From: Lydia Wegman/RTP/USEPA/US

To: nfrey@omb.eop.gov

Cc: Richard Wayland/RTP/USEPA/US@EPA, James Hemby/RTP/USEPA/US@EPA, 

martin.karen@epa.gov, Joseph-J Dougherty/DC/USEPA/US@EPA

Date: 06/11/2012 04:24 PM

Subject: Fw: Responses to monitoring and implementation comments

Nathan, 

Attached are the edits to the language we were just discussing on our call, concerning comment 26. We 

hope this addresses the issues raised by the commenters. 

Lydia
c. Surrogacy Approach for the Proposed New Secondary PM2.5 Visibility Index NAAQS

As summarized in section VI.F of this preamble, the EPA is proposing a distinct 

secondary NAAQS for PM2.5 that will provide protection against visibility impairment, measured 

in terms of a visibility index using a calculated PM2.5 light extinction indicator (see section 

VI.D.1 above). The PM2.5 visibility index values are determined using a six-step procedure

involving 24-hour speciated PM2.5 concentration data together with climatological relative 

humidity factors. The EPA plans to calculate design values for the proposed new secondary 

PM2.5 visibility index NAAQS using the procedures described in section VII.A.5 above, relying 

upon ambient PM2.5 speciation measurement data available through the CSN or IMPROVE 

methods and spatial interpolation of historical relative humidity data. 

As explained above, the PSD program requires individual new or modified stationary 

sources to carry out an air quality analysis to demonstrate that their proposed emissions increases 

will not cause or contribute to a violation of any NAAQS. Such a demonstration for the proposed 

new secondary PM2.5 visibility index NAAQS could require each PSD applicant to predict, via 

air quality modeling, the visibility impairment that will result from its proposed emissions in 

conjunction with an assessment of existing air quality (visibility impairment) conditions. Under 

40 CFR § 51.166 (l)(1) and 40 CFR § 52.21(l)(1), all applications of air quality models for 

purposes of determining compliance with a NAAQS, including the proposed secondary 

NAAQS for PM2.5, must be based upon air quality models specified in Appendix W to 40 

CFR § 51. Currently there are no air quality models identified in Appendix W that are 

recommended for regulatory applications (Appendix W to 40 CFR § 51, Section 3.1.1(b)) 

for addressing the atmospheric chemistry associated with secondary formation of PM2.5 .

Thus, if this demonstration were to be attempted using the six-step procedure that the 
EPA is proposing to use for calculating PM2.5 visibility index design values, significant technical 

and regulatory issues with the modeling procedures could arise. Those technical difficulties 

include the current limitations on speciated source-specific emissions data for model input; the 

lack of an EPA-approved air quality model with the capability to address the atmospheric 

chemistry associated with secondary formation of PM2.5; and the lack of PSD screening tools for 

streamlining the air quality analysis process. In addition, due to the limited monitoring network 

for speciated PM2.5, some sources may not be able to rely on existing speciated monitoring data 

to adequately represent the background air quality and thereby satisfy preconstruction 

monitoring requirements. Consequently, those prospective PSD sources could be required to 

collect new data in order to determine the representative background concentrations of PM2.5

species (i.e., those required for calculating the PM2.5 visibility index values as described in 

section VII.A.5 above).

Recognizing these difficult technical issues, the EPA believes that there is an essential 

need to provide alternative approaches to enable prospective PSD sources to demonstrate that 

they will not cause or contribute to a violation of the secondary PM2.5 visibility index NAAQS, if 

finalized as proposed. To meet this need, the EPA believes that it is reasonable to allow the use 

of a surrogacy approach, as discussed below, for at least the interim period while technical issues 

are being resolved, but which could potentially be continued beyond such time if shown to be 

appropriate. The EPA is providing notice of its intent to follow such an approach and is asking 

for comments on the approach as discussed in the remainder of this section. The Agency believes 

that following this approach will facilitate the transition to a workable PSD permitting approach 

under the proposed new secondary PM2.5 visibility index NAAQS.

