From: "Frey, Nathan J." [Nathan_J._Frey@omb.eop.gov]

Sent: 12/14/2012 06:21 AM GMT To: Lydia Wegman; "Mancini, Dominic J." <Dominic_J._Mancini@omb.eop.gov> Cc: Michael Ling; Anna Wood; Joseph

- J Dougherty; "Lee, Amanda I." <Amanda_I._Lee@omb.eop.gov> Subject: Summary of Interagency Comments Under EOs 12866 and 13563

-- PM NAAQs Final Rule RIA

Attached please find a summary of interagency comments under EOs 12866 and 13563 regarding the PM NAAQs preamble related to the secondary standard. Please let me know if you have any questions or if you would like to discuss further tomorrow.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

353 VI. Rationale for Final Decisions on the Secondary PM Standards This section presents the Administrator’s final decisions regarding the need to revise the current suite of secondary PM 2.5 and PM 10 standards to address visibility impairment and other welfare effects considered in this review. As discussed more fully below, Specifically, t his section describes the Administrator’s final decision to retain the current suite of secondary PM standards to address PM

- related visibility impairment as well as other PM

- related welfare effects, including ecological effects, effects on materials, and climate impacts. This suite of standards includes an annual PM 2.5 standard of 15 μg/m 3 , a 24

- hour PM 2.5 standard of 35 μg/m 3 , and a 24

- hour PM 10 standard of 150 μg/m 3

. The Administrator is revising only the form of the secondary annual PM 2.5 standard to remove the option for spatial averaging consistent with this change to the primary annual PM 2.5 standard. C ontrary to what was proposed, the Administrator has decided not to establish a distinct standard to address PM

- related visibility impairment

. The rationale for the final this decision is presented below

. T he Administrator’s final decision s on the secondary standards are based on a thorough review , in the Integrated Science Assessment, of the latest scientific information , published through mid

- 2009 , on welfare effects associated with fine and coarse particles in the ambient air , as presented in the Integrated Science Assessment

. The final decisions also take into account: (1) staff assessments of the most policy

relevant information presented and assessed in the Integrated Science Assessment and staff analyses of air quality and visibility effects p resented in the Visibility Assessment and the Policy Assessment, upon which staff conclusions regarding appropriate

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

354 considerations in this review are based; (2) CASAC advice and recommendations, as reflected in discussions of drafts of the Integrated Scien ce Assessment, Visibility Assessment, and Policy Assessment at public meetings, in separate written comments, and in CASAC’s letters to the Administrator; (3) the multiple rounds of public comments received during the development of these documents, either both in connection with CASAC meetings or and separately; and (4) public comments received on the proposal. In particular, this section presents background information on the EPA’s previous and current reviews of the secondary PM standards (section VI.A), a summary of the proposed decisions regarding the secondary PM standards (section VI.B), a
summary discussion of significant public comments received on those proposed decisions (section VI.C), and the Administrator’s final decisions on the secondary PM standards (section VI.D).
A. Background The current suite of secondary PM standards is identical to the current suite of primary PM standards set in 2006 , including 24

- hour and annual PM 2.5 standards and a
24

- hour PM 10 standard. The current secondary PM 2.5 st andards are intended to provide protection from PM

- related visibility impairment, whereas the entire suite of secondary PM standards is intended to provide protection from other PM

- related effects on public welfare, including effects on sensitive ecosystem s, materials damage and soiling, and climatic and radiative processes. The approach used for reviewing the current suite of secondary PM standards built upon and broadened the approaches used in previous PM NAAQS reviews. The following discussion focuses p articularly on the current secondary PM 2.5 standards

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

355 related to visibility impairment and provides a summary of the approaches used to review and establish secondary PM 2.5 standards in the last two reviews (section VI.A.1); judicial review of the 2006 stan dards that resulted in the remand of the secondary annual and 24

- hour PM 2.5 NAAQS to the EPA (section VI.A.2); and the approach used in this review for evaluating the secondary PM 2.5 standards used in this review (section VI.A.3).
1. Approaches Used in Pre vious Reviews The original secondary PM 2.5 standards were established in 1997, and a revision to the 24

- hour standard was made in 2006. The approaches used in making final decisions on secondary standards in those reviews, as well as the current review, ut ilized different ways to consider the underlying body of scientific evidence. They also reflected an evolution in EPA’s understanding of the nature of the effect on public welfare from
PM

- related visibility impairment, from an approach focusing that focus ed only on Federal Class I area visibility impacts to a more multifaceted approach that also considers consider ed PM

- related impacts on visibility in non - Federal Class I area visibility area s , such as in urban areas. This evolution occurred in conjunction with the expansion of available PM data and information from associated visibility

- related studies of public perception, valuation, and personal comfort and well

- being. In 1997, the EPA revised the PM NAAQS in part by establishing new identical primary and sec ondary PM 2.5 standards. In revising the secondary standards, the EPA recognized that PM produces adverse effects on visibility and that impairment of visibility was being experienced throughout the U.S., in multi

- state regions, urban areas, and remote mand atory Federal Class I areas alike. However, in considering an appropriate level for a secondary standard to address adverse effects of PM 2.5 on

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

356 visibility, the EPA concluded that the determination of a single national level was complicated by important reg ional differences influenced by factors such as background and current levels of PM 2.5 , composition of PM 2.5 , and average relative humidity. Variations in these factors across regions could thus result in situations where attaining an appropriately protect ive concentration of fine particles in one region might or might not provide adequate protection in a different region. The EPA also determined that there was insufficient information at that time to establish a level for a national secondary standard that would represent a threshold above which visibility conditions would always be adverse and below which visibility conditions would always be acceptable. Based on an assessment of the potential visibility improvements that would result from reaching attainm ent with the new primary standards for PM 2.5 , the EPA concluded that attainment of the annual and 24

- hour PM 2.5 primary standards would lead to visibility improvements in the eastern U.S. at both urban and regional scales, but little or no change in the we stern U.S., except in and near certain urban areas. The EPA also considered the potential effectiveness of a regional haze program, required by sections 169A and 169B of the CAA 1 to address those effects of PM on visibility that would not be addressed thro ugh attainment of the primary PM 2.5 standards. The regional haze program would be designed to address the widespread, regionally uniform type of haze caused by a multitude of sources. The structure and requirements of sections 169A and 169B of the CAA prov ide for visibility protection programs that can

1 In 1977, Congress established as a national goal ‘‘the prevention of any future, and the remedying of any existing, impairment of visibility in mandatory Federal Class I areas which impairment results from manmade a ir pollution,” section 169A(a)(1) of the CAA. The EPA is required by section 169A(a)(4) of the CAA to promulgate regulations to ensure that ‘‘reasonable progress’’ is achieved toward meeting the national goal.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

357 be more responsive to the factors contributing to regional differences in visibility than can programs addressing the kinds of nationally applicable secondary NAAQS considered in the 1997 review. The regional haze visibility goal is more protective than a secondary NAAQS since the goal is to eliminate any anthropogenic impairment rather than to provide a level of protection from visibility impairment that is requisite to protect the public welfare. Thus, an im portant factor considered in the 1997 review was whether a regional haze program, in conjunction with secondary standards set identical to the suite of PM 2.5 primary standards, would provide appropriate protection for visibility in non

Federal Class I area s. The EPA concluded that the two programs and associated control strategies should provide such protection due to the regional approaches needed to manage emissions of pollutants that impair visibility in many of these areas. For these reasons, in 1997 th e EPA concluded that a national regional haze program, combined with a nationally applicable level of protection achieved through secondary PM 2.5 standards set identical to the primary PM 2.5 standards, would be more effective for addressing regional variat ions in the adverse effects of PM 2.5 on visibility than would be national secondary standards for PM with levels lower than the primary
PM 2.5 standards. The EPA further recognized that people living in certain urban areas may place a high value on unique s cenic resources in or near these areas and as a result might experience visibility problems attributable to sources that would not necessarily be addressed by the combined effects of a regional haze program and PM 2.5 secondary standards. The EPA concluded that in such cases, state or local regulatory approaches, such as past action in Colorado to establish a local visibility standard for the City of Denver, would be more appropriate and effective in addressing these special situations

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

35 8 because of the localiz ed and unique characteristics of the problems involved. Visibility in an urban area located near a mandatory Federal Class I area could also be improved through state implementation of the then

- current visibility regulations, by which emission limitations can be imposed on a source or group of sources found to be contributing to ‘‘reasonably attributable’’ impairment in the mandatory Federal Class I area. Based on these considerations, in 1997 the EPA set secondary PM 2.5 standards identical to the primary PM 2.5 standards, that would work in conjunction with a regional haze program the R egional H aze P rogram to be established under sections 169A and 169B of the CAA, as the most appropriate and effective means of addressing the public welfare effects associated with visibility impairment. Together, the two programs and associated control strategies were expected to provide appropriate protection against PM

related visibility impairment and enable all regions of the country to make reasonable progress toward the national visibility goal. In 2006, the EPA revised the suite of secondary PM 2.5 standards to address visibility impairment by making the suite of secondary standards identical to the revised suite of primary PM 2.5 stand ards. The EPA’s decision regarding the need to revise the suite of secondary PM 2.5 standards reflected a number of new developments that had occurred and sources of information that had become available following the 1997 review. First, the EPA promulgated a Regional Haze Program in 1999 (65 FR 35713, July 1, 1999) which required states to establish goals for improving visibility in Federal Class I areas and to adopt control strategies to achieve these goals. Second, extensive new information from visibilit y and fine particle monitoring networks had become available, allowing for updated characterizations of visibility trends and PM concentrations in urban

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

359 areas, as well as Federal Class I areas. These new data allowed the EPA to better characterize visibili ty impairment in urban areas and the relationship between visibility and PM 2.5 concentrations. Finally, additional studies in the U.S. and abroad provided the basis for the establishment of standards and programs to address specific visibility concerns in a number of local areas. These studies (Denver, Phoenix, and British Columbia) utilized photographic representations of visibility impairment and produced reasonably consistent results in terms of the visual ranges found to be generally acceptable by study participants. The EPA considered the information generated by these studies useful in characterizing the nature of particle

- induced haze and for informing judgments about the acceptability of various levels of visual air quality in urban areas across the U.S. Based largely on this information, the Administrator concluded that it was appropriate to revise the secondary PM 2.5 standards to provide increased protection from visibility impairment principally in urban areas, in conjunction with the regional haze program for protection of visual air quality in Federal Class I areas. In so doing, the Administrator recognized that PM

- related visibility impairment is principally related to fine particle concentrations and that perception of visibility impairment is most directly related to short

- term, nearly instantaneous levels of visual air quality. Thus, in considering whether the then

- current suite of secondary standards would provide the appropriate degree of protection, he concluded that it was appropriate to
f ocus on just the 24

- hour secondary PM 2.5 standard to provide requisite protection. The Administrator then considered whether PM 2.5 mass remained the appropriate indicator for a secondary standard to protect visibility, primarily in urban areas. The Adminis trator noted that PM

- related visibility impairment is principally related to fine

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

360 particle levels. H ygroscopic components of fine particles, in particular sulfates and nitrates, contribute disproportionately to visibility impairment under high humidity

con ditions. Particles in the coarse mode generally contribute only marginally to visibility impairment in urban areas. With the substantial addition to the air quality and visibility data made possible by the national urban PM 2.5 monitoring networks, an analy sis conducted for the 2006 review found that, in urban areas, visibility levels showed far less difference between eastern and western regions on a 24

- hour or shorter time basis than implied by the largely non

- urban data available in the 1997 review. In an alyzing how well PM 2.5 concentrations correlated with visibility in urban locations across the U.S., the 2005 Staff Paper concluded that clear correlations existed between 24

- hour average PM 2.5 concentrations and calculated (i.e., reconstructed) light exti nction, which is directly related to visual range (U.S. EPA, 2005, p. 7

- 6). These correlations were similar in the eastern and western regions of the U.S. These correlations were less influenced by relative humidity and more consistent across regions when PM 2.5 concentrations were averaged over shorter, daylight time periods (e.g., 4 to 8 hours) when relative humidity in eastern urban areas was generally lower and thus more similar to relative humidity in western urban areas. The 2005 Staff Paper noted that a standard set at any specific PM 2.5 concentration would necessarily result in visual ranges that vary somewhat in urban areas across the country, reflecting the variability in the correlations between PM 2.5 concentrations and light extinction. The 2005 S taff Paper concluded that it was appropriate to use PM 2.5 as an indicator for standards to address visibility impairment in urban areas, especially when the indicator is defined for a relatively short period (e.g., 4 to 8 hours) of daylight hours (U.S. EPA , 2005, p. 7

- 6). Based on their review of the Staff

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

361 Paper, most CASAC Panel members also endorsed such a PM 2.5 indicator for a secondary standard to address visibility impairment (Henderson, 2005a, p. 9). Based on the above considerations, the Administrato r concluded that PM 2.5 should be retained as the indicator for fine particles as part of a secondary standard to address visibility protection, in conjunction with averaging times from 4 to 24 hours. In considering what level of protection against PM

- relat ed visibility impairment would be appropriate, the Administrator took into account the results of the public perception and attitude surveys regarding the acceptability of various degrees of visibility impairment in the U.S. and Canada, state and local vis ibility standards within the U.S., and visual inspection of photographic representations of several urban areas across the U.S. In the Administrator’s judgment, these sources provided useful but still quite limited information on the range of levels approp riate for consideration in setting a national visibility standard primarily for urban areas, given the generally subjective nature of the public welfare effect involved. Based on photographic representations of varying levels of visual air quality, public perception studies, and local and state visibility standards, the 2005 Staff Paper had concluded that 30 to 20 μg/m 3 PM 2.5 represented a reasonable range for a national visibility standard primarily for urban areas, based on a sub

- daily averaging time (U.S. EPA, 2005, p. 7

- 13). The upper end of this range was below the levels at which illustrative scenic views are significant ly obscured, and the lower end was around the level at which visual air quality generally appeared to be good based on observation of the illustrative views. This concentration range generally corresponded to median visual ranges in urban areas within regi ons across the U.S. of approximately 25 to 35 km, a range that was bounded above by the visual range targets selected in specific areas

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

362 where state or local agencies placed particular emphasis on protecting visual air quality. In considering a reasonable r ange of forms for a PM 2.5 standard within this range of levels, the 2005 Staff Paper had concluded that a concentration

- based percentile form was appropriate, and that the upper end of the range of concentration percentiles for consideration should be cons istent with the 98 th percentile used for the primary standard and that the lower end of the range should be the 92 nd percentile, which represented the mean of the distribution of the 20 percent most impaired days, as targeted in the regional haze program ( U.S. EPA, 2005 pp. 7

- 11 to 7

- 13). While recognizing that it was difficult to select any specific level and form based on then

- currently available information (Henderson, 2005a, p. 9), the CASAC Panel was generally in agreement with the ranges of levels and forms presented in the 2005 Staff Paper. The Administrator also considered the level of protection that would be afforded by the proposed suite of primary PM 2.5 standards (71 FR 2681, January 17, 2006), on the basis that although significantly more infor mation was available than in the 1997 review concerning the relationship between fine PM levels and visibility across the country, there was still little available information for use in making the relatively subjective value judgment needed in selecting t he appropriate degree of protection to be afforded by such a standard. In so doing, the Administrator compared the extent to which the proposed suite of primary standards would require areas across the country to improve visual air quality with the extent of increased protection likely to be afforded by a standard based on a sub

- daily averaging time. Based on such an analysis, the Administrator observed that the predicted percent of counties with monitors not likely to meet the proposed suite of primary PM 2 .5 standards was actually somewhat greater than the predicted percent of

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

363 counties with monitors not likely to meet a sub

- daily secondary standard with an averaging time of 4 daylight hours, a level toward the upper end of the range recommended in the 2005 Staff Paper, and a form within the recommended range. Based on this comparison, the Administrator tentatively concluded that revising the secondary
24

- hour PM 2.5 standard to be identical to the proposed revised primary PM 2.5 standard (and retaining the the n - current annual secondary PM 2.5 standard) was a reasonable policy approach to addressing visibility protection primarily in urban areas. In proposing this approach, the Administrator also solicited comment on a sub

- daily (4

- to 8

- hour averaging time) seco ndary PM 2.5 standard (71 FR 2675 to 2781, January 17, 2006). In commenting on the proposed decision, the CASAC requested that a sub

- daily standard to protect visibility “be favorably reconsidered” (Henderson, 2006a, p.6). The CASAC noted three cautions re garding the proposed reliance on a secondary PM 2.5 standard identical to the proposed 24

- hour primary PM 2.5 standard: (1) PM 2.5 mass measurement is a better indicator of visibility impairment during daylight hours, when relative humidity is generally low; the sub

- daily standard more clearly matches the nature of visibility impairment, whose adverse effects are most evident during the daylight hours; using a 24

- hour PM 2.5 standard as a proxy introduces error and uncertainty in protecting visibility; and sub

- daily standards are used for other NAAQS and should be the focus for visibility; (2) CASAC and its monitoring subcommittees had repeatedly commended EPA’s initiatives promoting the introduction of continuous and near

continuous PM monitoring and recognized that an expanded deployment of continuous
PM 2.5 monitors would be consistent with setting a sub

- daily standard to protect visibility; and (3) the analysis showing a similarity between percentages of counties not likely to

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

364 meet what the CASAC Panel conside red to be a lenient 4

- to 8

- hour secondary standard and a secondary standard identical to the proposed 24

- hour primary standard was a numerical coincidence that was not indicative of any fundamental relationship between visibility and health. The CASAC Pan el further stated that “visual air quality is substantially impaired at PM 2.5 concentrations of 35 μg/m 3 ” and that “[i]t is not reasonable to have the visibility standard tied to the health standard, which may change in ways that make it even less appropri ate for visibility concerns” (Henderson, 2006a, pp. 5 to 6). In reaching a final decision, the Administrator focused on the relative protection provided by the proposed primary standards based on the above

- mentioned similarities in percentages of counties meeting alternative standards and on the limitations in the information available concerning studies of public perception and attitudes regarding the acceptability of various degrees of visibility impairment in urban areas, as well as on the subjective na ture of the judgment required. In so doing, the Administrator concluded that caution was warranted in establishing a distinct secondary standard for visibility impairment and that the available information did not warrant adopting a secondary standard that would provide either more or less protection against visibility impairment in urban areas than would be provided by secondary standards set equal to the proposed primary PM 2.5 standards.
2. Remand of 2006 Secondary PM 2.5 Standards As noted above in secti on II.B.2 above, several parties filed petitions for review challenging EPA’s decision to set the secondary NAAQS for fine PM identical to the primary NAAQS. On judicial review, the D.C. Circuit remanded to the EPA for

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

365 reconsideration the secondary NAAQS f or fine PM because the Agency’s decision was unreasonable and contrary to the requirements of section 109(b)(2). American Farm Bureau Federation v. EPA , 559 F. 3d 512 (D.C. Cir., 2009). The petitioners argued that the EPA’s decision lacked a reasoned basis . First, they asserted that the EPA never determined what level of visibility was “requisite to protect the public welfare.” They argued that the EPA unreasonably rejected the target level of protection recommended by its staff, while failing to provide a target level of its own. The court agreed, stating that “the EPA’s failure to identify such a level when deciding where to set the level of air quality required by the revised secondary fine PM NAAQS is contrary to the statute and therefore unlawful. Furth ermore, the failure to set any target level of visibility protection deprived the EPA’s decision

- making of a reasoned basis.” 559 F. 3d at 530. Second, the petitioners challenged EPA’s method of comparing the protection expected from potential standards. T hey contended that the EPA relied on a meaningless numerical comparison, ignored the effect of humidity on the usefulness of a standard using a daily averaging time, and unreasonably concluded that the primary standards would achieve a level of visibility roughly equivalent to the level the EPA staff and CASAC deemed “requisite to protect the public welfare.” The court found that the EPA’s equivalency analysis based on the percentages of counties exceeding alternative standards “failed on its own terms.” Th e same table showing the percentages of counties exceeding alternative secondary standards, used for comparison to the percentages of counties exceeding alternative primary standards to show equivalency, also included six other alternative secondary standa rds within the recommended CASAC range that would be

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

366 more “protective” under EPA’s definition than the adopted primary standards. Two

- thirds of the potential secondary standards within the CASAC’s recommended range would be substantially more protective th an the adopted primary standards. The court found that the EPA failed to explain why it looked only at one of the few potential secondary standards that would be less protective, and only slightly less so, than the primary standards. More fundamentally, ho wever, the court found that the EPA’s equivalency analysis based on percentages of counties demonstrated nothing about the relative protection offered by the different standards, and that the tables offered no valid information about the relative visibilit y protection provided by the standards. 559 F. 3d at 530

- 31. Finally, the Staff Paper had made clear that a visibility standard using PM 2.5 mass as the indicator in conjunction with a daily averaging time would be confounded by regional differences in humi dity. The court noted that the EPA acknowledged this problem, yet did not address this issue in concluding that the primary standards would be sufficiently protective of visibility. 559 F. 3d at 530. Therefore, the court granted the petition for review and remanded for reconsideration the secondary PM 2.5 NAAQS. 3. General Approach Used in the Policy Assessment for the Current Review The approach used in this review broadened the general approaches used in the last two PM NAAQS reviews by utilizing, to the e xtent available, enhanced tools, methods, and data to more comprehensively characterize visibility impacts. As such, the EPA took into account considerations based on both the scientific evidence (“evidence

based”) and a quantitative analysis of PM

- related impacts on visibility (“impact

- based”) to inform conclusions related to the adequacy of the current secondary PM 2.5 standards

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

367 and alternative standards that were appropriate for consideration in this review. As in past reviews, the EPA also considered tha t the secondary NAAQS should address PM

- related visibility impairment in conjunction with the Regional Haze Program, such that the secondary NAAQS would focus on protection from visibility impairment principally in urban areas in conjunction with the Regio nal Haze Program that is focused on improving visibility in Federal Class I areas. The EPA again recognized that such an approach remains the most appropriate and effective means of addressing the public welfare effects associated with visibility impairmen t in areas across the country. The Policy Assessment drew from the qualitative evaluation of all studies discussed in the Integrated Science Assessment (U.S. EPA, 2009a). Specifically, the Policy Assessment considered the extensive new air quality and sour ce apportionment information available from the regional planning organizations, long

- standing evidence of PM effects on visibility, and limited public preference studies stud y information from four urban areas (U.S. EPA, 2009a, chapter 9), as well as the integration of evidence across disciplines (U.S. EPA, 2009a, chapter 2). In addition, limited information that had become available regarding the characterization of public preferences in urban areas provided some new perspectives on the usefulness of this information in informing the selection of target levels of urban visibility protection. On these bases, the Policy Assessment again focused assessments on visibility conditions in urban a reas. The conclusions in the Policy Assessment reflected EPA staff’s understanding of both evidence

- based and impact

- based considerations to inform two overarching questions related to (1) the adequacy of the current suite of PM 2.5 standards and (2) what
p otential alternative standards, if any, should be considered in this review to provide

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

368 appropriate protection from PM

- related visibility impairment. In addressing these broad questions, the discussions in the Policy Assessment were organized around a serie s of more specific questions reflecting different aspects of each overarching question (U.S. EPA, 2011a, Figure 4

- 1). When evaluating the visibility protection afforded by the current or any alternative standards considered, the Policy Assessment took into account the four basic elements of the NAAQS: indicator, averaging time, level, and form.
B. Proposed Decisions on Secondary PM Standards At the time of proposal, the Administrator proposed to revise the suite of secondary PM standards by adding a distinct standard for PM 2.5 to address PM

- related visibility impairment, focused primarily on visibility in urban areas. This proposed standard was to be defined in terms of a PM 2.5 visibility index, which would use measured PM 2.5 mass concentration, in combination with speciation and relative humidity data, to calculate PM 2.5 light extinction, translated into the deciview (dv) scale; a 24

- hour averaging time; a 90 th percentile form, averaged over 3 years; and a level of 28

- 30 dv. To address other non

- visibility wel fare effects, the Administrator proposed to retain the current suite of secondary PM standards generally, while revising only the form of the secondary annual PM 2.5 standard to remove the option for spatial averaging consistent with this proposed change to the primary annual PM 2.5 standard. Each of these proposed decisions is described in more detail in the proposal and below.
1. PM

- Related Visibility Impairment As discussed in Section VI.B. of the proposal, the Administrator’s proposed decision regarding a distinct secondary standard to provide protection from visibility impairment reflected careful consideration of the following: (1) the latest scientific

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

369 information on visibility effects associated with PM as described in the Integrated Science Assessment (U.S. EPA, 2009a); (2) insights gained from assessments of correlations between ambient PM 2.5 and visibility impairment prepared by EPA staff in the Visibility Assessment (U.S. EPA, 2010b); and (3) specific conclusions regarding the need for revisions to the current standards (i.e., indicator, averaging time, form, and level) that, taken together, would be requisite to protect the public welfare from adverse effects on visual air quality. This section summarizes key information from the proposal regarding the nature of visibility impairment, including the relationship between ambient PM and visibility, temporal variations in light extinction, periods during the day of interest for assessing visibility conditions, and exposure durations of interest (section VI.B.1.a); limited public perceptions and attitudes about visibility impairment and the impacts of visibility impairment on public welfare (section VI.B.1.b); CASAC advice regarding the need for, and design of, secondary standards to protect visibility (se ction VI.B.1.c); and the Administrator’s proposed conclusions regarding setting a distinct standard to address visibility impairment (section VI.B.1.d).

a. Nature of PM

- related Visibility Impairment As noted at the time of proposal, the fundamental science characterizing the contribution of PM, especially fine particles, to visibility impairment is well understood. This science provides the basis for the Integrated Science Assessment designation of the relationship between PM and visibility impairment as ca usal. New research available in this review, discussed in chapter 9 of the Integrated Science Assessment, continues to support and refine EPA’s understanding of the effect of PM on visibility and the source
contributions to that effect in rural and remote locations. This research provides new

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

370 insights regarding the regional source contributions to urban visibility impairment and better characterization of the increment in PM concentrations and visibility impairment that occur in many cities (i.e., the urban excess) relative to conditions in the surrounding rural areas (i.e., regional background). Ongoing urban PM 2.5 speciated and aggregated mass monitoring has produced new information that has allowed for updated characterization of current visibility levels in urban areas. i. Relationship between Ambient PM and Visibility Visibility impairment is caused by the scattering and absorption of light by suspended particles and gases in the atmosphere. When PM is present in the air, its contribution to light extin ction typically greatly exceeds that of gases. The combined effect of light scattering and absorption by both particles and gases is characterized as light extinction, i.e., the fraction of light that is scattered or absorbed in the atmosphere. Light extin ction can be quantified by a light extinction coefficient with units of 1/distance, which is often expressed as 1/(1 million meters) or inverse megameters (abbreviated Mm - 1 ) or in terms of an alternative scale known as the deciview scale, defined by the fo llowing equation: 2 Deciview (dv) = 10 ln ( b ext / 10 Mm - 1 ) The deciview scale is frequently used in the scientific literature on visibility, as well as in the Regional Haze Program. In particular, the deciview scale is used in the public

2 As used in the Regional Haze Program, the term b ext refers to light extinction due to PM 2.5 , PM 10

- 2.5 , and “clean” atmospheric gases. In the Policy Assessment, in focusing on light extinction due to PM 2.5 , the deciview values include only the effects of P M 2.5 and the gases. The “Rayleigh” term associated with clean atmospheric gases is represented by the constant value of 10 Mm - 1 . Omission of the Rayleigh term would create the possibility of negative deciview values when the PM 2.5 concentration is very low.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

371 perception studies that were considered in the past and current reviews to in form judgments about an appropriate degree of protection to be provided by a secondary NAAQS. The amount of light extinction contributed by PM depends on the particle concentration as well as on the particle size distribution and composition and also on th e relative humidity. As described in detail in section VI.B.1.a of the proposal, visibility scientists have developed an algorithm, known as the IMPROVE algorithm, 3 to estimate light extinction using routinely monitored fine particle (PM 2.5 ) speciation and coarse particle mass (PM 10

– 2.5 ) data, as well as data on relative humidity. There is both an original and a revised version of the IMPROVE algorithm (Pitchford et al., 2007). The revised version was developed to address observed biases in the predictions using the original algorithm under very low and very high light extinction conditions. 4 These IMPROVE algorithms are routinely used to calculate light extinction levels on a 24

- hour basis in Federal Class I areas under the Regional Haze Program. In either version of the IMPROVE algorithm, the concentration of each of the major aerosol components is multiplied by a dry extinction efficiency value and, for the hygroscopic components (i.e., ammoniated sulfate and ammonium nitrate), also multiplied by an additi onal factor to account for the water growth to estimate these

3 The algorithm is referred to as the IMPROVE algorithm because it was developed specifically to use the aerosol monitoring data generated at network sites and with equipment specifically designed to support the IMPROVE program and was evaluated using IMPROVE optical measurements at the subset of sites that make those measurements (Malm et al., 1994). 4 These biases were detected by comparing light extinction estimates generated from the IMPROVE algorithm to direct optical measur ements in a number of rural Federal Class I areas.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

372 components’ contribution to light extinction. Summing the contribution of each component gives the estimate of total light extinction per unit distance denoted as the light extinction coefficien t ( b ext ), as shown below for the original IMPROVE algorithm. b ext ≈ 3 x f

(RH) x [Sulfate] + 3 x f (RH) x [Nitrate] + 4 x [Organic Mass] + 10 x [Elemental Carbon] + 1 x [Fine Soil] + 0.6 x [Coarse Mass] + 10 Light extinction ( b ext ) is in units of Mm - 1 , the mass concentrations of the components indicated in brackets are in units of μg/m 3 , and f (RH) is the unitless water growth term that depends on relative humidity. The final term of 10 Mm - 1 is known as the Rayleigh scattering term and accounts f or light scattering by the natural gases in unpolluted air. Despite the simplicity of this algorithm, it performs reasonably well and permits the contributions to light extinction from each of the major components (including the water associated with the s ulfate and nitrate compounds) to be separately approximated. Inspection of the PM component

- specific terms in the simple original IMPROVE algorithm shows that most of the PM 2.5 components contribute 5 times or more light extinction than a similar concentra tion of PM 10

– 2.5 . The f (RH ) term in the original algorithm reflects the increase in light scattering caused by particulate sulfate and nitrate under conditions of high relative humidity. Particles with hygroscopic components (e.g., particulate sulfate and nitrate) contribute more light extinction at higher relative humidity than at lower relative humidity because they change size in the atmosphere in response to ambient relative humidity conditions

. For relative humidity below 40 percent the f (RH) value is 1, but it increases to 2 at

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

373 approximately 66 percent, 3 at approximately 83 percent, 4 at approximately 90 percent, 5 at approximately 93 percent, and 6 at approximately 95 percent relative humidity. The result is that both particulate sulfate and nitrate are more efficient per unit mass in light extinction than any other aerosol component for relative humidity above approximately 85 percent where their total light extinction efficiency exceeds the 10 m 2 /g associated with elemental carbon (EC). PM containi ng elemental or black carbon (BC) absorbs light as well as scattering it, making it the component with the greatest light extinction contributions per unit of mass concentration, except for the hygroscopic components under these high relative humidity cond itions. 5 As noted above, s ubsequent to the development of the original IMPROVE algorithm, an alternative algorithm (variously referred to as the “revised algorithm” or the “new algorithm” in the literature) has been developed. It w as developed. The revised IMPROVE algorithm is different from the original algorithm in several important ways

. First, the revised algorithm employs a more complex split

- component mass extinction efficiency to correct biases believed to be related to particle size distributions , a sea salt term that can be important for remote coastal areas,

. 6 Specifically, t he revised algorithm incorporat es terms to account for particles representing the different dry extinction and

5 The IMPROVE algorithm does not explicitly separate the light

- scattering and light

absorbing effects of elemental carbon. 6 In either version of the IMPROVE algorithm, the concentration of each of the maj or aerosol components is multiplied by a dry extinction efficiency value and, for the hygroscopic components (i.e., ammoniated sulfate and ammonium nitrate), also multiplied by an additional factor to account for the water growth to estimate these componen ts’ contribution to light extinction. Both the dry extinction efficiency and water growth terms have been developed by a combination of empirical assessment and theoretical calculation using typical particle size distributions associated with each of the m ajor aerosol components.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

374 water uptake from two size modes of sulfate, nitrate and organic m ass . 7 Second, the revised algorithm uses a different multiplier for organic carbon for purposes of estimating organic carbonaceous material , to better represent aged aerosol found in remote areas

. 8 In addition, the revised algorithm includes a term for hygroscopic sea salt that can be important for remote coastal areas, and site

- specific Rayleigh light scattering terms in place of a universal Rayleigh light scattering value. As noted in section VI.B.1.a of the proposal, the revised IMPROVE algorithm can yield higher estimates of current light extinction levels in urban areas on days with relatively poor visibility as compared to the original algorithm (Pitchford, 2010). This difference is primarily attributable to the split

component mass extinction e fficiency treatment in the revised algorithm. Th is revised algorithm was evaluated at 21 remote locations and is generally used by RPOs and States for implementation of the Regional Haze Rule

. ii. Temporal Variations of Light Extinction Particulate matter concentrations and light extinction in urban environments vary from hour to hour throughout the 24

- hour day due to a combination of diurnal changes in meteorological conditions and systematic changes in emissions activity (e.g., rush hour traffic). Variou s factors combine to make early morning the most likely time for peak

7 The relative contributions of sulfate, nitrate and organic mass concentrations to visibility impairment with the revised algorithm, are different than with the original algorithm due to the combination of the dry extinction coeff icient and f(RH) functions for derived concentrations of small and large particles. The apportionment of the total fine particle concentration of each of the three PM 2.5 components into the concentrations of the small and large size fractions was empirical ly developed for remote areas. The fraction of the fine particle component that is in the large mode is estimated by dividing the total concentration of the component by 20 ug/m 3 . If the total concentration of a component exceeds 20 ug/m 3 , all of it is ass umed to be in the large mode

. 8 The revised IMPROVE algorithm uses a multiplier of 1.8 for rural areas instead of 1.4 as used in the original algorithm for the mean ratio of organic mass to organic carbon.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

375 urban light extinction; although the net effects of the systematic urban

- and larger

- scale variations mean that peak daytime PM light extinction levels can occur any time of day, in many areas they occur most often in early morning hours (U.S. EPA, 2010b, sections 3.4.2 and 3.4.3; Figures 3

- 9, 3

- 10, and 3

- 12). This temporal pattern in urban areas contrasts with the general lack of a strong diurnal pattern in PM concentrations and light

ex tinction in most Federal Class I areas, reflective of a relative lack of local sources as compared to urban areas. The use in the Regional Haze Program of 24

- hour average concentrations in the IMPROVE algorithm is consistent with this general lack of a str ong diurnal pattern in Federal Class I areas. iii. Periods during the Day of Interest for Assessment of Visibility As noted in sections VI.B.1.b and VI.B.1.c of the proposal, daytime visibility has dominated the attention of those who have studied the vi sibility effects of air pollution, particularly in urban areas. The EPA recognizes, however, that physically PM light extinction behaves the same at night as during the day and can contribute to nighttime visibility effects by enhancing the scattering of a nthropogenic light, contributing to the “skyglow” within and over populated areas, adding to the total sky brightness , and contributing to the reduction in contrast of stars against the background. However, l ittle research has been conducted on nighttime v isibility, and the state of the science is not comparable to that associated with daytime visibility impairment, particularly in terms of the impact on human welfare. T he Policy Assessment notes that the science is not available at this time to support ade quate characterization specifically of nighttime PM light extinction conditions and the related effects on public welfare (U.S. EPA, 2011a, p.
4

- 18). Therefore the EPA has focused its assessments of PM visibility impacts in urban

