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From: Lydia Wegman/RTP/USEPA/US

To: nfrey@omb.eop.gov, dmancini@omb.eop.gov

Cc: martin.karen@epa.gov, hannon.john@epa.gov, Beth Hassett-Sipple/RTP/USEPA/US@EPA, Joseph-J 

Dougherty/DC/USEPA/US@EPA, Janet McCabe/DC/USEPA/US@EPA, Gina 

McCarthy/DC/USEPA/US@EPA

Date: 06/14/2012 04:10 PM

Subject: Edits to the secondary standard section 

Nathan and Dom, 

Attached are edits to the secondary standard section addressing interagency comments received last night. 

Please let us know if you have any questions.
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Revisions to section VI.D.4 in draft PM NAAQS NPR

VI. Rationale for Proposed Decisions on the Secondary Standards

D. Consideration of Alternative Standards for Visibility Impairment

4. Level

In considering alternative levels for a new standard that would provide requisite protection 

against PM-related visibility impairment primarily in urban areas, the Policy Assessment has taken into 

account the evidence- and impact-based considerations discussed above and in section 4.2.1 of the 

Policy Assessment, 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 this information in the context of a 

standard defined in terms of a calculated PM2.5 light extinction indicator, discussed above and in the 

Policy Assessment section 4.3.1; with alternative averaging times of 24-hours or multi-hour, sub-daily 

periods (e.g., 4-hours), discussed above and in Policy Assessment section 4.3.2; and a 90th percentilebased form, discussed above and in section 4.3.3 of the Policy Assessment.

As part of the Policy Assessment’s assessment of the adequacy of the current standards, 

summarized in section VI.B. above and in Policy Assessment section 4.2.1, it 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 notes that CASAC agreed that this was an appropriate range of levels to consider for such a 
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standard (Samet, 2010d, p. 11).1The Policy Assessment also recognizes 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 is 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.

As an initial matter, in considering alternative levels for a sub-daily standard based directly on 

the four preference study results, the Policy Assessment notes that the individual low and high 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. As discussed 

above and in section 4.2.1 of the Policy Assessment, the Phoenix study was by far the best of the 

studies, providing somewhat more support for the middle CPL. In considering the results from these 

studies, the Policy Assessment recognizes that the 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 

recognizes 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 concludes, as did CASAC, that these studies are 

appropriate to use for this purpose (US EPA, 2011a, p. 4-61).

In considering potential alternative levels for a 24-hour standard, the Policy Assessment explores 

 

1

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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various approaches to adjusting the CPLs derived directly from the preference studies, as presented and 

discussed in Appendix G of the Policy Assessment, especially section G-5. 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 so doing, staff recognized that 

there are multiple approaches for estimating generally equivalent levels on a city-specific or national 

basis, and that the inherent spatial and temporal variability in relative humidity and fine particle 

composition across cities leads to a set of alternative estimates of levels that may be construed as being 

generally equivalent on a national basis.

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 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 Table G-6 of Appendix G, 2in considering estimates aggregated or 

averaged over all 15 cities as well as the range of city-specific estimates for the various approaches 

considered, 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.3

 

2Note 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 G6 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

3As 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 
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Some of the approaches used in these analyses focused on comparing 24-hour and 4-hour light 

extinction values in each of the 15 urban areas, whereas other approaches focused on comparisons based 

on using aggregated data across the urban areas. Two of these approaches, which used regressions of 

city-specific annual 90th percentile light extinction values or 3-year light extinction design values, gave 

nearly identical results and were considered by staff to be most appropriate for further consideration. 

These approaches (shown in US EPA, 2011a, Appendix G, Figures G-7 and G-8, referred to as 

Approaches A and B) were preferred by staff based on the high R2values of the regressions and because 

the regressions were determined by data from days with PM2.5 light extinction conditions in the range of 

20 to 40 dv. This contrasted with the other approaches that were influenced by PM2.5 light extinction 

conditions well below this range. Based on these analyses (presented in Appendix G of the Policy 

Assessment), the Policy Assessment notes that the single approach thought by staff to be more 

appropriate for further consideration (referred to as Approach B in Appendix G) yielded adjusted 24-

hour CPLs of 21, 25, and 28 dv as being levels that are generally equivalent in an aggregate or central 

tendency sense to 4-hour CPLs of 20, 25, and 30 dv.4

Two of the approaches yielded not only estimates of generally equivalent levels on an aggregated 

basis but also city-specific estimates (referred to as Approaches C and E in Appendix G) that showed 

greater variability than the aggregated estimates. In all cases, the range of city-specific estimates of 

generally equivalent 24-hour levels included the 4-hour level for each of the CPLs of 20, 25, and 30 dv 

(as shown in Tables G-7 and G-8, Appendix G of the Policy Assessment, for Approaches C and E, 

respectively). Looking more broadly at these results could support consideration of using the same CPL 

 

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.

