From: Lydia Wegman/RTP/USEPA/US

To: "Frey, Nathan J." <Nathan_J._Frey@omb.eop.gov>, "Schwab, Margo" 

<Margo_Schwab@omb.eop.gov>

Cc: Joseph-J Dougherty/DC/USEPA/US@EPA, Beth Hassett-Sipple/RTP/USEPA/US@EPA, 

Karen Martin/RTP/USEPA/US@EPA, Erika Sasser/RTP/USEPA/US@EPA, John 

Hannon/DC/USEPA/US@EPA, Vicki Sandiford/RTP/USEPA/US@EPA

Date: 12/13/2012 04:39 PM

Subject: Edits to secondary standard section

Nathan. 

Attached is a revised section on the secondary standards which reflects changes made to 

address interagency comments from yesterday as well as other edits and additions necessary to 

clarify and strengthen this section. The pages which address comments from our discussion 

yesterday are as follows:

p. 352 -- upfront clarification of decision not to set distinct secondary

p. 366, 368 -- word "limited" added here and in other locations to further emphasize limitations on 

evidence

p. 379

p. 383, footnote 14

p. 388, footnote 19-- CASAC quote inserted

p. 417 CASAC quote inserted

p. 437 -- again providing reader preview of final decision

pp. 461-475 Preference Study Section

p. 520 -- added mention of specific standards being retained
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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 PM2.5 and PM10 standards to address visibility 

impairment and other welfare effects considered in this review. As discussed more fully 

below,Specifically, this 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 PM2.5 standard 

of 15 μg/m3, a 24-hour PM2.5 standard of 35 μg/m3, and a 24-hour PM10 standard of 150 

μg/m3. The Administrator is revising only the form of the secondary annual PM2.5

standard to remove the option for spatial averaging consistent with this change to the 

primary annual PM2.5 standard. Contrary to what was proposed, the Administrator has 

decided not to establish a distinct standard to address PM-related visibility impairment.

The rationale for the finalthis decision is presented below.

The Administrator’s final decisions 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 policyrelevant information presented and assessed in the Integrated Science Assessment and 

staff analyses of air quality and visibility effects presented in the Visibility Assessment 

and the Policy Assessment, upon which staff conclusions regarding appropriate 

considerations in this review are based; (2) CASAC advice and recommendations, as 
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reflected in discussions of drafts of the Integrated Science 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, eitherboth in connection with 

CASAC meetings orand 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 

summarydiscussion 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 PM2.5 standards and a 

24-hour PM10 standard. The current secondary PM2.5 standards 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 ecosystems, 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 particularly on the current secondary PM2.5 standards 

related to visibility impairment and provides a summary of the approaches used to review 
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and establish secondary PM2.5 standards in the last two reviews (section VI.A.1); judicial 

review of the 2006 standards that resulted in the remand of the secondary annual and 24-

hour PM2.5 NAAQS to the EPA (section VI.A.2); and the approach used in this review 

for evaluating the secondary PM2.5 standards used in this review (section VI.A.3).

1. Approaches Used in Previous Reviews

The original secondary PM2.5 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, utilized 

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 focusingthat focused only on Federal 

Class I area visibility impacts to a more multifaceted approach that also 

considersconsidered PM-related impacts on visibility in non-Federal Class I area 

visibilityareas, such as in urban areas. This evolution occurred in conjunction with the 

expansion of available PM data and information from associatedvisibility-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 secondary PM2.5 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 mandatory Federal Class I areas alike. However, in considering an 

appropriate level for a secondary standard to address adverse effects of PM2.5 on 

visibility, the EPA concluded that the determination of a single national level was 
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complicated by important regional differences influenced by factors such as background 

and current levels of PM2.5, composition of PM2.5, and average relative humidity. 

Variations in these factors across regions could thus result in situations where attaining 

an appropriately protective 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 attainment with the new primary standards for PM2.5, the EPA concluded 

that attainment of the annual and 24-hour PM2.5 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 western 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 CAA1to address those effects of PM on 

visibility that would not be addressed through attainment of the primary PM2.5 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 provide for visibility protection programs that can 

be more responsive to the factors contributing to regional differences in visibility than 

can programs addressing the kinds of nationally applicable secondary NAAQS 

 

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 air 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.
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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 important factor considered in the 1997 review was whether 

a regional haze program, in conjunction with secondary standards set identical to the suite 

of PM2.5 primary standards, would provide appropriate protection for visibility in nonFederal Class I areas. 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 the EPA concluded that a national regional haze 

program, combined with a nationally applicable level of protection achieved through 

secondary PM2.5 standards set identical to the primary PM2.5 standards, would be more 

effective for addressing regional variations in the adverse effects of PM2.5 on visibility 

than would be national secondary standards for PM with levels lower than the primary 

PM2.5 standards. The EPA further recognized that people living in certain urban areas 

may place a high value on unique scenic 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 PM2.5 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 

because of the localized and unique characteristics of the problems involved. Visibility in 

an urban area located near a mandatory Federal Class I area could also be improved 
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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 PM2.5 standards 

identical to the primary PM2.5 standards, that would work in conjunction with a regional 

haze programthe Regional Haze Program 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 PMrelated 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 PM2.5 standards to address 

visibility impairment by making the suite of secondary standards identical to the revised 

suite of primary PM2.5 standards. The EPA’s decision regarding the need to revise the 

suite of secondary PM2.5 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 visibility and fine particle monitoring networks had become available, 

allowing for updated characterizations of visibility trends and PM concentrations in urban 

areas, as well as Federal Class I areas. These new data allowed the EPA to better 

characterize visibility impairment in urban areas and the relationship between visibility 
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and PM2.5 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 PM2.5 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 

focus on just the 24-hour secondary PM2.5 standard to provide requisite protection.

The Administrator then considered whether PM2.5 mass remained the appropriate 

indicator for a secondary standard to protect visibility, primarily in urban areas. The 

Administrator noted that PM-related visibility impairment is principally related to fine 

particle levels. Hygroscopic components of fine particles, in particular sulfates and 

nitrates, contribute disproportionately to visibility impairment under high humidity 
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conditions. 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 PM2.5 monitoring networks, an analysis 

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 analyzing how 

well PM2.5 concentrations correlated with visibility in urban locations across the U.S., the 

2005 Staff Paper concluded that clear correlations existed between 24-hour average PM2.5

concentrations and calculated (i.e., reconstructed) light extinction, 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 PM2.5 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 PM2.5

concentration would necessarily result in visual ranges that vary somewhat in urban areas 

across the country, reflecting the variability in the correlations between PM2.5

concentrations and light extinction. The 2005 Staff Paper concluded that it was 

appropriate to use PM2.5 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 

Paper, most CASAC Panel members also endorsed such a PM2.5 indicator for a secondary 

standard to address visibility impairment (Henderson, 2005a, p. 9). Based on the above 
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considerations, the Administrator concluded that PM2.5 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-related 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 visibility 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 appropriate 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/m3PM2.5 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 significantly 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 regions 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 

where state or local agencies placed particular emphasis on protecting visual air quality. 

In considering a reasonable range of forms for a PM2.5 standard within this range of 
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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 consistent with the 98th percentile used for the primary standard 

and that the lower end of the range should be the 92nd 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 PM2.5 standards (71 FR 2681, January 17, 2006), on the 

basis that although significantly more information 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 the 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 PM2.5 standards was actually somewhat greater than the predicted percent of 

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 
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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 PM2.5 standard to be identical to the proposed revised primary PM2.5 standard 

(and retaining the then-current annual secondary PM2.5 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) secondary PM2.5 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 regarding the proposed reliance on a secondary PM2.5

standard identical to the proposed 24-hour primary PM2.5 standard: (1) PM2.5 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 PM2.5 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 nearcontinuous PM monitoring and recognized that an expanded deployment of continuous 

PM2.5 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 

meet what the CASAC Panel considered to be a lenient 4- to 8-hour secondary standard 

and a secondary standard identical to the proposed 24-hour primary standard was a 
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numerical coincidence that was not indicative of any fundamental relationship between 

visibility and health. The CASAC Panel further stated that “visual air quality is 

substantially impaired at PM2.5 concentrations of 35 μg/m3” 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 appropriate 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 nature 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 PM2.5 standards.

2. Remand of 2006 Secondary PM2.5 Standards 

As noted above in section 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 

reconsideration the secondary NAAQS for fine PM because the Agency’s decision was 

unreasonable and contrary to the requirements of section 109(b)(2). American Farm 
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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. Furthermore, 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. They 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.” The 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 standards within the recommended CASAC range that would be 

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 
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substantially more protective than 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, however, 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 visibility protection provided by the standards. 559 F. 3d at 

530-31.

Finally, the Staff Paper had made clear that a visibility standard using PM2.5 mass 

as the indicator in conjunction with a daily averaging time would be confounded by 

regional differences in humidity. 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 PM2.5 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 extent 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 (“evidencebased”) and a quantitative analysis of PM-related impacts on visibility (“impact-based”) 

to inform conclusions related to the adequacy of the current secondary PM2.5 standards 

and alternative standards that were appropriate for consideration in this review. As in past 

reviews, the EPA also considered that the secondary NAAQS should address PM-related 
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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 Regional 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 impairment 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 source apportionment 

information available from the regional planning organizations, long-standing evidence 

of PM effects on visibility, and limited public preference studiesstudy 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 areas.

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 PM2.5 standards and (2) what 

potential alternative standards, if any, should be considered in this review to provide 

appropriate protection from PM-related visibility impairment. In addressing these broad 

questions, the discussions in the Policy Assessment were organized around a series of 
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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 PM2.5 to address PM-related 

visibility impairment, focused primarily on visibility in urban areas. This proposed 

standard was to be defined in terms of a PM2.5 visibility index, which would use 

measured PM2.5 mass concentration, in combination with speciation and relative humidity 

data, to calculate PM2.5 light extinction, translated into the deciview (dv) scale; a 24-hour 

averaging time; a 90th percentile form, averaged over 3 years; and a level of 28-30 dv. To 

address other non-visibility welfare effects, the Administrator proposed to retain the 

current suite of secondary PM standards generally, while revising only the form of the 

secondary annual PM2.5 standard to remove the option for spatial averaging consistent 

with this proposed change to the primary annual PM2.5 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 

information on visibility effects associated with PM as described in the Integrated 

Science Assessment (U.S. EPA, 2009a); (2) insights gained from assessments of 
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correlations between ambient PM2.5 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 (section 

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 causal. 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 

insights regarding the regional source contributions to urban visibility impairment and 

better characterization of the increment in PM concentrations and visibility impairment 
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that occur in many cities (i.e., the urban excess) relative to conditions in the surrounding 

rural areas (i.e., regional background). Ongoing urban PM2.5 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 extinction 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 extinction 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 following equation:2

Deciview (dv) = 10 ln (bext / 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 

perception studies that were considered in the past and current reviews to inform 

judgments about an appropriate degree of protection to be provided by a secondary 

NAAQS.