To support consideration of alternative approaches that could be used by prospective PSD 
sources, the EPA conducted a two-pronged technical analysis of the relationships between the 

proposed new PM2.5 visibility index standard and the proposed primary annual and 24-hour 

PM2.5 standards (Wayland, 2012). The first prong of the analysis addressed aspects of a PSD 

significant impact analysis by evaluating whether an individual source’s impact resulting in a 

small increase in PM2.5 concentration would produce a comparably small increase in visibility 

impairment. This analysis included estimates of PM2.5 speciation profiles based on direct PM2.5

emission profiles for a broad range of source categories and for theoretical upper and lower 

bound scenarios. The analysis indicated that small increases in PM2.5 concentrations caused by 

individual sources produce similarly small changes in visibility impairment for ambient 

conditions near the proposed standard level of either 30 dv or 28 dv. The second prong of the 

analysis addressed aspects of a PSD cumulative impact analysis by exploring the relationship 

between the 3-year design values for the proposed primary annual and 24-hour PM2.5 standards 

and coincident design values for the proposed new PM2.5 visibility index standard based on 

recent air quality data. This analysis showed that visibility generally decreases when daily PM2.5

concentrations increase, and vice versa. This analysis further explored the appropriateness of 

using a demonstration of attainment with the primary 24-hour PM2.5 standard as a surrogate for a 

demonstration of attainment with the proposed new secondary PM2.5 visibility index standard. 

The Wayland (2012) analysis was based on 2008 to 2010 air quality data and on the proposed 

retention of the primary 24-hour PM2.5 standard with a level of 35 μg/m3in conjunction with a 

98th percentile form (section III.F) and the proposed new secondary PM2.5 visibility index 

standard with a level of either 30 dv or 28 dv in conjunction with 24-hour averaging time and a 

90th percentile form (see section VI.F).1This analysis indicated that all or nearly all areas in 

 

1 As identified in section IX.E above, the relationships between design values characterized in 

the Wayland (2012) analysis and summarized here are dependent upon the specific level and 
attainment of the primary 24-hour PM2.5 standard would also likely be in attainment of the 

proposed new secondary PM2.5 visibility index standard. 

The EPA believes that this technical analysis is robust and will have broad national 

application; thus, sources may choose to rely on the surrogacy demonstration even after 

resolution of the issues and limitations described previously, consistent with the case law on the 

use of surrogates. Based on this technical analysis, the EPA currently believes that there is 

sufficient evidence that a demonstration of compliance with the mass-based primary 24-hour 

PM2.5 NAAQS serves as a suitable surrogate for demonstrating compliance with the proposed 

secondary 24-hour PM2.5 visibility index NAAQS under the PSD program. As such, many or all 

sources undergoing PSD review for PM2.5 would be able to rely upon their analysis 

demonstrating compliance with the mass-based 24-hour primary PM2.5 NAAQS to also show 

compliance with the proposed secondary PM2.5 visibility index NAAQS, if finalized. The 

described surrogate approach would thus serve to overcome the technical challenges discussed 

above and minimize otherwise burdensome and costly air quality analyses associated with 

individual sources being required to perform separate and distinct analyses with regard to the 

proposed new secondary PM2.5 visibility index standard.

The surrogate approach is not intended to replace or otherwise undermine the 

validity of the analytical techniques employed for air quality related value assessments 

(AQRV’s), including visibility, required under 40 CFR § 51.166 (p) and 40 CFR § 52.21 (p). 

The Federal Land Managers (FLM) and the Federal official with direct responsibility for 

management of Federal Class I parks and wilderness areas have an affirmative 

responsibility to protect the AQRVs of such lands, and to provide the appropriate 

procedures and analysis techniques for assessing AQRV’s (Appendix W to 40 CFR § 51, 
Sections 6.1 (b) and 6.2.3(a)). The FLM’s have developed specific modeling approaches for 

AQRV assessments that are not specifically governed under the requirements set forth in 

40 CFR § 51.166 (l)(1) and 40 CFR § 52.21(l)(1), thus the surrogate approach is not 

applicable to the AQRV assessments under the PSD program. 

The surrogate approach could be incorporated into the PSD program in any of three 

alternative ways. First, the decision as to whether the surrogate approach is adequate could be 

handled on a case-by-case basis in consultation with the permitting authority, similar to the 

existing consultation process under the EPA’s Guideline on Air Quality Models for ozone and 

secondary PM2.5 impacts (40 CFR part 51, appendix W, section 5.2.1.c), with no presumption 

regarding its adequacy. Second, the EPA could establish a rebuttable presumption that the 

surrogate approach is applicable for all permits through either guidance or a notice-and-comment 

rulemaking. In either the first or second alternative, there would be a possibility that reliance on a 

surrogate-based demonstration could be subjected to challenge for any particular permit analysis. 

Third, the EPA could establish that the surrogate approach is applicable for all permits, also 

through a notice-and-comment rulemaking. The EPA seeks comment on all of the identified 

issues and proposed alternative implementation mechanisms associated with the proposed 

surrogate approach. It is the Agency’s intention to issue either guidance or new regulatory 

provisions as just described for a surrogacy approach by the time any final revisions to the PM 

NAAQS become effective, so that sources seeking permits will not be unnecessarily delayed.