UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON D.C. 20460 

OFFICE OF THE ADMINISTRATOR 

 SCIENCE ADVISORY BOARD 

November 24, 2009 

EPA-CASAC-10-002 

The Honorable Lisa P. Jackson 

Administrator 

U.S. Environmental Protection Agency 

1200 Pennsylvania Avenue, N.W. 

Washington, D.C. 20460 

Subject: Review of Particulate Matter Urban-Focused Visibility Assessment (External 

Review Draft, September 2009)

Dear Administrator Jackson: 

The Clean Air Scientific Advisory Committee (CASAC) Particulate Matter (PM) 

National Ambient Air Quality Standards (NAAQS) Review Panel met on October 5 - 6, 2009 to 

review the Particulate Matter Urban-Focused Visibility Assessment (External Review Draft, 

September 2009). The Panel also held a public teleconference on November 12, 2009 to discuss 

and finalize its draft report. In this letter, CASAC offers general comments on the Visibility 

Assessment, followed by consensus responses to the Agency’s charge questions. Comments 

from individual panelists are also attached. 

The Urban-Focused Visibility Assessment provides a sound basis for EPA to consider a 

new secondary PM standard based on visibility as a welfare endpoint. CASAC was impressed 

by the comprehensive approach, quality of analysis, and clarity of presentation evident in the 

draft assessment. While the Visibility Assessment focused only on selected urban areas, a new 

secondary PM standard would apply to all urban and rural areas of the country, and provide an 

effective complement to the protection currently provided in Class 1 Federal National Parks and 

Wilderness Areas by the 1999 Regional Haze Rule. CASAC strongly supports the introduction 

of a new PM light extinction indicator for a secondary standard to protect against adverse effects 

on visibility. We also concur that an hourly averaging time most appropriately represents the 

nearly instantaneous nature of human perception of impaired visibility. The 20 to 30 deciview 

(DV) range of light extinction levels evaluated in the assessment is a good representation of the 

levels of visibility impairment considered to be unacceptable based on the urban visibility 

preference studies currently available. CASAC also considers this to be a reasonable range for 

the Administrator to consider in selecting a new secondary PM NAAQS. 

To evaluate the PM light extinction indicator, EPA chose the 90th and 95th percentiles of 

the daily maximum daylight hours as the form for this indicator. These percentiles were used to 

evaluate relationships among various levels for primary PM2.5 mass standards and new secondary 
. 

PM light extinction standards under consideration as “candidate protection levels.” However, 

neither of these percentiles can be supported as the single best choice of the form of the standard, 

and CASAC recommends that other percentiles be considered, up to and including the 98th

percentile used for the current 24-hour primary and secondary standards. CASAC also 

recommends lowering the relative humidity (RH) limit from 95% to 90% and using it as a screen 

(i.e., hours above it should be discarded) rather than a cap, to more clearly exclude weather 

events like fog or precipitation and to minimize effects of measurement error and spatial 

variability. 

Visibility varies between the more humid and often more polluted Eastern U.S. and the 

Western U.S.; however, the Visibility Assessment demonstrates that current visibility conditions 

in all the urban study areas would sometimes fail to meet even the most lenient candidate 

protection level under consideration (a PM extinction level of 30 DV). In general, the more 

humid Eastern urban areas would have more days that fail to meet the standard. In fact, Eastern 

urban areas would exceed a 30 DV threshold on an average of 20% or more of the days even 

after assuming full compliance with the most stringent primary PM standard under consideration 

(12μg/m3 and 25μg/m3). The limited available urban preference studies showed a stronger 

preference for higher visibility in Denver, Colorado than in Washington, D.C. and considered 

reasons that might explain this phenomenon such as the inherent differences in the landscape 

scenes used in each location. We encourage the Agency to conduct additional visibility 

preference studies prior to the next PM NAAQS review to support refinements of this standard in 

future PM NAAQS review cycles. In these future studies, it will be important to continue 

exploring whether there are differences in urban visibility preferences by location, the reasons 

for these differences and what implications, if any, such differences might have for policy 

options and implementation strategies. 