 

2 As used in the Regional Haze Program, the term bext refers to light extinction due to 

PM2.5, PM10-2.5, and “clean” atmospheric gases. In the Policy Assessment, in focusing on 

light extinction due to PM2.5, the deciview values include only the effects of PM2.5 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 PM2.5 concentration is very low.
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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 the 

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,3to estimate 

light extinction using routinely monitored fine particle (PM2.5) speciation and coarse 

particle mass (PM10–2.5) 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.4These 

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 additional factor to account for the water growth to estimate these 

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 coefficient (bext), as shown below for the original IMPROVE algorithm. 

bext ≈ 3 x f(RH) x [Sulfate]

+ 3 x f(RH) x [Nitrate]

 

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 measurements in a number of rural Federal Class I 

areas.
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+ 4 x [Organic Mass]

+ 10 x [Elemental Carbon]

+ 1 x [Fine Soil]

+ 0.6 x [Coarse Mass]

+ 10 

Light extinction (bext) is in units of Mm-1, the mass concentrations of the 

components indicated in brackets are in units of μg/m3, 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 for 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 sulfate 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 PM2.5 components contribute 5 times or 

more light extinction than a similar concentration of PM10–2.5. 

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 

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 m2/g associated 
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with elemental carbon (EC). PM containing 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 conditions.5

As noted above, subsequent 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. Itwas 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, the revised algorithm 

incorporates terms to account for particles representing the different dry extinction and 

water uptake from two size modes of sulfate, nitrate and organic mass.

7

Second, the 

revised algorithm uses a different multiplier for organic carbon for purposes of estimating 

 

5

The IMPROVE algorithm does not explicitly separate the light-scattering and lightabsorbing effects of elemental carbon.

6

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 additional factor to account for the water growth to estimate these 

components’ 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 

major aerosol components.

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 coefficient 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 PM2.5 components into the concentrations of 

the small and large size fractions was empirically 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/m3. If the total concentration of a 

component exceeds 20 ug/m3, all of it is assumed to be in the large mode. 
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374

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 splitcomponent mass extinction efficiency treatment in the revised algorithm. This 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). Various factors combine to make early morning the most likely time for peak 

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 

extinction 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 

 

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.
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concentrations in the IMPROVE algorithm is consistent with this general lack of a strong 

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 visibility 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 anthropogenic 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, little 

research has been conducted on nighttime visibility, 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. The Policy Assessment notes that the science is not 

available at this time to support adequate 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 

areas on daylight hours during 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 unacceptability 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 
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haze-obscured scene, 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 preferences 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 considered 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 infrequent 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. However, 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 Assessment, information regarding the fraction of the public that has only one or a 

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 visibility 

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 
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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 current 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 conditions. 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 special treatment to periods of fog and rain when considering whether 

current PM2.5 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 impaired visibility due to fog or precipitation should not be treated as having 

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 

CASAC, 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 

in 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 
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removed from consideration, yet those hours had on average ten times the likelihood 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 years 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 Perception 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 studies. 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 preference 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 

improve visibility” (U.S. EPA, 2009a, p. 9-66). Because 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. Contingent 
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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 

implementation costs. Nonetheless, the EPA recognizes that valuation studies do provide 

additional evidence that the public is experiencing losses in welfare due to visibility 

impairment.9The public perception studies are described in detail below.

In order to identify levels of visibility impairment appropriate for consideration in 

setting secondary PM NAAQS to protect the public welfare, the Visibility Assessment 

comprehensively examined information that was available in this review regarding 

people’s stated 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 visibility 

 

9

In the regulatory impact analysis (RIA) accompanying 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 because 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 benefits 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 consideration of the information or analyses in 

the RIA.
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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 scenes 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 characteristics 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. 

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

survey 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 visibility 

preference studies for four urban areas were reviewed in The Visibility Assessment (U.S. 

EPA, 2010b, chapter 2) reviewed the limited number of urban visibility preference 

studies currently available (i.e., four studies) to assess the light extinction levels judged 

by the participant to have acceptable visibility for those particular scenes.

The reanalysis of urban preference studies conducted in the Visibility Assessment 

 

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 background, and the inherent interest of the objects to the 

viewer. Distance is particularly important because at a given value of light 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.
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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, British 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 

scenes, 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 

responding to a series of questions while viewing a set of images of various urban visual 

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” exercises 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 (excellent). They were then shown the 
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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 impairment 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 the 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 unreasonable, 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 within 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 

of light extinction than they do images representing higher levels. 

Given the similarities in the approaches used, it isthe 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 the 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 presentation methods 

(e.g., projected slides of actual photos, projected images generated using WinHaze (a 
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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 results. 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 by 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 graphically to allow for visual comparison. This 

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

 

11 Only 47 percent of the British Columbia participants rated a 19.2 dv photograph as 

acceptable.
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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

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

Range of the 50% Acceptance Criteria13

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 

 

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.

13 Top scale shows light extinction in inverse megameter units; bottom scale in 

deciviews. Logit analysis estimated response 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 whether the 

range identified by combining the results of the studies depicted in Figure 6 is more 

broadly representative.
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385

result of a logistical regression analysis 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 acceptability 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, 

2011a; U.S. EPA, 2010b, Table 2-4). This table shows that the logit model results also 

support the upper and lower ends of the range of 50th 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 50th 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 provisionally 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 limited 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 50th percentile criterion value from the Phoenix study (i.e., 24.2 dv), 

which is by far the best of the four studies in terms of the fit of the data to the response 

curve and the representativeness of study participants. Based on the currently available 

 

15 These values were rounded from 74 Mm-1

and 201 Mm-1to avoid an implication of 

greater precision than is warranted. Note that the middle value of 25 dv when converted 

to light extinction is 122 Mm-1is rounded to 120 Mm-1for 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 Policy Assessment reflected the unrounded values.
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386

information, the Policy Assessment concluded that the use of 25 dv to represent the 

middle of the distribution of 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 

conditions that have been judged 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 correspond 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 proposal 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 

applied 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 standard, 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, 2011a, 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 

 

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

Mm-1.
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for urban visibility conditions between western and eastern 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 

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 important 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 

otherwise 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 less 

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 

 

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 residents of the Houston, Texas area. 
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preference results. 

Another question considered was whether the high CPL value that is based on the 

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 areas 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 British 

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 inherent in using ambient 

 

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 more 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.
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measurements to represent sight path-averaged light extinction values. Reducing these 

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 value 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 

on 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 iswas “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 areis 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 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 broad 

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)
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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 just 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 regarding 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 

extinction20 levels for 15 urban areas21 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 ofand 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 PM10 and PM2.5 light 

extinction. For reasons discussed above, hours with relative humidity greater than 90 

percent were excluded from consideration. Recent visibility conditions in these urban 

areas were then compared to the CPLs identified above. The Visibility Assessment, 

 

20 PM-related light extinction is used here to refer to the light extinction caused by PM 

regardless of particle size; PM10 light extinction refers to the contribution by particles 

sampled through an inlet with a particle size 50 percent cutpoint of 10 μm diameter; and 

PM2.5 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, Birmingham, Atlanta, Detroit, Pittsburgh, Baltimore, Philadelphia, 

and New York. 
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which focused on PM10 light extinction in 14 of the 15 urban areas during the 2005 to 

2007 time period,22 found that all 14 areas had daily maximum hourly PM10 light 

extinction values estimated to exceed even the highest CPL some of the days. Except for 

the two Texas areas and the non-California western urban areas, all of the other urban 

areas were estimated to have maximum hourly PM10 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 hourly PM10 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 compared with those in the western U.S. 

The Policy Assessment repeated the Visibility Assessment-type modeling based 

on PM2.5 light extinction and data from the more recent 2007 to 2009 time period for the 

same 15 study areas (including St. Louis). While the estimates of the percentage of daily 

maximum hourly PM2.5 light extinction values exceeding the CPLs were somewhat lower 

than for PM10 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 

PM2.5 concentrations that exceeded the high CPL on about 10 percent up to about 50 

percent of the days based on PM2.5 light extinction, while all 15 areas were estimated to 

 

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 PM10

monitoring, although less than a mile from the site of the PM2.5 data, was not 

representative of the urban area and resulted in unrealistically large PM10-2.5 values. The 

EPA staff considered these comments credible and set aside the St. Louis assessment 

results for PM10 light extinction. Thus, results and statements in the Policy Assessment 

regarding PM10 light extinction applied to only the other 14 areas. However, results 

regarding PM2.5 light extinction in most cases applied to all 15 study areas because the St. 

Louis estimates for PM2.5 light extinction were not affected by the PM10 monitoring issue.
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have maximum hourly PM2.5 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 PM2.5 secondary standards, the Policy Assessment applied the 

proportional rollback approach described in section VI.C.2 of the proposal to all the 

PM2.5 monitoring sites in each study area.23 After adjusting for composition, the Policy 

Assessment applied the original IMPROVE algorithm to calculate the PM10 light 

extinction, using “rolled back” PM2.5 component concentrations, the current conditions 

PM10-2.5 concentration for the day and hour, and relative humidity for the day and hour.

In these analyses, the Policy Assessment estimated both PM2.5 and PM10 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 PM2.5 light extinction and the percentage of daily 

maximum hourly PM2.5 light extinction values estimated to exceed the CPLs when just 

meeting the current suite of PM2.5 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 

PM2.5 light extinction values in all of the study areas other than the three western nonCalifornia 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 

 

23 Phoenix and Salt Lake City met the current PM2.5 NAAQS under current conditions 

and required no reduction.
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maximum 1-hour average PM2.5 light extinction values that exceeded the low CPL on 

over 15 percent to about 90 percent of the days. Figure 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 PM2.5 light extinction values in all 

of the study areas other than the three western non-California 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 estimated to have daily maximum 4-hour 

average PM2.5 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 PM10 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 question the adequacy of the current suite of 

PM2.5 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 simulations of conditions that would just meet the current suite of 

PM2.5 secondary standards. In particular, for air quality that was simulated to just meet 

the current PM2.5 standards, greater than 10 percent of the days were estimated to have 

peak light extinction values that exceed the highest, least protective CPL of 30 dv in 
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terms of PM2.5 light extinction for 9 of the 15 urban areas, based on 1-hour average 

values, and would thus likely fail to meet a 90th percentile-based standard at that level. 

For these areas, 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 the days were estimated to have peak light extinction that exceeded that CPL 

in terms of PM2.5 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 

10 percent of the days with peak light extinction exceeding the highest CPL in terms of 

PM2.5 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 PM2.5 light 

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 extinction 

estimated to be associated with just meeting the current suite of PM2.5 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 

PM2.5 mass, in conjunction with the current 24-hour and annual averaging times, is 

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 
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differences in relative humidity and species composition of PM2.5, 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 decision to remand the PM2.5 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 current suite of PM2.5 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 standards 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 protect 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 PM2.5 standards, 

that “[T]he levels are too high, the averaging times are too long, and the PM2.5 mass 

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 available evidence and the advice of CASAC, the 
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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 visibility 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 visibility 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 advice 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 studies. 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 recognizing 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 appropriate 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 

CASAC, the Administrator 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-
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related visibility 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 PM2.5 mass, in conjunction with the current 24-hour 

and annual averaging times, is not 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 ambient 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 PM2.5 standards. Based on the above 

considerations, at the time of proposal the Administrator provisionally concluded that the 

current secondary PM2.5 standards, taken together, are neither sufficiently protective nor 

suitably structured to provide an appropriate degree of public welfare protection from 

PM-related visibility 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 

PM2.5 standard designed to protect against visibility impairment: the current PM2.5 mass 

indicator; directly measured PM2.5 light extinction; and calculated PM2.5 light extinction. 

