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From: Lydia Wegman/RTP/USEPA/US

To: nfrey@omb.eop.gov

Cc: Janet McCabe/DC/USEPA/US@EPA, martin.karen@epa.gov, Joseph-J 

Dougherty/DC/USEPA/US@EPA, hannon.john@epa.gov, Beth HassettSipple/RTP/USEPA/US@EPA

Date: 06/07/2012 04:35 PM

Subject: Revisions to the secondary standard level section

Nathan, 

Please find attached some revisions to the level section for the secondary standard for visibility. We can 

discuss when I call you, which I will do shortly. Thanks. 

Lydia
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4. Level (p. 295)

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 percentile-based 

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 standard (Samet, 2010d, p. 11).1The Policy Assessment also 

 

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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 decisionmaking 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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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 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. 
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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 

 

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 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
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the 4-hour level for each of the CPLs.3

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 

 

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 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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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 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 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 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. The Policy Assessment was also 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.

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 
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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 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 
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appropriate in conjunction with the Regional Haze Program to achieve appropriate levels of 

protection against PM-related 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. 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 
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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.

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 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 
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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.

F. Administrator’s Proposed Decisions on Secondary PM Standards (p. 324)

With regard to the secondary PM standards, the Administrator proposes to set a new 

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

urban areas. The new secondary standard is defined in terms of a calculated PM2.5 light 

extinction indicator, translated into the deciview scale, which is referred to as a PM2.5 visibility 

index; a 24-hour averaging time; a 90th percentile form, averaged over 3 years; and a level set at 

one of two options – either 30 dv or 28 dv. The Administrator solicits comment on a range of 

levels for such a standard down to 25 dv, as well as on alternative standards to address PMrelated visibility impairment, including a sub-daily averaging time (e.g., 4 hours) and associated 

alternative levels in the range of 30 to 25 dv. To address other non-visibility welfare effects, the 

Administrator proposes to revise the form of the secondary annual PM2.5 standard to remove the 

option for spatial averaging and to retain all other elements of the current suite of secondary PM 

standards.