 

EPA staff have advised that implementation strategies are separate from the CASAC 

review activities; however, we note that scientific information indicates that a “progress based” 

metric rather than a “threshold based” national standard would best ensure progress toward 

protecting the urban visibility welfare value under the PM NAAQS indicator in both Western 

and Eastern states. A progress-based secondary PM NAAQS would incorporate elements of the 

traditional standard setting process, retaining the indicator, averaging time, form and level. 

Compliance with the standard would then be based on maintaining a minimal percentage 

reduction over time in the frequency of daylight hours or days exceeding the level. A standard 

requiring a uniform rate of progress over time would encourage a reasonable rate of 

improvement in the East, while still providing protection and incentives for improvement in the 

West. An ongoing, steady, progress-based approach would be more directly consistent with, and 

could be much more efficiently coordinated with, the Regional Haze Rule requirements for 

reasonable rates of progress in improving visibility in all (Eastern and Western) National Parks 

and Wilderness Areas with Class I status, regardless of the current baseline levels of impairment. 

Moreover, it would be consistent with the absence of fixed time requirements for attaining 

secondary standards, which are to be achieved “as expeditiously as practicable.” 

Another approach would be to set the standard at a relatively stringent level and 

percentile, such that it would be protective in the West, with the clearly stated intention to 

specify guidance on future rates of progress that will be considered reasonable and which would 

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be intentionally less rapid for urban areas than for the protected National Parks and Wilderness 

areas covered by the Regional Haze Rule. For the upcoming Policy Assessment, we ask that 

EPA consider methods for assessing progress and achieving the various secondary PM NAAQS 

options, in addition to the form, level, averaging time, and indicator options. As EPA has not 

recently promulgated any secondary NAAQS that are more stringent than the associated primary 

NAAQS, and §109 of the Clean Air Act outlines broad review and advice responsibilities for the 

CASAC, we believe both “progress based” and “threshold based” approaches would be useful to 

consider in setting the next secondary PM NAAQS, as well as anticipating the needs of future 

PM NAAQS reviews. 

 Finally, EPA should carefully consider the new monitoring program that will be required 

to implement a visibility standard. While there are a variety of commercially available 

instruments capable of measuring hourly light extinction and/or its separate components of light 

scattering and absorption, such measurements are currently not being made in most urban areas 

or in large-scale network operations. For this reason, CASAC strongly encourages the Agency 

to move quickly to establish a small, urban visibility pilot monitoring network to test, evaluate 

and refine current and evolving light extinction measurement methods. The Agency is also 

encouraged to seek additional advice from the CASAC Ambient Air Monitoring and Methods 

Subcommittee (AAMMS) on the most suitable methods for a national urban visibility monitoring 

network. 

We thank the Agency for the opportunity to provide advice on the PM Urban-Focused 

Visibility Assessment and look forward to the review of EPA’s second draft Visibility 

Assessment and the draft Policy Assessment early next year. 

 Sincerely, 

 

 /Signed/ 

Dr. Jonathan M. Samet, Chair 

Clean Air Scientific Advisory Council 

Enclosures: A. CASAC Particulate Matter Review Panel Roster 

 B. CASAC Responses to Charge Questions 

 C. Individual Panelists’ Responses to Charge Questions 

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Enclosure A

Clean Air Scientific Advisory Committee 

Particulate Matter Review Panel 

CHAIR 

Dr. Jonathan M. Samet, Professor and Chair, Department of Preventive Medicine, University of 

Southern California, Los Angeles, CA 

CASAC MEMBERS (FY 2009) 

Dr. Joseph Brain, Philip Drinker Professor of Environmental Physiology, Department of 

Environmental Health, Harvard School of Public Health, Harvard University, Boston, MA 

Dr. Ellis B. Cowling, University Distinguished Professor At-Large Emeritus, Colleges of Natural 

Resources and Agriculture and Life Sciences, North Carolina State University, Raleigh, NC 

Dr. James Crapo, Professor of Medicine, Department of Medicine, National Jewish Medical and 

Research Center, Denver, CO 

Dr. H. Christopher Frey, Professor, Department of Civil, Construction and Environmental 

Engineering, College of Engineering, North Carolina State University, Raleigh, NC 