Directly measured PM2.5 light extinction is a measurement (or combination of 
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measurements) of the light absorption and scattering caused by PM2.5 under ambient 

conditions. Calculated PM2.5 light extinction uses the IMPROVE algorithm to calculate 

PM2.5 light extinction using measured PM2.5 mass, speciated PM2.5 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 PM2.5 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 PM2.5 light extinction or 

calculated PM2.5 light extinction as an indicator is justified because light extinction is a 

physically meaningful measure of the characteristic of ambient PM2.5 that is most 

relevant and directly related to PM-related visibility effects (U.S. EPA, 2011a, p. 4-41). 

Further, as noted above, PM2.5 is the component of PM responsible for most of the 

visibility impairment in most urban areas. In these areas, the contribution of PM10–2.5 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 PM2.5 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 PM2.5

mass indicator. The Policy Assessment recognized that while in the future it would be 

appropriate to consider a direct measurement of PM2.5 light extinction it was not an 

appropriate option in this review because a suitable specification of the equipment and 

associated performance verification procedures cannot be developed in the time frame for 

this review.

(a) PM2.5 Mass 
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In terms of utilizing a PM2.5 mass indicator, the proposal noted that PM2.5 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 

PM2.5 that contributes, often significantly, to visibility impacts. Further, the PM2.5 mass 

concentration monitors do not provide information on the composition of the ambient 

PM2.5, which plays a central role in the relationship between PM-related visibility 

impairment and ambient PM2.5 mass concentrations. Additional analyses discussed in the 

proposal that looked at the contribution of PM2.5 to total PM-related light extinction 

(defined in terms of hourly PM10 calculated light extinction) indicate that there is a poor 

correlation between hourly PM10 light extinction and hourly PM2.5 mass principally due 

to the impact of the water content of the particles on light extinction, which depends on 

both the composition of the PM2.5 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 PM2.5 composition conditions that make it 

impossible to select a PM2.5 concentration that generally 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 PM2.5 mass as the indicator. 

(b) Directly Measured PM2.5 Light Extinction

PM light extinction has a nearly one-to-one relationship to light extinction, unlike 

PM2.5 mass concentration. As explained above, PM2.5 is the component responsible for 
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the large majority of PM light extinction in most places and times. PM2.5 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 PM2.5 light extinction (light 

scattering and absorption) or to jointly measure both as total light extinction (from which 

Rayleigh scattering is subtracted to get PM2.5 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 secondary standard relative to estimates of PM2.5 light extinction calculated using 

PM2.5 mass and speciation data. These include greater accuracy of direct measurements 

with shorter averaging times and overall greater simplicity when compared to the need 

for measurements of multiple parameters to calculate PM light extinction.

In evaluating whether direct measurement of PM2.5 or PM10 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 PM2.5 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 discourage development of other potentially superior 

approaches. Instead, the CASAC AAMMS recommended that the EPA develop 

performance-based approval criteria for direct measurement methods in order to put all 

approaches on a level playing field.

At the present time, the EPA has not undertaken to develop and test such 
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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 Assessment concluded that while in the future it would be appropriate to 

consider a direct measurement of PM2.5 light extinction, or the sum of separate 

measurements of light scattering and light absorption, as the indicator for the secondary 

PM2.5 standard, this 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 PM2.5 Light Extinction 

For the reasons discussed above, the Policy Assessment concluded that a 

calculated PM2.5 light extinction indicator would be the preferred approach. PM2.5 light 

extinction can be calculated from PM2.5 mass, combined with speciated PM2.5 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 

routinely collected from the urban CSN24 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 Assessment for calculating hourly PM2.5 light 

 

24 About 200 sites in the CSN routinely measure 24-hour average PM2.5 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).
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extinction25 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 provided reasonably good estimates of PM2.5

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 identified 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 algorithm26

(2) the use of more refined value(s) for the organic carbon multiplier27

(3) the use of the reconstructed 24-hour PM2.5 mass (i.e., the sum of the five PM2.5

components from speciated monitoring) as a normalization value for the 

hourly measurements from the PM2.5 instrument as a way of better reflecting 

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 indicateindicated that the use of a calculated PM2.5 light extinction indicator 

would provide a much higher degree of uniformity in terms of the degree of protection 

 

25 As noted at the time of proposal, the sheer size of the ambient 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 approach 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 PM2.5 massbased 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 PM2.5

mass on the same day or averaged across a month and then applied to measurements of 

PM2.5 mass on each day of the month. 

27 An organic carbon (OC)-to-organic mass (OM) multiplier of 1.6 was used for the 

assessment, which was found to produce a value of OM comparable to the one derived 

with the original, albeit more complex Visibility Assessment method.
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from visibility impairment across the country than a PM2.5 mass indicator, because a 

calculated PM2.5 light extinction indicator would directly incorporate the effects of 

humidity and PM2.5 composition differences between various regions. Further, the 

proposal noted that the Policy Assessment concluded that consideration could be given to 

defining a calculated PM2.5 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, PM2.5

mass. In addition, there are mixed results of data quality assessments 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 PM2.5 continuous FEMs 

on a 1-hour or multi-hour (e.g., 4-hour) basis, these factors increase the uncertainty of 

utilizing continuous methods to support 1-hour or 4-hour PM2.5 mass measurements as an 

input to the light extinction calculation. Therefore, as noted at the time of proposal, until 

issues regarding the comparability of 24-hour PM2.5 mass values derived from continuous 

FEMs and filter-based FRMs28 are resolved, there is reason to be cautious about relying 

on a calculation procedure that uses hourly PM2.5 mass values reported by continuous 

FEMs in combination with speciated PM2.5 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 atmosphere that most directly relates to visibility 

 

28 Filter-based FRMs are designed to adequately quantify the amount of PM2.5 collected 

over 24-hours. They cannot be presumed to be appropriate for quantifying average 

concentrations over 1-hour or 4-hour periods.
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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 concluded that the calculated PM2.5 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 this indicator, CASAC agreed with the staff 

preference presented in the second draft Policy Assessment for a calculated PM2.5 light 

extinction indicator. CASAC noted that “[I]ts reliance on procedures that have already 

been implemented in the CSN and routinely collected continuous PM2.5 data suggest that 

it could be implemented much sooner than a directly measured indicator” (Samet, 2010d, 

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 CASAC 

advice into account, the Administrator provisionally concluded that a new calculated 

PM2.5 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 appropriate 

 

29 In commenting on the second draft Policy Assessment, CASAC did not have an 

opportunity to review the assessment of continuous PM2.5 FEMs compared to collocated 

FRMs (Hanley and Reff, 2011) as presented and discussed in the final Policy Assessment 

(U.S. EPA, 2011a, p. 4-50).
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indicator to replace the current PM2.5 mass indicator. Such an indicator, referred to as a 

PM2.5 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 PM2.5 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 PM2.5 standards. The Administrator also 

noted that such a PM2.5 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 PM2.5 composition and relative humidity across 

the country. 

Based on these above considerations, the Administrator proposed to set a distinct 

secondary standard for PM2.5 defined in terms of a PM2.5 visibility index (i.e., a 

calculated PM2.5 light extinction indicator, 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 PM2.5 visibility index rather than a PM10 visibility index 

which 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 IMPROVE algorithm;

(4) the use of historical monthly, seasonal, or regional speciation averages;
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(5) alternative approaches to determining relative humidity; and

(6) simplified approaches to generating hourly PM2.5 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 PM2.5 visibility index was 

informed by information related to the nature of PM visibility effects and the nature of 

inputs to the calculation of PM2.5 light extinction, as discussed above. The EPA 

considered both sub-daily (1- and 4-hour averaging times) and 24-hour averaging times. 

In considering sub-daily averaging times, 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. 

Taking 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 conditions, 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, CASAC 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 
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averaging time is well within the instrument response times of the various currently 

available and developing optical monitoring methods. 

However, CASAC also advised that if a PM2.5 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 PM2.5 light extinction 

indicator relies in part on measured PM2.5 mass, it would be appropriate to consider a 

multi-hour averaging time on the order of a few hours (e.g. 4-hours). A multi-hour 

averaging time might reasonably characterize the visibility effects experienced by the 

segment of the population who have access to visibility conditions often or continuously 

throughout 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 Assessment 

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 different 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 

averaging time. Thus, the Policy Assessment concluded that it is appropriate to consider a 
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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 PM2.5 light extinction indicator (U.S. EPA, 2011a, 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 PM2.5 instruments operating at state/local monitoring 

sites, the Policy Assessment noted that the occurrence of questionable outliers in 1-hour 

data submitted to AQS from continuous FEM PM2.5 instruments had been observed at 

some of these sites (Evangelista, 2011). Some of these outliers were questionable simply 

by virtue of their extreme magnitude, as high as 985 μg/m3, 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 concentrations 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 validation 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 

PM2.5 light extinction calculated on a 24-hour basis to reduce the various data quality 

 

30 Similarly questionable hourly data were not observed in the 2005 to 2007 continuous 

PM2.5 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. Therefore, there were 

more opportunities for very infrequent measurement errors to be observed in the larger 

FEM data set. 
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concerns described above with respect to relying on continuous PM2.5 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 those 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 light extinction 

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 PM2.5 indicator. For these 

analyses, 4-hour average PM2.5 light extinction was calculated based on using the 

Visibility Assessment approach. The 24-hour average PM2.5 light extinction was 

calculated using the original IMPROVE algorithm and long-term relative humidity 

conditions to calculate PM2.5 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 PM2.5 light extinction were constructed for each of the 15 

cities included in the Visibility Assessment and for all 15 cities pooled 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 dayspecific 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 

 

31 These analyses are also based on the use of a 90th percentile form, averaged over 3 

years, as discussed below in section VI.D.3 and in section 4.3.3 of the Policy Assessment 

(U.S. EPA, 2011a).
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average PM2.5 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 This suggested that PM2.5 light 

extinction calculated on a 24-hour basis is a reasonable and appropriate surrogate to 

PM2.5 light extinction calculated on a sub-daily basis. 

Taking the above considerations and CASAC’s advice into account, the Policy 

Assessment concluded that it would be appropriate to consider a 24-hour averaging time, 

in conjunction with a calculated PM2.5 light extinction indicator and an appropriately 

specified standard level, as discussed below. By using site-specific daily data on PM2.5

composition and site-specific long-term relative humidity conditions, this 24-hour 

average indicator would provide more consistent protection from PM2.5-related visibility 

impairment than would a secondary PM2.5 NAAQS based only on 24-hour or annual 

average PM2.5 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 PM2.5 light extinction indicator and with further consideration of the 

data quality issues discussed above. Such an averaging time, to the extent that data 

quality issues can be appropriately addressed, would be more directly related to the shortterm nature of the perception of visibility impairment, short-term variability in PMrelated visual air quality, and the short-term nature (hourly to multiple hours) of 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 

 

32 The EPA staff noted that the R2

value (0.44) for Houston was notably lower than for 

the other cities.
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instrument variability with respect to PM2.5 mass measurement. In conjunction 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 hours 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 

sensitive to data quality issues, including short-term variability 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 PM2.5 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 time 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 PM2.5 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). 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 
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multi-hour averaging time would also apply to the calculated PM2.5 light extinction 

indicator since hourly PM2.5 mass measurements are also required 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 PM2.5

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 

PM2.5 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 period of 4 hours, which is consistent with CASAC’s advice 

concerning an averaging time associated with the use of a PM2.5 mass indicator.

