See the latest EHS federal and state regulatory updates due to COVID-19

On August 1, 2016, the Environmental Protection Agency (EPA) posted a draft memorandum titled, "Guidance on SILs for Ozone and Fine Particles (PM2.5) in the Prevention of Significant Deterioration (PSD) Permitting Program," along with corresponding technical support documents on the NSR Permitting website.  As part of any PSD permit application, the applicant must perform air dispersion modeling to demonstrate compliance with the National Ambient Air Quality Standards (NAAQS) and PSD increments.  In order to do so, a proposed project must not have a significant impact on ambient air, i.e., modeled concentrations of a particular pollutant must not exceed the corresponding SIL for that pollutant and averaging period.  Should a cumulative analysis be needed at which point a NAAQS violation is discovered, then the SIL is once again relied upon to demonstrate that the applicant's project does not "cause or contribute" a violation of the NAAQS or PSD increment.

EPA's guidance establishes separate SILs for PM2.5 NAAQS and PSD Increment, lowers the SIL for PM2.5 annual NAAQS, and lowers the Class I SIL for PSD Increment for PM2.5 annual.  The following are the recommended SILs as proposed in the draft guidance document:


Table 1. Recommended SIL Values for Ozone and PM2.5 NAAQS

Pollutant (averaging period)NAAQS StandardNAAQS SIL Concentration
Ozone (8-hour)70 ppb1.0 ppb*
PM2.5 (24-hour)35 µg/m31.2 µg/m3**
PM2.5 (annual)12 (primary) or 15 µg/m3 (secondary)0.2 µg/m3***
* The ozone 8-hour SIL is brand new.  There has not previously been an ozone SIL established.
** Already established in 40 CFR 51.165(b)(2).
*** The PM2.5 annual SIL currently codified under §51.165(b)(2) is 0.3 µg/m3.  Until the code of federal regulations is updated, this draft guidance recommends 0.2 µg/m3 be used in lieu of 0.3 µg/m3.


Table 2. Recommended SIL Values for PM2.5 Increment

(averaging period)
PSD Increment SIL Concentration (µg/m3)
Class IClass IIClass III**
PM2.5 (24-hour)
PM2.5 (annual)
* There are no PSD increments established for ozone.
** There are currently no Class III areas.

With this draft guidance, EPA intends to first assess how permitting authorities use the recommended SILs, and EPA plans to codify the SIL values in a future rulemaking.  This guidance could prove to be problematic for anyone currently performing a PM2.5 modeling assessment while relying on the annual SIL since there are two different values now established:  0.3 µg/m3 as codified under 40 CFR 51.165(b)(2) and 0.2 µg/m3 presented in the draft guidance.

For more information and to submit comments on the draft guidance and supporting documents, you can visit EPA's website.  Public comments will be accepted through September 30, 2016.

To assess current and/or future compliance with the PM2.5 and ozone standards, please call the Trinity Oklahoma City office at (405) 848-3724.