Dr. Donna Kenski,∗

 Data Analysis Director, Lake Michigan Air Directors Consortium, 

Rosemont, IL 

Dr. Armistead (Ted) Russell, Professor, Department of Civil and Environmental Engineering, 

Georgia Institute of Technology, Atlanta, GA 

CONSULTANTS 

Dr. Lowell Ashbaugh, Associate Research Ecologist, Crocker Nuclear Lab, University of 

California, Davis, Davis, CA 

Prof. Ed Avol, Professor, Preventive Medicine, Keck School of Medicine, University of 

Southern California, Los Angeles, CA 

Dr. Wayne Cascio, Professor, Medicine, Cardiology, Brody School of Medicine at East Carolina 

University, Greenville, NC 

Dr. David Grantz, Director, Botany and Plant Sciences and Air Pollution Research Center, 

Riverside Campus and Kearney Agricultural Center, University of California, Parlier, CA 

Dr. Joseph Helble, Dean and Professor, Thayer School of Engineering, Dartmouth College, 

Hanover, NH 

 

∗

 / Unable to participate in this review. 

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Dr. Rogene Henderson, Senior Scientist Emeritus, Lovelace Respiratory Research Institute, 

Albuquerque, NM 

Dr. Philip Hopke, Bayard D. Clarkson Distinguished Professor, Department of Chemical 

Engineering, Clarkson University, Potsdam, NY 

Dr. Morton Lippmann, Professor, Nelson Institute of Environmental Medicine, New York 

University School of Medicine, Tuxedo, NY 

Dr. Helen Suh MacIntosh, Associate Professor, Environmental Health, School of Public Health, 

Harvard University, Boston, MA 

Dr. William Malm, Research Physicist, National Park Service Air Resources Division, 

Cooperative Institute for Research in the Atmosphere, Colorado State University, Fort Collins, 

CO 

Mr. Charles Thomas (Tom) Moore, Jr., Air Quality Program Manager, Western Governors' 

Association, Cooperative Institute for Research in the Atmosphere, Colorado State University, 

Fort Collins, CO 

Dr. Robert F. Phalen, Professor, Department of Community & Environmental Medicine; 

Director, Air Pollution Health Effects Laboratory; Professor of Occupational & Environmental 

Health, Center for Occupation & Environment Health, College of Medicine, University of 

California Irvine, Irvine, CA 

Dr. Kent Pinkerton, Professor, Regents of the University of California, Center for Health and the 

Environment, University of California, Davis, CA 

Mr. Richard L. Poirot, Environmental Analyst, Air Pollution Control Division, Department of 

Environmental Conservation, Vermont Agency of Natural Resources, Waterbury, VT 

Dr. Frank Speizer, Edward Kass Professor of Medicine, Channing Laboratory, Harvard Medical 

School, Boston, MA 

Dr. Sverre Vedal, Professor, Department of Environmental and Occupational Health Sciences, 

School of Public Health and Community Medicine, University of Washington, Seattle, WA 

SCIENCE ADVISORY BOARD STAFF 

Dr. Holly Stallworth, Designated Federal Officer, EPA Science Advisory Board Staff Office 

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NOTICE 

This report has been written as part of the activities of the EPA's Clean Air Scientific Advisory 

Committee (CASAC), a federal advisory committee independently chartered to provide 

extramural scientific information and advice to the Administrator and other officials of the EPA. 

CASAC provides balanced, expert assessment of scientific matters related to issues and 

problems facing the Agency. This report has not been reviewed for approval by the Agency and, 

hence, the contents of this report do not necessarily represent the views and policies of the EPA, 

nor of other agencies within the Executive Branch of the federal government. In addition, any 

mention of trade names or commercial products does not constitute a recommendation for use. 

CASAC reports are posted on the EPA Web site at: http://www.epa.gov/casac. 

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Enclosure B 

CASAC Responses to Charge Questions on the Urban-Focused Visibility Assessment

(External Review Draft, September 2009) 

1) After careful consideration of the evidence provided in the second draft ISA, and in 

particular the significant body of work that has been conducted by the Regional 

Planning Organizations under the Regional Haze Rule, (i.e. information on urban and 

rural PM concentrations and compositions), we have decided to continue to focus this 

assessment on the PM induced visibility impairment that is occurring in urban areas. 