Taking into account the information discussed above with regard to analyses and 

conclusions presented in the final Policy Assessment the Administrator recognized 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 the 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 recognized that data quality uncertainties have recently 

been associated with currently available instruments that would be used to provide the 

hourly PM2.5 mass measurements that would be needed in conjunction with an averaging 

time shorter than 24-hours. As a result, while the Administrator recognized the 
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desirability of a sub-daily averaging time, she had 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 

PM2.5 light extinction calculated on a 24-hour averaging basis is a reasonable and 

appropriate surrogate for sub-daily PM2.5 light extinction calculated on a 4-hour average 

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 PM2.5 visibility 

index. 

iv. Form

As discussed in section 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 PM2.5 NAAQS is such 

that the level of the standard is compared to the 3-year average of the annual 98th

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 can result in unusually high pollution levels for a particular year. The use of a multiyear percentile form, among other things, makes the standard less subject to the 

possibility of transient violations caused by statistically unusual indicator values, thereby 

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 
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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 air 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 

frequency of time that visibility levels should 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 eliminating 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, 90th, 95th, and 98th

percentile forms were assessed 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 80th 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 90th 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 
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or below the level of the standard would reasonably 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 

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 Assessment 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 wellbeing effects associated with those visibility conditions. However, it is logical to 
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conclude that people with limited opportunities to experience visibility conditions on a 

daily basis would experience the entire impact associated with visibility based on their 

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 part of the Visibility 

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 daily daylight 1-hour 

approach using the 90th percentile form and an all daylight hours approach combined with 

the 98th percentile form (U.S. EPA, 2010b, section 4.1.4). This suggestssuggested 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 PM2.5 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 concluded that it would be appropriate to give primary consideration to a 90th

percentile form, averaged over three years (U.S. EPA, 2011a, p. 4-60). Further, in the 
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case of a multi-hour, sub-daily alternative standard, the Policy Assessment concluded that 

it would be appropriate to give primary consideration to a form based on the maximum 

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

4-60). This sub-daily form would be expected to provide appropriate protection for 

various segments of the population, including those with limited opportunities during a 

day and those with more extended opportunities over the daylight hours to experience 

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 considering 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 90th

percentile form, averaged over 3 years, is appropriate, and proposed such a form in 

conjunction with a PM2.5 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 PM2.5 visibility index and an 90th 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 various 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 
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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-daily standard (e.g., 1- to 4-hour averaging time) of 20, 25, and 30 dv, which are 

approximately equivalent to PM2.5 light extinction of values of 65, 110, and 190 

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 PM2.5 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 

 

33 In 2009, the D.C. Circuit remanded the secondary PM2.5 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 failure to identify a target level 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.
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CPLs are in fact generally reflective of the results from the Denver and Washington, DC 

studies respectively, and the middle CPL is very near to the 50th 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 visibility preference studies, the Policy 

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 uncertainty 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 standard with CPLs 

of 20, 25, and 30 dv. In conducting these analyses, staff initially expected that the values 

of 24-hour average PM2.5 light extinction and daily maximum daylight 4-hour average 

PM2.5 light extinction would differ on any given day, with the shorter term peak value 
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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 fully 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 TableTables G-67 and G-8 of 

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 ranged 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 cityspecific estimates of generally equivalent 24-hour levels included the 4-hour level for 

each of the CPLs of 20, 25, and 30 dv. As noted 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 

 

34Note 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 Tables G-7 and G-8, 

respectively, 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.

35As discussed in more detail 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 

greater 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 values of f(RH) used to determine daily maximum 4-hour daylight light extinction 

on the same day. Second, PM2.5 concentrations may be greater during non-daylight 

periods than during daylight hours.
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identify a generally equivalent 24-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 observed in urban areas across the country, so as to reduce the 

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. These 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 specific 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 between 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 
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achieving appropriate levels of protection against PM-related visibility impairment in 

urban, non-urban, and Federal Class I areas across 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 nonurban 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 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.

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 PM2.5 light extinction indicator, a 90th percentile form, 

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 
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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 conjunction with the Regional Haze Program to achieve appropriate levels of 

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 

proposal, 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 PM2.5 visibility index; a 90th

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 information 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 PM2.5 visibility index and a 90th

percentile form, averaged over 3 years) to provide appropriate protection from PMrelated 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 PM2.5 visibility index in any given urban area, such that there is a relatively high 
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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. 

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 

would be appropriate 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 number 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 second 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, provide 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 Washington, 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 uncertainties. 
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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 levels are based.

(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 with 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. indicator, 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 distinct 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 considerations described above, the 

Administrator proposed to revise the suite of secondary PM standards by adding a 

distinct standard for PM2.5 to address PM-related visibility impairment, focused primarily 

on visibility in urban areas. This proposed visibility standard was to be defined in terms 

of a PM2.5 visibility index, which would use measured PM2.5 mass, combined with PM2.5

speciation data and relative humidity data, to calculate PM2.5 light extinction, translated 

into the deciview (dv) scale; a 24-hour averaging time; a 90th percentile form, averaged 
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over 3 years; and a level of 28-30 dv. 

vii. Related Technical Analysis

At the time of proposal, the EPA conducted a two-pronged technical analysis of 

the relationships between the proposed PM2.5 visibility index standard and the current 24-

hour PM2.5 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. Specifically, 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 standard under the Prevention of Significant 

Deterioration (PSD) program. Such a demonstration for the proposed secondary PM2.5

visibility index NAAQSstandard could require each PSD applicant to predict, via air 

quality modeling, the increase in visibility impairment, in terms of the proposed PM2.5

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 PM2.5 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 PM2.5

visibility index design values from monitored air concentrations of PM2.5 components, 

significant technical issues 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/m3 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 
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PSD significant impact analysis by evaluating whether an individual source’s impact 

resulting in a small increase in the ambient PM2.5 concentration would produce a 

comparably small increase in visibility impairment. This analysis included estimates of 

PM2.5 speciation profiles based on direct PM2.5 emission profiles for a broad range of 

source categories and for theoretical upper and lower bound scenarios.

The 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 PM2.5 NAAQSstandard and coincident design values for the proposed 

PM2.5 visibility index NAAQSstandard based on recent air quality data. This aspect of the 

analysis indicated that increases in 24-hour PM2.5 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 PM2.5 NAAQSstandard as a surrogate for a demonstration that 

a source does not cause or contribute to a violation of the proposed secondary PM2.5

visibility index NAAQSstandard. This analysis iswas based on 2008 to 2010 air quality 

data, and comparescompared the proposed level of 35 μg/m3for the 24-hour PM2.5

standard and for illustrative purposes an alternative standard level of 12 μg/m3for the 

annual PM2.5 standard with the proposed levels of 28 or 30 dv for the secondary PM2.5

visibility index standard in conjunction with a 24-hour averaging time and a 90th 

percentile form. The results indicateindicated that all (for the 30 dv level) or nearly all 

(for the 28 dv level) areas in attainment of the 24-hour PM2.5 standard would also behave 

been in attainment of the proposed secondary PM2.5 visibility index standard.

Based on this technical analysis, the EPA proposed that there is sufficient 
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evidence that a demonstration that a source does not cause or contribute to a violation of 

the mass-based 24-hour PM2.5 NAAQSstandard serves as a suitable surrogate for 

demonstrating that a source does not cause or contribute to a violation of the proposed 

secondary 24-hour PM2.5 visibility index NAAQSstandard under the PSD program. As 

such, the EPA proposed to conclude that many or all sources undergoing PSD review for 

PM2.5 could rely upon their analysis for demonstrating that they do not cause or 

contribute to a violation of the mass-based 24-hour PM2.5 NAAQSstandard to also show 

that they do not cause or contribute to a violation of the proposed secondary PM2.5

visibility index NAAQSstandard, if a distinct visibility standard 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 PM2.5

NAAQSstandard and the proposed PM2.5 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 PM2.5 visibility index standard, the existing 

24-hour PM2.5 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 PM2.5 visibility index explored the 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 PM2.5 NAAQSstandard and the proposed secondary visibility index standard is not 

obvious a priori because of differences in design value calculations for the standards. 
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However, closer examination of this relationship indicated that increases or decreases in 

24-hour PM2.5 design values correspond, respectively, to increases or decreases in 

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 PM2.5 design values and the visibility index design values with r2values ranging 

from 0.65 to 0.98 across these sites, with an average r2value of 0.75 across all U.S. sites. 

Moreover, the data indicated that no design value existed where the visibility index 

design value exceeded 30 dv, but the 24-hour PM2.5 NAAQSstandard level of 35 μg/m3

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 PM2.5 design values for 

these sites were below 35 μg/m3. This was attributed to the combination of high nitrate 

and sulfate fractions, substantial RH adjustment factors, and PM2.5 distribution 

characteristics that led to relatively high visibility index design values for a given 24-hour 

PM2.5 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 PM2.5 NAAQS level of 35 μg/m3 would attain the proposed secondary 

PM2.5 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 PM2.5 level and the secondaryproposed visibility index 30 dv level, the 

secondary standardproposed level of 30 dv would likely be attained if PM2.5

concentrations were reduced such that the 24-hour PM2.5 level of 35 μg/m3 was attained.

 

36 Kelly et al. (2012a) also noted that “Regional reductions in sulfate PM2.5 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).
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(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 thea visibility index NAAQSstandard (28-30 

dv), the 24-hour PM2.5 standard of 35 μg/m3 would be controlling in almost all (at 28 dv) 

or all (at 30 dv) 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 soiling, and climatic and radiative processes (71 FR 61144, 

October 17, 2006). Specifically, there was a lack of evidence linking various nonvisibility welfare effects to specific levels of ambient PM. In that review, to provide a 

level of protection for these 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 outlines 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 

particulate matter components of NOx and SOx and their transformation products which 

 

37 The analysis also showed that attaining the 24-hour PM2.5 standard level of 35 μg/m3

would result in achieving a lower PM2.5 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 composition of ambient PM2.5 and the lower relative humidity 

in those areas.
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are addressed fully in the joint review of the secondary NO2 and SO2 NAAQS.

a. Evidence of Other Welfare Effects 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 Science 

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 

current mass-based PM2.5 and PM10 secondary standards were not an appropriate or 

effective means of focusing protection against PM-associated climate effects due to these 

 

38 Atmospheric PM is referred to as aerosols in the remainder of this section to be 

consistent with the Integrated Science Assessment.
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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 associated 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. 

Ecological 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 biological 

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 

findings related to:

(1) Impacts on plants and the ecosystem services they 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;

 

39 This conclusion would apply for both the secondary (welfare-based) and the primary 

(health-based) standards. 
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(2) Impacts on ecosystem support 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 PBDEs; 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 

industrial point sources (quarries, cement kilns, metal smelting) (U.S. EPA, 2009a, 

sections 9.4.3 and 9.4.5.7).