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

376 areas on daylight hours du ring this review. iv. Exposure Durations of Interest As noted in section VI.B.1.d of the proposal, the roles that exposure duration and variations in visual air quality within any given exposure period play in determining the acceptability or unacceptabil ity of a given level of visual air quality have not been investigated via preference studies. In the preference studies available for this review, subjects were simply asked to rate the acceptability or unacceptability of each image of a haze

- obscured scen e, without being provided any suggestion of assumed duration or of assumed conditions before or after the occurrence of the scene presented. Preference and/or valuation studies show that atmospheric visibility conditions can be quickly assessed and prefere nces determined. The EPA is unaware of any studies that characterize the extent to which different frequencies and durations of exposure to visibility conditions contribute to the degree of public welfare impact that occurs. The Policy Assessment consider ed a variety of circumstances that are commonly expected to occur in evaluating the potential impact of visibility impairment on the public welfare based on available information (U.S. EPA, 2011a, pp. 4

- 19 to 4

- 20). In some circumstances, such as infrequen t visits to scenic vistas in natural or urban environments, people are motivated specifically to take the opportunity to view a valued scene and are likely to do so for many minutes to hours to appreciate various aspects of the vista they choose to view. H owever, the public has many more opportunities to notice visibility conditions on a daily basis in settings associated with performing daily routines (e.g., during commutes and while working, exercising, or recreating outdoors). As noted in the Policy Asse ssment, information regarding the fraction of the public that has only one or a

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

377 few opportunities to experience visibility during the day, or on the role the duration of the observed visibility conditions has on wellbeing effects associated with those visi bility conditions, is not available (U.S. EPA, 2011a, p. 4

- 20). However, it is possible that people with limited opportunities to experience visibility conditions on a daily basis would receive the entire impact of the day’s visual air quality based on the visibility conditions that occur during the short time period when they can see it. Since this group could be affected on the basis of observing visual air quality conditions for periods as short as one hour or less, and because during each daylight hour there are some people outdoors, commuting, or near windows, the Policy Assessment judged that it would be appropriate to use the maximum hourly value of PM light extinction during daylight hours for each day for purposes of evaluating the adequacy of the c urrent suite of secondary standards. Other observers may have access to visibility conditions throughout the day. For this group, it might be that an hour with poor or “unacceptable” visibility can be offset by one or more other hours with clearer conditio ns. Therefore, the proposal acknowledged that it might also be appropriate to consider a multi

- hour daylight exposure period.

v. Periods of Fog and Rain As discussed in section VI.C of the proposal, the EPA also recognized that it is appropriate to give s pecial treatment to periods of fog and rain when considering whether
current PM 2.5 standards adequately protect public welfare from PM

- related visibility impairment. Visibility impairment occurs during periods with fog or precipitation irrespective of the presence or absence of PM. Therefore, it is logical that periods with naturally im paired visibility due to fog or precipitation should not be treated as having

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

378 PM

- impaired visibility. There are multiple ways to adjust visibility data to reduce the effects of fog and precipitation. In the Visibility Assessment, following the advice of
CA SAC, the EPA evaluated the effect of excluding daylight hours for which relative humidity was greater than 90 percent from analyses in order to avoid precipitation and fog confounding estimates of PM visibility impairment. For the 15 urban areas included
i n the Visibility Assessment, the EPA found that a 90 percent relative humidity cutoff criterion was effective in that on average less than 6 percent of the daylight hours were removed from consideration, yet those hours had on average ten times the likelih ood of rain, six times the likelihood of snow/sleet, and 34 times the likelihood of fog compared with hours with 90 percent or lower relative humidity. In the Regional Haze program, the EPA utilizes monthly average relative humidity values based on 10 year s of climatological data to reduce the effect of fog and precipitation. This approach focuses on longer

- term averages for each monitoring site and thereby eliminates the effect of very high humidity conditions on visibility at those locations. b. Public P erception of Visibility Impairment As described in section VI.B.2 of the proposal, there are two main types of studies that evaluate the public perception of urban visibility impairment: urban visibility preference studies and urban visibility valuation st udies. As noted in the Integrated Science Assessment, “[b]oth types of studies are designed to evaluate individuals’ desire (or demand) for good visual air quality (VAQ) where they live, using different metrics to evaluate demand. Urban visibility preferen ce studies examine individuals’ demand by investigating what amount of visibility degradation is unacceptable while economic studies examine demand by investigating how much one would be willing to pay to

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

379 improve visibility” (U.S. EPA, 2009a, p. 9

- 66). Bec ause of the limited number of new studies on urban visibility valuation, the Integrated Science Assessment cites to the discussion in the 2004 Criteria Document of the various methods one can use to determine the economic valuation of changes in visibility , which include hedonic valuation, contingent valuation and contingent choice, and travel cost. Contingent valuation studies are a type of stated preference study that measures the strength of preferences and expresses that preference in dollar values. Con tingent valuation studies often include payment vehicles that require respondents to consider implementation costs and their ability to pay for visibility improvements in their responses. This study design aspect is critical because the EPA cannot consider implementations costs in setting either primary or secondary NAAQS. Therefore in considering the information available to help inform the standard

- setting process, the EPA has focused on the public perception studies that do not embed consideration of imp lementation costs. Nonetheless, the EPA recognizes that valuation studies do provide additional evidence that the public is experiencing losses in welfare due to visibility impairment. 9 The public perception studies are described in detail below. In order to identify levels of visibility impairment appropriate for consideration in

9 In the regulatory impact analysis (RIA) accompa nying this rulemaking, the EPA describes a revised approach to estimate urban residential visibility benefits that applies the results of several contingent valuation studies. The EPA is unable to apply the public perception studies to estimate benefits be cause they do not provide sufficient information on which to develop monetized benefits estimates. Specifically, the public perception studies do not provide preferences expressed in dollar values, even though they do provide additional evidence that the b enefits associated with improving residential visibility are not zero. As previously noted in this preamble, the RIA is done for informational purposes only, and the proposed decisions on the NAAQS in this rulemaking are not in any way based on considerati on of the information or analyses in the RIA.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

380 setting secondary PM NAAQS to protect the public welfare, the Visibility Assessment comprehensively examined information that was available in this review regarding people’s state d preferences regarding acceptable and unacceptable visual air quality. Light extinction is an atmospheric property that by itself does not directly translate into a public welfare effect. Instead, light extinction becomes meaningful in the context of the impact of differences in visibility on the human observer. This has been studied in terms of the acceptability or unacceptability expressed for the visibility impact of a given level of light extinction by a human observer. The perception of the visibilit y impact of a given level of light extinction occurs in conjunction with the associated characteristics and lighting conditions of the viewed scene. 10 Thus, a given level of light extinction may be perceived differently by observers looking at different sce nes or the same scene with different lighting characteristics. Likewise, different observers looking at the same scene with the same lighting may have different preferences regarding the associated visual air quality. When scene and lighting characteristic s are held constant, the perceived appearance of a scene (i.e., how well the scenic features can be seen and the amount of visible haze) depends only on changes in light extinction. This has been demonstrated using the WinHaze model (Molenar et al., 1994) that uses image processing technology to apply user

- specified changes in light extinction values to the same base photograph with set scene and lighting characteristics.

10 By “characteristics of the scene” the EPA means the distance(s) between the viewer and the object(s) of interest, the shapes and colors of the objects, the contrast between objects and the sky or other backgr ound, and the inherent interest of the objects to the viewer. Distance is particularly important because at a given value of l ight extinction, which is a property of air at a given point(s) in space, more light is actually absorbed and scattered when light passes through more air between the object and the viewer.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

381 Much of what is known about the acceptability of levels of visibility comes from

surv ey studies in which participants were asked questions about their preference or the value they place on various visibility levels as displayed to them in scenic photographs and/or WinHaze images with a range of known light extinction levels. Urban visibili ty preference studies for four urban areas were reviewed in The Visibility Assessment (U.S. EPA, 2010b, chapter 2 ) revi ew ed the limited number of u rban visibility preference studies currently available (i.e., four studies ) to assess the light extinction le vels judged by the participant to have acceptable visibility for those particular scenes. The reanalysis of urban preference studies conducted in the Visibility Assessment for this review included three completed western urban visibility preference survey studies plus a pair of smaller focus studies designed to explore and further develop urban visibility survey instruments. The three western studies included one in Denver, Colorado (Ely et al., 1991), one in the lower Fraser River valley near Vancouver, B ritish Columbia (BC), Canada (Pryor, 1996), and one in Phoenix, Arizona (BBC Research & Consulting, 2003). A pilot focus group study was also conducted for Washington, DC (Abt Associates Inc., 2001). In response to an EPA request for public comment on the Scope and Methods Plan (74 FR 11580, March 18, 2009), comments were received (Smith, 2009) about the results of a new focus group study of scenes from Washington, DC, that had been conducted on subjects from both Houston, Texas, and Washington, DC, using

s cenes, methods and approaches similar to the method and approach employed in the EPA pilot study (Smith and Howell, 2009). When taken together, these studies from the four different urban areas included a total of 852 individuals, with each individual
resp onding to a series of questions while viewing a set of images of various urban visual

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

382 air quality conditions. The approaches used in the four studies were similar and were all derived from the method first developed for the Denver urban visibility study. In particular, the studies all used a similar group interview type of survey to investigate the level of visibility impairment that participants described as “acceptable.” In each preference study, participants were initially given a set of “warm up” exerc ises to familiarize them with how the scene in the photograph or image appears under different VAQ conditions. The participants next were shown 25 randomly ordered photographs (images), and asked to rate each one based on a scale of 1 (poor) to 7 (excellen t). They were then shown the same photographs or images again, in the same order, and asked to judge whether each of the photographs (images) would violate what they would consider to be an appropriate urban visibility standard (i.e. whether the level of i mpairment was “acceptable” or “unacceptable”). The term “acceptable” was not defined, so that each person’s response was based on his/her own values and preferences for VAQ. However, when answering this question, participants were instructed to consider th e following three factors: (1) The standard would be for their own urban area, not a pristine national park area where the standards might be stricter; (2) The level of an urban visibility standard violation should be set at a VAQ level considered to be un reasonable, objectionable, and unacceptable visually; and (3) Judgments of standards violations should be based on visibility only, not on health effects. While the results differed among the four urban areas, results from a rating exercise show that withi n each preference study, individual survey participants consistently distinguish between photos or images representing different levels of light extinction, and that more participants rate as acceptable images representing lower levels

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

383 of light extinction than they do images representing higher levels. Given the similarities in the approaches used, it is the EPA staff concluded that it

was reasonable to compare the results to identify overall trends in the study findings and to conclude that this comparison can usefully inform the selection of a range of levels for use in further analyses . However, the staff also noted that variations in th e specific materials and methods used in each study introduce uncertainties that should also be considered when interpreting the results of these comparisons. Key differences between the studies include the following: 1) scene characteristics; 2) image pre sentation methods (e.g., projected slides of actual photos, projected images generated using WinHaze (a significant technical advance in the method of presenting visual air quality conditions), or use of a computer monitor screen; 3) number of participants in each study; 4) participant representativeness of the general population of the relevant metropolitan area; and 5) specific wording used to frame the questions used in the group interview process. In the Visibility Assessment, each study was evaluated separately and figures developed to display the percentage of participants that rated the visual air quality depicted in each photograph as “acceptable.” Ely et al. (1991) introduced a “50% acceptability” criterion analysis of the Denver preference study r esults. The 50 percent acceptability criterion is designed to identify the visual air quality level (defined in terms of deciviews or light extinction) that best divides the photographs into two groups: those with a visual air quality rated as acceptable b y the majority of the participants, and those rated not acceptable by the majority of participants. The Visibility Assessment adopted this criterion as a useful index for comparison between studies. The results of each analysis were then combined graphical ly to allow for visual comparison. This

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

384 information was then carried forward into the Policy Assessment. Figure 6 presents the graphical summary of the results of the studies in the four cities and draws on results previously presented in Figures 2

- 3, 2

- 5, 2

- 7, and 2

- 11 of chapter 2 in the Visibility Assessment. Figure 6 also contains lines at 20 dv and 30 dv that generally identify a range where the 50 percent acceptance criteria occur across all four of the urban preference studies (U.S. EPA, 2011a, p. 4

- 24). Out of the 114 data points shown in Figure 6, only one photograph (or image) with a visual air quality below 20 dv was rated as acceptable by less than 50 percent of the participants who rated that photograph. 11 Similarly, only one image with a visual air quality above 30 dv was rated acceptable by more than 50 percent of the participants who viewed it. 12

11 Only 47 percent of the British Columbia participants rated a 19.2 dv photograph as acceptable. 12 In the 2001 Washington, DC study, a 30.9 dv image was used as a repeated slide. The first time it was shown 56 percent of the participants rated it as acceptable, but only 11 percent rated it as acceptable the second time it was shown. The same visual air quality level was rated as acceptable by 4 percent of the participants in the 2009 study (Test 1). All three points are shown in Figure 6.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

385 Figure 6. Summary of Results of Urban Visibility Studies in Four Cities, Showing the Identified

Range of the 50% Acceptance Criteria 13

Source: U.S. EPA, 2011a, Figure 4

- 2; U.S. EPA 2010b, Figure 2

- 16 As Figure 6 above shows, each urban area has a separate and unique response curve that appears to indicate that it is distinct from the others. 14 These curves are the result of a logistical regression ana lysis using a logit model of the greater than 19,000 ratings of haze images as acceptable or unacceptable. The model results can be used to estimate the visual air quality in terms of dv values where the estimated response functions cross the 50 percent ac ceptability level, as well as any alternative criteria levels. Selected examples of these are shown in Table 4

- 1 of the Policy Assessment (U.S. EPA,

13 Top scale shows light extinction in inverse megameter units; bottom scale in deciviews. Logit analysis estimated r esponse functions are shown as the color

- coded curved lines for each of the four urban areas. 14 At present , data is only available for four urban areas, as presented in Figure 6 and discussed throughout this section. Additional research could help inform w hether the range identified by combining the results of the studies depicted in Figure 6 is more broadly representative

.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

386 2011a; U.S. EPA, 2010b, Table 2

- 4). This table shows that the logit model results also support the upper an d lower ends of the range of 50 th percentile acceptability values (e.g., near 20 dv for Denver and near 30 dv for Washington, DC) already identified in Figure 6. Based on the composite results and the effective range of 50 th percentile acceptability across the four urban preference studies shown in Figure 6 and Table 4

- 1 of the Policy Assessment, benchmark levels of (total) light extinction were selected in a range from 20 dv to 30 dv (75 to 200 Mm - 1 ) 15 for the purpose of prov isionally assessing whether visibility conditions would be considered acceptable (i.e., less than the low end of the range), unacceptable (i.e., greater than the high end of the range), or potentially acceptable (within the range ). ) based on the very limit ed public preference information

. A midpoint of 25 dv (120 Mm - 1 ) was also selected for use in the assessment. This level is also very near to the 50 th percentile criterion value from the Phoenix study (i.e., 24.2 dv), which is by far the best of the four s tudies in terms of the fit of the data to the response curve and the representativeness of study participants. Based on the currently available information, the Policy Assessment concluded that the use of 25 dv to represent the middle of the distribution o f results seemed well supported (U.S. EPA, 2011a, p. 4

- 25). These three benchmark values provide a low, middle, and high set of light extinction conditions that are used to provisionally define daylight hours with urban haze

15 These values were rounded from 74 Mm - 1 and 201 Mm - 1 to avoid an implication of greater precision than is warranted. Note that th e mid dle value of 25 dv when converted to light extinction is 122 Mm - 1 is rounded to 120 Mm - 1 for the same reason. Assessments conducted for the Visibility Assessment and the first and second drafts of the Policy Assessment used the unrounded values. The Policy Assessment considered the results of assessment using unrounded values to be sufficiently representative of what would result if the rounded values were used that it was unnecessary to redo the assessments. That is why some tables and figures in the Polic y Assessment reflected the unrounded values.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

387 conditions that have been judge d unacceptable by at least 50 percent of the participants in one or more of these preference studies. As discussed above, PM light extinction is taken to be (total) light extinction minus the Rayleigh scatter, 16 such that the low, middle, and high levels co rrespond to PM light extinction levels of about 65 Mm - 1 , 110 Mm - 1 , and 190 Mm - 1 . In the Visibility Assessment, these three light extinction levels were called Candidate Protection Levels (CPLs). This term was also used in the Policy Assessment and in the p roposal notice. It is important to note, however, that the degree of protection provided by a secondary NAAQS is not determined solely by any one component of the standard but by all the components (i.e., indicator, averaging time, form, and level) being

a pplied together. Therefore, the Policy Assessment noted that the term CPL is meant only to indicate target levels of visibility within a range that the EPA staff felt appropriate for consideration that could, in conjunction with other elements of the stand ard, including indicator, averaging time, and form, potentially provide an appropriate degree of visibility protection. In characterizing the Policy Assessment’s confidence in each CPL and across the
range, a number of issues were considered (U.S. EPA, 201 1a, p. 4

- 26). Looking first at the two studies that define the upper and lower bounds of the range, the Policy Assessment considered whether they represent a true regional distinction in preferences for urban visibility conditions between western and easte rn U.S. There was little information available to help evaluate the possibility of a regional distinction especially given that there have been preference studies in only one eastern urban area. Smith and

16 Rayleigh scatter is light scattering by atmospheric gases which is on average about 10 Mm - 1

.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

388 Howell (2009) found little difference in preference response to Washington, DC, haze photographs between the study participants from Washington, DC, and those from Houston, Texas. 17 This provides some limited evidence that the value judgment of the public in different areas of the country may not be an impo rtant factor in explaining the differences in these study results. In further considering what factors could explain the observed differences in preferences across the four urban areas, the Policy Assessment noted that the urban scenes used in each study had different characteristics (U.S. EPA, 2011a, p. 4

- 26). For example, each of the western urban visibility preference study scenes included mountains in the background while the single eastern urban study did not. It is also true that each of the western scenes included objects at greater distances from the camera location than in the eastern study. There is no question that objects at a greater distance have a greater sensitivity to perceived visibility changes as light extinction is changed compared to

o therwise similar scenes with objects at a shorter range. This alone might explain the difference between the results of the eastern study and those from the western urban studies. Having scenes with the object of greatest intrinsic value nearer and hence l ess sensitive in the eastern urban area compared with more distant objects of greatest intrinsic value in the western urban areas could further explain the difference in preference results. Another question considered was whether the high CPL value that i s based on the

17 The first preference study using WinHaze images of a scenic vista from Washington, DC was conducted in 2001 using subjects who were residents of Washington, DC. More recently, Smith and Howell (2009) interviewed additional subjects using the same images and interview procedure. The additional subjects included some residents of the Washington, DC area and some reside nts of the Houston, Texas area.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

389 eastern preference results is likely to be generally representative of urban areas that do not have associated mountains or other valued objects visible in the distant background. Such areas would include the middle of the country, many area s in the eastern U.S., and possibly some areas in the western U.S. as well. 18 Based on the currently available information, the Policy Assessment concluded that the high end of the CPL range (30 dv) is an appropriate level to consider ( U.S. EPA, 2011a, p. 4

- 27). With respect to the low end of the range, the Policy Assessment considered factors that might further refine its understanding of the robustness of this level. The Policy Assessment concluded that additional urban preference studies, especially with a greater variety in types of scenes, could help evaluate whether the lower CPL value of 20 dv is generally supportable (U.S. EPA, 2011a, p. 4

- 27). Further, the reason for the noisiness in data points around the curves apparent in both the Denver and Briti sh Columbia results compared to the smoother curve fit of Phoenix study results could be explored. One possible explanation discussed in the Policy Assessment is that these older studies use photographs taken at different times of day and on different days to capture the range of light extinction levels needed for the preference studies. In contrast, the use of WinHaze in the Phoenix (and Washington, DC) study reduced variations that affect scene appearance preference rating and avoided the uncertainty inhe rent in using ambient measurements to represent sight path

- averaged light extinction values. Reducing these

18 In order to examine this issue, an effort would have to be made to see if scenes in such areas could be found that would be generally comparable to the western scenes (e.g., scenes that contain valued scenic elements at m ore sensitive distances than that used in the eastern study). This is only one of a family of issues concerning how exposure to urban scenes of varying sensitivity affects public perception for which no preference study information is currently available.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

390 sources of noisiness and uncertainty in the results of future studies of sensitive urban scenes could provide more confidence in the selection of a low CPL value. Based on the above considerations, and recognizing the limitations in the currently available information, the Policy Assessment concluded that it is reasonable to consider a range of CPL values including a high value of 30 dv, a mid

- range v alue of 25 dv, and a low value of 20 dv (U.S. EPA, 2011a, p. 4

- 27). Based on its review of the second draft Policy Assessment, CASAC also supported this set of CPLs for consideration by the EPA in this review. CASAC noted that these CPL values were based

o n all available visibility preference data and that they bound the study results as represented by the 50 percent acceptability criteria. While recommending that further visibility preference studies be conducted to reduce remaining uncertainties, 19 CASAC concluded that this range of levels is was “adequately supported by the evidence presented” (Samet, 2010d, p. iii).

c. Summary of Proposed Conclusions i. Adequacy of the Current Standards for PM

- Related Visibility Impairment At the time of proposal, the Administrator provisionally concluded that the current suite of secondary PM standards are is not sufficiently protective of visual air quality, and that consideration should be given to an alternative secondary standard that

wo uld provide additional protection against PM

- related visibility impairment, with a focus primarily in urban areas. This proposed conclusion was based on the information

19 “CASAC has also identified needs for the next review cycle in terms of further research on a number of topics related to urban visibility; .... In particular, there is a need for the Agency to conduct additional urban visibility preference studies over a b road range of urban areas and viewing conditions, to further evaluate and refine the range of visibility levels considered to be acceptable in the current assessment.” (Samet, 2010a)

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

391 presented in the proposal with regard to the nature of PM

- related visibility impairment , the results of public perception surveys on the acceptability of varying degrees of visibility impairment in urban areas, analyses of the number of days that are estimated to exceed a range of candidate protection levels under conditions simulated to jus t meet the current standards, and the advice of CASAC. This section summarizes key points from section VI.C of the proposal regarding visibility under current conditions, the degree of protection afforded by the current standards, and CASAC’s advice regard ing the adequacy of the current standards. As discussed in section VI.C.1 of the proposal, to evaluate visibility under current conditions the Visibility Assessment and Policy Assessment estimated PM

- related light extinction 20 levels for 15 urban areas 21 in the United States. Consistent with the emphasis in this review on the hourly or multi

- hour time periods that might reasonably characterize the visibility effects experienced by various segments of the population, these analyses focused on using maximum 1

- hour of and 4

- hour values of PM light extinction during daylight hours for purposes of evaluating the degree of visibility impairment. Hourly average PM

- related light extinction was analyzed in terms of both PM 10 and PM 2.5 light extinction. For reasons disc ussed above, hours with relative humidity greater than 90 percent were excluded from consideration. Recent visibility conditions in these urban

20 PM

- related light extinction is used here to refer to the light extincti on caused by PM regardless of particle size; PM 10 light extinction refers to the contribution by particles sampled through an inlet with a particle size 50 percent cutpoint of 10 μm diameter; and PM 2.5 light extinction refers to the contribution by particles sampled through an inlet with a particle size 50 percent cutpoint of 2.5 μm diameter. 21 The 15 urban areas are Tacoma, Fresno, Los Angeles, Phoenix, Salt Lake City, Dallas, Houston, St. Louis, Birmi ngham, Atlanta, Detroit, Pittsburgh, Baltimore, Philadelphia, and New York.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

392 areas were then compared to the CPLs identified above. The Visibility Assessment, which focused on PM 10 light ex tinction in 14 of the 15 urban areas during the 2005 to 2007 time period, 22 found that all 14 areas had daily maximum hourly PM 10 light extinction values estimated to exceed even the highest CPL some of the days. Except for the two Texas areas and the non

- C alifornia western urban areas, all of the other urban areas were estimated to have maximum hourly PM 10 concentrations that exceeded the high CPL on about 20 percent to over 60 percent of the days. All 14 of the urban areas were estimated to have maximum ho urly PM 10 concentrations that exceeded the low CPL on about 40 percent to over 90 percent of the days. In general, areas in the East and in California tend to have a higher frequency of hourly visibility conditions estimated to be above the high CPL compar ed with those in the western U.S. The Policy Assessment repeated the Visibility Assessment

- type modeling based
on PM 2.5 light extinction and data from the more recent 2007 to 2009 time period for the same 15 study areas (including St. Louis). While the es timates of the percentage of daily maximum hourly PM 2.5 light extinction values exceeding the CPLs were somewhat lower than for PM 10 light extinction, the patterns of these estimates across the study areas was found to be similar. More specifically, except for the two Texas and the non

- California western urban areas, all of the other urban areas were estimated to have maximum hourly

22 Comments on the second draft Visibility Assessment from those familiar with the monitoring sites in St. Louis indicated that the site selected to provide continuous PM 10 monitor ing, although less than a mile from the site of the PM 2.5 data, was not representative of the urban area and resulted in unrealistically large PM 10

- 2.5 values. The EPA staff considered these comments credible and set aside the St. Louis assessment results for PM 10 light extinction. Thus, results and statements in the Policy Assessment regarding PM 10 light extinction applied to only the other 14 areas. However, results regarding PM 2.5 light extinction in most cases applied to all 15 study areas because the S t. Louis estimates for PM 2.5 light extinction were not affected by the PM 10 monitoring issue.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

393 PM 2.5 concentrations that exceeded the high CPL on about 10 percent up to about 50 percent of the days based on PM 2.5 light ex tinction, while all 15 areas were estimated to have maximum hourly PM 2.5 concentrations that exceeded the low CPL on over 10 percent to over 90 percent of the days. To evaluate how PM

- related visibility would be affected by just meeting the current suite of PM 2.5 secondary standards, t he Policy Assessment applied the proportional rollback approach described in section VI.C.2 of the proposal to all the
PM 2.5 monitoring sites in each study area. 23 After adjusting for composition, the Policy Assessment applied the original IMPROVE algorithm to calculate the PM 10 light extinction, using “rolled back” PM 2.5 component concentrations, the current conditions
PM 10

- 2.5 concentration for the day and hour, a nd relative humidity for the day and hour. In these analyses, the Policy Assessment estimated both PM 2.5 and PM 10 light extinction in terms of both daily maximum 1

- hour average values and multi

- hour (i.e., 4

- hour) average values for daylight hours. Figure 4

- 7 and Table 4

- 6 of the Policy Assessment displayed the results of the rollback procedures as a box and whisker plot of daily maximum daylight 1

- hour PM 2.5 light extinction and the percentage of daily maximum hourly PM 2.5 light extinction values estimated to exceed the CPLs when just meeting the current suite of PM 2.5 secondary standards for all 15 areas considered in the Visibility Assessment (including St. Louis) (excluding hours with relative humidity greater than 90 percent). These displays showed that the daily maximum 1

- hour average
PM 2.5 light extinction values in all of the study areas other than the three western non

-

23 Phoenix and Salt Lake City met the current PM 2.5 NAAQS under current conditions and required no reduction.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

394 California areas were estimated to exceed the high CPL on about 8 percent up to over 30 percent of the days and to exceed the middle CPL on about 30 percent up to about 70 percent of the days, while all areas except Phoenix were estimated to have daily maximum 1

- hour average PM 2.5 light extinction values that exceeded the low CPL on over 15 percent to about 90 percent of the days. Figur e 4

- 8 and Table 4

- 7 of the Policy Assessment present results based on daily maximum 4

- hour average values. These displays show that the daily maximum 4

- hour average PM 2.5 light extinction values in all of the study areas other than the three western non

- Ca lifornia areas and the two areas in Texas were estimated to exceed the high CPL on about 4 percent up to over 15 percent of the days and to exceed the middle CPL on about 15 percent up to about 45 percent of the days, while all areas except Phoenix were es timated to have daily maximum 4

- hour average PM 2.5 light extinction values that exceeded the low CPL on over 10 percent to about 75 percent of the days. A similar set of figures and tables were developed in terms of PM 10 light extinction (U.S. EPA, 2011a, Figures 4

- 5 and 4

- 6, Tables 4

- 4 and 4

- 5). Taking the results of these analyses focusing on 1

- hour and 4

- hour maximum light extinction values into account, the Policy Assessment concluded that the available information in this review clearly called into qu estion the adequacy of the current suite of
PM 2.5 standards in the context of public welfare protection from visibility impairment, primarily in urban areas, and supported consideration of alternative standards to provide appropriate protection (U.S. EPA, 2011a, p. 4

- 39). This conclusion was based in part on the large percentage of days, in many urban areas, that were estimated to have maximum
1

- hour or 4

- hour light extinction values that exceed the range of CPLs identified for consideration under simulatio ns of conditions that would just meet the current suite of

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

395 PM 2.5 secondary standards. In particular, for air quality that was simulated to just meet the current PM 2.5 standards, greater than 10 percent of the days were estimated to have peak light extincti on values that exceed the highest, least protective CPL of 30 dv in terms of PM 2.5 light extinction for 9 of the 15 urban areas, based on 1

- hour average values, and would thus likely fail to meet a 90 th percentile

- based standard at that level. For these ar eas, the percent of days estimated to have maximum 1

- hour values that exceed the highest CPL ranged from over 10 percent to over 30 percent. Similarly, when the middle CPL of 25 dv was considered, greater than 30 percent up to approximately 70 percent of t he days were estimated to have peak light extinction that exceeded that CPL in terms of PM 2.5 light extinction, for 11 of the 15 urban areas, based on 1

- hour average values. Based on a 4

- hour averaging time, 5 of the areas were estimated to have at least
1 0 percent of the days with peak light extinction exceeding the highest CPL in terms of
PM 2.5 light extinction, and 8 of the areas were estimated to have at least 30 percent of the days with peak light extinction exceeding the middle CPL in terms of PM 2.5 l ight extinction. For the lowest CPL of 20 dv, the percentages of days with 4

- hour maximum light extinction estimated to exceed that CPL are even higher for all cases considered. Based on all of the above, the Policy Assessment concluded that PM light extin ction estimated to be associated with just meeting the current suite of PM 2.5 secondary standards in many areas across the country exceeded levels and percentages of days that could reasonably be considered to be important from a public welfare perspective (U.S. EPA, 2011a, p. 4

- 40). Further, the Policy Assessment concluded that use of the current indicator of
PM 2.5 mass, in conjunction with the current 24

- hour and annual averaging times, is

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

396 clearly called into question for a national standard intended to protect public welfare from
PM

- related visibility impairment (U.S. EPA, 2011a, p. 4

- 40). This is because such a standard is inherently variable in the degree of protection provided because of regional differences in relative humidity and species compositio n of PM 2.5 , which are critical factors in the relationship between the mix of fine particles in the ambient air and the associated impairment of visibility. The Policy Assessment noted that this concern was one of the important elements in the court’s deci sion to remand the PM 2.5 secondary standards set in 2006 to the Agency. Thus, in addition to concluding that the available information clearly calls into question the adequacy of the protection against PM

- related visibility impairment afforded by the curr ent suite of PM 2.5 standards, the Policy Assessment also concluded that it clearly calls into question the appropriateness of each of the current standard elements: indicator, averaging time, form, and level (U.S. EPA, 2011a, p. 4

- 40). After reviewing the information and analysis in the second draft Policy Assessment, CASAC concluded that the “currently available information clearly calls into question the adequacy of the current standards and that consideration should be given to revising the suite of stan dards to provide increased public welfare protection” (Samet, 2010d, p. iii). CASAC noted that the detailed estimates of hourly PM light extinction associated with just meeting the current standards “clearly demonstrate that current standards do not protec t against levels of visual air quality which have been judged to be unacceptable in all of the available urban visibility preference studies.” Further, CASAC stated, with respect to the current suite of secondary PM 2.5 standards, that “[T]he levels are too high, the averaging times are too long, and the PM 2.5 mass

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

397 indicator could be improved to correspond more closely to the light scattering and absorption properties of suspended particles in the ambient air” (Samet, 2010d, p. 9). After considering the avai lable evidence and the advice of CASAC, the Administrator concluded at the time of proposal that such information did provide an appropriate basis to inform a conclusion as to whether the current standards afford adequate protection against PM

- related visi bility impairment in urban areas. The Administrator took into account the information discussed above with regard to the nature of PM

- related visibility impairment, the results of public perception surveys on the acceptability of varying degrees of visibil ity impairment in urban areas, analyses of the number of days on which peak 1

- hour or 4

- hour light extinction values are estimated to exceed a range of candidate protection levels under conditions simulated to just meet the current standards, and the advic e of CASAC. She noted the clear causal relationship between PM in the ambient air and impairment of visibility, the evidence from the visibility preference studies, and the rationale for determining a range of candidate protection levels based on those stu dies. She also noted the relatively large number of days when maximum 1

- hour or 4

- hour light extinction values were estimated to exceed the three candidate protection levels, including the highest level of 30 dv, under the current standards. While recogniz ing the limitations in the available information on public perceptions of the acceptability of varying degree of visibility impairment and the information on the number of days estimated to exceed the CPLs, she concluded that such information provided an a ppropriate basis to inform a conclusion as to whether the current standards provide adequate protection against PM

- related visibility impairment in urban areas. Based on these considerations, and placing great importance on the advice of

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

398 CASAC, the Adminis trator provisionally concluded that the current standards are not sufficiently protective of visual air quality, and that consideration should be given to an alternative secondary standard that would provide additional protection against PM

related visibil ity impairment, with a focus primarily in urban areas. Having reached this conclusion, the Administrator also stated at the time of proposal that the current indicator of PM 2.5 mass, in conjunction with the current 24

- hour and annual averaging times, is no t well suited for a national standard intended to protect public welfare from PM

- related visibility impairment. As noted in the proposal, the current standards do not incorporate information on the concentrations of various species within the mix of ambien t particles, nor do they incorporate information on relative humidity, both of which play a central role in determining the relationship between the mix of PM in the ambient air and impairment of visibility. Such considerations were reflected both in CASAC ’s advice to set a distinct secondary standard that would more directly reflect the relationship between ambient PM and visibility impairment and in the court’s remand of the current secondary PM 2.5 standards. Based on the above considerations, at the time of proposal the Administrator provisionally concluded that the current secondary PM 2.5 standards, taken together, are neither sufficiently protective nor suitably structured to provide an appropriate degree of public welfare protection from
PM

- related vis ibility impairment, primarily in urban areas. This led the EPA to consider alternative standards by looking at each of the elements of the standards

— indicator, averaging time, form, and level

— as discussed below. ii. Indicator At the time of proposal, the EPA considered three alternative indicators for a

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

399 PM 2.5 standard designed to protect against visibility impairment: the current PM 2.5 mass indicator; directly measured PM 2.5 light extinction; and calculated PM 2.5 light extinction. Directly measured PM 2.5 light extinction is a measurement (or combination of measurements) of the light absorption and scattering caused by PM 2.5 under ambient conditions. Calculated PM 2.5 light extinction uses the IMPROVE algorithm to calculate
PM 2.5 light extinction using measu red PM 2.5 mass, speciated PM 2.5 mass, and measured relative humidity. The Policy Assessment evaluated each of these alternatives, finally concluding that consideration should be given to establishing a new calculated PM 2.5 light extinction indicator (U.S. EPA, 2011a, p. 4

- 51). As discussed in section VI.D.1 of the proposal, the Policy Assessment concluded that consideration of the use of either directly measured PM 2.5 light extinction or calculated PM 2.5 light extinction as an indicator is justified because light extinction is a physically meaningful measure of the characteristic of ambient PM 2.5 that is most relevant and directly related to PM