What is the Panel’s view on this approach? Is the rationale supporting the selected 

approach clear and appropriate? 

CASAC found the rationale for pursuing a secondary PM NAAQS based on daytime urban 

visibility to be well justified, based on previous work associated with the Regional Haze Rule, on 

the existing ability to model this parameter based on available PM mass measurements, and on 

the concentration of impacted populations in these areas. There was some concern that visibility 

should not be assumed to represent all of welfare effects of PM, even though it is analytically 

more accessible at this time. 

Because urban areas represent only a small fraction of the land mass of the U.S., a meaningful 

secondary standard, complementary to the Regional Haze Rule, should extend to rural non-Class 

I areas. 

Some aspects of the assessment require more rigorous presentation. Uncertainties surrounding 

nitrate mass, including volatilization, presence in coarse and fine modes, and degree of 

neutralization, for example, require further evaluation and explanation in the document. The 

specific quantitative application of measured relative humidity, the method of determining 

daylight hours, and the actual method used to calculate policy relevant background could be 

more fully presented. 

2) After further considering the nature of urban versus more remote area PM, and in light 

of discussions with CASAC at the April 2, 2009 meeting, we have decided not to 

develop an urban optimized algorithm at this time, but instead to rely on the original 

IMPROVE algorithm to relate urban PM to local haze (PM light extinction). Is the 

Panel generally supportive of this approach? Is the rationale supporting this decision 

appropriate and clearly presented? 

The decision to use the original IMPROVE (Interagency Monitoring of Protected Visual 

Environments) algorithm to relate urban PM to local haze is suitable. Given the underlying 

uncertainties such as Roc factors for urban environments, a lack of detailed PM composition 

information and other biases in estimating species concentrations in urban areas, and the lack of 

optical measurements in urban areas, the approach taken and presented in Chapter 1 is 

reasonable. 

It is recommended that the original IMPROVE equation be modified to include sea salt, and 

consideration should be given as to what the appropriate Roc factor might be. 

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The rationale supporting this approach is appropriate and clearly articulated in the document. 

3) In a change from the planned approach presented in the Scope and Methods Plan, we 

have decided to conduct a reanalysis of the urban visibility preference studies available 

at the time of the 2006 PM NAAQS review, rather than conducting new public 

preference studies since it is highly unlikely that the results of new studies could be 

completed in time to inform this review. This reanalysis was designed to explore the 

similarities and differences (comparability) between the current studies and to assess 

what information could be drawn from these results to inform the selection of visual air 

quality (VAQ) candidate protection levels (CPLs) to be used in subsequent impact 

assessments. This reanalysis also includes a recent study by Smith and Howell (2009) 

for Washington, D.C. which was presented to the CASAC during the public comment 

phase of the April 2, 2009 meeting and later provided to EPA staff. Does the Panel 

agree that the information provided by this reanalysis is useful to inform the selection 

of CPL? Does the Panel agree that inclusion of the Smith and Howell (2009) is 

appropriate in both the ISA and Visibility Assessment? To what extent does the Panel 

consider that the reanalysis of the urban visibility preference studies is clearly and 

appropriately characterized? 

For the purposes of evaluating a secondary PM NAAQS protective of non-Class I area visual air 

quality in this PM NAAQS review cycle, we support the reanalysis effort in lieu of conducting 

additional preference studies, given the time available. We support the inclusion of the Smith 

and Howell data. As noted in the answer to Charge Question 8, to better understand and bound 

uncertainty, a logistic distribution could be fitted to the data and percentile values extracted from 

the curve fit, along with associated confidence intervals. This analysis would provide an 

understanding as to whether the values derived from various studies are indeed statistically 

different from each other. 

While the Agency is encouraged to allocate sufficient funds and resources to support the planned 

new urban visibility preference studies, there would not have been time to complete the new 

studies quickly enough to support the current assessment, and the results of those new studies 

will be more useful if they are carefully planned and executed, and not rushed to meet an 

accelerated NAAQS review schedule. The reanalysis shows a relatively strong degree of 

convergence in the identification of unacceptable levels of visual air quality across the different 

study areas, and provides a sufficient basis for use of the proposed CPLs in the current risk 

assessment, and for considering alternative levels of secondary PM standards based on PM light 

extinction. 