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.
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(2) Ecosystem services can be adversely impacted by PM in the environment, 

including supporting, provisioning, 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 appropriate 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 ecosystems 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 NOx/SOx secondary review (e.g., metals, organics) (U.S. 

EPA, 2011a, p. 5-24). 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 damage (materials damage effects) and 

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 between 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 
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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 Assessment 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 role 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 increased 

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 processes 

and contributions of black crust to soiling are not sufficient for quantitative 

analysis.

(5) While several studies in the PM Integrated Science Assessment and NOx/SOx

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 degradation. 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 evidence in this review 

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).
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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 forcing 

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). CASAC noted that the best-established effects are related to 

particles containing nitrogen and sulfur, which are being considered in the EPA’s 

ongoing review of the secondary NAAQS for NOx/SOx. 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 
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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 standard.

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 distinct 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 

generally to retain the current suite of secondary PM standards40 to address non-visibility 

welfare effects. Specifically, the Administrator proposed to retain all aspects of the 

current secondary 24-hour PM2.5 and PM10 standards. With regard to the secondary 

annual PM2.5 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 PM2.5 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 standard to protect against visibility impairment, discussed 

 

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

standards includes annual and 24-hour PM2.5 standards and a 24-hour PM10 standard. 
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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 discussed in section 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 PM2.5 standard to remove the option for spatial averaging consistent 

with this change to the primary annual PM2.5 standard. The Administrator has also 

decided, contrary to what was proposed, not to establish a distinct secondary standard to 

address PM-related visibility impairment. This section discusses EPA’s responses to the 

comments EPA received on its proposal, and the 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 to set a distinct secondary PM2.5

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, NCBA, NEDA-CAP, NMA, 

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 PM2.5 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 expressed the view that it was 

premature to set such a standard pending collection of additional visibility preference 
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study data and the resolution of a number of key technical issues. Support for setting such 

a distinct secondary standard for visibility at this 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 

proposed 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 

other implementation challenges.

A discussion of the significant comments outlined above, 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 discussed 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 
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setting a secondary standard based on the proposed visibility 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 PM2.5

standards, the appropriateness of the protection that would be provided by the proposed 

PM2.5 visibility index standard, and the relationship between this degree of protection and 

that provided by the current secondary 24-hour secondary PM2.5 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 

PM2.5 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 

calculated PM2.5 light extinction as the basis for the indicator rather than relying on direct 

measurements of PM2.5 light extinction, commenters from both groups expressed concern 

over specific elements of the proposed method of calculating PM2.5 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 relative humidity and PM2.5

 

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 are comments regarding 

monitoring issues related to the proposed distinct visibility index standard.
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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 stated 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 extinction 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 review and had concluded 

that relying on a calculated PM2.5 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 calculated PM2.5 light extinction 

indicator, at least as an interim approach.

Commenters opposed to setting a distinct standard generally argued that it was 

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 PM2.5 speciation 

filter analysis, rather 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 

 

42 Some commenters expressed concern about the omission of other contributors to 

visibility impairment from the visibility index, as discussed in the Response to Comments 

document.
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stated that relying 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 

CASAC 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 proposal 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 developed in the 

time frame of this review. Moreover, CASAC concluded that relying on a calculated 

PM2.5 light extinction indicator based on PM2.5 chemical speciation and relative humidity

data represented a reasonable approach. The inputs that are necessary include 

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 PM2.5 light extinction and PM-related 

visibility impairment. Furthermore, CASAC agreed, noting that the proposed calculated 

PM2.5 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 PM2.5 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 
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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 algorithm has been widely 

incorporated into Regional Haze plans, and urged the EPA to use this same equation in 

the visibility index for the sake of consistency: “EPA approved this approach for regional 

haze and does not dispute its greater accuracy. Therefore, 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 original 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 
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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 coarse 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 “acceptable” 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 “deciview” calculated for purposes of the 

proposed PM2.5 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 PM2.5 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. 

Third, some commenters 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 relative 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 the prescribed monitoring regime to identify periods when 
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PM2.5 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 sensitive 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 humidity 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 PM2.5 visibility index values could 

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 with coincident 

optical measurements. As discussed above in section VI.B.1.a.i, the 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 

 

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 using 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 measurements in a number of rural Class I areas.
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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, the 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 are discussed further below, along 

with adjustments that the EPA believes are likely appropriate based on the current 

(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 SANDWICH 

mass closure approach in the Urban Focused Visibility Assessment (U.S. EPA, 2010b)

for purposes of calculating maximum daylight hourly PM2.5 light extinction and 

 

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 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.
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evaluated which multiplier would produce 24-hour results most similar to the 

SANDWICH approach using 24-hour PM2.5 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 PM2.5 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 approach 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 

whether this value was appropriate. Comments received by the Agency generally indicate 

that the 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 

inappropriate 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 rural 

 

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 Procedures in the Speciation Network,”

http://www.epa.gov/ttn/amtic/files/ambient/pm25/spec/faqcarbon.pdf)
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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). 

Although the newrevised 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 newrevised algorithm to best represent the more complex and different urban 

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 

CSN network, the EPA agrees with commentersjudges that a multiplier of 1.4 is likely 

too low, and 6 for urban areas would be most appropriate for purposes of calculating 

PM2.5 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 

 

46 The difference between higher PM2.5 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).

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, VI.C.1.e, and VI.C.1.f, respectively.
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analyses the EPA conducted for Appendix F of the Policy Assessment, afor the OC to 

OM multiplier of 1.6 for urban areas is judged 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 hygroscopic 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 contributor to light extinction in urban areas. Moreover, 

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 PM2.5 components into the concentrations of 

the small and large size 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 

emissions. This is a particular concern not only for organic mass, which as explained 

previously has a large contribution from nearby urban emission sources, but also for 

PM2.5 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 compared 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 
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of calculating light extinction in urban areas for purposes of calculating the 90th

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 IMPROVE 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 

the original algorithm for purposes of calculating urban light extinction. 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 with commenters who suggested 

that coarse particles should be included in the calculated light extinction indicator. As 

noted in the proposal, PM2.5 is the component of PM responsible for most of the visibility 

impairment in most urban areas. Currently available data suggest that PM10–2.5 is a minor 

contributor to visibility impairment most of the time, although at some locations (U.S. 

EPA, 2010b, Figure 3-13 for Phoenix) PM10–2.5 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 PM10-2.5 monitoring. This 

significantly increases the difficulty of 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 PM10 data allowing calculation of hourly PM10-

2.5 data for 2005 to 2007. In addition, PM10-2.5 is generally less homogenous in urban 
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areas than PM2.5: in that coarse particlesparticle 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 PM10–2.5 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 includes coarse particles at this time, and the calculated indicator should be based on 

PM2.5 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 

and its effects of visibility impairment, and this approach focuses 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 of climatological data appropriately 
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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 approach 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 continues to conclude that a calculated PM2.5 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 PM2.5 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 conjunction 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 PM2.5

visibility index defined in this way would appropriately reflect the relationship between 

ambient 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 PM2.5 speciation 

concentrations and 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 PM2.5

standards. Such a PM2.5 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 PM2.5 composition and relative humidity across 
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the country. 

c. Averaging time

Few commenters specifically addressed the issue 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 with the emphasis among all 

commenter groupscommenters 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 averaging 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 light extinction values to the 

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 relationship, 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 values was as low as r2= 0.42 in Houston, and stated 

that the EPA was being overly “optimistic” in concluding that city-specific and pooled r2

values in the range of 0.6 to 0.8 showed good correlation (UARG, Attachment 2, p. 27).
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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 have 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 averaging time is needed for an urban-focused PM2.5 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 visibility 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 

have the information or infrastructure 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 
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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 averaging 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 response 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 PM2.5 light extinction indicator would be most appropriate, 

the EPA has 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 

humidity, 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’ general 

point that, as a starting premise, a sub-daily averaging time would generally be 

preferable. 
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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 association 

with currently available instruments that would be used to provide the hourly PM2.5 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 reservations 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, effectively 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 concerns preclude adoption of 

a sub-daily averaging time.

Moreover, analyses conducted for the Policy Assessment indicate that PM2.5 light 

extinction calculated on a 24-hour average basis would be a reasonable and appropriate 

surrogate for PM2.5 light extinction calculated on a 4-hour basis. The scatter plots 

comparing 24-hour and 4-hour calculated PM2.5 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 24-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 
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individual days in 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 of 90th 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 considered 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-

hour values were drawn not just on the basis of the city-specific results but also on the 

more robust 90th 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 90th 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 state that a 

24-hour averaging time cannot serve as an appropriate surrogate for sub-daily periods of 
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visibility impairment. On the contrary, the EPA continues to conclude, on the basis of 

this analysis, that PM2.5 light extinction calculated on a 24-hour basis is a reasonable and 

appropriate surrogate for sub-daily PM2.5 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.) Hourspecific 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 variability 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 that is sufficiently long to ensure that 

hour-specific influences are balanced against more typical conditions. Perhaps even more 

important is the concern that many peak hourly measurements may be significantly 

influenced by atypical instrument performance; this 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 

EPA concludes that a 24-hour averaging time would be appropriate for a distinct 

secondary standard based on a calculated PM2.5 light extinction indicator. 

d. Form
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The EPA received very few comments with regard to the proposal to adopt a 90th

percentile form, averaged over 3-years, in conjunction with a PM2.5 visibility index and a 

24-hour averaging time. One commenter stated that it was inappropriate to use a 90th

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 intent 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 90th 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 90th percentile form advocated averaging the 90th percentile values over longer 

time periods, arguing that averaging over only 3 years would not provide a stable 

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 

high 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 
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the standard. Furthermore, this approach is consistent with the approach taken in other 

NAAQS, including the current 24-hour PM2.5 NAAQS, which has a 98th percentile form 

averaged over 3 years.. With regard to the comment opposing the 90th 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 should 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 number 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 80th percentile of visibility impairment), the EPA continues to 

believe, as noted in the proposal, that a percentile well above the 80th percentile would be 

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 90th

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 90th percentile 

of visibility impairment values.
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With regard to comments requesting the EPA adopt a longer multi-year averaging 

period for the 90th percentile values, the EPA disagrees that it would be appropriate to 

average the 90th percentile values over periods longer than 3 years. The EPA recognizes 

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. Utilizing a 3-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. The Agency has adopted this

approach in other NAAQS, including the current secondary 24-hour PM2.5 NAAQS, 

which has a 98th 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 protectiveness of a visibilitythe standard. Based on this the EPA does not 

believe it would be appropriate to average 90th percentile values over a period as long as 

five years. Therefore, the EPA continues to conclude that a 90th percentile form, averaged 

over 3 years, would be appropriate, in conjunction with a calculated PM2.5 visibility 

indexlight 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 addressed 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 preference studies as the basis for the selection of a range of appropriate levels 
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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 

studies 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. These 

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 levels 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 highlighted 

a number of limitations and uncertainties (enumerated below) and uncertainties

associated with these studies as support for this view. In contrast, other commenters felt 

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 select a single level for a national 

standard. 
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(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 comments. 