- related visibility effects (U.S. EPA, 2011a, p. 4

- 41). Further, as noted above, PM 2.5 is the component of PM respon sible for most of the visibility impairment in most urban areas. In these areas, the contribution of PM 10

– 2.5 is a minor contributor to visibility impairment most of the time. The Policy Assessment also indicated that the available evidence demonstrated a strong correspondence between calculated PM 2.5 light extinction and PM

- related visibility impairment, as well as the significant degree of variability in visibility protection across the U.S. allowed by a PM 2.5 mass indicator. The Policy Assessment recogni zed that while in the future it would be appropriate to consider a direct measurement of PM 2.5 light extinction it was not an appropriate option in this review because a suitable specification of the equipment and

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

400 associated performance verification proced ures cannot be developed in the time frame for this review. (a) PM 2.5 Mass In terms of utilizing a PM 2.5 mass indicator, the proposal noted that PM 2.5 mass monitoring methods are in widespread use, including the FRM involving the collection of periodic ( usually 1

- day

- in

- 6 or 1

- day

- in

- 3) 24

- hour filter samples. However, these routine monitoring activities do not include measurement of the full water content of the ambient
PM 2.5 that contributes, often significantly, to visibility impacts. Further, the PM 2. 5 mass concentration monitors do not provide information on the composition of the ambient
PM 2.5 , which plays a central role in the relationship between PM

- related visibility impairment and ambient PM 2.5 mass concentrations. Additional analyses discussed i n the proposal that looked at the contribution of PM 2.5 to total PM

- related light extinction (defined in terms of hourly PM 10 calculated light extinction ) indicate that there is a poor correlation between hourly PM 10 light extinction and hourly PM 2.5 mass principally due to the impact of the water content of the particles on light extinction, which depends on both the composition of the PM 2.5 and the ambient relative humidity. Both composition and especially relative humidity vary during a single day, as well as from day

- to

- day, at any site and time of year. Also, there are systematic regional and seasonal differences in the distribution of ambient humidity and PM 2.5 composition conditions that make it impossible to select a PM 2.5 concentration that ge nerally would correspond to the same
PM

- related light extinction levels across all areas of the nation. Analyses discussed in the proposal quantify the projected uneven protection that would result from the use of 1

- hour average PM 2.5 mass as the indicator

.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

401 (b) Directly Measured PM 2.5 Light Extinction PM light extinction has a nearly one

- to

- one relationship to light extinction, unlike
PM 2.5 mass concentration. As explained above, PM 2.5 is the component responsible for the large majority of PM light extinc tion in most places and times. PM 2.5 light extinction can be directly measured using several instrumental methods, some of which have been used for decades to routinely monitor the two components of PM 2.5 light extinction (light scattering and absorption) or to jointly measure both as total light extinction (from which Rayleigh scattering is subtracted to get PM 2.5 light extinction). As noted at the time of proposal, there are a number of advantages to direct measurements of light extinction for use in a se condary standard relative to estimates of PM 2.5 light extinction calculated using
PM 2.5 mass and speciation data. These include greater accuracy of direct measurements with shorter averaging times and overall greater simplicity when compared to the need fo r measurements of multiple parameters to calculate PM light extinction. In evaluating whether direct measurement of PM 2.5 or PM 10 light extinction is appropriate to consider in the context of this PM NAAQS review, the EPA solicited comment from the Ambient Air Monitoring and Methods Subcommittee (AAMMS) of CASAC. The CASAC AAMMS recommended that consideration of direct measurement should be limited to PM 2.5 light extinction, and that although instruments suitable for this purpose are commercially available at present, research is expected to produce even better instruments in the near term. The CASAC AAMMS advised against choosing any currently available commercial instrument, or even a general measurement approach, as an FRM because to do so could discourag e development of other potentially superior approaches. Instead, the CASAC AAMMS recommended that the EPA develop

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

402 performance

- based approval criteria for direct measurement methods in order to put all approaches on a level playing field. At the present tim e, the EPA has not undertaken to develop and test such performance

- base approval criteria. The EPA anticipates that if an effort were begun it would take at least several years before such criteria would be ready for regulatory use. Thus, the Policy Assess ment concluded that while in the future it would be appropriate to consider a direct measurement of PM 2.5 light extinction, or the sum of separate measurements of light scattering and light absorption, as the indicator for the secondary
PM 2.5 standard, thi s is not an appropriate option in this review because a suitable specification of the equipment or appropriate performance

- based verification procedures cannot be developed in the time frame for this review (U.S. EPA, 2011a, p. 4

- 51,

- 52). (c) Calculated PM 2.5 Light Extinction For the reasons discussed above, the Policy Assessment concluded that a calculated PM 2.5 light extinction indicator would be the preferred approach. PM 2.5 light extinction can be calculated from PM 2.5 mass, combined with speciated PM 2.5 mass concentration data plus relative humidity data, as is presently routinely done on a 24

- hour average basis under the Regional Haze Program using data from the rural IMPROVE monitoring network. This same calculation procedure, using a 24

- hour average basis, could be used for a NAAQS focused on protecting against PM

- related visibility impairment primarily in urban areas. This approach would use the type of data that is

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

403 routinely collected from the urban CSN 24 in combination with monthly average relative humidity data based on long

- term climatological means as used in the Regional Haze Program (U.S. EPA, 2011a, Appendix G, section G.2). The proposal discussed the complex approach utilized in the Visibility Asse ssment for calculating hourly PM 2.5 light extinction 25 and discussed various simplified approaches for calculating these hourly values that were analyzed in the Policy Assessment. The Policy Assessment concluded that each of these simplified approaches prov ided reasonably good estimates of PM 2.5 light extinction and each would be appropriate to consider as the indicator for a distinct hourly or multi

- hour secondary standard (U.S. EPA, 2011a, p. 4

- 48). The proposal also recognized that the Policy Assessment i dentified a number of variations on these simplified approaches that it would be appropriate to consider, including: (1) the use of the split

- component mass extinction efficiency approach from the revised IMPROVE algorithm 26

24 About 200 sites in the CSN routinely measure 24

- hou r average PM 2.5 chemical components using filter

- based samplers and chemical analysis in a laboratory, on either a 1

- day

- in

- 3 or 1

- day

- in

- 6 schedule (U.S. EPA, 2011a, Appendix B, section B.1.3). 25 As noted at the time of proposal, the sheer size of the amb ient air quality, meteorological, and chemical transport modeling data files involved with the Visibility Assessment approach would make it very difficult for state agencies or any interested party to consistently apply such an app roach on a routine basis for the purpose of implementing a national standard defined in terms of the Visibility Assessment approach. 26 If the revised IMPROVE algorithm were used to define the calculated PM 2.5 mass based indicator, it would not be possible to algebraically reduce the revised algorithm to a two

- factor version as described above and in Appendix F of the Policy Assessment for the simplified approaches. Instead, five component fractions would be determined from each day of speciated sampling, and then either applied to hourly measurements of PM 2.5 mass on the same day or averaged across a month and then applied to measurements of PM 2.5 mass on each day of the month.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

404 (2) the use of more refined value(s) fo r the organic carbon multiplier 27 (3) the use of the reconstructed 24

- hour PM 2.5 mass (i.e., the sum of the five PM 2.5 components from speciated monitoring) as a normalization value for the hourly measurements from the PM 2.5 instrument as a way of better reflec ting ambient nitrate concentrations (4) the use of historical monthly or seasonal, or regional, speciation averages Overall, the analyses conducted for the Visibility Assessment and Policy
Assessment indicate indicate d that the use of a calculated PM 2.5 light e xtinction indicator would provide a much higher degree of uniformity in terms of the degree of protection from visibility impairment across the country than a PM 2.5 mass indicator, because a calculated PM 2.5 light extinction indicator would directly incorp orate the effects of humidity and PM 2.5 composition differences between various regions. Further, the proposal noted that the Policy Assessment concluded that consideration could be given to defining a calculated PM 2.5 light extinction indicator on either a 24

- hour or a sub

- daily basis (U.S. EPA, 2011a, p. 4

- 52). However, the Policy Assessment noted that approval of continuous FEM monitors has been based only on 24

- hour average, not hourly, PM 2.5 mass. In addition, there are mixed results of data quality as sessments on a 24

- hour basis for these monitors, as well as the near absence of performance data for sub

- daily averaging periods. Thus, while it is possible to utilize data from PM 2.5 continuous FEMs
on a 1

- hour or multi

- hour (e.g., 4

- hour) basis, these fa ctors increase the uncertainty of utilizing continuous methods to support 1

- hour or 4

- hour PM 2.5 mass measurements as an input to the light extinction calculation. Therefore, as noted at the time of proposal, until

27 An organic carbon (OC)

- to

- organic mass (OM) multiplier of 1.6 was used for the assessment, which was f

ound to produce a value of OM comparable to the one derived with the original, albeit more complex Visibility Assessment method.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

405 issues regarding the comparability of 24

- hour PM 2.5 mass values derived from continuous FEMs and filter

- based FRMs 28 are resolved, there is reason to be cautious about relying on a calculation procedure that uses hourly PM 2.5 mass values reported by continuous FEMs in combination with speciated PM 2.5 mass values from 24

- hour filter

- based samplers. (d) CASAC Advice In reviewing the second draft Policy Assessment, CASAC stated that it “overwhelmingly ... would prefer the direct measurement of light extinction,” recognizing it as the property of the at mosphere that most directly relates to visibility effects (Samet, 2010d, p. iii). CASAC noted that “[I]t has the advantage of relating directly to the demonstrated harmful welfare effect of ambient PM on human visual perception.” However, CASAC also conclu ded that the calculated PM 2.5 light extinction indicator “appears to be a reasonable approach for estimating hourly light extinction” (Samet, 2010d, p. 11). Further, based on CASAC’s understanding of the time that would be required to develop an FRM for th is indicator, CASAC agreed with the staff preference presented in the second draft Policy Assessment for a calculated PM 2.5 light extinction indicator. CASAC noted that “[I]ts reliance on procedures that have already been implemented in the CSN and routine ly collected continuous PM 2.5 data suggest that it could be implemented much sooner than a directly measured indicator” (Samet, 2010d,

28 Filter

- based FRMs are designed to adequately quantify the amount of PM 2.5 collected over 24

- hours. Th ey cannot be presumed to be appropriate for quantifying average concentrations over 1

- hour or 4

- hour periods.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

406 p. iii). 29 (e) Administrator's Proposed Conclusions on Indicator At the time of proposal, while agreeing with CASAC that a directly measured PM light extinction indicator would provide the most direct link between PM in the ambient air and PM

- related light extinction, the Administrator provisionally concluded that this
was not an appropriate option in this review because a suitable specification of currently available equipment or performance

- based verification procedures cannot be developed in the time frame of this review. Taking all of the above considerations and CAS AC advice into account, the Administrator provisionally concluded that a new calculated
PM 2.5 light extinction indicator, similar to that used in the Regional Haze Program (i.e., using an IMPROVE algorithm as translated into the deciview scale), was the ap propriate indicator to replace the current PM 2.5 mass indicator. Such an indicator, referred to as a
PM 2.5 visibility index, would appropriately reflect the relationship between ambient PM and PM

- related light extinction, based on the analyses discussed in the proposal and incorporation of factors based on measured PM 2.5 speciation concentrations and relative humidity data. In addition, selection of this type of indicator would address, in part, the issues raised in the court’s remand of the 2006 PM 2.5 stan dards. The Administrator also noted that such a PM 2.5 visibility index would afford a relatively high degree of uniformity of visual air quality protection in areas across the country by virtue of directly incorporating the effects of differences in PM 2.5 composition and relative humidity across

29 In commenting on the second draft Policy Assessment, CASAC did not have an opportunity to review the assessment of continuous PM 2.5 FEMs compared to collocated FRMs (Ha nley and Reff, 2011) as presented and discussed in the final Policy Assessment (U.S. EPA, 2011a, p. 4

- 50).

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

407 the country. Based on these above considerations, the Administrator proposed to set a distinct secondary standard for PM 2.5 defined in terms of a PM 2.5 visibility index (i.e., a calculated PM 2.5 light extinction in dicator, translated into the deciview scale) to protect against PM

- related visibility impairment primarily in urban areas. The Administrator proposed that such an index be based on the original IMPROVE algorithm in conjunction with monthly average relative humidity data based on long

- term climatological means as used in the Regional Haze Program. The EPA solicited comment on all aspects of the proposed indicator, especially: (1) the proposed use of a PM 2.5 visibility index rather than a PM 10 visibility index wh ich would include an additional term for coarse particles; (2) using the revised IMPROVE algorithm rather than the original IMPROVE algorithm; (3) the use of alternative values for the organic carbon multiplier in conjunction with either the original or revised IM PROVE algorithm; (4) the use of historical monthly, seasonal, or regional speciation averages; (5) alternative approaches to determining relative humidity; and (6) simplified approaches to generating hourly PM 2.5 light extinction values for purposes of calculating an hourly or multi

- hour indicator. iii. Averaging Times In this review, as discussed in section VI.D.2 of the proposal, consideration of appropriate averaging times for use in conjunction with a PM 2.5 visibility index was informed by information related to th e nature of PM visibility effects and the nature of inputs to the calculation of PM 2.5 light extinction, as discussed above. The EPA considered both sub

- daily (1

- and 4

- hour averaging times) and 24

- hour averaging times.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

408 In considering sub

- daily averaging t imes, the EPA has also considered what diurnal periods and ambient relative humidity conditions would be appropriate to consider in conjunction with such an averaging time. As an initial matter, the Policy Assessment considered sub

- daily averaging times.
T aking into account what is known from available studies concerning how quickly people experience and judge visibility conditions, the possibility that some fraction of the public experiences infrequent or short periods of exposure to ambient visibility con ditions, and the typical rate of change of the path

- averaged PM light extinction over urban areas, the initial analyses conducted as part of the Visibility Assessment focused on a 1

- hour averaging time. In its review of the first draft Policy Assessment, C ASAC agreed that a
1

- hour averaging time would be appropriate to consider, noting that PM effects on visibility can vary widely and rapidly over the course of a day and such changes are almost instantaneously perceptible to human observers (Samet, 2010c, p . 19). The Policy Assessment noted that this view related specifically to a standard defined in terms of a directly measured PM light extinction indicator, in that CASAC also noted that a 1

- hour averaging time is well within the instrument response times o f the various currently available and developing optical monitoring methods. However, CASAC also advised that if a PM 2.5 mass indicator were to be used, it would be appropriate to consider “somewhat longer averaging times

– 2 to 4 hours

– to

assure a more stable instrumental response” (Samet, 2010c, p. 19). In considering this advice, the Policy Assessment concluded that since a calculated PM 2.5 light extinction indicator relies in part on measured PM 2.5 mass, it would be appropriate to consider a multi

- ho ur averaging time on the order of a few hours (e.g. 4

- hours). A multi

- hour

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

409 averaging time might reasonably characterize the visibility effects experienced by the segment of the population who have access to visibility conditions often or continuously throu ghout the day. For this segment of the population, it may be that their perception of visual air quality reflects some degree of offsetting an hour with poor visual air quality with one or more hours of clearer visual conditions. Further, the Policy Assess ment recognized that a multi

- hour averaging time would have the effect of averaging away peak hourly visibility impairment, which can change significantly from one hour to the next (U.S. EPA, 2011a, p. 4

- 53; U.S. EPA, 2010b, Figure 3

- 12). In considering either 1

- hour or multi

- hour averaging times, the Policy Assessment recognized that no data are available with regard to how the duration and variation of time a person spends outdoors during the daytime impacts his or her judgment of the acceptability of d ifferent degrees of visibility impairment. As a consequence, it is not clear to what degree, if at all, the protection levels found to be acceptable in the public preference studies would change for a multi

- hour averaging time as compared to a 1

- hour

avera ging time. Thus, the Policy Assessment concluded that it is appropriate to consider a
1

- hour or multi

- hour (e.g., 4

- hour) averaging time as the basis for a sub

- daily standard defined in terms of a calculated PM 2.5 light extinction indicator (U.S. EPA, 2011 a, p. 4

- 53). In addition, as discussed above, some data quality uncertainties have been observed with regard to hourly data collected by FEMs. Specifically, as part of the review of data from all continuous FEM PM 2.5 instruments operating at state/local mo nitoring sites, the Policy Assessment noted that the occurrence of questionable outliers in 1

- hour data submitted to AQS from continuous FEM PM 2.5 instruments had been observed at

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

410 some of these sites (Evangelista, 2011). Some of these outliers were questio nable simply by virtue of their extreme magnitude, as high as 985 μg/m 3 , whereas other values were questionable because they were isolated to single hours with much lower values before and after, a pattern that is much less plausible than if the high conce ntrations were more sustained. 30 The Policy Assessment noted that any current data quality problems might be resolved in the normal course of monitoring program evolution as operators become more adept at instrument operation and maintenance and data valida tion or by improving the approval criteria and testing requirements for continuous instruments. Regardless, the Policy Assessment noted that multi

- hour averaging of FEM data could serve to reduce the effects of such outliers relative to the use of a 1

- hour averaging time. The Policy Assessment noted that there are significant reasons to consider using
PM 2.5 light extinction calculated on a 24

- hour basis to reduce the various data quality concerns described above with respect to relying on continuous PM 2.5 monitoring data. However, the Policy Assessment recognized that 24 hours is far longer than the hourly or multi

- hour time periods that might reasonably characterize the visibility effects experienced by various segments of the population, including both th ose who do and do not have access to visibility conditions often or continuously throughout the day. Thus, the Policy Assessment concluded that the appropriateness of considering a 24

- hour averaging time would depend upon the extent to which PM

- related lig ht extinction

30 Similarly questionable hourly data were not observed in the 2005 to 2007 continuous PM 2.5 data used in the Visibility Assessment, all of which came from early

- generation continuous instruments that had not been approved as FEMs. However, only 15 sites and instruments were involved in the Visibility Assessment analyses, versus about 180 currently operating FEM instruments submitting data to AQS. There fore, there were more opportunities for very infrequent measurement errors to be observed in the larger FEM data set.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

411 calculated on a 24

- hour average basis would be a reasonable and appropriate surrogate for PM

- related light extinction calculated on a sub

- daily basis. To examine this relationship, the EPA conducted comparative analyses of 24

- hour and 4

- hour averaging times in conjunction with a calculated PM 2.5 indicator. For these analyses, 4

- hour average PM 2.5 light extinction was calculated based on using the Visibility Assessment approach. The 24

- hour average PM 2.5 light extinction was calculated using t he original IMPROVE algorithm and long

- term relative humidity conditions to calculate PM 2.5 light extinction. Based on these analyses, 31 which are presented and discussed in Appendix G of the Policy Assessment, scatter plots comparing
24

- hour and 4

- hour calculated PM 2.5 light extinction were constructed for each of the 15 cities included in the Visibility Assessment and for all 15 cities p ooled together (U.S. EPA, 2011a, Figures G

- 4 and G

- 5). Though there was some scatter around the regression line for each city because the calculated 4

- hour light extinction values included day

specific and hour

- specific influences that are not captured by the simpler 24

- hour approach, these analyses generally showed good correlation between 24

- hour and 4

- hour average PM 2.5 light extinction, as evidenced by reasonably high city

- specific and pooled
R 2 values, generally in the range of over 0.6 to over 0.8. 32 T his suggested that PM 2.5 light extinction calculated on a 24

- hour basis is a reasonable and appropriate surrogate to
PM 2.5 light extinction calculated on a sub

- daily basis. Taking the above considerations and CASAC’s advice into account, the Policy

31 These analyses are also based on the use of a 90 th percentile form, averaged over 3 years, as discussed below i n section VI.D.3 and in section 4.3.3 of the Policy Assessment (U.S. EPA, 2011a). 32 The EPA staff noted that the R 2 value (0.44) for Houston was notably lower than for the other cities.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

412 Assess ment concluded that it would be appropriate to consider a 24

- hour averaging time, in conjunction with a calculated PM 2.5 light extinction indicator and an appropriately specified standard level, as discussed below. By using site

- specific daily data on PM 2. 5 composition and site

- specific long

- term relative humidity conditions, this 24

- hour average indicator would provide more consistent protection from PM 2.5 - related visibility impairment than would a secondary PM 2.5 NAAQS based only on 24

- hour or annual

aver age PM 2.5 mass. In particular, this approach would account for the systematic difference in humidity conditions between most eastern states and most western states. The Policy Assessment also concluded that it would also be appropriate to consider a multi

- hour, sub

- daily averaging time, for example a period of 4 hours, in conjunction with a calculated PM 2.5 light extinction indicator and with further consideration of the data quality issues discussed above. Such an averaging time, to the extent that data
qu ality issues can be appropriately addressed, would be more directly related to the short

term nature of the perception of visibility impairment, short

- term variability in PM

related visual air quality, and the short

- term nature (hourly to multiple hours) o f relevant exposure periods for segments of the viewing public. Such an averaging time would still result in an indicator that is less sensitive than a 1

- hour averaging time to short

- term instrument variability with respect to PM 2.5 mass measurement. In co njunction with consideration of a multi

- hour, sub

- daily averaging time, the Policy Assessment concluded that consideration should be given to including daylight hours only and to applying a relative humidity screen of approximately 90 percent to remove hou rs in which fog or precipitation is much more likely to contribute to the observed visibility impairment (U.S. EPA, 2011a, p. 4

- 58). Recognizing that a 1

- hour averaging time would be even more

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

413 sensitive to data quality issues, including short

- term variabil ity in hourly data from currently available continuous monitoring methods, the Policy Assessment concluded that it would not be appropriate to consider a 1

- hour averaging time in conjunction with a calculated PM 2.5 light extinction indicator in this review (U.S. EPA, 2011a, p. 4

- 58). As noted above, in its review of the first draft Policy Assessment, CASAC concluded that PM effects on visibility can vary widely and rapidly over the course of a day and such changes are almost instantaneously perceptible to human observers (Samet, 2010c, p. 19). Based in part on this consideration, CASAC agreed that a 1

- hour averaging time would be appropriate to consider in conjunction with a directly measured PM light extinction indicator, noting that a 1

- hour averaging tim e is well within the instrument response times of the various currently available and developing optical monitoring methods. At that time, CASAC also advised that if a PM 2.5 mass indicator were to be used, it would be appropriate to consider “somewhat long er averaging times

– 2

- to 4

- hours

– to assure a more stable instrumental response” (Samet, 2010c, p. 19). Thus, CASAC’s advice on averaging times that would be appropriate for consideration was predicated in part on the capabilities of monitoring methods that were available for the alternative indicators discussed in the draft Policy Assessment. CASAC’s views on a
multi

- hour averaging time would also apply to the calculated PM 2.5 light extinction indicator since hourly PM 2.5 mass measurements are also requ ired for this indicator when calculated on a sub

- daily basis. It is important to note that at the time it provided advice on suitable averaging times, CASAC did not have the benefit of EPA’s subsequent assessment of the data quality issues associated with the use of continuous FEMs as the basis for hourly PM 2.5

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

414 mass measurements. Furthermore, since CASAC only commented on the first and second drafts of the Policy Assessment, neither of which included discussion of a calculated
PM 2.5 indicator based on a 24

- hour averaging time, CASAC did not have a basis to offer advice regarding a 24

- hour averaging time. In addition, the 24

- hour averaging time is not based on consideration of 24

- hours as a relevant exposure period, but rather as a surrogate for a sub

- daily p eriod of 4 hours, which is consistent with CASAC’s advice concerning an averaging time associated with the use of a PM 2.5 mass indicator. Taking into account the information discussed above with regard to analyses and conclusions presented in the final Pol icy Assessment the Administrator recognize d that hourly or sub

- daily, multi

- hour averaging times, within daylight hours and excluding hours with relative humidity above approximately 90 percent, are more directly related than a 24

- hour averaging time to th e short

- term nature of the perception of PM

- related visibility impairment and the relevant exposure periods for segments of the viewing public. On the other hand, she recognize d that data quality uncertainties have recently been associated with currently a vailable instruments that would be used to provide the hourly PM 2.5 mass measurements that would be needed in conjunction with an averaging time shorter than 24

- hours. As a result, while the Administrator recognize d the desirability of a sub

- daily averagin g time, she ha d strong reservations about proposing to set a standard at this time in terms of a sub

- daily averaging time. In considering the information and analyses related to consideration of a 24

- hour averaging time, the Administrator recognized that the Policy Assessment concluded that
PM 2.5 light extinction calculated on a 24

- hour averaging basis is a reasonable and appropriate surrogate for sub

- daily PM 2.5 light extinction calculated on a 4

- hour average

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

415 basis. In light of this finding and the views of CASAC based on its reviews of the first and second drafts of the Policy Assessment, the Administrator proposed to set a distinct secondary standard with a 24

- hour averaging time in conjunction with a PM 2.5 visibility index. iv. Form As discussed in sec tion VI.D.3 of the proposal, the “form” of a standard defines the air quality statistic that is to be compared to the level of the standard in determining whether the standard is achieved. The form of the current 24

- hour PM 2.5 NAAQS is such that the level of the standard is compared to the 3

- year average of the annual 98 th percentile value of the measured indicator. The purpose in averaging for three years is to provide stability from the occasional effects of inter

- annual meteorological variability that ca n result in unusually high pollution levels for a particular year. The use of a multi

year percentile form, among other things, makes the standard less subject to the possibility of transient violations caused by statistically unusual indicator values, the reby providing more stability to the air quality management process that may enhance the practical effectiveness of efforts to implement the NAAQS. Also, a percentile form can be used to take into account the number of times an exposure might occur as part of the judgment on protectiveness in setting a NAAQS. For all of these reasons, the Policy Assessment concluded it would be appropriate to consider defining the form of a distinct secondary standard in terms of a 3

- year average of a specified percentile a ir quality statistic (U.S. EPA, 2011a, p. 4

- 58). The urban visibility preference studies that provided results leading to the range of CPLs being considered in this review offer no information that addresses the

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

416 frequency of time that visibility levels sho uld be below those values. Given this lack of information, and recognizing that the nature of the public welfare effect is one of aesthetics and/or feelings of well

- being, the Policy Assessment concluded that it would not be appropriate to consider elimina ting all exposures above the level of the standard and that allowing some number of hours/days with reduced visibility can reasonably be considered (U.S. EPA, 2011a, p. 4

- 59). In the Visibility Assessment, 90 th , 95 th , and 98 th percentile forms were assesse d for alternative PM light extinction standards (U.S. EPA, 2010b, section 4.3.3). In considering these alternative percentiles, the Policy Assessment noted that the Regional Haze Program targets the 20 percent most impaired days for improvements in visual air quality in Federal Class I areas. If improvement in the 20 percent most impaired days were similarly judged to be appropriate for protecting visual air quality in urban areas, a percentile well above the 80 th percentile would be appropriate to increase the likelihood that all days in this range would be improved by control strategies intended to attain the standard. A focus on improving the 20 percent most impaired days suggests that the 90 th percentile, which represents the median of the distribution of the 20 percent worst days, would be an appropriate form to consider. Strategies that are implemented so that 90 percent of days have visual air quality that is at or below the level of the standard would reason ably be expected to lead to improvements in visual air quality for the 20 percent most impaired days. Higher percentile values within the range assessed could have the effect of limiting the occurrence of days with peak PM

- related light extinction in urban areas to a greater degree. In considering the limited information available from the public preference studies, the Policy Assessment found no basis to conclude that it would be appropriate to consider limiting the

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

417 occurrence of days with peak PM

- related light extinction in urban areas to a greater degree. Another aspect of the form discussed in the proposal for a sub

- daily averaging time was whether to include all daylight hours or only the maximum daily daylight hour(s). The maximum daily daylight 1

- hour or multi

- hour form would be most directly protective of the welfare of people who have limited, infrequent or intermittent exposure to visibility during the day (e.g., during commutes), but spend most of their time without an outdoor view. For such people a view of poor visibility during their morning commute may represent their perception of the day’s visibility conditions until the next time they venture outside during daylight, which may be hours later or perhaps the next day. Other people have exposure to visibility conditions throughout the day. For those people, it might be more appropriate to include every daylight hour in assessing compliance with a standard, since it is more likely that each daylight hour could affect their welfare. The Policy Ass essment did not have information regarding the fraction of the public that has only one or a few opportunities to experience visibility during the day, nor did it have information on the role the duration of the observed visibility conditions has on wellbe ing effects associated with those visibility conditions. However, it is logical to conclude that people with limited opportunities to experience visibility conditions on a daily basis would experience the entire impact associated with visibility based on t heir short

- term exposure. The impact of visibility for those who have access to visibility conditions often or continuously during the day may be based on varying conditions throughout the day. In light of these considerations, the analyses conducted as p art of the Visibility

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

418 Assessment analyses included both the maximum daily hour and the all daylight hours forms. The Policy Assessment noted that there is a close correspondence between the level of protection afforded for all 15 urban areas by a maximum d aily daylight 1

- hour approach using the 90 th percentile form and an all daylight hours approach combined with the 98 th percentile form (U.S. EPA, 2010b, section 4.1.4). This suggests suggest ed that reductions in visibility impairment required to meet either form of the standard would provide protection to both fractions of the public (i.e., those with limited opportunities and those with greater opportunities to view PM

- related visibility conditions). CASAC generally supported consideration of both types of forms without expressing a preference based on its review of information presented in the second draft Policy Assessment (Samet, 2010d, p. 11). In conjunction with a calculated PM 2.5 light extinction indicator and alternative
24

- hour or sub

- daily (e.g., 4

- hour) averaging times, based on the above considerations, and given the lack of information on and the high degree of uncertainty over the impact on public welfare of the number of days with visibility impairment over a year, the Policy Assessment conclude d that it would be appropriate to give primary consideration to a 90 th percentile form, averaged over three years (U.S. EPA, 2011a, p. 4

- 60). Further, in the case of a multi

- hour, sub

- daily alternative standard, the Policy Assessment concluded that it woul d be appropriate to give primary consideration to a form based on the maximum daily multi

- hour period in conjunction with the 90 th percentile form (U.S. EPA, 2011a, p.
4

- 60). This sub

- daily form would be expected to provide appropriate protection for vario us segments of the population, including those with limited opportunities during a day and those with more extended opportunities over the daylight hours to experience

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

419 PM

- related visual air quality. Though CASAC did not provide advice as to a specific form that would be appropriate, it took note of the alternative forms considered in that document and encouraged further analyses in the final Policy Assessment that might help to clarify a basis for selecting from within the range of forms identified. In cons idering the available information and the conclusions in the final Policy Assessment in light of CASAC’s
comments, at the time of proposal the Administrator provisionally concluded that a 90 th percentile form, averaged over 3 years, is appropriate, and pro posed such a form in conjunction with a PM 2.5 visibility index and a 24

- hour averaging time.

v. Level As discussed in section VI.D.4 of the proposal, in considering appropriate levels for a 24

- hour standard defined in terms of a PM 2.5 visibility index and an 90 th percentile form, averaged over 3 years, the Policy Assessment took into account the evidence

- and impact

- based considerations discussed above, with a focus on the results of public perception and attitude surveys related to the acceptability of var ious levels of visual air quality and on the important limitations in the design and scope of such available studies

. The Policy Assessment considered a variety of approaches for identifying appropriate levels for such a standard, including utilizing both adjusted and unadjusted CPLs derived from the visibility preference studies. The Policy Assessment interpreted the results from the visibility preferences studies conducted in four urban areas to define a range of low, middle, and high CPLs for a sub

- dail y standard (e.g., 1

- to 4

- hour averaging time) of 20, 25, and 30 dv, which are approximately equivalent to PM 2.5 light extinction of values of 65, 110, and 190

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

420 Mm - 1

. The Policy Assessment noted that CASAC agreed that this was an appropriate range of levels to consider for such a standard The CASAC generally supported this approach, noting that the “ EPA staff’s approach for translating and presenting the technical evidence and assessment results is logically conceived and clearly presented. The 20

- 30 deciview range of levels chosen by EPA staff as “Candidate Protection Levels” is adequately supported by the evidence presented” (Samet, 2010d, p. 11). 33 The Policy Assessment also recognized that to define a range of alternative levels that would be appropriate to consider for a 24

- hour calculated PM 2.5 light extinction standard, it would be appropriate to consider whether some adjustment to these CPLs is warranted since these preference studies cannot be directly interpreted as applying to a 24

- hour exposure period (as noted above and in Policy Assessment section 4.3.1). Considerations related to such adjustments are more specifically discussed below.
In considering alternative levels for a sub

- daily standard based directly on the four preference study results, the Policy Assessment noted that the individual low and high CPLs are in fact generally reflective of the results from the Denver and Washington, D C studies respectively, and the middle CPL is very near to the 50 th percentile criteria result from the Phoenix study, which was by far the best of the studies, providing somewhat more support for the middle CPL. In considering the results from the four vi sibility preference studies, the Policy

33 In 2009, the D.C. Circuit remanded the secondary PM 2.5 standards to the EPA in part because the Agency failed to identify a target level of protection, even though EPA staff and CASAC had identified a range of target levels of protection that were appropriate for consideration. The court determined that the Agency’s failur e to identify a target l evel of protection as part of its final decision was contrary to the statute and therefore unlawful, and that it deprived EPA’s decision

- making of a reasoned basis. See 559F. 3d at 528

- 31; see also section VI.A.2 above and the Policy Assessment, section 4. 1.2.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

421 Assessment recognized that currently available studies are limited in that they were conducted in only four areas, three in the U.S. and one in Canada. Further, the Policy Assessment recognized that available studies provide no information on how the duration and variation of time a person spends outdoors during the daytime may impact their judgment of the acceptability of different degrees of visibility impairment

. As such, there is a relatively high degree of uncert ainty associated with using the results of these studies to inform consideration of a national standard for any specific averaging time. Nonetheless, the Policy Assessment concluded, as did CASAC, that these studies are appropriate to use for this purpose (U.S. EPA, 2011a, p. 4

- 61). Using approaches described in section VI.C.4 of the proposal, the Policy Assessment explored various approaches to adjusting the CPLs derived from the preference studies to inform alternative levels for a 24

- hour standard. These various approaches, based on analyses of 2007

– 2009 data from the 15 urban areas assessed in the Visibility Assessment, focused on estimating CPLs for a 24

- hour standard that would provide generally equivalent protection as that provided by a 4

- hour stan dard with CPLs of 20, 25, and 30 dv. In conducting these analyses, staff initially expected that the values of 24

- hour average PM 2.5 light extinction and daily maximum daylight 4

- hour average
PM 2.5 light extinction would differ on any given day, with the s horter term peak value generally being larger. This would mean that, in concept, the level of a 24

- hour standard should include a downward adjustment compared to the level of a 4

- hour standard to provide generally equivalent protection. As discussed more f ully in section G.5 of Appendix G and summarized below, this initial expectation was not found to be the case across the range of CPLs considered. In fact, as shown in Table Table s G

- 6 7 and G

- 8 of

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

422 Appendix G , and in the corrected version of Table G

- 6 found in Frank et al. (2012b) , 34 in considering estimates aggregated or averaged over all 15 cities as well as the range of city

- specific estimates for the various approaches considered, these analyses indicated that the generally equivalent 24

- hour levels range d from somewhat below the 4

- hour level to just above the 4

- hour level for each of the CPLs. 35 In all cases, the range of city

specific estimates of generally equivalent 24

- hour levels included the 4

- hour level for each of the CPLs of 20, 25, and 30 dv. As n oted in the proposal, looking more broadly at these results could support consideration of using the same CPL for a 24

- hour standard as for a 4

- hour standard, recognizing that there is no one approach that can most closely identify a generally equivalent 2 4

- hour standard level in each urban area for each CPL. The use of such an unadjusted CPL for a 24

- hour standard would place more emphasis on the relatively high degree of spatial and temporal variability in relative humidity and fine particle composition o bserved in urban areas across the country, so as to reduce the

