

EL CENTRO, CA 92243-2850 

August 21, 2012 

Gerardo Rios 

AIR POLL 

Chief, Air Permits Office (Air-3) 

EPA Region 9 

75 Hawthorne Street 

San Francisco, CA 94105 

DISTRICT 

SUBJECT: Additional Clarifications for Rule 904 and 40 CFR 51.166 

Dear Mr. Rios: 

TELEPHONE: (760) 482-4606 

FAX: (760) 353-9904 

This letter is in response to your request for additional information regarding District Rule 904. 

In a letter dated July 10, 2012, we responded to your initial request for clarifications regarding 

Ru1e 904 and the requirements for a Prevention of Significant Deterioration (PSD) permitting 

program. Below we provide an additional clarification regarding Rule 904. 

To implement the PSD permitting program under the Clean Air Act, Imperial County Air 

Pollution Control District (District) incorporated section 40 CFR 52.21(k)(2) of EPA's 

regu1ations by reference into the district's regulations and included this provision as part of a SIP 

revision submitted to EPA for approval. The District understands that EPA has recently stated, in 

response to litigation, that EPA intends to revise its PSD permitting regulations at 40 CFR 

51.166(k)(2) and 40 CFR 52.2l(k)(2) to ensure that these provisions are consistent with EPA 

statements in the supporting rulemaking record and the Clean Air Act. In light of EPA's 

response to the litigation and in order to ensure that permitting actions by the District are 

consistent with the Clean Air Act, I am writing to clarify that the District interprets and intends 

to apply the PSD program provisions in the District's portion of the California SIP in a manner 

that is consistent with certain EPA statements included in the preamble of the 2010 final rule that 

added section 52.2l(k)(2) to the federal regulations. 

Section 52.2l(k)(2) establishes significant impact levels (SILs) for PM2.s and reads as follows: 

(k)(2) Significant impact levels. For purposes of PM2.s, the demonstration required in 

paragraph (k)(l) of this section is deemed to have been made if the emissions increase 

from the new stationary source alone or from the modification alone would cause, in all 

areas, air quality impacts less than the following amounts: 

Pollutant Averaging time Class I area Class II area Class III area 

PM2.s ................. Annual ............ 0.06 !!g/m' 0.3 !!g/m' 0.3 !!g/m' 

24-hour ........... 0.07 !!g/m' 1.2 !!g/m' 1.2 !!g/m' 

AN EQUAL OPPORTUNITY f AFFIRMATIVE ACTION EMPLOYER 
Paragraph (k)(l) requires that sources applying for a new PSD permit demonstrate that any 

allowable emission increases from the proposed source or modification, in conjunction with all 

other applicable emissions increases or reductions, will not cause or contribute to a violation of 

any NAAQS or any applicable increment. 40 CFR 52.21(k)(l). In the preamble to the 2010 final 

rule adding the (k)(2) provision, EPA advised 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." Prevention of Significant Deterioration (PSD) 

for Particulate Matter Less than 2.5 Micrometers (PM2.s) -Increments, Significant Impact levels 

(SILs) and Significant Monitoring Concentration (SMC), 75 Fed. Reg. 64,864, 64,892 (Oct. 20, 

201 0). In another passage of the preamble, EPA also observed that "the use of a SIL may not be 

appropriate when a substantial portion of any NAAQS or increment is known to be consumed." 

Id. at 64,894. 

Consistent with these statements by EPA in the preamble to the 2010 final rule, the District does 

not interpret section 52.21 (k)(2) to preclude the District from exercising the discretion to 

determine when it may be appropriate to conclude that even a de minimis impact on air quality 

(an impact below the PM2.s SIL values) will cause or contribute to an air quality problem and to 

seek remedial action from the proposed new source or modification. Based on this 

interpretation, the District will not read section 52.21(k)(2) as an absolute "safe harbor," but will 

exercise discretion to determine whether a particular application of the PM2.s SILs is appropriate 

when a substantial portion of the PM2.s NAAQS or increment is known to be consumed. The 

District retains the discretion to require additional information from a permit applicant as needed 

to assure that the source will not cause or contribute to a violation of any NAAQS or applicable 

increment pursuant to paragraph 52.21 (k)(l ). 

Our expectation is that the need for the District to exercise this discretion will only arise in 

limited circumstances. The District looks forward to receiving additional guidance from EPA on 

how to identif'y when a permitting authority should exercise this type of discretion before 

applying the PM2.

5 SILs in section 52.21(k)(2) to support a demonstration that a source will not 

cause or contribute to a violation of the PM2.s NAAQS and increments. 

Sincerely,