4) We have chosen to use the range that represents the 50th acceptability criteria across 

the four cites studied (i.e., the VAQ level that best divides the photographs shown in 

into two groups: those with a VAQ rated as acceptable by the majority of the 

participants, and those rated not acceptable by the majority of participants) as CPLs to 

characterize the nature of the impact on urban VAQ associated with current PM levels. 

Please comment on the clarity and appropriateness of the rationale supporting this 

decision. Does the Panel have suggestions for alternative ranges to consider? 

CASAC agreed that the presentation is clear, and that the 50th percentile for the acceptability 

criteria is logical, given the noted similarities in methodologies employed in the 4 study areas. 

The reanalysis could also be done at the 90th percentile for acceptable visibility, as has been done 

8
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in various perception threshold studies. 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. It may miss shifts in other parts of the distributions for different parts of the country as 

emissions controls are applied. It is likely that these “acceptable” and “unacceptable” VAQ 

levels divided at the 50th percentile will be further refined in the future, as results of additional 

studies become available. For use in the current NAAQS review, the identified range of VAQ 

levels provides a sound and defensible basis for establishment of a separate PM secondary 

standard based on achieving and maintaining acceptable levels of VAQ in urban areas. 

The Phoenix study with the large sample size confirms the results from the more limited sample 

sizes in the other studies. Each study incorporated various visibility ranges to arrive at the 

unique 50th percentile values. Future visibility preference studies of large unbiased sample 

sizes would assist in further confirming and refining these results for future NAAQS review 

cycles. These studies should include variation in urban and rural locations, contributing sources, 

meteorology, climate, population size, aerosol characteristics, and monitoring/viewing site 

representativeness across the U.S. 

For all the visibility preference studies reviewed here, respondents were specifically directed to 

exclude consideration of potential health effects in making their judgments. In reality, human 

perception is complexly related to feelings of psychological wellbeing and/or emotional stress, 

which in turn can predispose or enhance the direct health effects from exposures to visibilityimpairing pollutants. More work is needed to better understand these complex visibility/health 

relationships, as their artificial exclusion from past preference studies has likely resulted in 

apparent tolerance for poorer levels of VAQ than would otherwise be considered acceptable.

5) A new indicator relating ambient PM to urban VAQ (i.e., PM light extinction) has been 

evaluated to improve our characterization of the relationship between ambient PM and 

visibility impairment. Is the Panel generally supportive of this approach? To what 

extent have we provided an adequate justification for the new indicator used? 

The use of visual air quality as an indicator of urban VAQ is reasonable, and is an approach 

CASAC strongly supports. Adequate justification for the use of a direct measurement is 

provided. Light extinction is an easily measured quantity that can be related in a reasonable way 

to observed visibility so that it provides an objective measure of VAQ. CASAC recommends 

that an RH limit be used as a screen (hours above it should be discarded) rather than a cap 

(where RH levels for hours above the cap are set equal to the cap), and also recommends that the 

RH screen be lowered to 90%. This lower RH limit would unavoidably eliminate consideration 

of some of the worst visibility conditions, but would provide greater confidence that degraded 

visibility was a result of pollution and not weather. 

6) An averaging time of one hour as a practical minimum time period was used in this 

assessment in recognition that, while the visibility impacts are nearly instantaneous, the 

urban VAQ does not generally change significantly from minute to minute, but does 

vary from hour to hour. To what extent does the Panel support this approach? Does 

the Panel consider the rationale supporting this approach to be clearly and 

appropriately presented?

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One hour averaging is a reasonable compromise, and the presentation in the document is 

credible. It is likely that in most circumstances, such an averaging period would be reasonable 

and matches the time frame for measurement of other pollutants. As indicated in the response to 

question # 7 below, a focus on the single worst hour of the day is not necessarily the best way to 

employ an hourly averaging time as the basis for a regulatory metric.