34 Note that the city

- specific ranges shown in Table G

- 6, Appendix G of the Policy Assessment are incorrectly stated for Approaches C and E. Drawing from the more detailed and correct results for Approaches C and E presented in T ables G

- 7 and G

- 8, respe ctively, the city

- specific ranges in Table G

- 6 for Approach C should be 17

– 21 dv for the CPL of 20 dv; 21

– 25 dv for the CPL of 25 dv; and 24

– 30 dv for the CPL of 30 dv; the city

- specific ranges in Table G

- 6 for Approach E should be 17

– 21 dv for the CPL of 20 dv; 21

– 26 dv for the CPL of 25 dv; and 25

– 31 dv for the CPL of 30 dv. In the EPA’s reanalysis comparing 4

- vs. 24

- hour values, Frank et al. (2012b) recreated Table G

- 6 using the correct values from Tables G

- 7 and G

- 8. 35 As discussed in more d etail in Appendix G of the Policy Assessment, some days have higher values for 24

- hour average light extinction than for daily maximum 4

- hour daylight light extinction, and consequently an adjusted "equivalent" 24

- hour CPL can be gr eater than the original 4

- hour CPL. This can happen for two reasons. First, the use of monthly average historical RH data will lead to cases in which the f (RH) values used for the calculation of 24

- hour average light extinction are higher than all or some of the four hourly value s of f

(RH) used to determine daily maximum 4

- hour daylight light extinction on the same day. Second, PM 2.5 concentrations may be greater during non

- daylight periods than during daylight hours.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

423 potential of setting a 24

- hour standard level that would require more than the intended degree of protection in some areas. In considering the appropriate level of a secondary standard focused on protection from PM

- related urban visibility impairment based on either a 24

- hour or a multi

- hour, sub

- daily (e.g., 4

- hour) averaging time, the EPA has been mindful of the important limitations in the available evidence from public preference studies. Th ese uncertainties and limitations are due in part to the small number of stated preference studies available for this review; the relatively small number of study participants and the extent to which the study participants may not be representative of the broader study area population in some of the studies; and the variations in the specific materials and methods used in each study such as scene characteristics, the range of VAQ levels presented to study participants, image presentation methods and specifi c wording used to frame the questions used in the group interviews. In addition the EPA has noted that the scenic vistas available on a daily basis in many urban areas across the country generally may not have the inherent visual interest or the distance b etween viewer and object of greatest intrinsic value as in the Denver and Phoenix preference studies, and that there is the possibility that there could be regional differences in individual preferences for VAQ. It is also important to note that as in past reviews, the EPA is considering a national visibility standard in conjunction with the Regional Haze Program as a means of achieving appropriate levels of protection against PM

- related visibility impairment in urban, non

- urban, and Federal Class I areas a cross the country. The EPA recognizes that programs implemented to meet a national standard focused primarily on the visibility problems in urban areas can be expected to improve visual air quality in surrounding non

-

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

424 urban areas as well, as would programs now being developed to address the requirements of the Regional Haze Program established for protection of visual air quality in Federal Class I areas. The EPA also believes that the development of local programs, such as those in Denver and Phoenix, can c ontinue to be an effective and appropriate approach to provide additional protection, beyond that afforded by a national standard, for unique scenic resources in and around certain urban areas that are particularly highly valued by people living in those a reas. The Policy Assessment concluded that it is appropriate to give primary consideration to alternative standard levels toward the upper end of the ranges identified above for 24

- hour and sub

- daily standards, respectively (U.S. EPA, 2011a, p. 4

- 63). Thus, the Policy Assessment concluded it is appropriate to consider the following alternative levels: a level of 28 dv or somewhat below, down to 25 dv, for a standard defined in terms of a calculated PM 2.5 light extinction indicator, a 90 th percentile for m, and a 24

- hour averaging time; and a standard level of 30 dv or somewhat below, down to 25 dv, for a similar standard but with a 4

- hour averaging time (U.S. EPA, 2011a, p. 4

- 63). The Policy Assessment judged that such standards would provide appropriate protection against PM

- related visibility impairment primarily in urban areas. The Policy Assessment noted that CASAC generally supported consideration of the 20

– 30 dv range as CPLs and, more specifically, that support for consideration of the upper part of the range of the CPLs derived from the public preference studies was expressed by some CASAC Panel members during the public meeting on the second draft Policy Assessment. The Policy Assessment concluded that such a standard would be appropriate in conj unction with the Regional Haze Program to achieve appropriate levels of

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

425 protection against PM

- related visibility impairment in areas across the country (U.S. EPA, 2011a, p. 4

- 63). Based on the considerations discussed above and in section VI.D.4 of the
pro posal, and taking into account the advice of CASAC, at the time of proposal the Administrator provisionally concluded that it would be appropriate to establish a target level of protection

– for a standard defined in terms of a PM 2.5 visibility index; a 90 th percentile form averaged over 3 years; and a 24

- hour averaging time

– equivalent to the protection afforded by such a sub

- daily (i.e., 4

- hour) standard at a level of 30 dv, which is the upper end of the range of CPLs identified in the Policy Assessment and generally supported by CASAC. More specifically, the Administrator provisionally concluded that
a 24

- hour level of either 30 dv or 28 dv could be construed as providing such a degree of protection, and that either level was supported by the available i nformation and was generally consistent with the advice of CASAC. Thus, the EPA proposed two options for the level of a new 24

- hour standard (defined in terms of a PM 2.5 visibility index and a 90 th percentile form, averaged over 3 years) to provide appropr iate protection from PM

related visibility impairment: either 30 dv or 28 dv. As noted in the proposal, the option of setting such a 24

- hour standard at a level of 30 dv would reflect recognition that there is considerable spatial and temporal variability in the key factors that determine the value of the PM 2.5 visibility index in any given urban area, such that there is a relatively high degree of uncertainty as to the most appropriate approach to use in selecting a 24

- hour standard level that would be gen erally equivalent to a specific 4

- hour standard level. Selecting a 24

- hour standard level of 30 dv would reflect a judgment that such substantial degrees of variability and uncertainty should be reflected in a higher standard level than

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

426 would be appropriat e if the underlying information were more consistent and certain. Alternatively, the option of setting such a 24

- hour standard at a level of 28 dv would reflect placing more weight on statistical analyses of aggregated data from across the study cities and not placing as much emphasis on the city

- to

- city variability as a basis for determining an appropriate degree of protection on a national scale. The information available for the Administrator to consider when setting the secondary PM standard raises a nu mber of uncertainties. While CASAC supported moving forward with a new standard on the basis of the available information, CASAC also recognized these uncertainties, referencing the discussion of key uncertainties and areas for future research in the secon d draft of the Policy Assessment. In discussing areas of future research, CASAC stated that: “The range of 50% acceptability values discussed as possible standards are based on just four studies (Figure 4

- 2), which, given the large spread in values, provid e only limited confidence that the benchmark candidate protection levels cover the appropriate range of preference values. Studies using a range of urban scenes (including, but not limited to, iconic scenes

– “valued scenic elements”
such as those in the W ashington, DC study), should also be considered” (Samet, 2010d, p. 12). The EPA solicited comment on how the Administrator should weigh those uncertainties as well as any additional comments and information to inform her consideration of these uncertaintie s. In addition, the EPA solicited comment on a number of other issues related to the level of the standard, including: (1) Both of the proposed levels and the various approaches to identifying generally equivalent levels upon which the alternative proposed l evels are based.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

427 (2) A broader range of levels down to 25 dv in conjunction with a 24

- hour averaging time. (3) A range of alternative levels from 30 to 25 dv in conjunction with a sub

- daily (e.g., 4

- hour) averaging time. (4) The strengths and limitations associated w ith the public preference studies and the use of these studies to inform the selection of a range of levels that could be used to provide an appropriate degree of public welfare protection when combined with the other elements of the standard (i.e. indicat or, form and averaging time). (5) Specific aspects of the public preference studies, including the extent to which the 50 percent acceptability criterion is an appropriate basis for establishing target protection levels in the context of establishing a distinc t secondary NAAQS to address PM

- related visibility impairment in urban areas; how the variability among preference studies in the extent to which study participants may be representative of the broader study area population should be weighed in the context of considering these studies in reaching proposed conclusions on a distinct secondary NAAQS; and the extent to which the ranges of VAQ levels presented to participants in each of the studies may have influenced study results and on how this aspect of the study designs should appropriately be weighed in the context of considering these studies in the context of this review. vi. Administrator’s Proposed Conclusions Regarding PM Standards to Protect Visibility At the time of proposal, based on the consider ations described above, the Administrator proposed to revise the suite of secondary PM standards by adding a distinct standard for PM 2.5 to address PM

- related visibility impairment, focused primarily on visibility in urban areas. This proposed visibility s tandard was to be defined in terms of a PM 2.5 visibility index, which would use measured PM 2.5 mass, combined with PM 2.5 speciation data and relative humidity data, to calculate PM 2.5 light extinction, translated into the deciview (dv) scale; a 24

- hour ave raging time; a 90 th percentile form, averaged over 3 years; and a level of 28

- 30 dv. vii. Related Technical Analysis

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

428 At the time of proposal, the EPA conducted a two

- pronged technical analysis of the relationships between the proposed PM 2.5 visibility in dex standard and the current 24

- hour PM 2.5 mass - based standard (Kelly, et al., 2012a). This analysis was designed to provide technical information to inform key issues related to implementing a distinct secondary standard for visibility as proposed. Specif ically, the EPA recognized that significant technical issues were likely to arise for new or modified emissions sources conducting air quality analyses for purposes of demonstrating that they would not cause or contribute to a violation of the visibility s tandard under the Prevention of Significant Deterioration (PSD) program. Such a demonstration for the proposed secondary PM 2.5 visibility index NAAQS standard could require each PSD applicant to predict, via air quality modeling, the increase in visibility impairment, in terms of the proposed PM 2.5 visibility index, that would result from the proposed source's emissions in conjunction with an assessment of existing air quality (visibility impairment) conditions in terms of the proposed PM 2.5 visibility index . The EPA noted that if this demonstration were to be attempted using the six

- step procedure that the EPA proposed to use for calculating PM 2.5 visibility index design values from monitored air concentrations of PM 2.5 components, significant technical issu es with the modeling procedures could arise. To address these technical issues, the EPA sought to explore whether sources that met the requirements pertaining to the 24

- hour mass

- based standard of 35 μg/m 3 would also meet the requirements pertaining to the proposed visibility index standard. As described in Kelly et al. (2012a), 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 the ambie nt PM 2.5 concentration would produce a

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

429 comparably small increase in visibility impairment. This analysis included estimates of
PM 2.5 speciation profiles based on direct PM 2.5 emission profiles for a broad range of source categories and for theoretical uppe r and lower bound scenarios. The second prong of the analysis addressed aspects of a PSD cumulative impact analysis by exploring the relationship between the three

- year design values for the existing 24

- hour PM 2.5 NAAQS standard and coincident design values for the proposed
PM 2.5 visibility index NAAQS standard based on recent air quality data. This aspect of the analysis indicated that increases in 24

- hour PM 2.5 design values generally correspond to increases in visibility index design values, and vice

- versa . The analysis further explored the appropriateness of using a demonstration that a source does not cause or contribute to a violation of the 24

- hour PM 2.5 NAAQS standard as a surrogate for a demonstration that a source does not cause or contribute to a vio lation of the proposed secondary PM 2.5 visibility index NAAQS standard . This analysis is wa s based on 2008 to 2010 air quality data, and compares compare d the proposed level of 35 μg/m 3 for the 24

- hour PM 2.5 standard and for illustrative purposes an alternati ve standard level of 12 μg/m 3 for the annual PM 2.5 standard with the proposed levels of 28 or 30 dv for the secondary PM 2.5 visibility index standard in conjunction with a 24

- hour averaging time and a 90th percentile form. The results indicate indicate d tha t all (for the 30 dv level) or nearly all (for the 28 dv level) areas in attainment of the 24

- hour PM 2.5 standard would also be have been in attainment of the proposed secondary PM 2.5 visibility index standard. Based on this technical analysis, the EPA proposed that there is sufficient evidence that a demonstration that a source does not cause or contribute to a violation of the mass

- based 24

- hour PM 2.5 NAAQS standard serves as a suitable surrogate for

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

430 demonstrating that a source does not cause or contrib ute to a violation of the proposed secondary 24

- hour PM 2.5 visibility index NAAQS standard under the PSD program. As such, the EPA proposed to conclude that many or all sources undergoing PSD review for
PM 2.5 could rely upon their analysis for demonstrating that they do not cause or contribute to a violation of the mass

- based 24

- hour PM 2.5 NAAQS standard to also show that they do not cause or contribute to a violation of the proposed secondary PM 2.5 visibility index NAAQS standard , if a distinct visibility sta ndard were finalized. Although this proposed “surrogacy policy” was designed to address an implementation

- related issue, the second prong of the technical analysis addresses the broader technical question of the relationship between the existing 24

- hour P M 2.5 NAAQS standard and the proposed PM 2.5 visibility index standard in terms of the degree of protection likely to be afforded by each standard. Specifically, the analysis indicated that depending on the level of the proposed PM 2.5 visibility index standar d, the existing
24

- hour PM 2.5 mass - based standard would be as protective or in some areas more protective of visibility than a distinct secondary standard set within the range of levels proposed. Commenters on the proposed PM 2.5 visibility index explored t he implications of this analysis at length, as discussed further below in section VI.C.1.f. For this reason, the analysis is described in some detail here. Kelly et al. (2012a) noted that the relationship between design values for the 24

- hour PM 2.5 NAAQS st andard and the proposed secondary visibility index standard is not obvious a priori because of differences in design value calculations for the standards. However, closer examination of this relationship indicated that increases or decreases in
24

- hour PM 2 .5 design values correspond, respectively, to increases or decreases in

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

431 visibility index values. Specifically, based on measurements from 102 sites with complete data from 2008

- 2010, Kelly et al. (2012a) found linear correlations between the
24

- hour PM 2.5 design values and the visibility index design values with r 2 values ranging from 0.65 to 0.98 across these sites, with an average r 2 value of 0.75 across all U.S. sites. Moreover, the data indicated that no design value existed where the visibility index d esign value exceeded 30 dv, but the 24

- hour PM 2.5 NAAQS standard level of 35 μg/m 3 was attained. Visibility index design values for certain sites in the Industrial Midwest were shown to exceed 28 dv despite the fact that the 24

- hour PM 2.5 design values for these sites were below 35 μg/m 3 . This was attributed to the combination of high nitrate and sulfate fractions, substantial RH adjustment factors, and PM 2.5 distribution characteristics that led to relatively high visibility index design values for a given 24

- hour
PM 2.5 design value for counties in the Industrial Midwest. 36 Kelly et al. (2012a) concluded that the “overall, design values based on 2008

- 2010 data suggest that counties that attain 24

- hour PM 2.5 NAAQS level of 35 μg/m 3 would att ain the proposed secondary
PM 2.5 visibility index NAAQS level of 30 dv and generally attain the level of 28 dv” (pp.
17

- 18). In addition, the Kelly et al. analysis indicated that at sites that violated both the
24

- hour PM 2.5 level and the secondary proposed visibility index 30 dv level, the secondary standard proposed level of 30 dv would likely be attained if PM 2.5 concentrations were reduced such that the 24

- hour PM 2.5 level of 35 μg/m 3 was attained

.

36 Kelly et al. (2012a) also noted that “Regional reductions in sulfate PM 2.5 due to emission controls planned as part of national rules as well as emission reductions associated with potential annual standard violations are expected to improve visibility in this region” (p. 17).

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

432 ( Kelly et al. , 2012a, p.15 )

. 37 A key implication of this analysis, therefore, was that within the range of levels proposed by the EPA for the a visibility index NAAQS standard (28

- 30 dv), the 24

- hour PM 2.5 standard of 35 μg/m 3 would be controlling in almost all (at 28 dv) or all (at 30 d v) instances.
2. Other (Non

- Visibility) PM

- related Welfare Effects In the 2006 review, the EPA concluded that there was insufficient information to consider a distinct secondary standard based on PM

- related impacts to ecosystems, materials damage and soil ing, and climatic and radiative processes (71 FR 61144, October 17, 2006). Specifically, there was a lack of evidence linking various non

visibility welfare effects to specific levels of ambient PM. In that review, to provide a level of protection for thes e welfare

- related effects, the secondary standards were set equal to the revised primary standards to directionally improve the level of protection afforded vegetation, ecosystems, and materials (71 FR 61210, October 17, 2006). This section briefly outline s key conclusions discussed more fully in section VI.E of the proposal regarding the non

- visibility welfare effects of PM. These conclusions relate to the climate, ecological (including effects on plants, soil and nutrient cycling, wildlife and water) and materials damage effects of PM. For all of these effects, the Policy Assessment concluded that there is insufficient information at this time to revise the current suite of secondary standards. It is important to note that the Policy Assessment explicitly excluded discussion of the effects associated with deposited

37 The analysis also showed that attaini ng the 24

- hour PM 2.5 standard level of 35 μg/m 3 would result in achieving a lower PM 2.5 visibility index level in certain areas of the country, largely western areas, than would be achieved in other areas of the country. This is due to differences in the c omposition of ambient PM 2.5 and the lower relative humidity in those areas.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

433 particulate matter components of NO x and SO x and their transformation products which are addressed fully in the joint review of the secondary NO 2 and SO 2 NAAQS.

a. Evidence of Other Welfare Effe cts Related to PM With regard to the role of PM in climate, the proposal noted that there is considerable ongoing research focused on understanding aerosol contributions to changes in global mean temperature and precipitation patterns. The Integrated Scien ce Assessment concluded “that a causal relationship exists between PM and effects on climate, including both direct effects on radiative forcing and indirect effects that involve cloud feedbacks that influence precipitation formation and cloud lifetimes” ( U.S. EPA, 2009a, section 9.3.10). These effects are discussed in more detail in section VI.E.1 of the proposal, which provides information on the major aerosol components of interest for climate processes, including black carbon (BC), organic carbon (OC), sulfates, nitrates, and mineral dusts, and the nature, magnitude, and direction (e.g., cooling vs. warming) of various aerosol impacts on climate. 38 The Policy Assessment concluded that aerosols alter climate processes directly through radiative forcing and by indirect effects on cloud brightness, changes in precipitation, and possible changes in cloud lifetimes (U.S. EPA, 2011a, p. 5

- 10). Further, the Policy Assessment noted that the major aerosol components that contribute to climate processes (i.e. BC, OC , sulfate, nitrate and mineral dusts) vary in their reflectivity, forcing efficiencies and even in the direction of climate forcing, though there is an overall net climate cooling associated with aerosols in the global atmosphere (U.S. EPA, 2009a, section 9.2.10). The Policy Assessment concluded that the

38 Atmospheric PM is referred to as aerosols in the remainder of this section to be consistent with the Integrated Science Assessment.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

434 current mass

- based PM 2.5 and PM 10 secondary standards were not an appropriate or effective means of focusing protection against PM

- associated climate effects due to these differences in components (U.S. EPA , 2011a, p. 5

- 11). In addition, in light of the significant uncertainties in current scientific information and the lack of sufficient data, the Policy Assessment concluded it is not currently feasible to conduct a quantitative analysis for the purpose of informing revisions of the current secondary PM standards based on climate (U.S. EPA, 2011a, p. 5

- 11). Overall the Policy Assessment concluded that there is insufficient information at this time to base a national ambient standard on climate impacts associ ated with current ambient concentrations of PM or its constituents (U.S. EPA, 2011a, p. 5

- 11,

- 12). 39 With regard to ecological effects, the proposal noted that several ecosystem components (e.g., plants, soils and nutrient cycling, wildlife and water) are impacted by PM air pollution, which may alter the services provided by affected ecosystems. Ecologica l effects include both direct effects due to deposition (e.g., wet, dry or occult) to vegetation surfaces and indirect effects occurring via deposition to ecosystem soils or surface waters where the deposited constituents of PM then interact with biologica l organisms. Some of the ecological effects considered in this review include direct effects to metabolic processes of plant foliage; contribution to total metal loading resulting in alteration of soil biogeochemistry and microbiology, and plant and animal growth and reproduction; and contribution to total organics loading resulting in bioaccumulation and biomagnification across trophic levels. Section VI.E.2 of the proposal summarizes key

39 This conclusion would apply for both the sec ondary (welfare

- based) and the primary (health

- based) standards.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

435 findings related to: (1) Impacts on plants and the ecosystem services th ey provide due to deposition of PM to vegetative surfaces, which alters the radiation received by the plant, and uptake of deposited PM components by plants from soil or foliage, which can lead to stress and decreased photosynthesis; (2) Impacts on ecosystem s upport services such as nutrient cycling, products such as crops and the regulation of flooding and water quality; (3) Impacts on wildlife, especially due to biomagnification of heavy metals (especially Hg) up the food chain and bioconcentration of POPs and PB DEs; and (4) Impacts of deposited PM, especially metals and organics, on the ecosystem services provided by water bodies, including primary production, provision of fresh water, regulation of climate and floods, recreational fishing and water purification. The proposal noted that the Integrated Science Assessment had concluded that ecological evidence is sufficient to conclude that a causal relationship is likely to exist between deposition of PM and a variety of effects on individual organisms and ecosystems ( U.S. EPA, 2009a, sections 2.5.3 and 9.4.7), and also noted that vegetation and other ecosystem components are affected more by particulate chemistry than size fraction. However, the proposal also pointed to the Integrated Science Assessment conclusion that it is generally difficult to characterize the nature and magnitude of effects and to quantify relationships between ambient concentrations of PM and ecosystem response due to significant data gaps and uncertainties as well as considerable variability that exists in the components of PM and their various ecological effects. There are few studies that link ambient PM concentrations to observed effect. Most direct ecosystem effects associated with particulate pollution occur in severely polluted areas near
in dustrial point sources (quarries, cement kilns, metal smelting) (U.S. EPA, 2009a, sections 9.4.3 and 9.4.5.7).

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

436 Based on the evidence available at this time, the proposal noted the following key conclusions in the Policy Assessment: (1) A number of significant environmental effects that either have already occurred or are currently occurring are linked to deposition of chemical constituents found in ambient PM. (2) Ecosystem services can be adversely impacted by PM in the environment, including supporting, provision ing, regulating and cultural services. (3) The lack of sufficient information to relate specific ambient concentrations of particulate metals and organics to a degree of impairment of a specific ecological endpoint hinders the identification of a range of appr opriate indicators, levels, forms and averaging times of a distinct secondary standard to protect against associated effects. (4) Data from regionally

- based ecological studies can be used to establish probable local, regional and/or global sources of deposited PM components and their concurrent effects on ecological receptors. The proposal noted that the Policy Assessment had concluded that the currently available information is insufficient for purposes of assessing the adequacy of the protection for ecosystem s afforded by the current suite of PM secondary standards or establishing a distinct national standard for ambient PM based on ecosystem effects of particulates not addressed in the NO x /SO x secondary review (e.g., metals, organics) (U.S. EPA, 2011a, p. 5

- 2 4). Furthermore, the Policy Assessment had concluded that in the absence of information providing a basis for specific standards in terms of particle composition, the observations continue to support retaining an appropriate degree of control on both fine and coarse particles to help address effects to ecosystems and ecosystem components associated with PM (U.S. EPA, 2011a, p. 5

- 24). With regard to materials damage, the proposal discussed effects associated with deposition of PM, including both physical dam age (materials damage effects) and

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

437 impaired aesthetic qualities (soiling effects). As with the other categories of welfare effects discussed above, the Integrated Science Assessment concluded that evidence is sufficient to support a causal relationship bet ween PM and effects on materials (U.S. EPA, 2009a, sections 2.5.4 and 9.5.4). The deposition of PM can physically affect materials, adding to the effects of natural weathering processes, by potentially promoting or accelerating the corrosion of metals, by degrading paints and by deteriorating building materials such as stone, concrete and marble (U.S. EPA, 2009a, section 9.5). In addition, the deposition of ambient PM can reduce the aesthetic appeal of buildings and objects through soiling. The Policy Asses sment made the following observations: (1) Materials damage and soiling that occur through natural weathering processes are enhanced by exposure to atmospheric pollutants, most notably sulfur dioxide and particulate sulfates. (2) While ambient particles play a ro le in the corrosion of metals and in the weathering of materials, no quantitative relationships between ambient particle concentrations and rates of damage have been established. (3) While soiling associated with fine and course particles can result in increas ed cleaning frequency and repainting of surfaces, no quantitative relationships between particle characteristics and the frequency of cleaning or repainting have been established. (4) Limited new data on the role of microbial colonizers in biodeterioration pro cesses and contributions of black crust to soiling are not sufficient for quantitative analysis. (5) While several studies in the PM Integrated Science Assessment and NO x /SO x Integrated Science Assessment suggest that particles can promote corrosion of metals there remains insufficient evidence to relate corrosive effects to specific particulate levels or to establish a quantitative relationship between ambient PM and metal de gradation. With respect to damage to calcareous stone, numerous studies suggest that wet or dry deposition of particles and dry deposition of gypsum particles can enhance natural weathering processes. The Policy Assessment concluded that none of the new ev idence in this review

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

438 called into question the adequacy of the current standards for protecting against material damage effects, that such effects could play no quantitative role in determining whether revisions to the secondary PM NAAQS are appropriate at this time, and that observations continue to support retaining an appropriate degree of control on both fine and coarse particles to help address materials damage and soiling associated with PM (U.S. EPA, 2011a, p. 5

- 29). b. CASAC Advice In advising the EPA regarding the non

- visibility welfare effects, CASAC stated that it “ concurs with the Policy Assessment’s conclusions that while these effects are important, and should be the focus of future research efforts, there is not currently a strong technical basis to support revisions of the current standards to protect against these other welfare effects” (Samet, 2010c). More specifically, with regard to climate impacts, CASAC concluded that while there is insufficient information on which to base a national standard, the causal relationship is established and the risk of impacts is high, so further research on a regional basis is urgently needed (Samet, 2010c, p. 5). CASAC also noted that reducing certain aerosol components could lead to increased radiative f orcing and regional climate warming while having a beneficial effect on PM

- related visibility. As a consequence, CASAC noted that a secondary standard directed toward reducing
PM

- related visibility impairment has the potential to be accompanied by regional warming if light scattering aerosols are preferentially targeted. With regard to ecological effects, CASAC concluded that the published literature is insufficient to support a national standard for PM effects on ecosystem services (Samet, 2010c, p.23) . CA SAC noted that the best

- established effects are related to

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

439 particles containing nitrogen and sulfur, which are being considered in the EPA’s ongoing review of the secondary NAAQS for NO x /SO x . With regard to PM

- related effects on materials, CASAC concluded that the published literature, including literature published since the last review, is insufficient either to call into question the current level of the standard or to support any specific national standard for PM effects on materials (Samet, 2010c, p.23 ) . Nonetheless, with regard to both types of effects, CASAC noted the importance of maintaining an appropriate degree of control of both fine and coarse particles to address such effects, even in the current absence of sufficient information to develop a s tandard.

c. Summary of Proposed Decisions Regarding Other Welfare Effects Based on the above considerations and the advice of CASAC, at the time of proposal the Administrator provisionally concluded that it would not be appropriate to establish any distin ct secondary PM standards to address other non

- visibility PM

- related welfare effects, including ecological effects, effects on materials, and climate impacts. Nonetheless, the Administrator concurred with the conclusions of the Policy Assessment and CASAC advice that it is important to maintain an appropriate degree of control of both fine and coarse particles to address such effects. Noting that there is an absence of information that would support any different standards, the Administrator proposed
genera lly to retain the current suite of secondary PM standards 40 to address non

- visibility welfare effects. Specifically, the Administrator proposed to retain all aspects of the current secondary 24

- hour PM 2.5 and PM 10 standards. With regard to the secondary

40 As summarized in section VI.A and Table 1 above, the current suite of secondary PM standards includes annual and 24

- hour PM 2.5 standards and a 24

- hour PM 10 standard.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

440 ann ual PM 2.5 standard, the Administrator proposed to retain the level of the current standard and to revise the form of the standard by removing the option for spatial averaging consistent with this change to the primary annual PM 2.5 standard

. C. Public Comments on Proposed Decisions regarding Secondary PM Standards The EPA received a large number of comments on its proposed decisions with regard to secondary PM standards, with the large majority of those comments focusing on the proposal to set a distinct standa rd to protect against visibility impairment, discussed below in section VI.C.1. Very few commenters addressed the proposal to retain the existing secondary standards for non

- visibility welfare effects, discussed below in section
VI.C.2

. As di s cussed in sec tion VI.D. below, the Administrator has decided to retain the current suite of secondary PM standards generally, while revising only the form of the secondary annual PM 2.5 standard to remove the option for spatial averaging consistent with this change to t he primary annual PM 2.5 standard

. The Administrator has also decided, contrary to what was proposed, not to establish a distinct secondary standard to address PM

- related visibility impairment. T his section discusses EPA’s responses to the
comments EPA rec eived on its proposal, and t he rationale behind the Administrator’s
final decisions is discussed in section VI. D below

. 1. Comments on Proposed Secondary Standard for Visibility Protection

a. Overview of Comments Among those commenting on the proposal t o set a distinct secondary PM 2.5 visibility index standard, a large majority of commenters, including more than 25 state and local agencies; regional organizations such as NACAA, NESCAUM, and WESTAR; and industry commenters, such as ACC, API, BP, EPRI, NCB A, NEDA

- CAP, NMA,

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

441 NSSGA, and UARG, opposed setting a distinct secondary standard for visibility at this time. Many commenters in this group expressed the view that such a standard was not needed, primarily on the basis that adequate protection was provided by the existing 24

- hour secondary PM 2.5 standard. Some of these commenters also expressed legal concerns with the nature of the proposed standard. Other commenters in this group supported a distinct secondary standard for visibility in concept, but expres sed the view that it was premature to set such a standard pending collection of additional visibility preference study data and the resolution of a number of key technical issues. Support for setting such a distinct secondary standard for visibility at thi s time came from a second group of commenters, including the Department of the Interior (National Park Service), several states, the Mid

- Atlantic/Northeast Visibility Union (MANE

- VU), the National Tribal Air Association (NTAA), environmental organizations such as the Appalachian Mountain Club, National Parks Conservation Association, Earthjustice (AMC, et al.) and the League of Women Voters of Texas. These commenters argued that the existing secondary standards are not sufficiently protective of visual air quality, and that a distinct secondary standard similar to the proposed visibility index standard is both necessary and appropriate to ensure adequate protection of visibility. Commenters in both groups expressed concerns about various aspects of the
propo sed distinct secondary standard, including the indicator, averaging time, level, and form. In addition, a large number of commenters, including commenters from both groups, expressed concern and/or confusion over the relationship between the Regional
Haze Program and the proposed distinct secondary standard for visibility, raising issues such as analytical differences in methods between the programs, monitoring issues, and

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

442 other implementation challenges. A discussion of the significant comments outlined ab ove, including EPA’s responses to the comments, is presented here, with more detailed discussion in the Response to Comments document. Comments relating to the specific elements of the proposed standard

– indicator, averaging time, form and level

– are disc ussed in sections VI.C.1.b

- e, respectively. Comments related to the need for a distinct secondary standard at this time are discussed in section VI.C.f. Legal issues raised by commenters opposed to setting a secondary standard based on the proposed visibil ity index are discussed in section VI.C.g. Finally, comments related to the relationship between a distinct secondary standard and the Regional Haze Program are discussed in section VI.C.h. 41 While the EPA concludes in section VI.D below to retain the current suite of secondary PM 2.5 standards, the appropriateness of the protection that would be provided by the proposed
PM 2.5 visibility index standard, and the relationship between this degree of protection and that provided by the current secondary 24

- hour secondary PM 2.5 standard, are key elements in the Administrator's decision, and are discussed below. b. Indicator Numerous commenters, both those supporting a distinct secondary standard and those opposed to setting such a standard, expressed views on the suitability of utilizing a
PM 2.5 calculated light extinction indicator for the standard as proposed. While these groups of commenters differed in terms of their views on the appropriateness of using

41 Comments pertaining to implementation issues, which the Administrator may not consider in making decisions about setting national ambient air quality standards, are discussed in the Response to Comments document, as a re comments regarding monitoring issues related to the pro posed distinct visibility index standard.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

443 calculated PM 2.5 light extinction as the basis for the indicator rather than relying on direct measurements of PM 2.5 light extinction, commenters from both groups expressed concern over specific elements of the proposed method of calculating PM 2.5 light extinction. In particular, commenters expressed differing views on which IMPROVE algorithm should be utilized; whether it is appropriate to exclude coarse particles from the indicator; and whether the proposed protocols for incorporating data on rel ative humidity and PM 2.5 species are appropriate. 42
i. Comments on Calculated vs. Directly Measured Light Extinction The majority of commenters in both groups noted the uncertainties associated with relying on a calculated light extinction indicator and st ated a preference for utilizing direct light extinction measurements. However, recognizing the limitations on applying direct measurements at present, commenters supporting the proposal to set a distinct standard argued that relying on “calculated light ex tinction is a reasonable first approach” (DOI, p. 2). These commenters pointed to the advice of CASAC, which had acknowledged that it was not possible for the EPA to develop an FRM for direct measurement of light extinction within the time frame of this re view and had concluded that relying on a calculated PM 2.5 light extinction indicator represented a reasonable approach that could be implemented sooner than a directly measured indicator. These commenters generally supported the proposal to adopt a calcula ted PM 2.5 light extinction indicator, at least as an interim approach. Commenters opposed to setting a distinct standard generally argued that it was

42 Some commenters expressed concern about the omission of other contributors to visibility impairment from the visibility index, as discussed in the Respons e to Comments document.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

444 inappropriate to rely on a calculated light extinction indicator rather than direct measurements. Some of these commenters argued that the proposed calculated light extinction indictor is ill suited for a bright line standard because the method uses average humidity and a reconstructed visibility measurement calculated from PM 2.5 speciation filter analysis, ra ther than measuring what is actually observed by individuals. A number of commenters advocated postponing setting a distinct standard until an approach based on direct light extinction measurements can be adopted. Many of these commenters stated that relyi ng on direct light extinction measurements would enable a standard to be based on a shorter averaging time, either 1

- hour or sub

- daily (4 to 6 hours), consistent with the more instantaneous nature of perceptions of visual air quality and the advice of
CASA C in this review. The EPA generally agrees with commenters that an indicator based on directly measured light extinction would provide the most direct link between PM in the ambient air and PM

- related light extinction. However, as noted at the time of prop osal and in accordance with the advice of CASAC, the EPA has concluded that this is not an appropriate option in this review because a suitable specification of currently available equipment or performance

- based verification procedures could not be develop ed in the time frame of this review. Moreover, CASAC concluded that relying on a calculated
PM 2.5 light extinction indicator based on PM 2.5 chemical speciation and relative humidity data represented a reasonable approach . The inputs that are necessary incl ude measurements that are available through existing monitoring networks and approved protocols. Thus, the EPA remains confident that the available evidence demonstrates that a strong correspondence exists between calculated PM 2.5 light extinction and PM