4 To provide some perspective in considering these results (US EPA, 2011a, Appendix G, Table G-6), 

the Policy Assessment notes that 1 dv is about the amount that persons can distinguish when viewing 

scenic vistas, and that a difference of 1 dv is equivalent to about a 10 percent difference in light 

extinction expressed in Mm-1.
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for a 24-hour standard as for a 4-hour standard, recognizing that there is no one approach that can most 

closely identify a generally equivalent 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 more broadly considering alternative standard levels that would be appropriate for a nationally 

applicable secondary standard focused on protection from PM-related urban visibility impairment based 

on either a 24-hour or multi-hour, sub-daily (e.g., 4-hour) averaging time, the Policy Assessment was 

mindful of the important limitations in the available evidence from public preference studies. While the 

Policy Assessment concluded, consistent with CASAC advice, that it is appropriate to consider a distinct 

secondary PM2.5 standard to address PM-related visibility impairment focused primarily in urban areas

based on the evidence from public preference studies, it also recognized that there are a number of 

uncertainties and limitations associated with the preference studies that have served as a basis for 

selecting an appropriate range of levels to consider, as discussed above in section VI.B.2. 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 Policy Assessment was 

mindful 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, and that there is the possibility that there could 

be regional differences in individual preferences for VAQ.
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Given these uncertainties and limitations noted above, the EPA broadly solicits comment on the 

strengths and limitations associated with these 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). In particular, the EPA solicits comment on the following specific aspects of the public preference 

studies and on how these studies should appropriately be considered in this review. Recognizing that all 

of these studies evaluated a 50 percent acceptability criterion as the basis for reaching judgments in the 

context of each study , the EPA requests comment on the extent to which this 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. Recognizing that these 

studies vary in the extent to which the study participants may be representative of the broader study area 

population, the EPA requests comment on how this aspect of the study designs should appropriately be 

weighed in the context of considering these studies in reaching proposed conclusions on a distinct 

secondary NAAQS. The EPA also solicits comment on 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.

As in past reviews, the EPA is considering a national visibility standard in conjunction with the 

Regional Haze Program as a means of achieving appropriate levels of protection against PM-related 

visibility impairment in urban, non-urban, and Federal Class I areas 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 non-urban areas as 

well, as would programs now being developed to address the requirements of the Regional Haze 

Program established for protection of visual air quality in Federal Class I areas. The EPA also believes 

that the development of local programs, such as those in Denver and Phoenix, can continue to be an 
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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.

Based on the above considerations, the Policy Assessment concludes 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 (US EPA, 2011a, p. 4-63). Thus, the Policy 

Assessment concludes 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 (US EPA, 

2011a, p. 4-63). The Policy Assessment judges that such standards would provide appropriate protection 

against PM-related visibility impairment primarily in urban areas. The Policy Assessment notes that 

CASAC generally supported consideration of the 20 – 30 dv range as CPLs and, more specifically, that 

support for consideration of the upper part of the range of the CPLs derived from the public preference 

studies was expressed by some CASAC Panel members during the public meeting on the second draft 

Policy Assessment. The Policy Assessment concludes that such a standard would be appropriate in 

conjunction with the Regional Haze Program to achieve appropriate levels of protection against PMrelated visibility impairment in areas across the country (US EPA, 2011a, p. 4-63).

Based on the above considerations, taking into account the conclusions in the Policy Assessment 

and the extent to which those conclusions reflected consideration of CASAC advice during the 

development of the Policy Assessment, as an initial matter, the Administrator provisionally concludes

that it is 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. 
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More specifically, the Administrator provisionally concludes 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 is supported by the 

available information and is generally consistent with the advice of CASAC. 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 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 cityto-city variability as a basis for determining an appropriate degree of protection on a national scale.

In light of these provisional conclusions, the Administrator proposes to set 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 PM-related visibility impairment based on one of two options. 

One option is to set the level of such a standard at 30 dv and the other option is to set the level at 28 dv. 

In so doing, the Administrator solicits comment on each of these levels and on the various approaches to 

identifying generally equivalent levels discussed above upon which the alternative proposed levels are 

based. Recognizing that there was some support for consideration of a broader range of levels, the 

Administrator also solicits comment on a range of levels down to 25 dv in conjunction with a 24-hour 

averaging time. Further, having solicited comment on a sub-daily (e.g., 4-hour) averaging time, the 

Administrator also solicits comment on a range of alternative levels from 30 to 25 dv in conjunction 

with such a sub-daily averaging time.

Finally, as we have indicated, the information available for the Administrator to consider when 
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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). We invite comment on how the 

Administrator should weigh those uncertainties as well as any additional comments and information to 

inform her consideration of these uncertainties.