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 the 

Response to Comments document. 

The primary or most frequent limitation(s) cited by many commenters 

relaterelates to the small number of preference studies that are available in this review. In 

particular, some commenters note that these preference studies 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 

were conducted in the West, while only one was 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, 
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AMC et al. state:

We believe that these studies provide sufficient results to inform setting a national 

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 different methods and respondents and yield a range of 20 – 24 dv, with one 

outlier of 29. (AMC, et al., pp. 6-7)

In responseRegarding 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 3897538973). 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 appropriately weighs the associated 

uncertainties. In this review of the PM standards, the EPA notes that The CASAC

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, 2010b, p. iii). As discussed below, in 

determining an appropriate degree of protection for visibility,in the proposed rule (77 FR 

38990), the Administrator has recognized and explicitly takentook 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 continues to be mindful of these uncertainties and 
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limitations in reaching her final determination 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 regarding 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, results represent 

an outlier. Rather, these results 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 

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 Rule 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 

available 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 serious 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 IVVI.A.1 of the proposal specifically identifies the 
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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 IVVI.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 the 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 perspectives on the 

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 

responses48; (2) an extensivea 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 

 

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/B55911DF9796E5E385257592006FB737/$F

ile/CRA+VAQ+Pref+Robustness+Study+3+30+09+final.pdf.
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studies and evaluated the potential significance of those differences on the study results49; 

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 Visibility 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 to an increase in the EPA’s current knowledge and understanding of 

the robustness of each survey studystudy’s results, the appropriateness of comparing each 

study’s results to the others, and the key uncertainties relevant to data interpretation. In 

addition, in the last review the decision to not adopt a distinct secondary standard was 

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 basis for a 

secondary NAAQS (see 77 FR 38975) and solicited comment on how these uncertainties 

 

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.
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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 Howell 

(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 identify 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 judgment 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 

artifacts 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) 

EPA also fails 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 analysisstudy 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 research would be useful to further inform our limited understanding of 

public preferences in urban areas. The 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 
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suggest: 1) the 2001 s results, while based on a small sample size of 9, may bewere 

consistent with results from a fair representation for the preferences forlarger 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) doesdid 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 PM2.5-related visibility effects on the 

public welfare are occurring. In this context, several 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 study 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 for 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 

PSpreference studies 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 
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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 

impairment 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 welfare. 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 would also be categorized 

as having an adverse effect on personal comfort and well-being. It is in the context of all 

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 "known 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-related 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 stress, 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 these 

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 
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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 review 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 sufficient 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 

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 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 impairment of VAQ. However a 

secondary standard is designed to provide protection from "known 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).

Another issue raised in the comments regarding adversity is the EPA’s decision to 

use the 50 percent acceptability criterion from the public preference studies in 

determining candidate protection levels of visibility impairment. In particular, some of 
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these commenters questioned the appropriateness of using the preference study 50 

percent acceptability 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 visibility 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 percent 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 

standard was set at the level of extinction that would divide the slides into two 

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 percent 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. TheThat same criterion was usedjudged 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 currently under 

consideration by British Columbia, Canada. ThusFurthermore, CASAC supported this 

approach, while recognizing the uncertainty associated with this issue. Specifically, 

CASAC agreed that “the 50th percentile for the acceptability criteria is logical, given the 
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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 50th 

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 served 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, consistent with the advice of CASAC, that it is 

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 results, concluding that these results 

were due to regionally varying factors and thus could not be used to set a national 

standard. In particularFor example, some commenters asserted that because the 

confidence intervals around the four 50 percent acceptability levels do not overlap at all, 

and because there are variations in preference study designs and inherent differences ofin

the visual setting among cities and panels, the four preference curves and their associated 

50 percent dv values are city-specific and statistically different. The commenters 

assertedconcluded, therefore, that it was inappropriate to aggregate the 50th percentile dv 

values from multiple studies and concluded that they should instead be evaluated 

individually. 
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Other commenters expressed 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...is 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 

dependent, it would be more appropriate to develop distinct visibility standards at the 

state or local level. Others pointed out that areas lackingwhich 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 relative 

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 uncertainty 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 

DOH/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 
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four unique urban scenes. However, the EPA does not agree that the study results 

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 not 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 significantly different views on 

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 study 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 participants 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 scenes and thus provide useful information about how people’s preferences for 
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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. These 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 include 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 

national scale, taking into account the variation across the country evidenced in the 

studies. This is discussed further in the next section, below.

The EPA agreesdoes 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. Section 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 gone 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 result in a uniform level of ambient PM2.5, 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 response as to 

how it is taking these variables into account. 

ii. Specific Comments on Level
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The EPA received relatively few comments endorsing a specific level for a 

distinct secondary standard for visibility. In general, commenters who opposed setting a 

distinct 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 time 

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 

advocating standards at the upper end of the range of proposed levels (i.e., 30 dv), while 

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 

neither 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 commenters 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 range 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 adjusted 4-hour values into 24-hour values. Based 
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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 of 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 

necessary to protect public welfare” (NSSGA, p. 15). Arizona DEQ noted that since the 

proposed calculated light extinction indicator excluded coarse 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 

lower 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 recent 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 
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averaging time. One commenter emphasized that the Policy Assessment had indicated 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 PM2.5

visibility index standard should be set at a level of 18.6-20 dv. The Department of the 

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 calculations 

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 PM2.5 standard of 35 

μg/m3and 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 the 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, 
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p. 2)

Similarly, the Colorado Department of Public 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 with 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 

standard, particularly 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 

CASAC, 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 the 

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 
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would be afforded by a 4-hour, 30 dv visibility index standard.

Moreover, the EPA disagrees with commenters who argued that the EPA’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 approaches for 

estimating generally equivalent levels on a city-specific or national basis, and that both of 

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 r2values 

(Approaches A and B in Appendix G of the Policy Assessment),51 which used regressions 

of 90th percentile light extinction values, the EPA determined it would also be appropriate 

to consider the city-specific estimates resulting from Approaches 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 approach 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 

 

51In 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 Policy 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.
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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 spatial and temporal variability in relative humidity and fine 

particle composition observed in urban areas across the country, consistent with EPA’s 

reanalysis discussed below. 

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 

analysis shows that, for all cases considered, the 24-hour equivalent level is 

evengenerally closer to the 4-hour values value at the upper end of the range of CPLs

than originally estimated, demonstratingas 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 dv53 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 90th percentile and pooled 

average values closer to the 4-hour CPL of 30 dv, the range of city-specific results shows 

 

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 adjusted 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 Assessment, 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.
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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 purposes of selecting a level 

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 

studies 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 visibility 

preference study scenes including mountains in the background and objects at greater 

 

56 The analysis in Appendix G of the Policy Assessment used the 4-hour light extinction 

value treated as the independent (x-axis) variable in an ordinary least squares regression. 

The 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 

conclusion 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 used 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.
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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 extinction changes 

compared to otherwise similar scenes with objects at a shorter range, this likely explains 

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 vistas 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. Therefore, 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 preference 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 well 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 locations. Making 
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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 

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 appropriate 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 the PM2.5 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 principally 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 
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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 

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 improvements in areas that may already be relatively clean. In fact, the 

evidence from the IMPROVE program that many urban areas have total 24-hour PMrelated 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 PM2.5 standard of 35 

μg/m3 would protect against visibility impacts exceeding the range of levels considered 

in the proposal (28-30 dv). While this analysis was conducted in support 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 PM2.5 standard of 35 μg/m3 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 PM2.5 NAAQS level of 35 μg/m3

would attain the proposed secondary PM2.5 visibility index NAAQS level of 30 dv and 
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generally attain the level of 28 dv” (pp. 17-18). 

Citing this conclusion, many state and local agencies and industry commenters 

argued that a visibility index standard in the range proposed (28-30 dv) would provide no 

additional protection beyond that afforded by the existing secondary 24-hour PM2.5

NAAQS, and therefore no distinct visibility standard was necessary. These commenters 

advocated retaining the current 24-hour PM2.5 mass-based standard to protect against 

visibility effects. “Since the 24-hour PM2.5 standard already protects the welfare the 24-

hour PM2.5 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 standard 

which had, in their view, no practical effect: “If the 24-hour PM2.5 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 agreed: “A PM2.5-

related Visibility Index appears redundant since the benefits achieved from the current 

primary and secondary annual and 24-hour PM2.5 standards already provide reductions 

that would improve visibility. Establishing a new PM2.5 secondary standard 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/m324-hour PM2.5 standard would 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 
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needed, 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 

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 visibility 

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 

Administrator also noted that the current indicator of PM2.5 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 

humidity, 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 PM2.5 standards, the Administrator provisionally concluded that the 

current 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 that 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 PM2.5 standards 

werewas better suited scientifically to the task of protecting fromagainst visibility 

impairment. Rather, the commenters’ position that a distinct secondary standard was not 
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needed for purposes of protecting visibility was based almost entirely on the relative 

degree of protection likely to be afforded by the existing standards (in particular, the 

existing 24-hour PM2.5 standard) as compared to the proposed visibility index, along with 

the relatively large uncertainties associated with the latter. Thus, for all the reasons 

discussed in the proposal with regard 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 provide 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 PM2.5 in the 

ambient 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 

expressed by commenters on the implications of this analysis for conclusions regarding 

the adequacy of the current secondary 24-hour PM2.5 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 defined in terms 
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of a PM2.5 visibility index (based on speciated PM2.5 mass concentrations and relative 

humidity data to calculate PM2.5 light extinction), a 24-hour averaging time, and a 90th

percentile form, averaged over 3 years, and a level of 30 dv, would provide sufficient but 

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 position to compare 

it specifically to the existing secondary 24-hour PM2.5 standard of 35 μg/m3for purposes 

of determining whether it would provide more, the same, or less protection from visibility 

impairment. The EPA must consider both whether the existing secondary 24-hour PM2.5

standard of 35 μg/m3is 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 PM2.5 standard 

provides sufficient 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 2006 

secondary standards, the EPA had argued that the 35 μg/m324-hour PM2.5 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 counties 

than the 35 μg/m324-hour PM2.5 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 standards. 

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, 
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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 PM2.5 standard of 35 μg/m3. Stated another 

way, all areas that met the 24-hour PM2.5 standard of 35 μg/m3 would have had visual air 

quality at least as good as 30 dv (24-hour average, based on 90th 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 PM2.5 standard of 35 μg/m3 would have 

had visual air quality better than 30 dv for the PM2.5 visibility index standard, and that at 

sites that violated both the 24-hour level and the secondary visibility index 30 dv level, 

the secondary standardvisibility index level of 30 dv would likely be attained if PM2.5

concentrations were reduced such that the 24-hour PM2.5 level of 35 μg/m3 was attained. 

The EPA has conducted a reanalysis (Kelly et al., 2012b) to update the area-byarea analysis in the original Kelly et al. (2012a) analysis in three respects. FirstFirst, 

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 PM2.5 design values as presented in Kelly et al., 

2012a. 