- r elated

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

445 visibility impairment. Furthermore, CASAC agreed, noting that the proposed calculated
PM 2.5 light extinction indicator based on the original IMPROVE algorithm “appears to be a reasonable approach for estimating hourly light extinction” (Samet, 2010d , p. 11) and “its reliance on procedures that have already been implemented in the CSN and routinely
collected continuous PM 2.5 data suggest that it could be implemented much sooner than a directly measured indicator” (Samet, 2010d, p. iii). Thus it would not be appropriate to postpone setting a distinct secondary standard until an approach based on direct light extinction measurements could be adopted. ii. Comments on Specific Aspects of Calculated Light Extinction Indicator Some commenters, even those supporting the adoption of a calculated light extinction indicator, also expressed concern over specific aspects of the proposed indicator. First, a number of commenters expressed concern over the proposal to use the original IMPROVE algorithm as the basis for the calculated light extinction indicator. These commenters noted that the original IMPROVE algorithm has been shown to have consistent biases at both low and high levels of light extinction. In particular, these commenters expressed concern with the algorithm’s bias at higher levels of light extinction, which they pointed out were the conditions that might be encountered on hazier days in urban areas. Some commenters supported use of the new ( revised ) IMPROVE algorithm. These commenters noted that the revised equation has been through a peer review which confirmed that it is based on the best science and corrects the biases inherent in the original algorithm. Commenters also noted that this revised alg orithm has been widely incorporated into Regional Haze plans, and urged the EPA to use this same equation in

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

446 the visibility index for the sake of consistency: “EPA approved this approach for regional haze and does not dispute its greater accuracy. Therefor e, a national secondary ambient air quality standard based on criteria that accurately reflect the latest scientific knowledge logically should not revert to the original IMPROVE algorithm” (Oklahoma DEQ, p. 2). Other commenters noted that both the origina l and the revised IMPROVE algorithms were designed in support of the Regional Haze Program which is focused on largely rural Class I areas, and that neither algorithm is necessarily suitable for urban areas. Noting that the EPA has not thoroughly evaluated the applicability of either IMPROVE algorithm in urban areas, these commenters urged additional research to evaluate the suitability of either algorithm (or an alternative approach) in urban areas. Second, a number of commenters argued that exclusion of c oarse PM from the calculated light extinction indicator was inappropriate. These commenters noted that coarse particulate matter is an important contributor to visibility impairment in many areas, particularly in the western U.S., and that the levels of “a cceptable” visual air quality derived from the visibility preference studies reflected total light extinction due to the full mix of particles (including coarse PM) in ambient air. A few commenters noted that due to the exclusion of coarse particles, a “de ciview” calculated for purposes of the proposed PM 2.5 visibility index is inconsistent with the unit as conventionally defined under the Regional Haze Program. Other commenters, however, supported the proposal to exclude coarse PM from the calculated light extinction indicator, noting the important role that PM 2.5 plays in urban visibility and arguing it would be more difficult to control the contribution of coarse particle sources such as wind

- blown dust to urban visibility impairment.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

447 Third, some comment ers questioned why the EPA was proposing to rely on monthly average relative humidity (f(RH)) values when hourly humidity data are widely available, particularly in urban areas. One commenter argued that the EPA’s proposed approach involves “guessing relat ive humidity” rather than relying on accurate, readily available measurements (Oklahoma DEQ, p. 1). The commenter stated that since relative humidity is highly variable and weather dependent, the proposed approach “effectively undermines the capacity of th e prescribed monitoring regime to identify periods when
PM 2.5 adversely affects visibility.” Other commenters supported this view, noting that relative humidity can vary substantially even within a 24

- hour period, and that light extinction can be very sens itive to these changes. These commenters recommended that hourly or daily humidity measurements should be utilized in place of the proposed monthly average f(RH) values. Some commenters also recommended that the EPA should utilize a 90 percent relative hu midity screen rather than 95 percent cap for purposes of eliminating periods in which visibility impairment is due to rain or fog. These commenters claimed that under a 95 percent cap, both the average f(RH) values and the PM 2.5 visibility index values cou ld be inflated in locations frequently affected by fog and/or precipitation. These commenters preferred the approach of excluding hours with relative humidity above 90 percent on the grounds that this approach would eliminate foggy/rainy hours irrespective of the frequency of occurrence. The EPA does not agree with commenters who advocated using the revised IMPROVE algorithm. Both the original and the revised IMPROVE algorithms have been evaluated by comparing the calculated estimates of light extinction w ith coincident

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

448 optical measurements. As discussed above in section VI.B.1.a.i, t he revised algorithm was developed to address observed biases in the predictions using the original algorithm under very low and very high light extinction conditions

. 43 The new IMPROVE algorithm (Pitchford et al., 1997) incorporates additional terms to account for particles representing the different dry extinction and water uptake (f(RH)) from two size modes of sulfate, nitrate and organic mass, as well as adding a term for hygroscopic sea salt. There are also adjustments for the calculation of OM as 1.8*OC compared to 1.4*OC in the original algorithm to better account for the more aged PM organic components found in remote areas , with further modifications and additions to better account for differences in particle composition and aging in remote areas

. 44 However, the EPA does not believe that these same modifications and additions would necessarily be appropriate for calculating light extinction in urban areas. Instead, t he EPA considers the original algorithm to be suitable for purposes of calculating urban light

- extinction, although some adjustments may be appropriate for urban environments as well. The reasons why the original algorithm is suited to urban environments a re discussed further below, along with adjustments that the EPA believes are likely appropriate based on the current

43 The original algorithm is referred to as the IMPROVE algorithm because it was developed specifically to use the aerosol monitoring data generated at network sites and with equipment specifically designed to support the IMPROVE program and was evaluated us ing IMPROVE optical measurements at the subset of sites that make those measurements (Malm et al., 1994). The biases in this algorithm were detected by comparing light extinction estimates generated from the original IMPROVE algorithm to direct optical mea surements in a number of rural Class I areas. 44 Specifically, the revised IMPROVE algorithm incorporates additional terms to account for particles representing the different dry extinction and water uptake (f(RH)) from two size modes of sulfate, nitrate an d organic mass, as well as adding a term for hygroscopic sea salt. There are also adjustments for the calculation of OM as 1.8*OC compared to 1.4*OC in the original algorithm to better account for the more aged PM organic components found in remote areas.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

449 (limited) state of knowledge

. First, the EPA considers that the multiplier of 1.8 used to convert OC to OM in the revised IMPROVE algorithm is too high for urban environments. The EPA is aware that there has been considerable debate within the research community about the appropriate multiplier to use to best represent urban environments. As discussed in Appendix F of the Policy Assessment (U.S. EPA, 2011a) , the EPA used the SANDW ICH mass closure approach in the Urban Focused Visibility Assessment (U.S. EPA, 2010b) for purposes of calculating maximum daylight hourly PM 2.5 light extinction and evaluated which multiplier would produce 24

- hour results most similar to the SANDWICH approach using 24

- hour PM 2.5 organic carbon derived from the new Chemical Speciation Network (CSN) carbon monitoring protocol established in 2007

. 45 Analyses presented in Appendix F of the Policy Assessment indicate that a multiplier of 1.6 is most appropriate for purposes of comparing the hourly PM 2.5 light extinction with calculated 24

- hour extinction (see Appendix F, section F.6 for a full explanation). The EPA also considers this higher multiplier to be a better appro ach for urban CSN monitoring sites where the new measurements of organic carbon tend to be lower than those produced by the older NIOSH

- type monitoring protocol (Malm, 2011). A multiplier of 1.6 is now used to calculate OM from OC measurements at CSN sites. At the time of proposal, the EPA proposed to use the original IMPROVE algorithm with its 1.4 multiplier for converting OC to OM, but requested comment on

45 Starting in 2007, the CSN adopted the IMPROVE monitoring protocol for the measurement of organic and elemental carbon using the IMPROVE analytical method and an IMPROVE

- like sampler. The transition was completed in 2009. ( “ Modification of Carbon Procedu res in the Speciation Network,” http://www.epa.gov/ttn/amtic/files/ambient/pm25/spec/faqcarbon.pdf)

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

450 whether this value was appropriate

. C omments received by the Agency generally indicate that t he OC

- to

- OM multiplier of 1.4 used in the original IMPROVE algorithm is too low for urban areas. Based on the analyses presented in Appendix F of the Policy Assessment, the EPA agrees with these commenters. However, the EPA also believes that it would be ina ppropriate to use a multiplier as high as 1.8 to convert OC to OM in urban areas. As noted by commenters, the organic mass contribution to visibility impairment can be large, and generally OM is significantly larger in urban areas compared to surrounding r ural

areas. 46 Because a large portion of the organic component of urban PM results from nearby emissions sources, the total OM mass is generally closer to the measured organic carbon ( OC ) from which it is derived. This means it is appropriate to use a smaller multiplier to convert OC to OM in urban areas as compared to the value of 1.8 used in the revised algorithm, which is tailored to remote areas. The CASAC noted that urban OM includes fresh emissions and the EPA concluded in the Visibility Assessment that “the original version is considered more representative of urban situations when emissions are still fresh rather than aged as at remote IMPROVE sites” (U.S. EPA, 2010b, p. 3

- 19). Alth ough the new revised algorithm represents the best science of estimating extinction in
remote areas with its aged aerosol, the commenters did not address how the EPA should modify the new revised algorithm to best represent the more complex and different urb an aerosol, particularly for OM. However, recognizing In light of all of these considerations, in particular the analyses the EPA conducted for Appendix F of the Policy Assessment and the fact that the monitoring method for organic carbon has recently changed in the

46 The difference between higher PM 2.5 mass in urban areas compared to surrounding regions, known as the urban excess, is largely attributed to organic mass (U.S. EPA Particle Pollution Report, 2004).

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

451 CSN network, the EPA agrees with commenters judges that a multiplier of 1. 4 is likely too low, and 6 for urban areas would be most appropriate for purposes of calculating
PM 2.5 light extinction in urban areas. 47 In formulating this judgment, the EPA recognizes that a somewhat higher neither the original nor the revised IMPROVE algorithm has been tested for suitability in urban areas and that additional research is necessary to reduce the uncertainties about the most appropriate value would be appropriate. Based on the analyses the EPA conducted for Appendix F of the Policy Assessment, a for th e OC to OM multiplier of 1.6 for urban areas is judg ed to in urban environments

. With regard to other changes between the original and revised IMPROVE algorithms, the EPA also does not believe that it would be appropriate for data produced with the new CSN carbon monitoring protocol. to include a term for hy groscopic sea salt for urban light extinction, or to differentiate between different size modes of sulfate, nitrate, and organic mass as empirically defined by the revised IMPROVE algorithm. Unlike in some remote coastal locations, sea salt is not major co ntribut or to light extinction in urban areas . Mor e over, urban sources of salt include sanding of roads during the winter and those re - entrained particles are mostly in the coarse size range

. Like in remote areas, small and large size modes of sulfate, nitrate and organic mass would exist in the urban environment. However, the apportionment of the total fine particle concentration of each of the three P M 2.5 components into the concentrations of the small and large s ize fractions would likely need a different approach than that used for remote areas. This is because of the closer proximity of urban sources to their

47 The implications of this shift to a 1.6 multiplier for OC in urban areas for decisions about averaging time, level, and need for a distinct secondary standard are discussed further below in sections VI.C.1.c, V I.C.1.e, and VI.C.1.f, respectively.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

452 emissions

. This is a particular concern not only for organic mass, which as explained previously has a l arge contribution from nearby urban emission sources, but also for
P M 2.5 nitrate whose concentrations are also higher in urban areas compared to the surrounding regions. Thus, a higher portion of the total urban concentration may be in the small mode compa red to remote areas and thus a different apportionment algorithm would be needed. Finally, the EPA does not consider it necessary to employ site

- specific Rayleigh light scattering terms in place of a universal Rayleigh light scattering value for purposes

o f calculating light extinction in urban areas for purposes of calculating the 90 th percentile values. The site

- specific Rayleigh value is most important to accurately estimate extinction on the best visibility days which is an essential metric for the regional haze program. For all of these reasons, the EPA considers the original IMPROV E algorithm better suited to the task of calculating urban light extinction than the revised IMPROVE algorithm. However, the EPA does consider it appropriate to make certain adjustments to th e original algorithm for purposes of calculating urban light exti nction. As discussed above, the EPA believes it is appropriate to use a 1.6 multiplier to convert OC to OM in urban areas . In addition, the EPA believes it is appropriate to exclude the term for coarse particles from the equation. The EPA does not agree wi th commenters who suggested that coarse particles should be included in the calculated light extinction indicator. As noted in the proposal, PM 2.5 is the component of PM responsible for most of the visibility impairment in most urban areas. Currently avail able data suggest that PM 10

– 2.5 is a minor contributor to visibility impairment most of the time, although at some locations (U.S.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

453 EPA, 2010b, Figure 3

- 13 for Phoenix) PM 10

– 2.5 can be a major contributor to urban visibility effects. While it is reasonable to assume that other urban areas in the desert southwestern region of the country may have conditions similar to the conditions shown for Phoenix, in fact few urban areas conduct continuous PM 10

- 2.5 monitoring. This significantly increases the difficulty o f assessing the role of coarse particles in urban visibility impairment. For example, among the 15 urban areas assessed in this review, only four areas had collocated continuous PM 10 data allowing calculation of hourly PM 10

-

2.5 data for 2005 to 2007. In ad dition, PM 10

- 2.5 is generally less homogenous in urban areas than PM 2.5 : in that coarse particles particle concentrations exhibit greater temporal variability and a steeper gradient across urban areas than fine particles (U.S. EPA, 2009a, p. 3

- 72). This makes it more challenging to select sites that would adequately represent urban visibility conditions. Thus, while it would be possible to include a PM 10

– 2.5 light extinction term in a calculated light extinction indicator, as was done in the Visibility Assessment, there is insufficient information available at this time to assess the impact and effectiveness of such a refinement in providing public welfare protection in areas across the country (U.S. EPA, 2011a, pp. 4

- 41 to 4

- 42). Therefore, the EPA concludes that it is not appropriate to set a standard based on a calculated light extinction indicator that inclu des coarse particles at this time, and the calculated indicator should be based on
PM 2.5 light extinction. With regard to the suggestion by some commenters that the calculated light extinction indicator should be calculated using hourly humidity data, the EPA disagrees that concurrent humidity measurements should be used. The use of longer

- term averages for each monitoring site adequately captures the seasonal variability of relative humidity

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

454 and its effects of visibility impairment, and this approach focus es more on the underlying aerosol contributions to visibility impairment and less on the day

- to

- day variations in humidity. This provides a more stable indicator for comparison to the NAAQS and one that is more directly related to the underlying emissions that contribute to visibility impairment. With regard to the comments advocating the use of a 90 percent humidity screen as opposed to a 95 percent humidity cap, the EPA believes that relying on monthly average relative humidity values based on 10 years o f climatological data appropriately reduces the effect of fog and precipitation. Although the approach of using a 95 percent humidity cap, as in the Regional Haze Program, includes some hours with relative humidity between 90

- 95 percent, the general approa ch of using a longer

- term average for each monitoring site effectively eliminates the effect of very high humidity conditions on visibility at those locations. Therefore, taking all of the above considerations and CASAC advice into account, the EPA continu es to conclude that a calculated PM 2.5 light extinction indicator, similar to that used in the Regional Haze Program (i.e., using an IMPROVE algorithm as translated into the deciview scale), would be the most appropriate indicator to replace the
current PM 2.5 mass indicator for a distinct secondary standard. Moreover, the EPA continues to conclude that this calculated indicator should based on the original IMPROVE algorithm , adjusted to use a 1.6 OC multiplier and exclude the term for coarse particles, in c onjunction with monthly average relative humidity data (i.e., f(RH) values) based on long

- term climatological means as used in the Regional Haze Program. A PM 2.5 visibility index defined in this way would appropriately reflect the relationship between

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

455 ambi ent PM and PM

- related light extinction, based on the analyses discussed in the proposal and reflecting the aerosol and relative humidity contributions to visibility impairment by incorporation of factors based on measured PM 2.5 speciation concentrations an d climatological average relative humidity data. In addition, this type of indicator would address, in part, the issues raised in the court’s remand of the 2006 PM 2.5 standards. Such a PM 2.5 visibility index would afford a relatively high degree of uniform ity of visual air quality protection in areas across the country by virtue of directly incorporating the effects of differences in PM 2.5 composition and relative humidity across the country.

c. Averaging time Few commenters specifically addressed the iss ue of averaging time. Those who did generally expressed the view that an hourly or sub

- daily averaging time would be the most appropriate approach, as supported by CASAC and the EPA’s own analyses in this review. These comments were generally consistent wi th the emphasis among all commenter groups commenters on the desirability of adopting a directly measured light extinction indicator that could be measured on an hourly or sub

- daily time scale. Some commenters noted that a standard based on a 4

- 6 hour avera ging time would better capture peak daily light extinction while allowing stable signal quality; others urged EPA to adopt a 1

- hour averaging time in conjunction with direct measurements. Commenters pointed to significant limitations associated with using a 24

- hour averaging time, including the uncertainties in translating hourly or sub

- daily visibility index values into
24

- hour equivalent values. Some commenters criticized the analysis presented in the Policy Assessment comparing the 24

- hour calculated lig ht extinction values to the

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

456 maximum daylight 4

- hour calculated light extinction values. These commenters stated that the scatter plots and regressions presented in the Policy Assessment indicate there is considerable variation in the 24

- hour vs. 4

- hour rel ationship, and interpreted this to mean that 24

- hour light extinction values are a poor surrogate for 4

- hour values. For example, several industry commenters cited an analysis which noted that the correlation coefficient between the 24

- hour and 4

- hour valu es was as low as r 2 = 0.42 in Houston, and stated that the EPA was being overly “optimistic” in concluding that city

- specific and pooled r 2 values in the range of 0.6 to 0.8 showed good correlation (UARG, Attachment 2, p. 27). In addition, some commenters expressed concern over potential bias and greater uncertainty introduced by the inclusion of nighttime hours, noting that because relative humidity tends to be higher at night, inclusion of these hours could cause areas to “record NAAQS exceedances that ha ve no corresponding visibility impairment value” (UARG, p. 36). Commenters also emphasized the poor fit of a 24

- hour averaging time with the near instantaneous judgments about visibility impairment reflected in the visibility preference studies. Commenters also noted that there is greater hourly variation in PM concentrations and resulting visibility conditions in urban areas than in Class I areas; thus, while the Regional Haze Program uses 24

- hour IMPROVE data, the commenters stated that a shorter averagin g time is needed for an urban

- focused PM 2.5 visibility standard. Some commenters objected to a 24

- hour averaging time as unsupported by the record in this review: “Because the science the Administrator relies on for the other elements of the proposed visib ility standard is tied to short

- term exposures to visibility impairment, the EPA has no basis for promulgating a standard that uses a 24

- hour averaging time” (API, p. 43). These commenters claimed that while the EPA may not

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

457 have the information or infrastr ucture in place to allow the Agency to set a standard based on a 1

- hour or other sub

- daily averaging time, this does not justify moving to a 24

- hour averaging time. Among commenters supporting the proposed distinct secondary standard for visibility, many commenters recognized the limitations on monitoring methods and currently available data that led to the EPA’s proposal to adopt a standard based on a 24

- hour averaging time. Most of these commenters acknowledged that the lack of reliable hourly speciation data means that a 24

- hour averaging time is the only workable approach for a standard based on calculated light extinction. Commenters advocating a distinct secondary standard for visibility therefore generally supported the proposal to adopt a 24

- hour av eraging time, at least as an interim approach until a directly measured light extinction indicator could be adopted in the future. This approach was also supported by a few industry commenters who noted that since a visibility index standard would be based on data from the IMPROVE and CSN monitors, which operate on a 24

- hour basis with 1

- in

- 3 (or 1

- in

- 6) day sampling, “it is imperative that EPA retain a 24

- hour averaging time if a secondary visibility standard is promulgated” (API, Attachment 2, p. 9).
In r esponse to comments supporting a 1

- hour or sub

- daily (4

- to 6

- hour) averaging time in conjunction with a direct light extinction measurements, the EPA notes that, as discussed above in the response to comments on indicator, the Agency has concluded that a directly measured light extinction indicator is not an appropriate option in this review, independent of the decision on averaging time. Having reached the conclusion that a calculated PM 2.5 light extinction indicator would be most appropriate,

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

458 the EPA ha s next considered what averaging time would be most desirable for such an indicator. As noted in the proposal, the EPA has recognized that hourly or sub

- daily (4

- to 6

- hour) averaging times, within daylight hours and excluding hours with high relative humi dity, are more directly related than a 24

- hour averaging time to the short

- term nature of the perception of PM

- related visibility impairment and the relevant exposure periods for segments of the viewing public. Thus, the Agency agrees with commenters’ gene ral point that, as a starting premise, a sub

- daily averaging time would generally be preferable. However, as noted at the time of proposal and discussed above in section VI.B.1.c, important data quality uncertainties have recently been identified in assoc iation with currently available instruments that would be used to provide the hourly PM 2.5 mass measurements that would be needed in conjunction with an averaging time shorter than 24 hours. As a result, at this time the Agency has strong technical reserva tions about a secondary standard that would be defined in terms of a sub

- daily averaging time. The data quality issues which have been identified, including short

- term variability in hourly data from currently available continuous monitoring methods, effec tively preclude adoption of a 1

- hour averaging time in this review, given the sensitivity of a 1

- hour averaging time to these data quality limitations. Even with regard to multi

- hour averaging times, the EPA continues to conclude that the data quality conc erns preclude adoption of a sub

- daily averaging time. Moreover, analyses conducted for the Policy Assessment indicate that PM 2.5 light extinction calculated on a 24

- hour average basis would be a reasonable and appropriate surrogate for PM 2.5 light extinction calculated on a 4

- hour basis. The scatter plots

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

459 comparing 24

- hour and 4

- hour calculated PM 2.5 light extinction in the Policy Assessment (U.S. EPA, 2011a, Figures G

- 4 and G

- 5) do show some scatter around the regression line for each city. This was to be expected, since the calculated 4

- hour light extinction includes day

- specific and hour

- specific influences that are not captured by the simpler
24

- hour approach. Overall, however, in the EPA’s view, both the city

- specific and pooled 15

- city 2 4

- hour vs. 4

- hour comparisons show strong correlation between the two averaging times. Moreover, the 90th percentile design values calculated for 4

- hour vs.
24

- hour light extinction are much more closely correlated than are the values for individual days i n particular urban areas calculated using these two approaches. Thus, while the EPA agrees with commenters who pointed out the relatively low correlation between 4

- and 24

- hour values in cities such as Houston, the Agency points out that the correlations o f 90 th percentile values are much higher, particularly when one considers the average values across urban areas. In general, the 90th percentile values line up better and demonstrate closer to a one

- to

- one relationship. The EPA has conducted a reanalysis (Frank et al., 2012b) of the relationships between estimated 24

- hour and 4

- hour visibility impairment based on the variety of metrics discussed in Appendix G of the Policy Assessment that further supports this finding. The reanalysis more appropriately con sidered the uncertainty of the calculated 4

- hour values. It also considered the effect of changing the OC to OM multiplier used in urban areas with the new CSN monitoring protocol from 1.4 to 1.6. The revised analysis shows that , for all cases considered, the 24

- hour values are generally closer to the 4

- hour values than originally estimated. Since conclusions in the proposal about the relationship between 4

- hour and 24

-

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

460 hour values were drawn not just on the basis of the city

- specific results but also on th e more robust 90 th percentile pooled values, the EPA disagrees with commenters who state that the Agency was overly optimistic in considering 24

- hour values an appropriate surrogate for 4

- hour values. Also, it is appropriate to focus on the 90 th percentile design value comparison since the design values would determine attainment status and the degree of improvement in air quality that could be expected in areas instituting controls to meet the NAAQS. Therefore the EPA does not agree with commenters who sta te that a
24

- hour averaging time cannot serve as an appropriate surrogate for sub

- daily periods of visibility impairment. On the contrary, the EPA continues to conclude, on the basis of this analysis, that PM 2.5 light extinction calculated on a 24

- hour bas is is a reasonable and appropriate surrogate for sub

- daily PM 2.5 light extinction calculated on a 4

- hour basis. The EPA recognizes that the effect of adopting a 24

- hour averaging time may be to smooth out some of the hour

- by

- hour variability in visibility index values. (Indeed, this is true if we compare a 4

- hour averaging time to a 1

- hour averaging time as well.) Hour

specific influences which would be evident if an hourly or sub

- daily averaging time were to be used will be masked to some extent when those hours are averaged together with other hours. This means, in part, that a 24

- hour averaging time may effectively reduce peak values by means of averaging them together with other hours, which may have lower values. However, given the well documented varia bility in hourly visibility conditions, especially in urban areas, as noted by commenters, it is reasonable to assume that in some cases peak hours may be significantly influenced by atypical conditions, making it appropriate to adopt an averaging time tha t is sufficiently long to ensure that hour

- specific influences are balanced against more typical conditions. Perhaps even more

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

461 important is the concern that many peak hourly measurements may be significantly influenced by atypical instrument performance; t his reinforces the conclusion that it is appropriate to adopt a longer averaging time, to ensure that hour

- specific uncertainties are balanced against more robust measurements. Thus, in agreement with commenters who supported a daily averaging time, the
EP A concludes that a 24

- hour averaging time would be appropriate for a distinct secondary standard based on a calculated PM 2.5 light extinction indicator. d. Form The EPA received very few comments with regard to the proposal to adopt a 90 th percentile form, averaged over 3

- years, in conjunction with a PM 2.5 visibility index and a
24

- hour averaging time. One commenter stated that it was inappropriate to use a 90 th percentile form, noting that this would result in the exclusion of a minimum of 36 days of data annually. The commenter expressed particular concern that this proposed approach, in combination with a 24

- hour standard based on an unadjusted CPL, would not capture the worst visibility impairment and that this would undermine “the inten t of setting a meaningful secondary visibility standard” (AMC, et al., p. 2). Another commenter argued that the EPA had provided no scientific basis for why the 90 th percentile form was suitable, and claimed that the Agency was making “a somewhat arbitrary judgment that people’s welfare would be affected only if adverse urban visibility were to occur more than 10 percent of the time” (API, Attachment 2, p. 4). On other hand, a few commenters who appeared to generally support the proposal to use a 90 th percentile form advocated averaging the 90 th percentile values over longer time periods, arguing that averaging over only 3 years would not provide a stable

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

462 assessment of visual air quality in the West because this time period is insufficient to properly account for western drought and fire cycles. These commenters pointed to the approach in the Regional Haze Program of averaging visibility impairment over 5 years, and noted that even within this longer time period data can be significantly influenced by
h igh emissions during significant fire years. The EPA disagrees with all of these comments , noting first that a multi

- year percentile form offers greater stability to the air quality management process by reducing the possibility that statistically unusual indicator values will lead to transient violations of the standard. Furthermore, this approach is consistent with the approach taken in other NAAQS, including the current 24

- hour PM 2.5 NAAQS, which has a 98 th percentile form averaged over 3 years.

. With re gard to the comment opposing the 90 th percentile form as inappropriately excluding the worst visibility days, the EPA notes that there is a significant lack of information on, and a high degree of uncertainty regarding, the impact on public welfare of the number of days with visibility impairment over the course of a year. For example, the visibility preference studies used to derive the range of CPLs considered in this review offered no information regarding the frequency of time that visibility levels sho uld be below those values. Based on this limitation, the EPA concluded in the Policy Assessment that it would not be appropriate to consider eliminating all exposures above the level of the standard and that it was reasonable to consider allowing some numb er of days with reduced visibility. Recognizing that the Regional Haze Program focuses attention on the 20 percent worst visibility days (i.e., those at or above the 80 th percentile of visibility impairment), the EPA continues to believe, as noted in the p roposal, that a percentile well above the 80 th percentile would be

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

463 appropriate to increase the likelihood that all days in this range would be improved by control strategies intended to help areas attain the standard. Focusing on the 90 th percentile, which represents the median of the distribution of the 20 percent worst visibility days, could be reasonably expected to lead to improvements in visual air quality on the 20 percent most impaired days. Thus, the EPA has made a reasoned judgment based on a full consideration of the upper end of the distribution of visibility impairment conditions and continues to conclude that it is appropriate to focus on the 90 th percentile of visibility impairment values. With regard to comments requesting the EPA adopt a long er multi

- year averaging period for the 90 th percentile values, the EPA disagrees that it would be appropriate to average the 90 th percentile values over periods longer than 3 years. The EPA recognizes that a multi

- year percentile form offers greater stabil ity to the air quality management process by reducing the possibility that statistically unusual indicator values will lead to transient violations of the standard. Utilizing a 3

- year average form provides stability from the occasional effects of inter

- ann ual meteorological variability that can result in unusually high pollution levels for a particular year. The Agency h as adopted this approach in other NAAQS, including the current secondary 24

- hour PM 2.5 NAAQS, which has a 98 th percentile form averaged over 3 years

. However, adopting a longer multi

- year averaging period longer than 3 years would increase even more the number of days with visibility impairment above the target level of protection and would therefore reduce the pr otectiveness of a visibility the standard. Based on this the EPA does not believe it would be appropriate to average 90 th percentile values over a period as long as five years. Therefore, the EPA continues to conclude that a 90 th percentile form, averaged

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

464 o ver 3 years, would be appropriate, in conjunction with a calculated PM 2.5 visibility index light extinction indicator and a 24

- hour averaging time.

e. Level With regard to level, commenters focused on two main themes. First, a large number of commenters a ddressed the information available from the public preference studies with regard to the acceptability of various levels of visual air quality. These comments, which are discussed in subsection VI.C.1.e.i below, address the EPA’s use of visibility preferen ce studies as the basis for the selection of a range of appropriate levels for the Administrator to consider. Many commenters challenged the use of these studies as the basis for setting a distinct secondary standard, arguing that limitations in these stud ies rendered them an unsuitable and insufficient basis on which to establish such a standard. Second, commenters expressed different views as to what level(s) of a distinct secondary standard would be appropriate, if the EPA were to set such a standard. Th ese comments reflected consideration of the results of the public preference studies as well as analyses conducted in the Visibility Assessment and the Policy Assessment, as discussed in the proposal. Comments addressing the appropriateness of specific lev els are discussed in subsection VI.C.1.e.ii below. i. Comments on Visibility Preference Studies A majority of commenters expressed the view that the existing preference studies provide an insufficient basis for selection by the Administrator of an appropriate level of public welfare visibility protection for a national standard. These commenters high lighted a number of limitations and uncertainties (enumerated below) and uncertainties associated with these studies as support for this view. In contrast, other commenters felt

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

465 that despite certain limitations, these studies do provide a sufficient basis on which the Administrator can select an appropriate level of a standard to provide national public welfare visibility protection. The remainder of this section organizes and discusses these comments under four broad topic areas, including: (a) limitations and uncertainties associated with the visibility preference studies; (b) preference study methods and design; (c) use of preference study results for determining adversity; (d) the appropriateness of using regionally varying preference study results to se lect a single level for a national standard. (a) Preference Study Limitations and Uncertainties A large and diverse number of limitations and uncertainties associated with the visibility preference studies have been identified and discussed in the public comm ents. Many of these same limitations and uncertainties were also identified and discussed by the EPA in the various documents developed throughout this review such as the Integrated Science Assessment, Visibility Assessment, and Policy Assessment, as well as in the proposed rule (see section VI.b.2 at 77 FR 38975 and also at 38990).