7) We have chosen to use the 90th and 95th percentile forms in our assessment of 

alternative secondary (welfare-based) standards. Please comment on the use of these 

alternative forms. 

Assessing the choice of a 90th, 95th, or 98th percentile form cannot really be done independently 

of the choice of Candidate Protection Levels (CPLs). We have taken the liberty of exploring the 

implications of the three candidate CPLs of 20 dv (74 Mm-1), 25 dv (122 Mm-1), and 30 dv (201 

Mm-1) in combination with the 90th, 95th, and 98th extinction percentiles, using extinction data 

that have been reconstructed from IMPROVE-type measurements at Atlanta, Georgia (see chart). 

The data are 24-hr averages, so the analysis is not directly applicable to setting a 1-hr standard 

but does serve to highlight a few key points. We have used a 95% RH cutoff with all RH values 

above 95% set equal to 95%. 

Table 1: Percentile Form of PM Secondary NAAQS and “Number of Days” Affected by 

Candidate Protection Levels (from example 24-hour Urban Light Extinction Data chart at end) 

Candidate Protection Levels1

20 deciviews 

(74 Mm-1)

25 deciviews 

(122 Mm-1)

30 deciviews 

(201 Mm-1)

# of days from example dataset that exceed 

the 20, 25, and 30 deciview CPL

(observations adjusted to 365 days)

299 172 51

# of days to be moved from Unacceptable 

to Acceptable at 90th Percentile

(observations adjusted to 365 days)

263 136 15

# of days to be moved from Unacceptable 

to Acceptable at 95th Percentile

(observations adjusted to 365 days)

281 154 33

# of days to be moved from Unacceptable 

to Acceptable at 98th Percentile

(observations adjusted to 365 days)

292 165 44

1 - From reanalysis of 4 visibility preference studies, applied to 15 cities’ fine mass data, converted to reconstructed light extinction 

Table 1 summarizes the results of the analysis. First, choosing the 90th, 95th, or 98th percentile 

corresponds to allowing 36, 18, and 7 days to exceed the target (standard) level. Currently, the 

24-hr extinction values exceed the 20 dv, 25 dv, and 30 dv on 299, 172, and 51 days of the year, 

based on the example. Keep in mind that these are for 24-hr extinction values, and undoubtedly 

the frequency of exceedances will be greater for 1-hr data. Notice that the number of days 

required to move from the unacceptable to acceptable for the 30-dv standard and for the 98th, 

95th, and 90th percentiles is 44, 33, and 15 days, while for the 20-dv standard it is 292, 281, and 

263 days. The 25-dv CPL is between these two extremes. Clearly, the choice of the level of the 

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standard has a more significant effect on implied emission reductions than the choice of 98th, 

95th, or 90th percentile. Secondly, if the bext values are lognormally distributed, the 98th

percentile will be more restrictive than the 95th, which in turn is more restrictive than the 90th

percentile. If the distribution is not lognormal, it is possible that the 98th percentile would target 

a single source or source area, while the 90th percentile choice would be responsive to more 

regional transport. As stated above, the analysis was carried out on a 24-hr dataset and with a 

95% RH round-down assumption. The same analysis using 1-hr data will push more extinction 

values above a chosen exceedance value, while choosing a 90% RH cutoff will reduce the 

extinction values and tend to reduce the number of exceedances. 

The assessment presented in the Urban Visibility Assessment document assumed a distribution 

of 1 hr maximum daylight hours. Another alternative would be to assume all daylight hours 

from which to pick the underlying distribution without concern for daily maximums. What are 

the potential effects of these choices on emission control strategies - will they be any different? 

Again, choosing the distribution from all daylight hours versus the maximum from each day may 

target single-source areas, while a control strategy responsive to improving the maximum 

extinction on each day may very well target sources having persistent regional contributions. 

Because the underlying distributions of extinction as well as the aerosol compositional structure 

of the distributions will vary significantly from urban area to urban area, it might be preferable to 

choose a rate-of-progress standard as opposed to an absolute standard. This would allow picking 

a relatively low dv level, such as 20 dv, that is not only protective of the eastern but also of the 

western cities. The standard could then be structured so as to require a 10% reduction per 

planning period in exceedances at all urban areas. This approach could require bringing 10 days 

below the exceedance value in an eastern city, while only requiring 1 day for a western urban 

area. All cities would have to make some ongoing progress over time toward improved 

visibility. 