Second, the EPA repeated the calculations comparing visibility index design

 

57 Some of the OC measurements were produced with CSN’s newer monitoring protocol 

and did not require a change in the computed OM.
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index values with 24-hour PM2.5 design values using 2009-2011 data, the most recent 

three years of air quality information currently available.58 Third, the EPA modified the 

area-by-area evaluation to ensure consistency with the data completeness criteria of 40 

CFR part 50, Appendix N, including the removal of data approved by EPA as exceptional 

events, for the current 24–hour PM2.5 NAAQSstandard and the proposed visibility index 

NAAQSstandard.

The results of this reanalysis, 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 arewere no areas with visibility impairment above 30 dv that dodid not also 

exceed the 24-hour PM2.5 standard of 35 μg/m3. 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 PM2.5 standard of 

35 μg/m3 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 reached a different conclusion, specifically 

that the 35 μg/m324-hour PM2.5 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 light 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.
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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 with 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, especially 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 range of 

public commenters, the EPA is persuadednow concludes that the 24-hour PM2.5 standard 

of 35 μg/m3provides sufficient protection in all areas against the effects of visibility 

impairment. The EPA concludes that the existing 24-hour PM2.5 standard provideswould 

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.
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However, the EPA also recognizes that it is important to evaluate whether such 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 

PM2.5 standard of 35 μg/m3 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/m3, 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 PM2.5 standard of 35 

μg/m3 would be over-protective of visibility in some areas. However, it is important to 

note that as long as the current secondary 24-hour PM2.5 standard of 35 μg/m3remains 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, which 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 PM2.5 visibility index. The proposed 

standard would use measured PM2.5 mass concentration, in combination with speciated 

PM2.5 mass concentration and relative humidity data, to calculate PM2.5 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 PM2.5 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 
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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 Resources 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) fail 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 concentration. (UARG, p. 22-23)

Other commenters raised similar or related issues, arguing that the EPA 

improperly set a visibility standard, and not a PM2.5 standard, and that NAAQS can only 

be set in terms of a level or concentration of the air pollutant. Commenters 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 PM2.5 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, the 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 

substances or matter in the ambient air” (NSSGA, p. 8). The commenters pointed to the 
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use of the term “air pollutant” in sections 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 108(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 not a PM2.5 standard as it does not limit the concentration of PM2.5 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 “specify 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 legislative history of the 

1970 amendments, referring to NAAQS as setting the “maximum permissible ambient air 

level” for an air pollutant. The commenters argue that the proposed standard is improper 

because it does not limit the concentration of PM2.5 or any 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 

determine the value of the visibility index. According to these commenters, the CAA 
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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 PM2.5 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 

treats 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 commenters 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 PM2.5 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 areas. The commenters noted that establishing a single 

national level for the PM2.5 visibility index would necessarily result in unequal acceptable 

PM2.5 levels in different areas of the country, with lowest allowable PM2.5 levels in urban 
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areas in the Southeast 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 alter 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 concentration 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 PM2.5 light 

extinction. According to these commenters, “EPA cannot use NAAQS to indirectly 

regulate multiple substances 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 air 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 particulate 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 

quality criteria document for those components” (NMA/NCBA, p. 29). According to 

these commenters, the approach the EPA adopted in promulgating a NAAQS for lead 
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supports this view: 

When EPA promulgated a NAAQS for lead, an individual substance in particle 

form, 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 do 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 NAAQS 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, including 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 PM2.5, and is not a 

“visibility standard.” The proposed secondary standard is based on the mass 

concentration of PM2.5 in the ambient air. The standard is defined in terms of calculated 

PM2.5 light extinction which is based on the measurement of the mass concentration of 

ambient PM2.5 over a 24-hour period. The measured mass concentration is adjusted based

on information on the speciated mass components of the PM2.5 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 

concentration of PM2.5 achieves the standard or not. Given any specific mass 

concentration of ambient PM2.5, combined with information on speciation and relative 
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humidity, it can be determined whether the specific mass concentration of ambient PM2.5

achieved the NAAQS. Hence, the proposed secondary NAAQS specifies acceptable 

levels of ambient mass concentration of PM2.5.

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 PM2.5. It does this by calculating the light 

extinction associated with ambient concentrations of PM2.5 and specifying the level of 

acceptable PM2.5 mass concentration in terms of this calculation. However this does not 

change the fact that the standard is for the air pollutant PM2.5, and defines acceptable 

ambient levels of this air pollutant. It does not transform the standard into a “visibility 

standard” and not a standard for PM2.5. 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 not change the fact 

that the proposed standard defined in terms of calculated PM2.5 light extinction is based 

on measurement of PM2.5 concentration in the ambient air, and is a NAAQS for PM2.5. 

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 unit 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 

such metrics in air pollution studies. For example, the current secondary standards for 

PM are defined in terms of the concentration of PM2.5 and PM10 in the ambient air, 
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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. 

Sections 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 

public 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 requirements, 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 

air quality for purposes of setting a NAAQS. The EPA is charged with setting air quality 

standards, and has the discretion under section 109(b) to choose the metric for defining 
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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 primary NAAQS for NO2 concentration over 3 hours. 

This addressed Congress’ concern over whether the then current NO2 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 provided 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. 

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 

 

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” listed 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.
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of protection of public welfare. The commenters’ interpretation 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 extinction caused by 

PM2.5. See 77 FR 38890, 38980-1 (June 29, 2012). There are several instrumental 

methods that directly measure PM2.5 light extinction – the amount of light extinction 

caused by the presence of PM2.5 in the ambient air. This is not a measure of the 

concentration of PM2.5 in the air, but a measure of the light extinction caused by PM2.5. 

This is clearly an effect associated with the presence of PM2.5 in the ambient air, and this 

atmospheric property is directly related to visibility effects. Unlike PM2.5 mass 

concentration, there is a close scientific relationship between directly measured PM2.5

light extinction and visibility effects.

It would appear straightforward to say that PM2.5 light extinction is a quality of 

the ambient air, and a secondary NAAQS that specified an acceptable level of PM2.5

based on directly measured PM2.5 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 visibility impairment. Unlike directly measured PM2.5 light extinction, 

the mass concentration of PM2.5 does not have the same direct relationship to light 

extinction, and specifying an acceptable level of mass concentration of PM2.5 would be a 

more indirect and less effective way to provide protection from visibility impairment 

caused by the presence of PM2.5 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 PM2.5 light extinction, the more scientifically appropriate and direct measure of 
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the effect PM2.5 has on visibility. Instead the EPA would be limited to the more indirect 

and less effective specification of a level of concentration of PM2.5.

The commenters also objected to the inclusion of relative humidity as an 

adjustment factor in the calculation of PM2.5 light extinction. Contrary to the claims of 

these commenters, the use of calculated PM2.5 light extinction does not regulate relative 

humidity. The proposed secondary standard would define acceptable levels of ambient 

PM2.5, 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 the 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 

into account in setting the standard. Including relative humidity as an adjustment factor in 

the calculation of PM2.5 light extinction is a reasonable and straightforward way to use 

the scientific information in the air quality criteria in establishing a standard 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 

(October 17, 2006). In the last review the EPA considered a distinct PM2.5 mass-based 

secondary standard. In that context, limiting the measurement of PM2.5 mass 

concentration to the mid-day hours when relative humidity had the least impact would 

promote the correlation between measured PM2.5 mass concentration and light extinction, 

 

60 UARG recognizes these provisions, but argues, as above, that this is limited by the 

requirement that EPA set a NAAQS based solely on ambient concentration. 
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which would promote achievement of a relatively consistent degree of visibility 

protection across the country. However in this rulemaking the proposed calculated PM2.5

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 PM2.5 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 PM2.5

that confounds the impact PM2.5 has on light extinction. As described in the Integrated 

Science Assessment, “PM 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 

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 absorb water and 

the resulting particle is larger in size and scatters more light, increasing the visibility 

 

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 exposure effect 

(which may be null)’ (Rothman and Greenland, 1998, 086599)” (U.S. EPA, 2009a, p. 1-

16).
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impact of the particle. But the particle is still a PM2.5 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 PM2.5 particles are what is in the air and impacting visibility, not just the dried 

mass of PM2.5 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 PM2.5, when in fact the proposed 

secondary standard accounts in a scientific manner for the fact that some PM2.5 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 PM2.5

light extinction, compared to a standard based on just PM2.5 mass concentration, 

improperly results in variable levels of acceptable PM2.5 mass concentrations across the 

country. This stems from the adjustments in the calculation for speciated components of 

PM2.5 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 

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. The proposed 

secondary standard would do just that – specify a level of air quality, defined in terms of 

calculated PM2.5 light extinction, that would achieve the desired degree of protection. The 
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fact that this results in varying allowable levels of PM2.5 mass concentrations is not 

inconsistent with the Act. The D.C. Circuit recently approved such a result. In the last 

review of the PM10 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/m3 standard for PM10 will 

result in arbitrarily varying levels of coarse PM, and that the agency should 

instead have used a PM10-2.5 indicator. The EPA does not dispute that using the 

PM10 indicator will result in coarse PM levels that vary within the limit of 150 

μg/m3. As the EPA explains: "Because the PM10 indicator includes both coarse 

PM (PM10-2.5) and fine PM (PM2.5), the concentration of PM10-2.5 allowed by a 

PM10 standard set at a single level declines as the concentration of PM2.5

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 PM10 indicator will result 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 the 

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 

other words: "The varying levels of coarse particles allowed by a PM10 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 EPA also offers a further rationale for tying the stringency of coarse PM 

regulation to increases in the level of PM2.5. ... EPA argues that it is "logical to 

allow lower levels of coarse particles when fine particle concentrations are high. . 

. . [I]nclusion of PM2.5 in the PM10 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, 559 F.3d 512, 534-5 (D.C. Cir. 2009). 

A similar result applies here. Under the proposed secondary standard there would 
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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 PM2.5 would be 

defined the same way across the nation, using the same method of calculating the 

allowable concentration of PM2.5. The same degree of protection from visibility 

impairment would apply across the country. While the allowable amount of PM2.5 could 

vary, this would be a reasoned way to achieve the desired degree of protection from 

visibility impairment. The requirements of section 109(b) would be satisfied.

Commenters also objected that the EPA could not set a NAAQS for the separate 

components of PM2.5 without listing the components of PM2.5 under section 108, based 

on an endangerment finding, and issuing air quality criteria for these components. 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 PM2.5. 

Information on both the speciated components of PM2.5 and the relative humidity affect 

how much light extinction is associated with any specific level of PM2.5, but the standard 

is for PM2.5. The D.C. Circuit has made it clear that PM2.5, just like PM10 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 Trucking Association et al. v. EPA, 175 F.3d 1027, 
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1055 (D.C. Cir. 1999). Commenters’ interpretation would apply to PM2.5 as well as to 

components of PM2.5, 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 PM2.5

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 inappropriate 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 supported setting a distinct secondary standard to protect 

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 visibility impairment in the exact same location. These 

commenters claimed that since data from the IMPROVE monitoring network would 
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inform nonattainment designations, as well as an area’s obligations under the Regional 

Haze Program, there could be considerable 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 

Federal Class I areas over the course of several decades, a distinct secondary standard for 

PM-related 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 

visibility 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 
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as measured 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 

visibility 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 

burden 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 and 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 stated that a distinct visibility 

standard as proposed would inherently conflict with the Regional Haze Program or that it 

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.
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2. Comments on the Proposed Decision Regarding Non-Visibility Welfare Effects

Relatively few commenters addressed the proposal 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 who 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 PM2.5 standard of 15 μg/m3in light of the 

proposal to revise the level of the primary annual PM2.5 standard to a level between 12-13 

μg/m3. 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 PM2.5 standards to be equivalent in all respects to the primary PM2.5 standards. 