. The most important and fundamental limitations and uncertainties will be discussed here in the preamble, while more specific, unique or detailed comments will be addressed in t he Response to Comments document. The primary or most frequent limitation (s) cited by many commenters relate relate s to the small number of preference studies that are available in this review. In particular, some commenters note that these preference stud ies cover just four locations, only three of which occur in the U.S., that the two studies conducted in Washington, DC were pilot studies, not full preference studies, and/or that three of the preference studies

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

466 were conducted in the West, while only one w as conducted in the East, providing only limited geographic coverage. Typically, these same commenters also pointed out that taken together, these limited studies only included a total of 852 participants, which they claimed was too small a sample size and unrepresentative nationally. These commenters thus concluded that there is insufficient information, both geographically and demographically, upon which to select a national level of a visibility index for purposes of visibility protection. In contrast, several commenters stated support for using the preference studies, concluding they provide an adequate basis, in spite of their limited nature. In particular, AMC et al. state: We believe that these studies provide sufficient results to inform setting a n ational visibility standard. While the number of studies is small, they do incorporate spatial variation and, in the case of Denver and Phoenix, varied populations.... EPA should have confidence, rather than uncertainty, in the fact that these studies used d ifferent methods and respondents and yield a range of 20

– 24 dv, with one outlier of 29. (AMC, et al., pp. 6

- 7) In response Regarding the first group of commenters , the EPA notes that it is well aware of the limited nature of the information, which it has described in great detail in the Integrated Science Assessment, Visibility Assessment, and Policy Assessment, as well as
in section VI.B.2 of the proposed rule (77 FR 38975 3897 3 ). The EPA further notes, however, that limited information does not preclude the Administrator from making judgments based on the best available science, taking into account the existing uncertainties and limitations associated with that available science. Thus, in reaching judgments based on the science, the Administrator appropri ately weighs the associated uncertainties. In this review of the PM standards, the EPA notes that The CASAC

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

467 supported this view and concluded that the available information provided a sufficient basis on which the Administrator could form a judgment about requisite PM

- related public welfare visibility protection . Specifically, CASAC stated “[t]he 20

- 30 deciview range of levels chosen by EPA staff as ‘Candidate Protection Levels’ is adequately supported by the evidence presented ” (Samet, 2 010b, p. iii). As d iscussed below, in determining an appropriate degree of protection for visibility, in the proposed rule (77 FR 38990), the Administrator has recognized and explicitly taken took into account the uncertainties and limitations in the science in determining an appropriate degree of protection when she proposed a level at the upper end of the recommended range. As discussed below, the Administrator continue s to be mindful of these uncertainties and limitations in reaching her final determinatio n regarding what constitutes an appropriate degree of protection with respect to visibility impairment

. With respect to the comments of AMC et al., the EPA agrees that these studies provide a sufficient basis to inform the Administrator’s judgments regar ding an appropriate level of protection from PM

- related visibility impairment , but she recognizes that these studies, which are the only studies before her, are a limited source of information . However, the EPA does not agree that the Washington, DC , resu lts represent an outlier

. Given the limited source of information, Rather, EPA believes these results should be included represent a reasonable 50% acceptability value for that type of urban

scene and thus are appropriately included in the range identified for the Administrator to consider

. Some commenters made the point that the EPA relied on much of this same evidence to reach the conclusion in 2006 that the information was too limited to allow

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

468 selection of a national standard. For example, API stated: [T]he bulk of the VAQ preference studies were available during the previous PM NAAQS review and were considered by the Agency in its establishment of the 2006 PM secondary NAAQS.... The Proposed Rul e does not mention this fact and does not explain why many of these same studies now compel EPA to propose this new secondary NAAQS....The Proposed Rule notes in passing that, since the last review of the PM NAAQS, ‘limited information that has become availa ble regarding the characterization of public preferences in urban areas has provided some new perspectives on the usefulness of this information in informing the selection of target levels of urban visibility protection.’ 77 Fed. Reg. at 38969/2. It is a s erious oversight that the Proposed Rule makes no attempt to explain what that information is or how it affects the interpretation of the VAQ preference studies. This ‘limited information’ is an apparent reference to information provided by Dr. Anne Smith. (API, p. 37) The EPA disagrees with these commenters. First, the EPA disagrees that it failed to distinguish between studies that were available in the previous review and the current review. The discussion in section IV VI .A.1 of the proposal specifically identifies the studies from Denver, Phoenix and British Columbia (77 FR 38967/2) as being considered in the last review. The EPA further disagrees with the implication that it is being circumspect about identifying the “limited information that has become available regarding the characterization of public preferences in urban areas.” Beginning in
section IV VI .A.3 of the proposed rule (77 FR 38969), the EPA was clear about what information, both preexisting and new, it relied upon in this review to inform its views and provide the basis for its proposal. In section VI.B.2, the EPA elaborates on the specific information, tools, methods and data which are considered in relation to the public preference studies, including the new information available since t he last review. As noted above and in the proposal, in addition to the substantial PM urban air quality information and analyses new to this review, there are three other noteworthy sources of information that have specifically “provided some new perspect ives on the

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

469 usefulness of” the preference studies “in informing the selection of target levels of urban visibility protection .” ” (77 FR 38969). They include: (1) results from additional urban visibility preference study experiments conducted for Washington , DC by Smith and Howell (2009) which added to the preference data for that location , provided proof that the 2001 study could be replicated, and shed light on the role of location in preference
responses 48 ; (2) an extensive a review and reanalysis of the urban visibility public preference studies from the four urban areas, including the newly available Smith and Howell (2009) experiments which examined the similarities and differences between the studies and evaluated the pote ntial significance of those differences on the study results 49 ; and (3) additional analyses conducted in response to questions raised by the CASAC, , including most importantly a logit analysis presented in (discussed in Chapter 2 and Appendix J of the Visibi lity Assessment (see Appendix J and discussion in Chapter 2) ) , which was requested, reviewed and approved by CASAC, which showed that each city’s responses represented unique and statistically different curves. 50 Taken together, these sources contributed t o an increase in the EPA’s current knowledge and understanding of the robustness of each survey study study ’s results, the appropriateness of comparing each study’s results to the others, and the key uncertainties relevant to data interpretation. In
additio n, in the last review the decision to not adopt a distinct secondary standard was

48 Smith, AE; Howell S (2009). An assessment of the robustness of visual air quality preference study results. CRA International. Washington, DC. http://yosemite.epa.gov/sab/sabproduct.nsf/B55911DF9796E5E385257592006FB73 7/$F ile/CRA+VAQ+Pref+Robustness+Study+3+30+09+final.pdf. 49 Stratus Consulting, 2009

. Review of Urban Visibility Public Preference Studies: Final Report

. EPA

- HQ

- OAR

- 2007

- 0492

- 0042. 50 Deck L., Lawson M. 2010

. Statistical Analysis of Existing Urban Visibility Preference Studies

. EPA

- HQ

- OAR

- 2007

- 0492

- 0089.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

470 remanded as contrary to law and failing to provide a reasoned explanation for the decision. As such it is not appropriate for purposes of comparison with the Administrator's judgment and reasoning in this review. (b) Preference Study Methods and Design In addition to the limitations and uncertainties noted above, many comments also asserted the methodologies used in the preference studies are fundamentally flawed. Many commenters cited some of the same issues that have already been identified by the
EPA as sources of uncertainty and potential factors in producing the statistically different study results (see section VI.B.1.b above). As noted above, the EPA is well aware of the issues raised regarding the adequacy of the preference studies to serve as a b asis for a secondary NAAQS (see 77 FR 38975) and solicited comment on how these uncertainties should be considered (see 77 FR 38990). Most of these same commenters also pointed to an assessment of the preference studies methodology provided by Smith and Ho well (2009) and others as the basis for their views, as indicated by the following comments: Smith and Howell (2009) show that VAQ preference study outcomes are malleable and depend entirely on the design of the study. Accordingly, such studies do not iden tify any meaningful threshold of acceptable visibility conditions. Despite Smith and Howell’s conclusions, EPA continues to assert that the VAQ preference studies can be used to identify minimally acceptable visibility conditions even though the Agency has never provided any valid scientific basis for discounting the Smith and Howell (2009) results. (API, p. 38) Well

- controlled preference studies discussed by Anne Smith of Charles River Associates at the March 2010 CASAC meeting demonstrated that the judgm ent of panel members was affected by the order in which photographs were presented and tendency to identify the middle of the range of visibility degredation as a threshold of acceptability. This points to a potential flaw in these studies and that artifac ts caused by these tendencies may have influenced study results. Dismissing these inherent flaws in the existing preference studies and then using these studies to set a secondary NAAQS is arbitrary and capricious. (API, Attachment 2, p. 12)

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

471 EPA also fai ls to acknowledge that the only study conducted since the last review rebuts the validity of the VAQ preference studies previously conducted. (UARG, Attachment 2, p. 28) As is explained in a more detailed discussion in the Response to Comments document, the EPA disagrees that the analysis study conducted by Smith and Howell (2009) supports the conclusion that the preference study methodologies were fundamentally flawed

. ; however , the EPA notes that their experiments do identify areas where additional resea rch would be useful to further inform our limited understanding of public preferences in urban areas

. T h e EPA views the Smith and Howell experiments as increasing the EPA’s knowledge and understanding of the findings of the 2001 Washington, DC focus group pilot study (Abt, 2001) in several important ways

. , although this information still remains limited overall

. Specifically, the Smith and Howell results suggest: 1) the 2001 s results, while based on a small sample size of 9, may be were consistent with resu lts from a fair representation for the preferences for larger sample of the general Washington, DC population; 2) an individual’s preferences for visibility in one location may not depend on whether they live in that location; and 3) presentation method (i. e., changing from slide projection to computer monitor) does did not appear to affect the reported preferences. The EPA provides detailed responses to specific comments on the design of the Smith and Howell experiments in the Response to Comments document. (c) Preference Study Results and Adversity A number of comments were received regarding the EPA’s use of preference study results to make the determination that adverse PM 2.5 - related visibility effects on the

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

472 public welfare are occurring. In this context, sev eral commenters questioned whether the EPA had made the case that unacceptable levels of visual air quality based on preference study results alone can be equated with an adverse public welfare effect. These commenters suggested that unless preference stud y information is linked to personal comfort and well

- being or other associated welfare effects, it cannot form the basis of a determination of adversity. For example, Kennecott Utah Copper LLC stated that: Thus, EPA seemingly was building the foundation f or a determination of what constitutes an adverse effect on visibility in the context of public welfare. However ... EPA subsequently veered toward an oversimplified focus on public acceptance of visibility conditions.... EPA’s discussion of visibility in the Policy Assessment and its proposed rule in the Federal Register focuses entirely on “acceptable” and “unacceptable” visual air quality and make no mention of an “adverse effect” in the context of visibility. EPA’s reliance on only 3 urban PS preference stud ies represents a paucity of data and a wholesale abandonment of any effort to seek a scientifically measurable adverse effect. (Kennecott Utah Copper LLC, p. 26) In response, the EPA first notes that the definition of effects on welfare included in section 302(h) of the CAA identifies both visibility and the broader category of effects on personal comfort and well

- being as effects on welfare. In setting a secondary standard to address visibility impairment, the EPA considers the effect on the public from
im pairment of visibility as a separate and distinct welfare effect in its own right. The EPA is not required to translate this into terms of personal comfort and well

- being, as visibility impairment is designated explicitly by Congress as an effect on welfar e. While there may be a large degree of overlap among these different welfare effects, the EPA properly focuses on evaluating all of the information before the Agency on the effect visibility impairment has on the public, whether or not this impairment wou ld also be categorized as having an adverse effect on personal comfort and well

- being. It is in the context of all

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

473 of this information that the EPA makes the judgment as to the appropriate degree of protection from known and anticipated adverse effects on the public from visibility impairment. The EPA recognizes that there is uncertainty about the degree of adversity to the public welfare associated with PM

- related visibility impairment. However a secondary standard is designed to provide protection from "k nown or anticipated" adverse effects, and a bright line determination of adversity is not required in judging the requisite degree of protection under section 109(b)(2). Furthermore, the EPA disagrees that it has abandoned its consideration of visibility

- re lated impacts on the welfare effect of personal comfort and well

- being, as is made clear in the following quote: Research has demonstrated that people are emotionally affected by low visual air quality, that perception of pollution is correlated with stres s, annoyance, and symptoms of depression, and that visual air quality is deeply intertwined with a ‘‘sense of place,’’ affecting people’s sense of the desirability of a neighborhood (U.S. EPA, 2009a, section 9.2.4). Though it is not known to what extent th ese emotional effects are linked to different periods of exposure to poor visual air quality, providing additional protection against short
- term exposures to levels of visual air quality considered unacceptable by subjects in the context of the preference studies would be expected to provide some degree of protection against the risk of loss in the public’s ‘‘sense of well
- being.’’ (77 FR 38973/1, emphasis added) The approach taken to address such qualitative, but policy

- relevant, information in this revie w is the same as in other NAAQS reviews. The review is initiated with a comprehensive assessment of all possible public health and welfare effects associated with PM in the Integrated Science Assessment. Then policy

- relevant effects for which there is suff

icient quantitative information to allow a determination of the change in risks associated with incremental changes in air quality are assessed (in this review, in the Visibility Assessment) and used to provide a quantitative basis to inform the selection of an appropriate range of levels for further consideration in the Policy Assessment. In the

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

474 Policy Assessment, the EPA considers all important policy

- relevant evidence and information, both quantitative and qualitative, in making recommendations regarding the range of policy options appropriate for the Administrator to consider. It is in the context of all of this information that the Administrator makes her final judgment as to the appropriate degree of protection from known and anticipated adverse effect s on the public from visibility impairment. The EPA recognizes that there is uncertainty about the degree of adversity to the public welfare associated with impairment of VAQ. However a secondary standard is designed to provide protection from "known or an ticipated" adverse effects, and a bright line determination of adversity is not required in judging the requisite degree of protection under section 109(b)(2). Another issue raised in the comments regarding adversity is the EPA’s decision to use the 50 per cent acceptability criterion from the public preference studies in determining candidate protection levels of visibility impairment . In particular, some of these commenters questioned the appropriateness of using the preference study 50 percent acceptabili ty criteria results as a basis for the selection of a national level of visibility protection. For example, AMC et al. recommended “a 75% acceptability criterion as a target that is in line with protecting the broader public from the negative effects of vi sibility impairment” (AMC, et al., p. 9). In the Visibility Assessment, the EPA noted that the use of the 50 percent acceptance level for urban visibility was first presented in Ely et al. (1991) (U.S. EPA, 2010b, p. 2

- 5). Ely discussed the use of the 50 p ercent acceptability criterion as a reasonable basis for setting an urban visibility standard. The standard was determined based on a 50% acceptability criterion, that is, the

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

475 standard was set at the level of extinction that would divide the slides into tw o groups: those judged acceptable and those judged unacceptable by a majority of the people in the study. The criterion is politically reasonable because it defines the point where a majority of the study participants begin to judge slides as representing unacceptable visibility. It is also consistent with psychological scaling theory which indicates that a “true score” exceeds a standard when more than 50% of the “observed scores” exceed that standard. (Ely et al., 1991, p. 11) As Ely described, the 50 pe rcent acceptability criterion and the preference study conducted by Ely were used as the basis for setting the level of the Denver Visibility Standard in 1990. The Th at same criterion was used judged appropriate and selected for

use in the Phoenix preference study (BBC research, 2003) and was also used as the basis for setting the level of the Phoenix Visibility Standard in 2003. Most recently, the 50 percent acceptability criterion has been recommended by the British Columbia Visibility Coordinating Committee as the basis for the visibility standard cu rrently under consideration by British Columbia, Canada. Thus Furthermore, CASAC supported this approach, while recognizing the uncertainty associated with this issue. Specifically, CASAC agreed that “the 50 th percentile for the acceptability criteria is lo gical, given the noted similarities in methodologies employed in the 4 study areas. ... In terms of choosing a specific percentile from the preference studies, we note that there may not be a “preferred” one, but in assessing preference studies to propose a PM secondary NAAQS, the 50 th percentile is sufficient, as it is the basis for existing visibility indexes used in the Denver/Colorado Front Range and Phoenix metropolitan areas” (Samet, 2009c, pp. 8

- 9). Therefore , after considering the information that ser ved as the original basis for its selection as described in Ely et al., 1991, and given its acceptance and use in existing visibility programs and the absence of information to support a different acceptability criterion , the EPA continues to conclude , con sistent with the advice of CASAC, that it is

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

476 reasonable to use the 50 percent acceptability criterion in determining target levels of protection from visibility impairment. (d) Appropriateness of using regionally varying preference study results to select a single level for a national standard. A number of commenters raised concerns regarding the bases for and implications of the differences observed in the preference study resu lts, concluding that these results were due to regionally varying factors and thus could not be used to set a national standard. In particular For example , some commenters asserted that because the confidence intervals around the four 50 percent acceptabili ty levels do not overlap at all, and because there are variations in preference study designs and inherent differences of in the visual setting among cities and panels, the four preference curves and their associated 50 percent dv values are city

- specific a nd statistically different. The commenters asserted conclud ed , therefore, that it was inappropriate to aggregate the 50 th percentile dv values from multiple studies and concluded that they should instead be evaluated individually. Other commenters expresse d the related view that the preference study results cannot be used to set a national standard for visibility impairment because the results show that visibility preferences vary regionally. For example, API stated that: The ‘one

- size

- fits

- all’ approach...i s not viable because it does not account for regional and city

- specific factors that have been made evident in the disparity of preference study data.... It is well known, for example, that the level of light extinction to which people in different areas of the country are accustomed, as well as the urban setting, are the primary factors that affect a person’s visual perception of an urban vista. Thus, the degree to which extinction threshold can be related to human welfare is inevitably regionally

- dependent. (API, Attachment 2, p. 4) Some of these commenters argued that because acceptable visual air quality is regionally

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

477 dependent, it would be more appropriate to develop distinct visibility standards at the state or local level. Others pointed out that areas lacking which lack “important visibility vistas” might not need standards at all, since flat areas without significant terrain have a limited maximum visual range (NEDA/CAP, p. 3). Other commenters stated that due to regionally varying factors, such as rel ative humidity, it is not possible to select a single level for a national standard to protect visibility across the United States

. In particular, these commenters pointed to differences between Eastern and Western areas, arguing that a single national standard could not offer the appropriate degree of protection in locations with distinct characteristics. For example: [T]he proposed method falls short because it is not temporally or geographically representative enough to have any meaning.... The uncertai nty evidenced in these studies and the non

- uniformity between the western and eastern vistas makes it impossible at this time to set an acceptable light extinction value that would appropriately address visibility concerns in non

- Class I areas. (New York D OH/DEC, pp. 5

- 6) The EPA agrees that the preference curves and the 50 percent dv levels are separate and distinct data points representing four different VAQ preference curves for four unique urban scenes. However, the EPA does not agree that the study res ults necessarily support a conclusion that preferences are regionally dependent, nor does the
EPA consider the fact that the four curves are distinct as a weakness of the approach or a reason that the results cannot be compared. In addition, the EPA does n ot agree that the study results necessarily support a conclusion that preferences are regionally dependent

. In particular, the EPA notes that the results of Smith and Howell (2009) which show that participants in Houston and Washington, DC did not have sig nificantly different views on

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

478 acceptable air quality in Washington, DC, provide limited support for the conclusion that people’s preferences differ less because of where they live and more because of the scene they are viewing. Indeed, these preference stu dy results seem to substantiate what is already known, On the other hand, the existing literature indicates that people’s preferences for VAQ depend in large part on the characteristics and sensitivity of the scene being viewed. It is to be expected that people looking at different scenes would have different preferences, measured using the 50% acceptability criterion, depending on how changes in VAQ impact the various scenes. Therefore, it might also be the case that if the same scene was shown to partic ipants in different regions of the country, a single preference curve could be drawn. The preference study results showing that people find less air quality degredation acceptable for sensitive scenes than for less sensitive scenes could be interpreted as validating the appropriateness of the approach for eliciting public preferences. It is the EPA’s view that the differences in the preference study results appropriately reflect the different sensitivities to visibility impairment among the four urban scene s and thus provide useful information about how people’s preferences for visual air quality would change when viewing urban scenes of differing sensitivities. The EPA understands there is a wide variety or range of urban scenes within the United States. T hese sensitive urban scenes include those with natural vistas such as the Colorado Rocky Mountains as well as those with iconic man

- made urban structures like the Washington Monument. The EPA believes that the scenes presented in the four urban areas inclu de important types of sensitive valued urban scenes and therefore, when considered together, can inform the selection of a level of acceptable urban VAQ at the

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

479 national scale, taking into account the variation across the country evidenced in the studies. T his is discussed further in the next section, below. The EPA agrees does agree with commenters that there are factors that vary regionally varying factors that are important to take into account when setting a national standard for visibility protection. Se ction VI.A above regarding the history of the secondary PM NAAQS review discusses the evolution of the EPA’s understanding regarding the regional differences in PM concentrations, relative humidity and other factors. As a result, the current review has gon e to great lengths to address these factors, leading to the EPA’s proposal to use the IMPROVE algorithm to calculate light extinction in order to take into account the varying effects of relative humidity and speciated PM. While this approach does not resu lt in a uniform level of ambient PM 2.5 , it does ensure a nationally uniform level of visibility protection. The EPA refers the reader to other sections of the final rule, including sections VI.B.1.a, VI.B.1.c, VI.C.1.b and VI.C.1.f, and the Response to Comments document for a more detailed res ponse as to how it is taking these variables into account. ii. Specific Comments on Level The EPA received relatively few comments endorsing a specific level for a distinct secondary standard for visibility. In general, commenters who opposed setting a di stinct secondary standard at this time did not address the question of what level would be appropriate if the EPA were to set a distinct secondary standard for visibility; similarly, commenters who supported adopting a distinct secondary standard at this t ime generally did not recommend a specific level. However, a few commenters did provide comments in support of a specific level or range of levels, with some commenters

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

480 advocating standards at the upper end of the range of proposed levels (i.e., 30 dv), wh ile others supported levels below the lower end of the proposed range (i.e., below 28 dv). As discussed above, a large number of commenters argued that the currently available data are insufficient to determine what constitutes a standard that would be

ne ither more nor less protective than necessary and that no standard should be set at this time. These commenters pointed to the limitations and uncertainties in the preference studies discussed above as the basis for this claim. These commenters pointed to significant variation in the results of the preference studies in support of their arguments that the studies should not be used to derive a level for a distinct secondary standard for visibility. For example, one consultant cited by several industry comme nters argued that the proposed level of 28 or 30 dv did not reflect the substantial difference in visibility preferences between the East and the West reflected in the preference studies (UARG, Attachment 2, p. 11), and that it did not reflect the full ran ge of preferences (i.e., potential 50 percent acceptability levels) likely to exist nationwide (UARG, Attachment 2, p. 19). This commenter further objected to the EPA’s proposal for a level of 28 or 30 dv on the grounds that the EPA had inaccurately adjust ed 4

- hour values into 24

- hour values. Based on his analysis, the consultant concluded that “a range of adjusted values from 28 to 32 dv is needed” to account for the majority of the spread between the 4

- hour vs. 24

- hour equivalent values at the upper end o f the distribution of values. A number of commenters questioned whether the proposed range of levels was appropriate. One industry commenter claimed that the EPA had not explicitly justified why a standard within the proposed range was requisite, stating that “EPA makes no attempt to explain how the proposed level of the standard is neither lower nor higher than

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

481 necessary to protect public welfare” (NSSGA, p. 15). Arizona DEQ noted that since the proposed calculated light extinction indicator excluded coar se particles and Rayleigh scattering, the proposed levels of 28 or 30 dv were inconsistent with the visibility preference studies, which considered total light extinction. Noting these perceived problems with the proposed range of levels, a few commenters noted that if the EPA were to set a distinct secondary standard, the level should be set no lower than 30 dv, “to account for inconsistent value judgments, a great deal of spatial and temporal variability, and a very high level of uncertainty” (Texas CEQ, p. 7). In contrast, some commenters supporting the EPA’s proposal for a distinct secondary standard for visibility stated that the proposed range of levels from 28

- 30 dv was insufficiently protective based on a 24

- hour averaging time, and recommended a lo wer level for the visibility index standard. These commenters expressed the view that the proposed levels of 28 or 30 dv represented neither adequate surrogates for equivalent
4

- hour values, as the EPA claimed, nor sufficiently protective levels based on r ecent air quality data. Several commenters stated that the EPA’s own analyses suggested that a standard set at a level of 28 or 30 dv was insufficiently protective based on a 24

- hour averaging time. One commenter emphasized that the Policy Assessment had i ndicated a level between 25

- 28 dv was appropriate for a standard calculated on a 24

- hour average, and encouraged the EPA to adopt a standard level of 25 dv. Several environmental groups provided comments stating that a 24

- hour average would underestimate a 4

- hour value by 13

- 42 percent and certain areas of the country

— particularly the Northeast

— would be affected disproportionately. These commenters suggested that a 24

- hour PM 2.5 visibility index standard should be set at a level of 18.6

- 20 dv. The Departmen t of the

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

482 Interior pointed to recent air quality data indicating that visibility on the 20% worst days in several large metropolitan areas, including Birmingham, Fresno, New York City, Phoenix, and Washington, DC was below 29 dv. While noting that these cal culations were based on IMPROVE calculations which include contributions from coarse PM mass, DOI expressed the view that the proposed level of 28 to 30 dv would not provide adequate visibility protection compared to the current 24

- hour PM 2.5 standard of 3 5 μg/m 3 and recommended that the standard be set at a level of 25 dv consistent with the results of the Phoenix visibility preference study. In contrast, the states of Arizona and Colorado submitted comments arguing that the visibility preference studies conducted in Phoenix and Denver, respectively, were designed to address a specific local problem and that the results of these studies were not an appropriate basis for selecting the level of a national standard. For example, Arizona DEQ noted: The cited studies were conducted considering total light extinction; including extinction resulting from particulate matter and Rayleigh scattering. Visibility impairment due to coarse particulate matter can be an important contributor in Arizona, specifically in th e Phoenix area where ongoing measurements have been made. Therefore, ADEQ believes that the proposed levels of the secondary visibility standard are inconsistent with applicable urban studies. (Arizona DEQ, p. 2) Similarly, the Colorado Department of Publi c Health and the Environment noted that the Denver visibility standard was designed to address “brown clouds”, i.e., strong inversions that occur in the Denver metropolitan area, and that this standard “is based on a specific view of Denver” associated wit h particular sight paths and direct measurement methods. The commenter stated that this standard “is applicable only to this location,” and that these limitations make it potentially unsuitable for application as “a national secondary

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

483 standard, particularl y a proposed standard that does not use a direct measurement method” (Colorado DPHE, p. 2). While acknowledging the uncertainties and limitations associated with the visibility preference studies as discussed above, the EPA continues to conclude, as did
C ASAC, that the preference studies are appropriate to use as the basis for selecting a target level of protection from visibility impairment. However, the EPA agrees with commenters who emphasize the high degree of variability in visibility conditions and t he potential variability in visibility preferences across different parts of the country. In light of the associated uncertainty, as noted in the proposal, the Administrator judged it appropriate to establish a target level of protection equivalent to the upper end of the range of Candidate Protection Levels (CPLs) identified in the Policy Assessment and generally supported by CASAC. Thus, the EPA proposed to set a 24

- hour visibility index standard that would provide protection equivalent to the protection afforded by a 4

- hour standard set at a level of 30 dv. In light of the comments received on the proposal, in particular comments emphasizing the uncertainty and variability in the results of the public preference studies, the EPA continues to conclude that this approach is warranted, and that it is appropriate to set a target level of protection equivalent to the protection that would be afforded by a 4

- hour, 30 dv visibility index standard. Moreover, t he EPA disagrees with commenters who argued that the EP A’s approach for translating 4

- hour CPLs into equivalent 24

- hour values was inappropriate. In adjusting 4

- hour values for purposes of defining an appropriate level for a 24

- hour standard, the EPA noted at the time of proposal that there were multiple appro aches for estimating generally equivalent levels on a city

- specific or national basis , and that both of

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

484 the two preferred approaches identified in the Policy Assessment . While expressing the view that it was appropriate to consider the two approaches with the highest r 2 values (Approaches A and B in Appendix G of the Policy Assessment), 51 which used regressions of 90 th percentile light extinction values, the EPA determined it would also be appropriate to consider the city

- specific estimates resulting from Ap proaches C and E which showed greater variability than the aggregated estimates. Approaches C and E generated a range of city

- specific estimates of generally equivalent 24

- hour levels that encompassed the range of levels considered appropriate for 4

- hour CPLs, including the CPL of 30 dv at the upper end of that range. This led to the conclusion that it would be appropriate to use This information provided support for using the same CPL for a 24

- hour standard as for a
4

- hour standard , since no single approa ch could generate an equivalent 24

- hour standard level in each urban area for each CPL. The EPA continues to conclude, as it did at the time of proposal, that using an unadjusted 4

- hour CPL for purposes of establishing a target level of protection for a 24

- hour standard . The EPA disagrees with commenters who argued that the EPA’s approach for translating 4

- hour CPLs into equivalent 24

- hour values was inappropriate. is appropriate because this approach places more emphasis on the relatively high degree of s patial and temporal variability in relative humidity and fine particle composition observed in urban areas across the country , consistent with EPA’s reanalysis discussed below

.

51 In particular, EPA staff expressed a preference for Approach B in the Policy Assessment. However, in light of the additional information provided by the other approaches explored in Appendix G of the Po licy Assessment and the reanalysis in Frank, et al. (2012b), the EPA judges it more appropriate to consider the range of values resulting from all five analytical approaches for purposes of informing decisions about the equivalent level of a 24

- hour standard.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

485 The EPA has conducted a reanalysis (Frank et al., 2012b) of the relationships between estimated 24

- hour and 4

- hour visibility impairment based on the variety of metrics discussed in Appendix G of the Policy Assessment. The reanalysis has more appropriately considered the uncertainty of the calculated 4

- hour values. The revised analy sis shows that , for all cases considered, the 24

- hour equivalent level is

even generally closer to the 4

- hour values value at the upper end of the range of CPLs than originally estimated , demonstrating as can be seen in the results for Approaches B, C, and D

. 52 For example, the reanalysis indicates that Approach B yields an adjusted 24

- hour CPL of 29 dv 53 as generally being equivalent to a 4

- hour CPL of 30 dv , while Approach C yields a 24

- hour equivalent CPL of 29 averaged across cities and a range of city

- specific values from 25

- 36 dv

. 54 , 55 Not only are the 90 th percentile and pooled average values closer to the 4

- hour CP L of 30 dv , the range of city

- specific results shows a wider spread that clearly encompasses the unadjusted 4

- hour value of 30 dv near the midpoint of the city

- specific range. This provides support for concluding that the EPA’s approach to translating of 4

- hour CPLs into equivalent 24

- hour values was appropriate , and that it is appropriate to use unadjusted 4

- hour values for purpose s of selecting a level

52 Approach E as presented in the Policy Assessment is based on the median values for each city ; these results are not affected by the regression analyses. Therefore, Approach E was not included in the reanalysis, and the results remain unchanged from those reported in the corrected Table G

- 6 as reported in Frank, et al., 2012b. 53 In Appendix G of the Policy Assessment, a 24

- hour ad justed CPL of 28 was estimated to be equivalent to a 4

- hour value of 30 under Approach B

. 54 In Appendix G of the Policy Assessmen t, a 24

- hour adjusted CPL of 27 dv was estimated to be equivalent to a 4

- hour CPL of 30 dv when averaged across cities, while city

- specific values were estimated to range from 24

- 30 dv. 55 In the reanalysis, Approach D (all days pooled regression) generated results of 28 dv for the 24

- hour CPL equivalent to a 4

- hour value of 30 dv as compared to a value of 27 dv in the original analysis described in Appendix G.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

48 6 for a standard based on a 24

- hour averaging time

. 56 Moreover, the EPA disagrees with commenters who argue that the currently available evidence is sufficient to justify establishing a target level of protection at 25 dv or below. The EPA recognizes that 25 dv represents the middle of the range of 50 percent acceptability levels from the 4 cities studied, and represents the 50 percent acceptability level from the Phoenix study, which the Agency has acknowledged as the best of the four

s tudies in terms of having the least noise in the preference study results and the most representative selection of participants

.. . The EPA also notes the caveats discussed in the proposal regarding whether it would be appropriate to interpret results from the western studies as generally representative of a broader range of scenic vistas in urban areas across the country. The Policy Assessment noted significant differences in the characteristics of the urban scenes used in each study, with western urban vis ibility preference study scenes including mountains in the background and objects at greater distances, while scenes in the eastern study did not. Since objects at a greater distance have a greater sensitivity to perceived visibility changes as light extin ction changes compared to otherwise similar scenes with objects at a shorter range, this likely explains

56 The analysis in Appendix G of the Policy Assessment used the 4

- hour light extinction value treat ed as the independent (x

- axis) variable in an ordinary least squares regression. T he EPA now concludes that this regression approach was inappropriate because that variable has error and in fact may be more uncertain than the calculated 24

- hour extinction values. The Frank et al. (2012b) reanalysis uses an orthogonal regression instead of ordinary least squares regression and results in slopes closer to the 1:1 line for all the results, particularly for Dallas, TX. Furthermore, consistent with the EPA’s con clusion that a higher multiplier for converting OC to OM would be appropriate (see section VI.C.1.b.ii above), the reanalysis substitutes a 1.6 multiplier for converting OC to OM in the calculation of 24

- hour values instead of the value of 1.4 that was use d in calculating 24

- hour values for Appendix G. The higher multiplier is more consistent with the SANDWICH approach used to calculate the 4

- hour values found in Appendix G. See Frank et al. (2012b) for a more detailed explanation.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

487 part of the difference between the results of the eastern study and results of the western studies. In the proposal, the EPA noted that the scenic vist as available on a daily basis in many urban areas across the country generally do not have the inherent visual interest or the distance between viewer and object of greatest intrinsic value as in the Denver and Phoenix preference studies. Also, the Agency takes note of the caution expressed by Colorado and Arizona about using the results of the Denver and Phoenix preference studies, which were aimed at addressing specific local visibility problems, to inform the choice of level for a national standard. Ther efore, the Agency considers it reasonable to conclude, especially in light of the significant uncertainties, that it is appropriate to place less weight on the western preference results and that the high CPL value (30 dv) that is based on the eastern pref

erence results is likely to be more representative of urban areas that do not have associated mountains or other valued objects visible in the distant background. These areas would include the middle of the country and many areas in the eastern U.S., as we ll as some western areas. As a result, the EPA concludes that it is more appropriate to establish a target level of protection at the upper end of the range of 24

- hour CPLs considered, recognizing that no one level will be "correct" for every urban area in the country. In considering the upper end of this range, the EPA must identify a target level of protection that is considered requisite to protect public welfare from a national perspective, recognizing that the same target level would apply in all locat ions. Making this judgment requires a balancing of the risks to the public welfare and the substantial uncertainties surrounding appropriate levels of visibility protection. As acknowledged in the proposal, the EPA recognizes that setting a target level of protection for a 24

- hour

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

488 standard at 30 dv would reflect a judgment that the current substantial degrees of variability and uncertainty inherent in the public preference studies should be reflected in a higher target protection level than would be appropr iate if the underlying information were more consistent and certain. Also, a 24

- hour visibility index at a level of 30 dv would reflect recognition that there is considerable spatial and temporal variability in the key factors that determine the value of t he PM 2.5 visibility index in any given urban area, such that there is a relatively high degree of uncertainty as to the most appropriate approach to use in selecting a 24

- hour standard level that would be generally equivalent to a specific 4

- hour standard level. In light of these uncertainties, the EPA continues to believe that it is appropriate to establish a target level of protection for visual air quality of 30 dv, averaged over 24

- hours, with a form as discussed above. In reaching this conclusion, the EPA notes that any national ambient air quality standard for visibility would be designed to work in conjunction with the Regional Haze Program as a means of achieving appropriate levels of protection against PM

- related visibility impairment in all areas of the country, including urban, non

- urban, and Federal Class I areas. While the Regional Haze Program is focused on improving visibility in Federal Class I areas and a secondary visibility index NAAQS would focus on protecting visual air quality principal ly in urban areas, both programs could be expected to provide benefits in surrounding areas. In addition, the development of local programs, such as those in Denver and Phoenix, can continue to be an effective and appropriate approach to provide additional protection, beyond that afforded by a national standard, for unique scenic resources in and around certain urban areas that are particularly highly valued by people living in those areas. With regard to comments from the Department of Interior

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

489 noting that many large metropolitan areas have 24

- hour IMPROVE values below 30 dv on the worst 20 percent of days already, the EPA notes that the purpose of establishing NAAQS is to ensure adequate protection of public welfare everywhere, not to mandate continuous im provements in areas that may already be relatively clean. In fact, the evidence from the IMPROVE program that many urban areas have total 24

- hour PM

related light extinction below 29 dv on the 20 percent worst visibility days suggests that many areas have relatively good visual air quality already. f. Need for a Distinct Secondary Standard to Protect Visibility Numerous commenters questioned whether a distinct secondary standard for visibility is necessary in light of the analysis described in section VI.B. 1.c.vii above (Kelly et al., 2012a) which indicated that a 24

- hour mass

- based PM 2.5 standard of 35 μg/m 3 would protect against visibility impacts exceeding the range of levels considered in the proposal (28

- 30 dv). While this analysis was conducted in supp ort of proposed implementation requirements for a distinct secondary standard (specifically, the modeling demonstrations that would be required under the PSD program), the second prong of the analysis showed that within the range of levels proposed by the EPA for the visibility index NAAQS (28

- 30 dv), the 24

- hour PM 2.5 standard of 35 μg/m 3 would generally be controlling. Kelly et al. (2012a) concluded that the “overall, design values based on 2008

- 2010 data suggest that counties that attain 24

- hour PM 2.5 NA AQS level of 35 μg/m 3 would attain the proposed secondary PM 2.5 visibility index NAAQS level of 30 dv and generally attain the level of 28 dv” (pp. 17

- 18). Citing this conclusion, many state and local agencies and industry commenters argued that a visibil ity index standard in the range proposed (28

- 30 dv) would provide no

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

490 additional protection beyond that afforded by the existing secondary 24

- hour PM 2.5 NAAQS, and therefore no distinct visibility standard was necessary. These commenters advocated retaining the current 24

- hour PM 2.5 mass - based standard to protect against visibility effects. “Since the 24

- hour PM 2.5 standard already protects the welfare th e 24

- hour PM 2.5 visibility standard is designed to protect, the new standard is duplicative and unnecessary” (South Dakota DENR, p. 2). Furthermore, a number of state commenters objected to the additional resource burden associated with implementing a stan dard which had, in their view, no practical effect: “If the 24

- hour PM 2.5 mass standard has the same effect as the visibility standard, crafting complex regulations to implement another standard seems redundant” (South Carolina DHEC, p. 3). Other states ag reed: “A PM 2.5 - related Visibility Index appears redundant since the benefits achieved from the current primary and secondary annual and 24

- hour PM 2.5 standards already provide reductions that would improve visibility. Establishing a new PM 2.5 secondary sta ndard for visibility would be an additional complication and burden to the states that is not warranted” (Indiana DEM, p. 5). In addition, several commenters submitted additional analyses supporting their position that a 35 μg/m 3 24

- hour PM 2.5 standard wo uld provide at least equivalent protection to a distinct 24

- hour visibility standard within the range of levels proposed (API, Attachment 2, p. 8 and Attachment 3, p. 1). In responding to these comments stating that a distinct visibility standard is not

n eeded, the EPA notes as an initial matter that the Administrator provisionally concluded at the time of proposal that the current PM standards were not sufficiently protective of visual air quality, and that consideration should be given to an alternative secondary

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

491 standard that would provide additional protection against PM

- related visibility impairment, especially in urban areas. This provisional conclusion was based on the results of public preference surveys on the acceptability of varying degrees of vi sibility impairment in urban areas, analyses of the number of days on which peak 1

- hour or 4

- hour light extinction values were estimated to exceed a range of CPLs under conditions simulated to just meet the current standards, and the advice of CASAC. The
A dministrator also noted that the current indicator of PM 2.5 mass, in conjunction with the
current 24

- hour and annual averaging times, was not well suited for purposes of protecting visibility, since it does not incorporate species composition or relative
h umidity, both of which play a central role in determining the impact of ambient PM on visibility. Taking into account the advice of CASAC and the court’s remand of the current secondary PM 2.5 standards, the Administrator provisionally concluded that the

cu rrent secondary standards were neither sufficiently protective nor suitably structured to provide an appropriate degree of public welfare protection from PM