 

We suggest exploring a number of reasonable alternative strategies in order to better characterize 

the outcomes with regard to control and enhancement of visibility over time. 

8) To what extent does the Panel support the graphical displays presented in this chapter? 

As currently presented, do these figures clearly summarize the assessment results? We 

have combined data from multiple studies for two locations - British Columbia and 

Washington, DC - (Abt, 2001; Smith and Howell, 2009 test 1) as presented in figure 2-

14. Does the Panel agree with developing a composite dataset for each of these two 

urban areas? 

We note that all these studies are somewhat different in methodologies and results, while sharing 

several noted similarities. While the emissions source mixes, ambient aerosol loadings and 

compositions, climate, and meteorology are quite different across these 4 study areas that 

produce the 20 to 30 deciview preference studies’ results, we conclude that this range of values 

is sufficiently tested and reasonable, for the purposes of selecting the Candidate Protection 

Levels defined at the end of Chapter 2. 

The graphic displays (such as those in Figures 2-2, 2-3, 2-5, 2-7, and 2-9 through 2-14) which 

compare visibility levels in deciviews with percentages of respondents rating the VAQs 

acceptable serve well in showing results from the different studies in common units, showing the 

range or distributions among different respondents and across the different study areas, and 

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showing the convergence of results for acceptable/unacceptable visibility in the range of 20 to 30 

dv. We would also like to see the absolute change in 1/Mm also shown on X axis below the 

deciview scale. The use of composite data sets based on combined study results for the British 

Columbia studies and for the Washington DC studies is reasonable, and seems appropriate for 

combining results from these study areas with those from the Phoenix and Denver studies. 

A composite of findings from the urban areas where studies were conducted and indices selected 

(Denver, Phoenix) with those having only study results (Lower Fraser Valley, Washington, DC) 

does not offer a direct comparison. The indices involve further analysis and 

technical/operational considerations beyond the visibility preference studies’ results. Combining 

of the various preference studies is acceptable, as long as the measure of the studies’ results is 

the value for the 50% acceptability criteria and not the ultimate index values selected. Further, a 

logistic distribution could be fitted to the data and percentile values extracted from the curve fit, 

along with associated confidence intervals. This analysis would provide insight as to whether 

the values derived from the various studies differ statistically. 

9) Despite significant differences in study characteristics (e.g., size, location), to what 

extent does the Panel support combining and comparing the results from the four cities, 

as shown in Figure 2-14? What is the Panel’s view on the clarity and adequacy of the 

descriptions of the uncertainties and limitations associated with such a combined 

assessment and the conclusions that can be drawn from the assessments? Please 

provide comments on additional insights, uncertainties, or caveats that should be 

considered. 

Combining and comparing the results from the four different study areas is an appropriate 

approach that takes maximum advantage of all the currently available data. To a certain extent, 

the relative convergence of results from the different study approaches and study areas strongly 

supports the use of these data collectively for use in identifying a relatively narrow range of 

acceptable VAQs across many locations and respondents. Although a formal uncertainty 

analysis was not conducted, the descriptive information on uncertainties and limitations of this 

approach conveys that information clearly. The analysis using a logistic distribution mentioned 

under #8, above, would assist in narrowing and bounding uncertainty. 

It should also be noted that the uncertainties associated with this approach for selecting a level or 

range of acceptable VAQ are likely to be offset by, and/or interactive with, the uncertainties 

associated with other aspects of a secondary PM standard - such as the averaging time, 

representing a day by its worst 1 (or more) VAQ hour(s), and the frequencies (percentiles) for 

which unacceptable VAQ conditions are acceptable. While the focus here has been on 

discerning a decision point or range of VAQ conditions considered acceptable versus 

unacceptable, it should also be recognized that there are welfare benefits associated with 

improvements across all parts of the visibility spectrum, including, for example, a shift of days 

with “good” to “excellent” VAQ. As a practical matter, efforts made to improve visibility 

conditions in any part of the distribution are likely to shift the entire distribution, with benefits 

that will be understated by a single 50% acceptability approach. The studies’ results inform the 

development of a consistent national secondary PM standard that may begin to protect VAQ 

using a light extinction metric (the quantitative value assessed in the preference studies). 