For example, the American Association of State Highway and Transportation Officials 

(AASHTO) supported “retaining secondary standards that are consistent with the primary 

standards in order to reduce the complexity 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 PM2.5 standard, the commenters 

recommended that the EPA adopt this same lower level for the primary secondary PM2.5

standard as well. 

In response to these comments, the EPA notes that the Agency lacks an 

appropriate scientific basis for revising the level of the secondary annual PM2.5 standard. 
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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 secondary 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 standards62 to address non-visibility welfare 

effects, while revising only the form of the secondary annual PM2.5 standard to remove 

the option for spatial averaging consistent with this change to the primary annual PM2.5

standard, as proposed.

Other commenters 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 “higher standards for gasoline quality” (Urban Air Initiative, p. 4). 

This commenter did not, however, advocate specific secondary NAAQS to address 

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 PM2.5 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 secondary NAAQS. Rather, the Act 

mandates promulgation of secondary standards requisite to protect public welfare 

from any “anticipated adverse effects associated with” regulated pollutants, 42 

 

62 As summarized in section VI.A and Table 1 above, the current suite of secondary PM 

standards includes annual and 24-hour PM2.5 standards and a 24-hour PM10 standard. 
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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 studies 

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 commenter 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 argued 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 change on public 

health adequately in the proposal.

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 Black 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 decisionmaking” (CBD, p. 13). The commenter also 

noted that the United StatesU.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 
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of aerosols generally, arguing that they “do not apply to the regulation of black carbon” 

(CBD, p. 14). Specifically, the commenter 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 due 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 aerosols”; 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 

commenter argued that “Black carbon must be regulated separately and in addition to 

PM2.5 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 Assessment 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 

PM2.5 and PM10 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 
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black carbon from existing engines, industrial and 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 carbon, 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 decrease 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 

EPA’s recent Report to Congress on Black Carbon (U.S. EPA, 2012c). In 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 information 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 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 lifetime” (U.S. 

EPA, 2009a, section 9.3.10), it also identified substantial remaining uncertainties with 
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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 pointed 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 that “few quantitative 

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. EPA, 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 the 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 
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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, nitrates, 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-induced 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 climate 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.

The Report to Congress on Black Carbon also stressed the importance of 

considering co-emitted PM species, such as SO2 and NOx, 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 the 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 is 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 carbon 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 
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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 levels 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 past century, due to changes in fuel use, more 

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. Clearly, 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 PM2.5 concentrations and/or direct

particle emissions” (U.S. EPA, 2012c, p. 161). The EPA has acknowledged the need to 

encourage PM mitigation strategies that focus on reducing directly emitted PM2.5 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 
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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 yet 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 reduction 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 reductions” 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 

the uncertainties associated with the spatial and 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 secondary 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

 

63 This conclusion applies for both the secondary (welfare-based) and the primary 

(health-based) standards. 
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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 PM2.5 standard of 15 μg/m3

a 24-hour PM2.5 standard of 35 μg/m3, and a 24-hour PM10 standard of 150 μg/m3, to 

address PM-related welfare effects, including visibility impairment as well as other PMrelated 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 PM2.5 standard to remove the option for 

spatial averaging consistent with this change to the primary annual PM2.5 standard. It also 

explains the Administrator’s decision, contrary to what was proposed, not to establish a 

distinct standard to address PM-related visibility 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 information 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 Administrator 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 protect against visibility 

impairment reflect, in part, her conclusions on the secondary standardPM standards for 

non-visibility welfare effects, section VI.D.1 first outlines her conclusions regarding the 
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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 

protectaddress 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 standards 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 

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 section 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 effects, 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 PM2.5 and PM10 standards. With regard to the secondary annual PM2.5

standard, the Administrator concludes that it is appropriate to retain a level of 15.0 μg/m3
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for this standard while revising only the form of the secondary annual PM2.5 standard to 

remove the option for spatial averaging consistent with this change to the primary annual 

PM2.5 standard. 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 PM2.5 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 would be requisite to 

protect public welfare with regard to visual air quality. The Administrator then 

determined whether to adopt a distinct secondary standard to achieve this target level of 

protection. In making this decision, the Administrator compared the 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 PM2.5 standard of 35 μg/m3.

64

Based on 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 PM2.5 visibility index as proposed, 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 PM2.5 in the ambient air and PM-related visibility impairment. Such a PM2.5

 

64 This focus on the 24-hour PM2.5 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. 
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visibility 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 PM2.5 composition and relative humidity across the country. 

In defining a target level of protection based on a PM2.5 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 PM2.5 light 

extinction indicator based on PM2.5 chemical speciation and relative humidity data 

represented a reasonable approach. Based on the analyses conducted in support of this 

rulemaking, as described above, as well as the advice of CASAC, the Administrator 

concludes that a calculated PM2.5 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 

PM2.5 visibility index standard. 

With regard to averaging time, the Administrator 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 short-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 PM2.5 mass measurements that would be needed in conjunction with 
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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 PM2.5 light extinction calculated on a 24-hour average basis would be a 

reasonable and appropriate surrogate for PM2.5 light extinction calculated on a 4-hour 

basis. Thus, the Administrator concludes that a 24-hour averaging time would be 

appropriate for a PM2.5 visibility index. The Administrator recognizes that a 24-hour 

averaging time would effectively reduce the influence of peak hours of visibility 

impairment on visibility index values, but concludes that in light of the concern that peak 

hourly measurements may be significantly influenced by atypical conditions and/or 

atypical instrument performance, 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 NAAQS, including the current 24-hour PM2.5 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 standard. 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 90th percentile, which represents the median of the 
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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 reasonably 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 PM-related light 

extinction in urban areas to a greater degree. Therefore, the Administrator concludes that 

a 90th percentile form, averaged over 3 years, is appropriate, for purposes of establishing 

a target level of protection in terms of a 24-hour PM2.5 visibility index.

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 the 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 PM2.5 visibility index with a 

24-hour averaging time that would provide protection equivalent to the protection 

afforded by a 4-hour PM2.5 visibility index with a level of 30 dv. Furthermore, the 

Administrator notes that for purposes of estimatingthe approaches used to estimate

generally equivalent levels for a 24-hour PM2.5 visibility index , both of the EPA’s two 

preferred approaches generated 90th 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 
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CPL of 30 dv at the upper end of 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 PM2.5 visibility index. 

In considering the alternative levels proposed 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 

preference studies and that 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 PM2.5 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 

PM2.5 visibility index (based on speciated PM2.5 mass concentrations and relative 

humidity data to calculate PM2.5 light extinction), a 24-hour averaging time, and a 90th

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 forto address PM-related visibility impairment would be designed to 

work in conjunction with the Regional Haze Program as a means of achieving 
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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 

NAAQS to address PM-related visibility NAAQSimpairment would focus on protecting 

visual air quality principally 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, could 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.

Having concluded that the protection provided by a standard defined in terms of a 

PM2.5 visibility index, with a 24-hour averaging time, and a 90th 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 determined 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 there is also a 24-hour 

PM2.5 standard of 35 μg/m3. 

In this context, the Administrator has considered the degree of protection from 

visibility impairment afforded by the existing secondary PM2.5 standards. The 
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Administrator has considered both whether the existing 24-hour PM2.5 standard of 35 

μg/m3is 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. (2012a; 

2012b) analyses indicate that based on 2008-2010 and 2009-2011 data, all areas meeting 

the 24-hour PM2.5 standard of 35 μg/m3had visual air quality at least as good as 30 dv 

(24-hour average, based on 90th percentile form averaged over 3 years). This means that 

it is highly likely that the secondary 24-hour PM2.5 standard of 35 μg/m3 would be 

controlling relative to a 24-hour standard based on a PM2.5 visibility index set at a level of 

30 dv, and highly unlikely that areas would exceed the target level of protection 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 PM2.5 standard of 35 μg/m3provides sufficient protection in all areas 

against the effects of visibility impairment—i.e., that the existing 24-hour PM2.5 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 PM2.5 standard of 35 μg/m3also would likely 

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/m3, some areas would have

levels of PM-related visibility impairment below 30 dv. Thus, the 24-hour PM2.5 standard 

of 35 μg/m3 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 
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mass-based standard does not account for variation in particle species and relative 

humidity. The 24-hour PM2.5 standard of 35 μg/m3 would provide more than the 

necessary protection 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 PM2.5 standard of 35 μg/m3for 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 PM2.5 visibility index. The issue is 

not whether to adopt a PM2.5 visibility index standard when viewed in isolation, but 

whether such a distinct secondary standard should be adopted in addition to the current 

secondary 24-hour PM2.5 standard of 35 μg/m3. The EPA notes that adoption of such a 

distinct secondary standard is not 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 PM2.5 standard of 35 μg/m3 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 secondary 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 adoption of a distinct secondary standard will not change the degree of overprotection 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 
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531

both 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 PM2.5 visibility index standard against the degree of protection provided 

by the current secondary 24-hour PM2.5 standard of 35 μg/m3. 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 PM2.5

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 withhas 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 

PM2.5 visibility index at a level of 30 dv based on a 24-hour averaging time, and a 90th

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 current secondary 24-hour PM2.5 secondary standard 

of 35 μg/m3, 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 protect public welfare consistent with section 

109(b)(2). This provides a clear basis for judging that the current secondary 24-hour 
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532

PM2.5 secondary standard of 35 μg/m3 would provide sufficient protection. The analysis 

also shows that the current 24-hour secondary 24-hour PM2.5 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 visibility, 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 provide requisite protection for both 

visibility and non-visibility related welfare effects. Thus the EPA’s decision has carefully 

considered and accounted for the views of the court in the remand of the 2006 NAAQS. 

E. Administrator’s Final Decisions on Secondary 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 form of the annual 

standard. Specifically, the EPA is retaining the current secondary 24-hour PM2.5 and 

PM10 standard to address PM-related non-visibility welfare effects including ecological 

effects, effects on materials, and climate impacts, the EPA is retaining the current 

secondary 24-hour PM2.5 and PM10 standard and is revising only the form of the 

secondary annual PM2.5 standard to remove the option for spatial averaging consistent 

with this change to the primary annual PM2.5 standard. With respect to PM-related 

visibility impairment, the Administrator has identified a target degree of protection, 

defined in terms of a PM2.5 visibility index (based on speciated PM2.5 mass 

concentrations and relative humidity data to calculate PM2.5 light extinction), a 24-hour 
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averaging time, and a 90th 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 basis for 

concluding that the current secondary 24-hour PM2.5 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 overprotection of visual air quality provided for some areas of the country by the secondary 

24-hour PM2.5 standard, the Administrator judges that adoption of a distinct secondary 

standard, in addition to the current suite of secondary standards, is not needed to provide 

requisite protection for both visibility and non-visibility related welfare effects.