- related visibility impairment. As a result, the EPA proposed a new, distinct secondary standard tha t was designed to address these deficiencies. The EPA notes that in critiquing the proposed secondary standard, commenters generally did not advocate that the form of the existing mass

- based PM 2.5 standards

were was better suited scientifically to the task of protecting from against visibility impairment. Rather, the commenters’ position that a distinct secondary standard was not needed for purposes of protecting visibility was based almost entirely on the relative degree of protection likely to be afforded b y the existing standards (in particular, the existing 24

- hour PM 2.5 standard) as compared to the proposed visibility index, along with

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

492 the relatively large uncertainties associated with the latter. Thus, for all the reasons discussed in the proposal with r egard to the scientific appropriateness of an indicator that takes into account both species composition and relative humidity, the EPA continues to conclude that the proposed standard based on a visibility index would be appropriate scientifically to prov ide targeted protection of visibility, since it would provide a measure of PM

- related light extinction that directly takes into account the factors (i.e., species composition and relative humidity) that influence the relationship between PM 2.5 in the ambie nt air and PM

- related visibility impairment. Furthermore, the EPA disagrees with commenters who stated that implementation concerns, in particular the additional resource burden associated with implementing a distinct secondary standard, should alter the Agency’s decision making with regard to a standard to protect visibility. The EPA may not take the costs of implementation into account in setting or revising the NAAQS. However, in light of the results of the Kelly et al. (2012a) analysis and the views

e xpressed by commenters on the implications of this analysis for conclusions regarding the adequacy of the current secondary 24

- hour PM 2.5 standard, the EPA has reconsidered some of the conclusions drawn at the time of proposal, in particular with regard to the degree of protection that would be provided by the current secondary standard. Based on a review of comments related to indicator, averaging time, form and level, the Agency has concluded that (as described in sections VI.C.1b

- e above) a standard defi ned in terms of a PM 2.5 visibility index (based on speciated PM 2.5 mass concentrations and relative humidity data to calculate PM 2.5 light extinction), a 24

- hour averaging time, and a 90 th percentile form, averaged over 3 years, and a level of 30 dv, would provide sufficient but

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

493 not more than necessary protection of the public welfare with regard to visual air quality. Having identified this target level of protection, the EPA is now in a p osition to compare it specifically to the existing secondary 24

- hour PM 2.5 standard of 35 μg/m 3 for purposes of determining whether it would provide more, the same, or less protection from visibility impairment. The EPA must consider both whether the exist ing secondary 24

- hour PM 2.5 standard of 35 μg/m 3 is sufficient (i.e. not under

- protective) and whether it is more stringent than necessary (i.e. over

- protective). With regard to the degree to which the existing secondary 24

- hour PM 2.5 standard provides suf

ficient but not more than necessary protection for visibility, the EPA first notes that the kind of area

- specific analysis conducted in Kelly et al. (2012a) is essential for addressing the court remand of the 2006 secondary standards. In the case of the 20 06 secondary standards , the EPA had argued that the 35 μg/m 3 24

- hour PM 2.5 standard was requisite because one part of the proposed range for a distinct secondary standard the Agency had considered would affect the attainment status of a somewhat fewer coun ties than the 35 μg/m 3 24

- hour PM 2.5 standard. The court rejected this kind of rough balancing, finding that the EPA’s equivalency analysis based on percentages of counties demonstrated nothing about the relative protection offered by the different standar ds. Based on this, an area

- by

- area evaluation of the relative degree of protection offered by different standards should be conducted to the extent air quality data is available. Kelly et al. (2012a) performed such an evaluation. Based on 2008

- 2010 data, there are no areas that would have exceeded a 30 dv, 24

- hour visibility index standard that would not also have exceeded a 24

- hour PM 2.5 standard of 35 μg/m 3 . Stated another way, all areas that met the 24

- hour PM 2.5 standard of 35 μg/m 3 would have had visu al air

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

494 quality at least as good as 30 dv (24

- hour average, based on 90 th percentile form averaged over 3 years). The Kelly (2012a) analysis also showed that for some areas, particularly in the West, areas that would have met a 24

- hour PM 2.5 standard of 35 μg/m 3 would have had visual air quality better than 30 dv for the PM 2.5 visibility index standard , and that at sites that violated both the 24

- hour level and the secondary visibility index 30 dv level, the secondary standard visibility index level of 30 dv would likely be attained if PM 2.5 concentrations were reduced such that the 24

- hour PM 2.5 level of 35 μg/m 3 was attained. The EPA has conducted a reanalysis (Kelly et al., 2012b) to update the area

- by

area analysis in the original Kelly et al. (2012a) analysis in t hree respects. First First, noting that the original Kelly at al. (2012a) analysis used a 1.4 multiplier to convert OC to OM at those monitors not using the new CSN monitoring protocol , the EPA recalculated the visibility index design values for 2008

- 2010 using a higher multiplier for converting OC to OM at monitors not already using the new CSN monitoring protocol SANDWICH approach, consistent with the Agency’s view that it is more appropriate to use a multiplier of 1.6 at such monitors as compared to 1.4, as described in section VI.C.1.a.ii, above. 57 The recomputed visibility design index values for 2008

- 2010 show the same overall relationship to 24

- hour PM 2.5 design values as presented in Kelly et al.,
2012a. Second, the EPA re peated the calculations comparing visibility index design index values with 24

- hour PM 2.5 design values using 2009

- 2011 data, the most recent

57 Some of the OC measurem ents were produced with CSN’s newer monitoring protocol and did not require a change in the computed OM.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

495 three years of air quality information currently available. 58 Third, the EPA modified the

area - by

- area evaluation t o ensure consistency with the data completeness criteria of 40
CFR p art 50, Appendix N , including the removal of data approved by EPA as exceptional
events , for the current 24

– hour PM 2.5 NAAQS standard and the proposed visibility index NAAQS standard

. The results of this re analysis, as presented in Kelly et al. (2012b), show a similar pattern to that described in the original Kelly memo. Specifically, the analysis indicates that there are were no areas with visibility impairment above 30 dv that do d id no t also exceed the 24

- hour PM 2.5 standard of 35 μg/m 3 . The updated memo concludes that the results for 2009

- 2011 corroborate the findings for 2008

- 2010. Based on these analyses (Kelly et al., 2012a; 2012b), the EPA concludes with a high degree of confidence that having air quality that meets the 24

- hour PM 2.5 standard of 35 μg/m 3 would be sufficient to ensure areas would not exceed 30 dv. The EPA notes that this conclusion from Kelly et al. (2012a) is supported by two analyses submitted by industry commenters (API, Attachments 2 and 3). At the time of proposal, the EPA had reache d a different conclusion, specifically that the 35 μg/m 3 24

- hour PM 2.5 standard was not sufficiently protective. This conclusion was based, in part, on the analyses conducted for the Visibility Assessment and Policy Assessment regarding 1

- to 4

- hour peak l ight extinction values based on 2007

- 2009 data. For the reasons outlined above in sections VI.B.1.c and VI.C.1.c, the EPA originally focused on hourly or sub

- daily timeframes for evaluating visibility conditions. However, data quality concerns effectively precluded adoption of a 1

- hour or sub

- daily averaging

58 The 2011 air quality data were not yet available at the time of proposal.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

496 time in this review, and ultimately the EPA has concluded that a 24

- hour averaging time can serve as an appropriate surrogate. In reaching this conclusion, the EPA has recognized that adopting a 24

- hour averaging time will likely smooth out some of the hour

- by

- hour variability in visibility index values, and will effectively reduce peak values by averaging them together with other hours. In concluding it is appropriate to adopt a
24

- hour averaging time, which limits the impact of hour

- specific influences, the Agency is now placing less weight on the results of the 1

- hour and 4

- hour analyses presented in the Visibility Assessment and the Policy Assessment which focused on identifying the percent of days wi th peak hourly light extinction above various CPLs. In light of the Agency’s conclusion that a 24

- hour averaging time would be appropriate, the Agency has determined to place more weight on analyses of visibility conditions over a 24

- hour time period, espe cially the results in Kelly et al. (2012a and 2012b). In addition, the EPA notes that the Kelly et al. analyses reflects updated air quality information from more recent years of data (2008

- 2010 for Kelly et al., 2012a; 2009

- 2011 for Kelly et al. 2012b) as compared to the air quality information used in the Visibility Assessment and Policy
Assessment

. In light of all of these considerations, including the results of the Kelly et al. (2012a; 2012b) analyses, and the supporting comments provided by a broad r ange of public commenters, the EPA is persuaded now concludes that the 24

- hour PM 2.5 standard of 35 μg/m 3 provides sufficient protection in all areas against the effects of visibility impairment. The EPA concludes that the existing 24

- hour PM 2.5 standard pr ovides would provide at least the target level of protection for visual air quality defined by a visibility index set at 30 dv, as described above, which the EPA judges appropriate.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

497 However, the EPA also recognizes that it is important to evaluate whether s uch a standard would be over

- protective (i.e. more stringent than necessary to protect public welfare). The analyses presented in Kelly et al. (2012a; 2012b) indicates that the 24

- hour
PM 2.5 standard of 35 μg/m 3 would achieve more than the target level of protection of visual air quality (30 dv) in some areas. That is, when meeting a mass

- based standard of 35 μg/m 3 , some areas would have levels of PM

- related visibility impairment far below 30 dv. Thus, when considered by itself and without consideration of the secondary standards adopted for purposes of non

- visibility welfare effects, the 24

- hour PM 2.5 standard of 35 μg/m 3 would be over

- protective of visibility in some areas. However, it is important to note that as long as the current secondary 24

- hour PM 2. 5 standard of 35 μg/m 3 remains in effect, this overprotection for visibility would occur, regardless of whether a distinct secondary standard based on a visibility index set at 30 dv were adopted. These issues are discussed more fully in section VI.D, whic h outlines the Administrator’s final conclusions on the secondary PM standards, below.
g. Legal Issues Some commenters opposed the proposal to establish a distinct secondary standard that would be defined in terms of a PM 2.5 visibility index. The propose d standard would use measured PM 2.5 mass concentration, in combination with speciated
PM 2.5 mass concentration and relative humidity data, to calculate PM 2.5 light extinction, translated to the deciview (dv) scale. The standard would also be defined in terms of a specified averaging time and form, and a level for the PM 2.5 visibility index set at one of two options

— either 30 dv or 28 dv. The commenters argued that the entire approach proposed by the EPA is inconsistent with the requirements of CAA section 109(b). They

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

498 pointed to a number of different aspects of the proposal which in their view made it incompatible with the CAA. For example, the Utility Air Reso urces Group (UARG) stated: In the past, EPA has always used a measure of PM mass as the indicator for both primary and secondary PM NAAQS. Such a standard is, as a general matter, consistent with the directive in the CAA that the NAAQS “specify a level of air quality” and targets for control the listed criteria air pollutant. CAA § 109(b)(2). The standard contained in EPA’s proposed rule does neither of these things. Instead, it would (1) regulate relative humidity, which is not a criteria pollutant; (2) fa il to “specify a level of air quality” as required by section 109(b)(2) of the CAA; and (3) result in a standard necessitating nationally variable PM concentrations instead of a standard establishing a nationally uniform, minimally acceptable PM concentrat ion. (UARG, p. 22

- 23) Other commenters raised similar or related issues, arguing that the EPA improperly set a visibility standard, and not a PM 2.5 standard, and that NAAQS can only be set in terms of a level or concentration of the air pollutant. Comment ers also argued that an endangerment finding and air quality criteria would be needed before the EPA could set a standard based on PM components. Each of these comments is discussed below. As an initial matter, the commenters argued that the proposed standard is unlawful because it is “not a PM 2.5 standard at all, but rather a visibility standard, and visibility is neither an air pollutant nor a criteria pollutant for which a NAAQS may be promulgated” (NMA/NCBA, p. 21). According to these commenters, t he CAA requires that NAAQS be established as limits on the concentration of an air pollutant in ambient air, not limits on the “identifiable effects” caused by that air pollutant. These commenters claimed that reduced visibility due to light extinction is not an air pollutant but instead is an effect, noting that “the Act’s definition of ‘air pollutant’ speaks in terms of specific

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

499 substances or matter in the ambient air” (NSSGA, p. 8). The commenters pointed to the use of the term “air pollutant” in section s 109(a)(1)(A) and (b)(2) as support for their argument, as these provisions refer to setting standards for the “air pollutant” to address the effects associated with the presence of the air pollutant in the ambient air. They likewise pointed to section 10 8(a)(2)’s reference to the presence of the air pollutant in the ambient air. Since reduced visibility is not an air pollutant, they argue the EPA cannot set a NAAQS that is a standard for visibility. They argue that the proposed secondary standard it is no t a PM 2.5 standard as it does not limit the concentration of PM 2.5 or any other fraction of particulate matter in the ambient air and therefore is not an “ambient air quality standard” for any pollutant. One commenter argued that the EPA is required to “sp ecify a level of air quality” under section 109(b)(2), which Congress intended as an acceptable concentration level of the air pollutant in the ambient air, noting that specification of acceptable visibility conditions is not the same as an acceptable air pollution concentration level. Citing
American Farm Bureau v. EPA , 559 F.3d at 516, one commenter claimed that the court had affirmed that “the NAAQS

— whether primary or secondary

— is a mass

- based standard” (Nevada DEP, p. 5). Commenters also refer to the le gislative history of the 1970 amendments, referring to NAAQS as setting the “ maximum permissible ambient air level” for an air pollutant. T he commenters argue that the proposed standard is improper because it does not limit the concentration of PM 2.5 or an y fraction of PM in ambient air, but improperly sets a limit on visibility effects. With regard to humidity, these commenters argued that the proposed standard improperly regulates relative humidity because it is included in the calculation to

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

500 determine the value of the visibility index. According to these commenters, the CAA allows the EPA to control criteria air pollutants through the NAAQS program, but not other various substances. The commenters stated that the EPA recognized this in the last review, treating humidity as a confounding factor and considering addressing it by measuring PM 2.5 mass - based concentration over the midday hours, when humidity would have the least effect. This would target the effects caused by PM, and not by humidity. Referring to American Farm Bureau v. EPA , 559 F.3d 512, 528 (D.C. Cir. 2009) and 77 Fed. Reg. at 38979 n.153. UARG contested the proposed calculated visibility index as it does not approach relative humidity as a confounding factor but instead “embraces it and

trea ts it as if it were a PM effect” (UARG p. 24). The commenters also stated that the use of a calculated visibility index, and the failure to exclude the effects of humidity, would result in acceptable PM concentrations that vary across the nation. These co mmenters claimed that such a standard is inconsistent with the requirements of the CAA because the proposed approach fails to establish a nationally uniform PM concentration standard. For example, API argued that the proposed visibility index approach is “ essentially specifying levels

— not a level

— of air quality” (API, p. 29). UARG agreed, and stated that the Act “requires that criteria pollutant concentrations throughout the nation reach, at the least, a single , specified ambient concentration level” (UARG, p. 25, emphasis in original). The commenters argue that a PM 2.5 visibility index standard cannot provide equal protection nationwide due to geographic variation in key factors such as relative humidity that affect level of particles allowed in different a reas. The commenters noted that establishing a single national level for the PM 2.5 visibility index would necessarily result in unequal acceptable

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

501 PM 2.5 levels in different areas of the country, with lowest allowable PM 2.5 levels in urban areas in the Sout heast and highest allowable levels in the arid West. UARG recognized that under section 108 the air quality criteria are to “address those variable factors (including atmospheric conditions) which of themselves or in combination with other factors may alte r the effects on public health or welfare of such air pollutant,” but stated that while section 108 “allows” this, it has no bearing on this issue. Instead, the commenter stated that the EPA may take such information into account in setting a permissible c oncentration of the pollutant that is uniform and national (UARG, p. 25). In addition, some commenters opposed to the proposed distinct secondary standard argued that in order to base a standard on measured levels of several speciated substances, the EPA must first make an endangerment finding and issue air quality criteria for each of the speciated substances included in the calculation of PM 2.5 light extinction. According to these commenters, “EPA cannot use NAAQS to indirectly regulate multiple substanc es which are not criteria pollutants under the guise of establishing a visibility standard” (NMA/NCBA, p. 21). Noting that air quality criteria for particulate matter were issued in 1969, NMA/NCBA argued that the 1969 Criteria Document “did not establish a ir quality criteria for individual constituents that occur in particle form, instead it established criteria for particulate matter as a whole” (p. 27). In light of the fact that criteria have never been issued for “individual speciated components of parti culate matter,” these commenters argued, “if EPA wishes to promulgate a rule such as its secondary visibility NAAQS, it first must make a finding that the speciated components listed in Appendix N endanger public health or welfare and then issue an air
qua lity criteria document for those components” (NMA/NCBA, p. 29). According to

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

502 these commenters, the approach the EPA adopted in promulgating a NAAQS for lead supports this view: When EPA promulgated a NAAQS for lead, an individual substance in particle for m, it did not assert that an endangerment finding or criteria document for lead was unnecessary because lead was already covered by the PM Criteria Document. Instead, EPA complied with the Section 108 and 109 NAAQS prerequisites for lead, just as it must d o for Appendix N substances if it intends to promulgate a NAAQS for those substances. ... [In 1976], EPA listed lead as an air pollutant that adversely affected public health or welfare, issued an air quality criteria document for lead, and promulgated a NAA QS for lead. 43 Fed. Reg. 46246 (Oct. 5, 1976). (NMA/NCBA, p. 29) Finally, UARG argued that the EPA has in the past recognized that the secondary NAAQS is an inappropriate vehicle for regulating PM

- related visibility, referring to 62 Fed. Reg. at 38680, in cluding fn 49. UARG claimed the same situation continues, and the EPA has not provided a valid basis for changing this conclusion. The EPA disagrees with the points raised by these commenters. While the EPA is not adopting the proposed secondary standard, as explained below, this decision is not based on concern over the EPA’s authority to adopt a secondary standard such as the one proposed. The proposed distinct secondary standard is a standard for PM 2.5 , and is not a “visibility standard.” The proposed secondary standard is based on the mass concentration of PM 2.5 in the ambient air. The standard is defined in terms of calculated
PM 2.5 light extinction which is based on the measurement of the mass concentration of ambient PM 2.5 over a 24

- hour period. The measured mass concentration is adjusted based on information on the speciated mass components of the PM 2.5 and the relative humidity, resulting in a calculated visibility index. The level of the visibility index, combined with the form of the standard and averaging time, identifies whether a level of ambient mass

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

503 concentration of PM 2.5 achieves the standard or not. Given any specific mass concentration of ambient PM 2.5 , combined with information on speciation and relative humidity, it can be determined whe ther the specific mass concentration of ambient PM 2.5 achieved the NAAQS. Hence, the proposed secondary NAAQS specifies acceptable levels of ambient mass concentration of PM 2.5 . The combination of indicator, averaging time, form, and level of the proposed NAAQS is designed to provide the appropriate degree of protection from visibility impairment caused by ambient levels of PM 2.5 . It does this by calculating the light extinction associated with ambient concentrations of PM 2.5 and specifying the level of

acc eptable PM 2.5 mass concentration in terms of this calculation. However this does not change the fact that the standard is for the air pollutant PM 2.5 , and defines acceptable ambient levels of this air pollutant. It does not transform the standard into a “v isibility standard” and not a standard for PM 2.5 . While the commenters had additional concerns over the use of relative humidity in the calculation, and the variation around the country of acceptable mass concentrations, those issues are separate and do no t change the fact that the proposed standard defined in terms of calculated PM 2.5 light extinction is based on measurement of PM 2.5 concentration in the ambient air, and is a NAAQS for PM 2.5 . With regard to the contention that section 109(b) limits the EPA to setting a standard that is based on the concentration of the pollutant in the ambient air, we note that the term “concentration” typically means some measure of relative content. For example, this would include relative measures such as mass per uni t of volume or parts per million. The EPA has often used such metrics to define the NAAQS, largely because the scientific evidence of health or welfare effects supporting the NAAQS typically use

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

504 such metrics in air pollution studies. For example, the curre nt secondary standards for PM are defined in terms of the concentration of PM 2.5 and PM 10 in the ambient air, measured as the dried mass of the particulate matter per unit of air. However section 109(b) does not require that a NAAQS be defined this way.
S ections 109(a) and (b) both use the general term “air quality” when discussing the EPA’s obligation to set NAAQS. The NAAQS are clearly national ambient “air quality” standards under section 109(b), which specifies that the primary NAAQS “shall
be ambient air quality standards” and the secondary NAAQS “shall specify a level of air quality.” Both the primary and secondary NAAQS are to be based on the “air quality criteria,” which are to accurately reflect the latest scientific knowledge on the effects on pub lic health and welfare associated with “the presence of such air pollutant in the ambient air, in varying quantities.” Section 109(b), 108(a)(2). Congress spoke in broad terms, tasking the EPA with assessing the latest scientific knowledge about the public health and welfare associated with the presence of the pollutant in the air, without limiting this to consideration of only those effects associated with one or more measures of concentration of the air pollutant. Congress referred to any and all effects associated with the presence of the pollutant in the ambient air, not just the effects associated with the concentration of the pollutant in the ambient air. Based on this knowledge, the EPA is required to set standards for the quality of the air that will provide the appropriate degree of protection from these health and welfare effects, without limitation on how to measure or define air quality. While concentration in the air has typically been an appropriate way to set a standard to achieve these require ments, the more general terms used in section 108(a) and 109(b) do not limit the EPA to using concentration as the only way to measure

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

505 air quality for purposes of setting a NAAQS. The EPA is charged with setting air quality standards, and has the discretio n under section 109(b) to choose the metric for defining air quality that is appropriate to address the health or welfare effect at issue. Congress did refer to “concentration” in certain situations. In section 109(c) Congress required the EPA to set a pri mary NAAQS for NO 2 concentration over 3 hours. This addressed Congress’ concern over whether the then current NO 2 standard, which used concentration as a metric, provided adequate protection. Congress also called on CASAC to advise the Administrator on the relative contribution to “air pollution concentrations” of natural and anthropogenic sources, under section 109(d)(2)(C)(iii). This information is in addition to the advice CASAC is required to provide concerning appropriate revisions to the “air quality criteria” and to the NAAQS under section 109(d)(2)(B). 59 While these provisions refer to ambient concentrations of pollutants, this reflects the EPA’s standard practice to date in setting NAAQS, and none of them change or limit the range of discretion provi ded under section 109(b) in setting NAAQS. They do not change the fact that the EPA is to set “air quality” standards, and is not limited to “air concentration” standards. The reference in the legislative history to a maximum permissible ambient air level for the pollutant also does not limit the EPA to a level of air pollutant concentration, as compared to a different metric for specifying the level of air quality, if that is judged to be appropriate.

59 In a provision that is not part of the CAA, in 1990 Congress required EPA to request a report from the National Academy of Sciences on the role of secondary national ambient air quality standards, including information on the “effects on welfare and the environment which are caused by ambient concentrations of pollutants” liste d under section 108, and the “ambient concentrations of each such pollutant which would be adequate to protect welfare and the environment from such effects.” Section 817(a) of the CAA Amendments of 1990, P.L. 101

- 549.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

506 The text of sections 108 and 109 does not support the limited interpretation commenters suggest. Instead these provisions provide the EPA with significant discretion in determining the metric for air quality that is appropriate to achieve the required degree of protection of public welfare. The commenters’ in terpretation would improperly limit this discretion, interfering with achieving the goals of section 109(b). For example, in this review the EPA considered whether it would be appropriate to base a secondary NAAQS on direct measurement of the light extinct ion caused by
PM 2.5 . See 77 FR 38890, 38980

- 1 (June 29, 2012). There are several instrumental methods that directly measure PM 2.5 light extinction

– the amount of light extinction caused by the presence of PM 2.5 in the ambient air. This is not a measure of the concentration of PM 2.5 in the air, but a measure of the light extinction caused by PM 2.5 . This is clearly an effect associated with the presence of PM 2.5 in the ambient air, and this atmospheric property is directly related to visibility effects. Unli ke PM 2.5 mass concentration, there is a close scientific relationship between directly measured PM 2.5 light extinction and visibility effects. It would appear straightforward to say that PM 2.5 light extinction is a quality of the ambient air, and a seconda ry NAAQS that specified an acceptable level of PM 2.5 based on directly measured PM 2.5 light extinction would be an “ambient air quality standard” for the air pollutant that specifies a “level of air quality” designed to provide protection against visibilit y impairment. Unlike directly measured PM 2.5 light extinction, the mass concentration of PM 2.5 does not have the same direct relationship to light extinction, and specifying an acceptable level of mass concentration of PM 2.5 would be a more indirect and le ss effective way to provide protection from visibility impairment

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

507 caused by the presence of PM 2.5 in the ambient air. Under the commenters’ interpretation, the EPA would be precluded from specifying a level of air quality in terms of directly measured PM 2. 5 light extinction, the more scientifically appropriate and direct measure of the effect PM 2.5 has on visibility. Instead the EPA would be limited to the more indirect and less effective specification of a level of concentration of PM 2.5 . The commenters al so objected to the inclusion of relative humidity as an adjustment factor in the calculation of PM 2.5 light extinction. Contrary to the claims of these commenters, the use of calculated PM 2.5 light extinction does not regulate relative humidity. The propos ed secondary standard would define acceptable levels of ambient
PM 2.5 , not acceptable levels of relative humidity. In addition, section 108 explicitly requires that the air quality criteria include information on the atmospheric conditions that can alter t he effects of the air pollutant on public health or welfare, and relative humidity certainly has this kind of impact. Section 109(b) requires that the standard be based on the air quality criteria, indicating that this information can and should be taken
i nto account in setting the standard. Including relative humidity as an adjustment factor in the calculation of PM 2.5 light extinction is a reasonable and straightforward way to use the scientific information in the air quality criteria in establishing a st andard to provide protection from visibility impairment. 60 Some commenters pointed to the EPA’s position in the last review, stating that the EPA properly treated relative humidity as a confounding factor, and in this review improperly moves away from that position. See 77 FR at 38979, 71 FR 61144, 61205

60 UARG recognizes these provisions, b ut argues, as above, that this is limited by the requirement that EPA set a NAAQS based solely on ambient concentration.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

508 (October 17, 2006). In the last review the EPA considered a distinct PM 2.5 mass - based secondary standard. In that context, limiting the measurement of PM 2.5 mass concentration to the mid

- day hours when rela tive humidity had the least impact would promote the correlation between measured PM 2.5 mass concentration and light extinction, which would promote achievement of a relatively consistent degree of visibility protection across the country. However in this rulemaking the proposed calculated PM 2.5 light extinction standard achieves a consistent degree of visibility protection by directly accounting for humidity, in a scientifically defensible manner. The goal has not changed

– achieving the desired degree of protection across the country. What has changed is that calculated PM 2.5 light extinction is a more direct and scientifically appropriate way to achieve that result. Finally, it should be made clear that water is not a separate compound from PM 2.5 that con founds the impact PM 2.5 has on light extinction. As described in the Integrated Science Assessment, “P M is the generic term for a broad class of chemically and physically diverse substances that exist as discrete particles (liquid droplets or solids)

over a wide range of sizes” (U.S. EPA, 2009a, p. 1

- 4). “ Particles composed of water soluble inorganic salts (i.e., ammoniated sulfate, ammonium nitrate, sodium chloride, etc.) are hygroscopic in that they absorb water as a function of relative humidity to form a liquid solution droplet. Aside from the chemical consequences of this water growth, the droplets become larger when relative humidity increases, resulting in increased light scattering. Hence, the same PM dry concentration produces more haze” ( U.S. EPA, 2009a, p. 9

- 6). Thus water is not a compound that is separate and apart from the particle

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

509 that acts as an extraneous confounding factor. 61 The effect of relative humidity occurs after the water becomes part of the particle. Certain water soluble salts absor b water and the resulting particle is larger in size and scatters more light, increasing the visibility impact of the particle. But the particle is still a PM 2.5 particle. The fact that the PM NAAQS traditionally uses a measurement of the dried mass of the particles as the metric for the standard does not change the fact that the particles in the air include liquid droplets and particles that have increased in size because of absorption of water. These ambient PM 2.5 particles are what is in the air and impa cting visibility, not just the dried mass of PM 2.5 that is measured in the laboratory and is currently used as the indicator for the PM NAAQS. Thus the commenters improperly claimed that the proposed secondary standard regulates water or relative humidity, and not PM 2.5 , when in fact the proposed secondary standard accounts in a scientific manner for the fact that some PM 2.5 particles are larger in size and have a greater impact on light extinction when the relative humidity increases. The commenters also raised concerns that a standard based on calculated PM 2.5 light extinction, compared to a standard based on just PM 2.5 mass concentration, improperly results in variable levels of acceptable PM 2.5 mass concentrations across the country. This stems from the adjustments in the calculation for speciated components of
PM 2.5 and relative humidity. According to commenters, this is improper as section 109(b) requires that the NAAQS set a single, specified ambient concentration that is nationally

61 According to the Integrated Science Assessment, “ Confounding is ‘...a confusion of effects. Specifically, the apparent effect of the exposure of interest is distorted because the effect of an extraneous factor is mistaken for or mixed with the actual ex posure effect (which may be null)’ (Rothman and Greenland, 1998, 086599)” (U.S. EPA, 2009a, p. 1

- 16).

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

510 uniform across the country. As discussed above, the text of section 109(b) does not specify this limitation of a single national acceptable concentration. Instead the secondary NAAQS is to specify a level of air quality that achieves the appropriate degree of protection. Th e proposed secondary standard would do just that

– specify a level of air quality, defined in terms of calculated PM 2.5 light extinction, that would achieve the desired degree of protection. The fact that this results in varying allowable levels of PM 2.5 m ass concentrations is not inconsistent with the Act. The D.C. Circuit recently approved such a result. In the last review of the PM 10 primary NAAQS, the court approved the EPA’s choice of an indicator that was designed to allow varying levels of acceptable coarse PM. The court stated that: The industry petitioners next argue that the 150 μg/m 3 standard for PM 10 will result in arbitrarily varying levels of coarse PM, and that the agency should instead have used a PM 10

- 2.5 indicator. The EPA does not dispute that using the PM 10 indicator will result in coarse PM levels that vary within the limit of 150 μg/m 3 . As the EPA explains: "Because the PM 10 indicator includes both coarse PM (PM 10

- 2.5 ) and fine PM (PM 2.5 ), the concentration of PM 10

- 2.5 allowed by a PM 10 standard set at a single level declines as the concentration of PM 2.5 increases. Thus, the level of coarse particles allowed varies depending on the level of fine particles present." Id. at 61,195

. Although the EPA acknowledges that a PM 10 indicator will r esult in varying coarse PM levels, it does not agree that the variance will be arbitrary. The EPA agrees with the industry petitioners that protection from coarse particles should be targeted at urban areas, where coarse particles have been shown to pose t he greatest danger. Id. at 61,194 . But the agency argues that targeting of urban areas is effectively accomplished by using an indicator that permits the varying levels that the industry petitioners challenge. ... Id. at 61,195

- 96 (citations omitted). In oth er words: "The varying levels of coarse particles allowed by a PM 10 indicator will therefore target protection in urban and industrial areas where the evidence of adverse health effects associated with exposure to coarse particles is strongest." Id

. The EP A also offers a further rationale for tying the stringency of coarse PM

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

511 regulation to increases in the level of PM 2.5 . ... EPA argues that it is "logical to allow lower levels of coarse particles when fine particle concentrations are high. . . . [I]nclusion of PM 2.5 in the PM 10 indicator for purposes of coarse particle protection would appropriately reflect the contribution that contaminants emitted in fine particle form can make to the overall health risk posed by coarse particles." Id

. In sum, we find that the EPA has provided a reasonable explanation for its decision[ ] ... to utilize a standard that allows targeted variance in coarse PM levels in an inverse relationship to the amount of fine PM in the air. American Farm Bureau v. EPA , 55 9 F.3d 512, 534

- 5 (D.C. Cir. 2009). A similar result applies here. Under the proposed secondary standard there would be a single level of air quality specified for the NAAQS. The standard would apply across the nation; it would not be a regional standard. The proposed standard would be the same standard everywhere

— the acceptable level of mass concentration of PM 2.5 would be defined the same way across the nation, using the same method of calculating the allowable concentration of PM 2.5 . The same degree of protection from visibility impairment would apply across the country. While the allowable amount of PM 2.5 could vary, this would be a reasoned way to achieve the desired degree of protection from visibility impairment. The requirements of section 109(b) wo uld be satisfied. Commenters also objected that the EPA could not set a NAAQS for the separate components of PM 2.5 without listing the components of PM 2.5 under section 108, based on an endangerment finding, and issuing air quality criteria for these compo nents. They argued that the issuance of air quality criteria for particulate matter starting in 1969 did not provide a lawful basis for a proposed secondary standard that is based on components of PM, as the 1969 air quality was for particulate matter “as a whole,” defining PM as particles smaller than 500 micrometers (NMA/NCBA, p. 27). However, as discussed above, the proposed standard sets the allowable limit on ambient concentrations of PM 2.5 .

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

512 Information on both the speciated components of PM 2.5 and the relative humidity affect how much light extinction is associated with any specific level of PM 2.5 , but the standard is for PM 2.5 . The D.C. Circuit has made it clear that PM 2.5 , just like PM 10 and TSP before that, is an appropriate subset of PM for the EPA to focus on in setting the NAAQS based on the scientific evidence before the EPA. This focus of the NAAQS does not make the subset a new pollutant that requires listing and new air quality criteria under section 108 before setting a NAAQS. American Trucki ng Association et al. v. EPA , 175 F.3d 1027, 1055 (D.C. Cir. 1999). Commenters’ interpretation would apply to PM 2.5 as well as to components of PM 2.5 , and is inconsistent with the ATA decision. In addition, it is clear that the current air quality criteria do address the scientific basis for calculating PM 2.5 light extinction as the EPA proposed (U.S. EPA, 2009a, pp. 9

- 5 to 9

- 8). Finally, at least one commenter argued that the EPA has concluded in prior reviews that the secondary NAAQS program is an inappr opriate vehicle for regulating PM related visibility impairment (UARG, p. 26). UARG mischaracterized the EPA’s past
decision

- making. In past reviews the EPA has been clear that the EPA should take into account the existence of the visibility program under section 169A, the regional haze program, when considering a secondary NAAQS and should not treat the secondary NAAQS as the sole mechanism to address visibility impairment across the country. That is the approach the EPA has taken in this and prior reviews . See 77 FR at 38990. h. Relationship with Regional Haze Program A large number of commenters expressed confusion and concern over differences between the proposed visibility index standard and the Regional Haze Program. This included commenters who suppo rted setting a distinct secondary standard to protect

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

513 visibility as well as those opposed to setting such a standard. A number of these commenters noted that visibility impairment would be assessed differently under the two approaches due to differences in the way light extinction is calculated, including different IMPROVE equations and differences in the inclusion and weighting of specific species and components. The commenters argued it would be inappropriate to have two different regimes for managing vis ibility impairment in the exact same location. These commenters claimed that since data from the IMPROVE monitoring network would inform nonattainment designations, as well as an area’s obligations under the Regional Haze Program, there could be considerab le confusion over how to draw nonattainment boundaries and what requirements would affect large sources in rural areas. These commenters also noted the resource burden associated with maintaining two different programs aimed at protecting visibility in the same geographic area. Some commenters argued that a visibility NAAQS should not apply to rural areas. The Department of the Interior requested that the EPA clearly define the geographic area to which the visibility index standard would be applicable, and suggested that Class I and Class II areas should generally be excluded from the standard. As discussed above, commenters questioned the need for a distinct visibility standard, arguing that the existing primary PM standards combined with the Regional Haze Program ensured adequate protection of visibility, even in urban areas. In response to these comments relating to the overlap between the Regional Haze program and a distinct secondary standard designed to protect visibility principally in urban areas, the EPA notes that the objectives of each program are distinct. While the Regional Haze program is designed to eliminate man

- made impairment of visibility in

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

514 Federal Class I areas over the course of several decades, a distinct secondary standard for
PM

- relate d visibility impairment would be focused on providing a nationally applicable level of protection for all areas, particularly urban areas which do not receive targeted protection under the Regional Haze Program. Moreover, the metric used to assess visibili ty impairment differs between the two programs precisely because each program is aimed at a different aspect of the problem. Recognizing the importance of fresh emissions for urban visibility, the Visibility Assessment focused on visibility impairment

as m easured by the original IMPROVE equation because “the original version is considered more representative of urban situations when emissions are still fresh rather than aged as at remote IMPROVE sites” (U.S. EPA, 2010b, p. 3

- 19). The Regional Haze Program, on the other hand, has shifted to a revised IMPROVE algorithm more suited to remote locations. While this difference is discussed in more detail in section VI.C.1.b above, the result is that each program would appropriately measure those aspects of visibil ity impairment most closely related to the problem the program is trying to prevent. Since the same data can be used to calculate both visibility impairment under the Regional Haze approach and the proposed visibility index, the additional calculation burd en for state and local agencies would be light. Also, to the extent that there is any difference in terms of the emissions control obligations the two different programs would impose upon state and local areas, this is likely appropriate given the extent a nd nature of visibility impairment in those areas. The EPA notes that in general, there is likely to be substantial overlap in the control strategies a state or local area would pursue under either program. Thus, the EPA disagrees with commenters who state d that a distinct visibility standard as proposed would inherently conflict with the Regional Haze Program or that it

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

515 would be appropriate to draw geographical distinctions that would explicitly exclude some areas (e.g., Class I areas) from the NAAQS. The EPA notes that the CAA requires that NAAQS be national in scope, and that the specific requirements laid out in the proposal for the distinct secondary standard would ensure that the protection it afforded would be appropriately targeted toward urban areas so that it could work in conjunction with

— not be in conflict with

— the Regional Haze Program under sections 169A and 169B of the CAA. 2. Comments on the Proposed Decision Regarding Non

- Visibility Welfare Effects Relatively few commenters addressed the prop osal to retain the existing suite of secondary PM standards to address non

- visibility welfare effects. A couple of states, including Mississippi and South Dakota, offered brief endorsements of the proposal , but the vast majority of commenters, even those w ho offered extensive comments on the proposed visibility index, did not comment on the proposal to retain the existing secondary standards.

. A few other commenters offered more extensive comments on the proposal to retain the existing secondary standards, and these commenters opposed this aspect of the proposal for one of two reasons. First, some commenters opposed the proposal to retain the current secondary annual PM 2.5 standard of 15 μg/m 3 in light of the proposal to revise the level of the primary annua l PM 2.5 standard to a level between 12

- 13 μg/m 3 . Expressing concern over the implications of this decision for the air quality planning obligations of states, these commenters argued that the EPA should revise the secondary PM 2.5 standards to be equivalent in all respects to the primary PM 2.5 standards. For example, the American Association of State Highway and Transportation Officials (AASHTO) supported “retaining secondary standards that are consistent with the primary

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

516 standards in order to reduce the com plexity of the transportation and air quality planning processes, as well as the transportation conformity process” (AASHTO, p. 3). Thus, if the EPA were to adopt a lower level for the primary annual PM 2.5 standard, the commenters recommended that the EPA adopt this same lower level for the primary secondary PM 2.5 standard as well. In response to these comments, the EPA notes that the Agency lacks an appropriate scientific basis for revising the level of t he secondary annual PM 2.5 standard. As noted above in section VI.B.2, there is an absence of information that would support any different secondary standards for PM. Comments related to the implementation challenges associated with distinct primary and sec ondary standards are not relevant to the Administrator’s final decisions regarding what standards are requisite to protect the public welfare. Therefore, the EPA continues to conclude that it would be appropriate to retain the current suite of secondary PM standards 62 to address non

- visibility welfare effects , while revising only the form of the secondary annual PM 2.5 standard to remove the option for spatial averaging consistent with this change to the primary annual PM 2.5 standard , as proposed. Other comme nters focused on the impacts of particulate matter on climate. One commenter cited a number of recent studies that considered mobile source black carbon emissions and associated climate impacts, and urged the EPA to protect the public welfare by setting “h igher standards for gasoline quality” (Urban Air Initiative, p. 4). This commenter did not, however, advocate specific secondary NAAQS to address

62 As summarized in section VI.A an d Table 1 above, the current suite of secondary PM standards includes annual and 24

- hour PM 2.5 standards and a 24

- hour PM 10 standard.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

517 climate impacts of PM. More extensive comments on this same subject were provided by the Center for Biological Diversity (CBD), which urged the EPA to “set a separate limit for black carbon within the overall PM 2.5 standard” to ensure that public welfare is fully protected “from the serious climate impacts of black carbon” (CBD, p. 2). This commenter argued that “ [p]recaution is required for secondary NAAQS,” citing American Trucking Associations, Inc. v. EPA , 283 F.3d 355, 369 (D.C. Cir. 2002): [N]othing in the Clean Air Act requires EPA to wait until it has perfect information before adopting a protective seconda ry NAAQS. Rather, the Act mandates promulgation of secondary standards requisite to protect public welfare from any “ anticipated adverse effects associated with” regulated pollutants, 42 U.S.C. § 7409(b)(2) (emphasis added), suggesting that EPA must act as soon as it has enough information (even if crude) to “anticipate[]” such effects[.] The commenter stressed the growing scientific evidence regarding the impacts of black carbon on climate, and argued that the EPA’s proposal ignores important research stud ies published within the last five years which provide improved estimates of the radiative forcing associated with black carbon, and the effects of black carbon on snow and ice, the Arctic climate, water availability and climate “tipping points.” The comme nter also noted that reductions in cooling aerosol species, particularly sulfate, due to pollution control programs are leading to an “unmasking” of the true extent of warming due to the accumulation of greenhouse gases in the atmosphere. The commenter arg ued that this unmasking effect can be offset by ensuring “that sufficient black carbon reductions accompany reductions in overall aerosol pollution” (CBD, p. 10). The commenter also argued that the EPA did not consider the negative impacts of climate chang e on public health adequately in the proposal.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

518 The commenter stated that the EPA had an obligation to address the impacts of black carbon in the PM NAAQS, despite the remaining uncertainties. The commenter pointed to the U.S. EPA’s report to Congress on Bl ack Carbon (U.S. EPA, 2012c), stating that the “report shows that EPA is aware of the climate science and public health information that point to the importance of addressing black carbon pollution. EPA must use this information in its relevant decisionmak ing” (CBD, p. 13). The commenter also
noted that the United States U

. S

. participates in a number of international forums that have recognized the need to take action on black carbon, and argued that the U.S. has “an obligation under the Gothenburg Protocol to address black carbon pollution.” The commenter challenged the uncertainties cited by EPA with regard to the climate impacts of aerosols generally, arguing that they “do not apply to the regulation of black carbon” (CBD, p. 14). Specifically, the comment er stated that “there are significant anthropogenic sources of black carbon that contribute a large proportion of total black carbon emissions”; that “there is enough information related to black carbon’s impact to know that global temperatures will rise d ue to black carbon emissions”; that spatial and temporal heterogeneity in black carbon emissions do not matter for estimating likely climate effects; that “[b]lack carbon’s negative climate impacts do not depend upon details of cloud interactions with aero sols”; and that the EPA does not need to be able to quantify the health or climate benefits precisely to know that it is appropriate to control black carbon as a specific component of PM under the CAA (CBD, pp. 14

- 15). As a result, the commenter concluded that the current size

- based PM mass standard “is insufficient to fully protect health and welfare,” and that the EPA was obligated to establish a specific limit on black carbon as a component of PM. The

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

519 commenter argued that “Black carbon must be regulate d separately and in addition to
PM 2.5 because absent separate standards sulfates and nitrates may be more likely to be mitigated than the black carbon component of PM” (CBD, p. 17). To support this point, the commenter cited the conclusion in the Policy As sessment that: The current standards that are defined in terms of aggregate size mass cannot be expected to appropriately target controls on components of fine and coarse particles that are related to climate forcing effects. Thus, the current mass

- based PM 2.5 and PM 10 secondary standards are not an appropriate or effective means of focusing protection against PM

- associated climate effects due to these differences in components. (U.S. EPA, 2011a, p. 5

- 11) The commenter also noted that existing regulations on diesel engines, which are the largest source of black carbon in the United States, do not affect existing engines and vehicles, and stated that “The NAAQS program is one of the few opportunities to reduce black carbon from existing engines, industrial a nd biofuel sources within the United States and rapidly reduce emissions from this pollutant” (CBD, p. 18). The EPA agrees with the commenters’ assertion that the scientific information about the impacts of aerosol species on climate is developing rapidly, and that understanding of the magnitude of aerosol effects on climate and the contribution of individual aerosol components to those effects has improved substantially over the past decade. The EPA also agrees that certain species, in particular black car bon, play a significant role in multiple aspects of climate. The Policy Assessment recognized that “Aerosols can impact glaciers, snowpack, regional water supplies, precipitation and climate patterns,” and may contribute to the melting of ice and snow, a d ecrease in surface albedo, and climate impacts in the Arctic and other locations (U.S. EPA, 2011a, p. 5

- 9). The contribution of black carbon to these effects is discussed in detail in the

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

520 EPA’s recent Report to Congress on Black Carbon (U.S. EPA, 2012c). I n particular, black carbon plays an important role in heating the lower atmosphere by absorbing incoming solar radiation and outgoing terrestrial radiation, i.e. via “direct” radiative forcing. However, the EPA disagrees that there is sufficient informatio n available at this time to establish a NAAQS to protect against the climate impacts associated with current ambient concentrations of black carbon or other PM constituents. While the Integrated Science Assessment concluded that “a causal relationship exis ts between PM and effects on climate, including both direct effects on radiative forcing and indirect effects that involve cloud feedbacks that influence precipitation formation and cloud lifetime” (U.S. EPA, 2009a, section 9.3.10), it also identified subs tantial remaining uncertainties with regard to the contribution of individual aerosol species to these climate effects. The contribution of individual aerosol components to total aerosol direct radiative forcing is more uncertain than the global average (U .S. EPA, 2009a, section 9.3.6.6), and the indirect effects of aerosols and aerosol components remain highly uncertain, in particular with regard to their complex interactions with clouds. With regard to black carbon, for example, the EPA disagrees with CBD ’s claims that “black carbon’s negative climate impacts do not depend upon details of cloud interactions with aerosols” and that the uncertainties associated with climate impacts of aerosols generally do not apply to black carbon. In fact, the EPA has poin ted to cloud interactions as the area of greatest uncertainty with regard to black carbon: recognizing that black carbon affects cloud reflectivity (albedo), lifetime, and stability as well as precipitation, the Report to Congress on Black Carbon noted tha t “few quantitative

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

521 estimates of these effects are available, and significant uncertainty remains. Due to all of the remaining gaps in scientific knowledge, it is difficult to place quantitative bounds on the forcing attributable to [black carbon] impacts on clouds at present” (U.S. EPA, 2012c, p. 4). The Report acknowledged that “most estimates of the forcing from aerosol indirect effects are based on all aerosol species (e.g. total PM) and are not estimated for individual species (e.g, BC alone)” (U.S. EP A, 2012c, p. 40). The Report concluded that it remains unclear the extent to which black carbon contributes to the overall aerosol indirect effect, and did not assign any central estimate or even a range of possible values to the role of black carbon in th e overall aerosol indirect effect. With regard to black carbon’s net contribution to climate, therefore, the Report concluded: The direct and snow/ice albedo effects of BC are widely understood to lead to climate warming. However, the globally averaged net climate effect of BC also includes the effects associated with cloud interactions, which are not well quantified and may cause either warming or cooling. Therefore, though most estimates indicate that BC has a net warming influence, a net cooling effect cannot be ruled out. It is also important to note that the net radiative effect of all aerosols combined (including sulfates, ni trates, BC and OC) is widely understood to be negative (cooling) on a global average basis. (U.S. EPA, 2012c, p. 3) Given the remaining uncertainties about the impact of aerosols on climate, there is even greater uncertainty with regard to how aerosol

- indu ced climate change will affect public health. At this time, it is not possible to estimate the extent to which aerosols in general, let alone particular aerosol components, contribute to the occurrence or exacerbation of adverse health outcomes due to clim ate change. The EPA therefore disagrees with CBD’s claim that the EPA should pursue black carbon reductions for purposes of reducing the impacts of climate change on public health.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

522 The Report to Congress on Black Carbon also stressed the importance of

cons idering co

- emitted PM species, such as SO 2 and NO x , in evaluating the benefits of black carbon mitigation options. Noting that many of these co

- emitted particles and gases have a cooling influence on climate, the Report noted the difficulty of estimating t he net effect of various mitigation measures on net radiative forcing or other climate variables. The EPA concluded that the location and timing of emissions reductions would be critically important for achieving climate benefits, and that “more research i s needed on the benefits of individual control measures in specific locations to support policy decisions made at the national level” (U.S. EPA, 2012c, p. 140). Thus, the EPA disagrees with CBD’s claim that spatial and temporal heterogeneity in black carbo n emissions do not matter for estimating likely climate effects, and continues to believe that being able to quantify the climate impacts of various aerosol species, alone and in combination, is essential for informing any possible revisions to the current secondary PM standards based on climate. Furthermore, while the EPA agrees with the commenter that a large percentage of black carbon emissions come from anthropogenic sources, including diesel engines and vehicles, the EPA notes that existing regulations on mobile diesel engines are already reducing these emissions substantially. Between 1990 and 2005, new engine requirements resulted in a 32 percent reduction in black carbon emissions from mobile sources, and a further 86 percent reduction from 2005 leve ls is projected to occur by 2030 as vehicles and engines meeting existing regulations are phased into the fleet (U.S. EPA, 2012c, p. 175). Long

- term historic data indicate that there has been a dramatic overall decline in black carbon emissions over the pa st century, due to changes in fuel use, more

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

523 efficient combustion practices, and implementation of PM controls. Therefore, the EPA disagrees with CBD’s claim that a distinct black carbon NAAQS is necessary to achieve reductions in black carbon emissions. C learly, U.S. emissions of black carbon are already declining substantially, suggesting that the existing mass

- based PM standards, though not targeting black carbon specifically, have been effective in achieving black carbon emissions reductions in practice . As acknowledged in the Report to Congress on Black Carbon , “While [black carbon] is not the direct target of existing programs, it has been reduced through controls aimed at reducing ambient PM 2.5 concentrations and/or direct particle emissions” (U.S. EP A, 2012c, p. 161). The EPA has acknowledged the need to encourage PM mitigation strategies that focus on reducing directly emitted PM 2.5 for purposes of reducing black carbon, and this is reflected in U.S. commitments under the Gothenburg Protocol: the new provisions in the Protocol pertaining to PM encourage parties to develop national inventories and projections for black carbon, and to “give priority” to black carbon when implementing measures to control PM. However, the EPA notes that the U.S. has not y et ratified the PM amendments to the Gothenburg Protocol, and furthermore, these amendments do not require action specifically to reduce black carbon, but rather encourage countries to take such actions voluntarily within the context of their broader PM re duction strategies. Thus the EPA disagrees with the commenter that the U.S. has an “obligation” to reduce black carbon under the Gothenburg Protocol, or that it has “agree[d] to choose mitigation options for particulate matter that focus on
black carbon re ductions” under the Protocol (CBD, p. 13). In sum, the EPA notes the substantial remaining the uncertainties and gaps with regard to the climate impacts of PM components, including black carbon. These include

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

524 the uncertainties associated with the spatial a nd temporal heterogeneity of PM components that contribute to climate forcing; the uncertainties associated with measurement of aerosol components; the inadequate consideration of aerosol impacts in climate modeling; and the currently insufficient data on local and regional microclimate variations and the heterogeneity of cloud formations. As a result, the EPA continues to conclude that it is not currently feasible to conduct a quantitative analysis for the purpose of informing revisions of the current seco ndary PM standards based on climate, and that there is insufficient information at this time to base a national ambient standard on climate impacts associated with current ambient concentrations of PM or any of its constituents. 63
D. Conclusions on Secondary PM Standards This section describes the Administrator’s conclusions regarding the secondary PM standards and the rationale leading to the Administrator’s final decision to retain the current suite of secondary PM standards , including an annual PM 2.5 standa rd of 15 μg/m 3
a 24

- hour PM 2.5 standard of 35 μg/m 3
, and a 24

- hour PM 10 standard of 150 μg/m 3
, to address PM

- related welfare effects, including visibility impairment as well as other PM

related welfare effects, including ecological effects, effects on materials, and climate impacts. Specifically, this section explains the Administrator’s decision, consistent with the proposal, to retain the current suite of secondary PM standards generally, while revising only the form of the secondary annual PM 2.5 stan dard to remove the option for spatial averaging consistent with this change to the primary annual PM 2.5 standard. It also

63 This conclusion applies for both the secondary (welfare

- based) and the primary (health

- based) standards.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

525 explains the Administrator’s decision, contrary to what was proposed, not to establish a
distinct standard to address PM

- related visib ility impairment. In reaching conclusions regarding the need to revise the secondary PM standards for both visibility and non

- visibility welfare effects, the Administrator has taken into account several key factors, including: (1) The latest scientific in formation on both visibility and non

- visibility welfare effects associated with PM, as previously described; 2) the advice of CASAC; and 3) the comments received during the public comment period, as discussed above. Based on this information, the Administr ator has reached final conclusions about the secondary PM standards and made final decisions about those standards, as outlined below. Because the Administrator’s final conclusions with regard to the need to establish a distinct secondary standard to prote ct against visibility impairment reflect, in part, her conclusions on the secondary standard PM standard s for

non - visibility welfare effects, section VI.D.1 first outlines her conclusions regarding the secondary PM standards to address non

- visibility welfare effects. This is followed by section VI.D.2 which outlines her conclusions regarding a secondary PM standard to protect address PM

- related visibility impairment . Finally, section VI.D.3 summarizes the Administrator’s final decisions with regard to the secondary PM standards for both visibility and non

- visibility welfare effects.
1. Conclusions Regarding Secondary PM Standards to Address Non

- Visibility Welfare Effects With regard to the secondary PM standa rds to address non

- visibility welfare effects, the Administrator concludes that it is generally appropriate to retain the existing secondary standards and that it is not appropriate to establish any distinct secondary PM

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

526 standards to address non

- visibility PM

- related welfare effects. This conclusion is based on the considerations discussed above in section VI.B.2, including the latest scientific information and the advice of CASAC, and the public comments received on the proposal, as discussed above in sect ion VI.C.2. The Administrator concurs with the advice of CASAC and the conclusions expressed at the time of proposal that it is important to maintain an appropriate degree of control of both fine and coarse particles to address non

- visibility welfare effec ts, including ecological effects, effects on materials, and climate impacts. In the absence of information that would support any different standards the Administrator concludes that it is appropriate to retain the existing suite of secondary standards to address non

- visibility welfare effects, as proposed. More specifically, the Administrator concludes it is appropriate to retain all aspects of the
current 24

- hour PM 2.5 and PM 10 standards. With regard to the secondary annual PM 2.5 standard, the Administrat or concludes that it is appropriate to retain a level of 15.0 μg/m 3 for this standard while revising only the form of the secondary annual PM 2.5 standard to remove the option for spatial averaging consistent with this change to the primary annual
PM 2.5 sta ndard. In reaching this conclusion, the Administrator notes that no areas in the country are currently using the option for spatial averaging to demonstrate attainment with the secondary annual PM 2.5 standard.
2. Conclusions Regarding Secondary PM Standards for Visibility Protection Having reached the conclusion that it is generally appropriate to retain the existing secondary standards to protect against non

- visibility welfare effects, the Administrator next considered the target level of protection that wou ld be requisite to protect public welfare with regard to visual air quality. The Administrator then

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

527 determined whether to adopt a distinct secondary standard to achieve this target level of protection. In making this decision, the Administrator compared th e degree of protection for visibility that would be provided by such a distinct secondary standard to the degree of protection provided by the existing secondary standards, focusing specifically on the secondary 24

- hour PM 2.5 standard of 35 μg/m 3

. 64
Based o n the considerations discussed above in section VI.B and VI.C, the Administrator first concludes that a target level of protection for a secondary standard is most appropriately defined in terms of a PM 2.5 visibility index as proposed, since it would provi de a measure of PM

- related light extinction that directly takes into account the factors (i.e., species composition and relative humidity) that influence the relationship between PM 2.5 in the ambient air and PM

- related visibility impairment. Such a PM 2.5

v isibility index standard would afford a relatively high degree of uniformity of visual air quality protection in areas across the country by virtue of directly incorporating the effects of differences in PM 2.5 composition and relative humidity across the c ountry. In defining a target level of protection based on a PM 2.5 visibility index, the Administrator has considered specific aspects of such an index, including the appropriate indicator, averaging time, level, and form. First, with regard to indicator, the Administrator notes the conclusion of CASAC that relying on a calculated PM 2.5 light extinction indicator based on PM 2.5 chemical speciation and relative humidity data represented a reasonable approach . Based on the analyses conducted in support of thi s

64 This focus on the 24

- hour PM 2.5 standard reflects the Administrator’s judgments that PM

- related visibility impairment is principally related to fine particle concentrations and that perception of visibility impairment is most directly related to short

- term levels of visual air quality.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

528 rulemaking, as described above, as well as the advice of CASAC, the Administrator concludes that a calculated PM 2.5 light extinction indicator that utilizes the original IMPROVE algorithm , adjusted to use a 1.6 OC multiplier and exclude the term for coarse particles, in conjunction with monthly average relative humidity data (i.e., f(RH) values) based on long

- term climatological means would be the most appropriate indicator for a
PM 2.5 visibility index standard. With regard to averaging time, the Adm inistrator notes that both CASAC and EPA staff have concluded that hourly or sub

- daily (4

- to 6

- hour) averaging times, within daylight hours and excluding hours with high relative humidity, are more directly related than a 24

- hour averaging time to the sho rt

- term nature of the perception of PM

- related visibility impairment and the relevant exposure periods for segments of the viewing public. However, in light of the important data quality uncertainties that have recently been identified in association with currently available instruments that would be used to provide the hourly PM 2.5 mass measurements that would be needed in conjunction with an averaging time shorter than 24 hours, the Administrator concludes it would not be appropriate at this time to set a standard based on a sub

- daily averaging time. Moreover, the Administrator notes that analyses conducted by the EPA during this review clearly indicate that PM 2.5 light extinction calculated on a 24

- hour average basis would be a reasonable and appropriate surrogate for PM 2.5 light extinction calculated on a 4

- hour basis. Thus, the Administrator concludes that a 24

- hour averaging time would be appropriate for a PM 2.5 visibility index. The Administrator recognizes that a 24

- hour averaging time would effective ly reduce the influence of peak hours of visibility impairment on visibility index values, but concludes that in light of the concern that peak

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

529 hourly measurements may be significantly influenced by atypical conditions and/or atypical instrument performanc e, it is appropriate to adopt a longer averaging time to ensure that hour

- specific influences and uncertainties are balanced against more robust
measurements. With regard to form, the Administrator notes that consistent with the approach taken in other NA AQS, including the current 24

- hour PM 2.5 NAAQS, a multi

- year percentile form offers greater stability to the air quality management process by reducing the possibility that statistically unusual indicator values will lead to transient violations of the sta ndard. Utilizing a three

- year average form provides stability from the occasional effects of inter

- annual meteorological variability that can result in unusually high pollution levels for a particular year. Moreover, considering the lack of information on and the high degree of uncertainty regarding the impact on public welfare of the number of days with visibility impairment over the course of a year, the Administrator considers it reasonable to focus on the 90 th percentile, which represents the median of the distribution of the 20 percent worst visibility days, a key focus of the Regional Haze program. The Administrator concludes that ensuring that 90 percent of days have visual air quality that is at or below the target level of protection could be reason ably expected to lead to improvements in visual air quality on the 20 percent most impaired days, and that the limited information available in this review provides no basis for adopting a different form which would limit the occurrence of days with peak P M

- related light extinction in urban areas to a greater degree. Therefore, the Administrator concludes that a 90 th percentile form, averaged over 3 years, is appropriate, for purposes of establishing a target level of protection in terms of a 24

- hour PM 2.5 visibility index.

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

530 With regard to level, the Administrator concludes that in light of the uncertainty associated with the high degree of variability in visibility conditions and the potential variability in visibility preferences across different parts of t he country, it is appropriate to establish a target level of protection based on the upper end of the range of Candidate Protection Levels (CPLs) identified in the Policy Assessment (i.e., 20

- 30 dv) and generally supported by CASAC. Thus, the Administrator concludes that it would be appropriate to set a target level of protection in terms of a PM 2.5 visibility index with a
24

- hour averaging time that would provide protection equivalent to the protection afforded by a 4

- hour PM 2.5 visibility index with a lev el of 30 dv. Furthermore, the Administrator notes that for purposes of estimating the approaches used to estimat e generally equivalent levels for a 24

- hour PM 2.5 visibility index , both of the EPA’s two preferred approaches generated 90 th percentile 24

- hour values similar to the 4

- hour values and a range of city

- specific estimates of generally equivalent 24

- hour levels that encompassed the range of levels considered appropriate for 4

- hour CPLs, including the CPL of 30 dv at the upper end o f that range. The Administrator thus concludes that it would be appropriate to use an unadjusted 4

- hour CPL for purposes of establishing a target level of protection in terms of a 24

- hour PM 2.5 visibility index. In considering the alternative levels propo sed for a 24

- hour standard, either 28 dv or 30 dv, the Administrator concludes that the current substantial degrees of variability and uncertainty inherent in the public preference studies should be reflected in a higher target protection level than would be appropriate if the underlying information were more consistent and certain. In addition, she concludes that, in light of the significant uncertainties, it is appropriate to place less weight on the results of western visibility

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

531 preference studies and th at the CPL value (30 dv) that is based on the eastern preference study results is likely to be more representative of urban areas that do not have associated mountains or other valued objects visible in the distant background For all of these reasons, the Administrator concludes that it is appropriate to set a target level of protection in terms of a 24

- hour PM 2.5 visibility index at 30 dv. In summary, in light of all the information available in this review, the Administrator concludes that the protection provided by a standard defined in terms of a
PM 2.5 visibility index (based on speciated PM 2.5 mass concentrations and relative humidity data to calculate PM 2.5 light extinction), a 24

- hour averaging time, and a 90 th percentile form, averaged over 3 years, set at a level of 30 dv, would be requisite to protect public welfare with regard to visual air quality. In reaching this conclusion, the Administrator notes that any national ambient air quality standard for to address PM

- related visibility impairment wo uld be designed to work in conjunction with the Regional Haze Program as a means of achieving appropriate levels of protection against PM

- related visibility impairment in all areas of the country, including urban, non

- urban, and Federal Class I areas. Whil e the Regional Haze Program is focused on improving visibility in Federal Class I areas and a secondary NAAQS to address PM

- related visibility NAAQS impairment would focus on protecting visual air quality principally in urban areas, both programs could be e xpected to provide benefits in surrounding areas. In addition, the development of local programs, such as those in Denver and Phoenix, could continue to be an effective and appropriate approach to provide additional protection, beyond that afforded by a na tional standard, for unique scenic resources in and around certain urban areas that are particularly highly valued by

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

532 people living in those areas. Having concluded that the protection provided by a standard defined in terms of a
PM 2.5 visibility index, with a 24

- hour averaging time, and a 90 th percentile form, averaged over 3 years, set at a level of 30 dv, would be requisite to protect public welfare with regard to visual air quality, the Administrator next has to determine whether to adopt such a visibility index as a distinct secondary standard. This determination requires considering such a secondary standard not in isolation but in the context of the full suite of secondary standards. As discussed above, the Administrator has deter mined to retain the current suite of secondary PM standards to address non

- visibility welfare effects (except for the form of the annual standard). A distinct secondary standard to address visibility impairment is properly considered in a context where the re is also a 24

- hour
PM 2.5 standard of 35 μg/m 3

. In this context, the Administrator has considered the degree of protection from visibility impairment afforded by the existing secondary PM 2.5 standards. The Administrator has considered both whether the e xisting 24

- hour PM 2.5 standard of 35 μg/m 3 is sufficient (i.e. not under

- protective) and whether it is not more stringent than necessary (i.e. not over

- protective). As discussed above in section VI.C.1. f , the results of the Kelly et al. (2012 a; 2012b ) analys es indicate that based on 2008

- 2010 and 2009

- 2011 data, all areas meeting the 24

- hour PM 2.5 standard of 35 μg/m 3 had visual air quality at least as good as 30 dv (24

- hour average, based on 90 th percentile form averaged over 3 years). This means that it is highly likely that the secondary 24

- hour PM 2.5 standard of 35 μg/m 3 would be controlling relative to a 24

- hour standard based on a PM 2.5 visibility index set at a level of

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

533 30 dv, and highly unlikely that areas would exceed the target level of protect ion for visibility of 30 dv without also exceeding the existing secondary 24

- hour standard. On the basis of this evidence, and the supporting public comments, the Administrator judges that the 24

- hour PM 2.5 standard of 35 μg/m 3 provides sufficient protecti on in all areas against the effects of visibility impairment

— i.e., that the existing 24

- hour PM 2.5 standard would provide at least the target level of protection for visual air quality of 30 dv which the Administrator judges appropriate. The Administrator also recognizes that the analyses presented in Kelly et al. (2012a; 2012b) indicate that the 24

- hour PM 2.5 standard of 35 μg/m 3 also would likely achieve more than the target level of protection of visual air quality (30 dv) in some areas. That is, when me eting a mass

- based standard of 35 μg/m 3 , some areas would have levels of PM

- related visibility impairment below 30 dv. Thus, the 24

- hour PM 2.5 standard of 35 μg/m 3 would be over

- protective in some areas (i.e. more stringent than necessary) relative to the target level of protection for visibility. This is not surprising, as the current

mass - based standard does not account for variation in particle species and relative humidity. The 24

- hour PM 2.5 standard of 35 μg/m 3 would provide more than the
necessary pro tection in the areas where this would be expected, for example western areas with lower relative humidity. In light of the Administrator’s conclusion that it is appropriate to retain the

current secondary 24

- hour PM 2.5 standard of 35 μg/m 3 for non

- visibility welfare effects, the Administrator notes that this standard will remain in place regardless of whether she elects to set a distinct secondary standard in terms of a PM 2.5 visibility index. The issue is not whether to adopt a PM 2.5 visib ility index standard when viewed in isolation, but

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

534 whether such a distinct secondary standard should be adopted in addition to the current secondary 24

- hour PM 2.5 standard of 35 μg/m 3 . The EPA notes that adoption of such a distinct secondary standard is no t needed to provide sufficient protection from visibility impairment with respect to the target level of protection determined above. In addition, adoption of such a distinct secondary standard would not change the fact that the current secondary 24

- hour P M 2.5 standard of 35 μg/m 3 would result in over

- protection from visibility impairment in certain areas of the country. Such over

- protection will occur whether or not such a distinct secondary standard is adopted. In effect, adopting such a distinct secondar y standard would have no impact on the degree of protection provided from visibility impairment. Since sufficient protection from visibility impairment would be provided for all areas of the country without adoption of a distinct secondary standard, and ad option of a distinct secondary standard will not change the degree of over

protection provided for some areas of the country, the Administrator judges that adoption of such a distinct secondary standard is not needed to provide requisite protection for bot h visibility and non

- visibility related welfare effects. It is important to note that this conclusion is based on the specific target level of protection determined above, and the specific set of current secondary standards. The Administrator’s conclusion with regard to the sufficiency of the protection provided by the current suite of secondary standards is based on comparing the a 30 dv target level of protection for a PM 2.5 visibility index standard against the degree of protection provided by the curre nt secondary 24

- hour PM 2.5 standard of 35 μg/m 3 . It is the combination of the specific target level of protection and the current suite of secondary standards that is the basis for the decision not to adopt a distinct secondary standard in terms of a PM 2.5

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

535 visibility index at this time. The EPA recognizes that, as in the last review, the final decision is to not adopt a distinct secondary standard to address visibility impairment. While the D.C. Circuit remanded the decision on a secondary standard in the last review, the EPA’s decision in this review fully comports with has addressed the issues raised in the court’s remand. Here the EPA has clearly identified the target degree of protection (defined in terms of a
PM 2.5 visibility index at a level of 30 dv b ased on a 24

- hour averaging time, and a 90 th percentile form, averaged over 3 years) that would be requisite to protect public welfare with regard to visual air quality. The EPA has carefully compared this degree of protection with that provided by the cur rent secondary 24

- hour PM 2.5 secondary standard of 35 μg/m 3 , based on an area - specific analysis of recent air quality data and concluded that the degree of protection from visibility impairment provided by the current secondary standard is sufficient to pr otect public welfare consistent with section 109(b)(2). This provides a clear basis for judging that the current secondary 24

- hour
PM 2.5 secondary standard of 35 μg/m 3 would provide sufficient protection. The analysis also shows that the current 24

- hour se condary 24

- hour PM 2.5 standard would provide more protection than is needed in some areas, largely because it does not take into account variable factors such as relative humidity. However, the EPA has recognized that adoption of a distinct secondary standard to address visibil ity, in addition to retaining the current secondary standard, would not change this result. The EPA has therefore concluded that adoption of such a distinct secondary standard, in addition to the current suite of secondary PM standards, is not needed to pr ovide requisite protection for both visibility and non

- visibility related welfare effects. Thus the EPA’s decision has carefully

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

536 considered and accounted for the views of the court in the remand of the 2006 NAAQS.
E. Administrator’s Final Decisions on Second ary PM Standards To address PM

- related welfare effects, including ecological effects, effects on materials, climate impacts, and visibility impairment, the Administrator is retaining the current suite of secondary PM standards, except for a change to the f orm of the annual standard. Specifically, the EPA is retaining the current secondary 24

- hour PM 2.5 and
PM 10 standard to address PM

- related non

- visibility welfare effects including ecological effects, effects on materials, and climate impacts , the EPA is re taining the current secondary 24

- hour PM 2.5 and PM 10 standard and is revising only the form of the secondary annual PM 2.5 standard to remove the option for spatial averaging consistent with this change to the primary annual PM 2.5 standard. With respect to PM

- related visibility impairment, the Administrator has identified a target degree of protection, defined in terms of a PM 2.5 visibility index (based on speciated PM 2.5 mass concentrations and relative humidity data to calculate PM 2.5 light extinction), a 24

- hour averaging time, and a 90 th percentile form, averaged over 3 years, and a level of 30 deciviews (dv), which she judges to be requisite to protect public welfare with regard to visual air quality. The EPA’s analysis of monitoring data provides the ba sis for concluding that the current secondary 24

- hour PM 2.5 standard would provide sufficient protection, and in some areas greater protection, relative to this target protection level. Adding a distinct secondary standard to address PM

- related visibility impairment would not affect this protection. Since sufficient protection from visibility impairment will be provided for all areas of the country without adoption of a distinct secondary standard, and adoption of a distinct secondary standard will not change the degree of over

-

### Summary of Interagency Working Comments on Draft Language under

EO12866/13563 Interagency Review. Subject to Further Policy Review.

537 protection of visual air quality provided for some areas of the country by the secondary
24

- hour PM 2.5 standard, the Administrator judges that adoption of a distinct secondary standard, in addition to the current suite of secondar y standards, is not needed to provide requisite protection for both visibility and non

- visibility related welfare effects.