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10) We have used the combined results presented in this chapter to develop a range of 

CPLs that are used in subsequent steps of the assessment. To what extent does the 

Panel support the range of CPLs used and the justification provided for selecting this 

range? Does the Panel recommend consideration of any alternative approaches or 

criteria for selecting CPLs? 

CASAC supports using combined results from existing urban visibility studies as a basis for 

selecting candidate protection levels (CPLs), which span the fairly narrow (20 to 30 dv) range of 

VAQ levels meeting the 50% acceptability criteria across the available study areas. As 

mentioned, the analysis could be done in a more elegant and statistically defensible way by 

fitting a logistic distribution to the data. Percentile values could then be extracted from the curve 

fit, along with associated confidence intervals. This analysis would help to show whether the 

values derived from various studies are indeed statistically different from each other, and 

percentiles other than the median could also be considered along with other elements of the 

proposed standard. It is probable that results from planned future visibility preference studies 

from other urban areas employing improved survey techniques may help narrow or otherwise 

refine the indicated 20 to 30 dv range of the CPLs proposed here. However, the currently 

proposed range is sufficiently well justified for use in the current standard setting process, in 

combination with other appropriate selections of indicator, averaging time and form.

11) Overall, we consider this assessment useful for providing information for the design of 

future urban visibility preference studies. Does the Panel support this conclusion and 

does the Panel have any recommendations for changes that could be made in the 

discussions of this information to enhance its usefulness for this purpose? 

CASAC concurs that this assessment provides a useful basis for identifying designs for, and 

priority information needs from, future urban visibility studies. Such studies should be designed 

to provide information to help refine several of the elements of a PM light extinction standard in 

addition to identifying the level(s) of VAQ considered unacceptable across different urban 

locations and viewing conditions. For example, what frequencies of unacceptable visibility 

conditions are “acceptable” over time, and should these frequencies be applied to the single 

worst hour of a day or should all daylight hours be considered? In addition to a single 

“threshold” of unacceptable visibility conditions, what are preferences for shifts in other 

percentiles of the visibility frequency distribution – such as an increase in the frequency of very 

clear days? Future studies should also evaluate the potential role of aerosols on sky color and 

visibility of clouds, which are both integral to good visibility in an urban setting. 

As always, there is a need to carefully balance the advocacy for future research needs with the 

confidence in using the best currently information to make sound decisions to better protect 

human health, welfare and environment. In this case, currently available information is adequate 

to establish a basic secondary standard based on PM light extinction, even while future research 

is clearly needed to refine such a standard in the future. It should also be noted that the more 

detailed information on hourly light scattering and absorption that will result from implementing 

a standard with a PM extinction indicator, will itself provide an invaluable information resource 

to support future urban visibility preference studies and refine extinction equations for different 

13
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mixes of urban aerosols. These data would also be available for near-real time reporting and 

public communication, will be useful for air quality forecasts and (in combination with other 

continuous measurements) will provide added information on highly time-resolved fine & coarse 

particle composition, including black carbon, of value for various health effects studies as well. 

12) Are the goals articulated in the first paragraph of this chapter achieved in the 

remainder of the assessment? If not, does the Panel have suggestions for additional 

assessments that should be done? 

13) Are the methods and approaches taken in these assessments, including those for 

monitor site selection, incomplete data adjustments, and the use of the CMAQ model to 

augment speciation data, appropriate and is the rationale for their selection clearly 

articulated? 

14)Is the approach used to estimate PRB as described in chapter 3 and Appendix C 

appropriate? 

Response to Questions 12 – 14: CASAC believes Chapter 3 provides a thorough assessment of 

the limitations of existing data and the uncertainties related to an assessment of visibility (light 

extinction). One addition may be appropriate – there are several monitoring networks that could 

serve as a validity check on the modeled results described in the assessment. Both SEARCH 

(Southeastern Aerosol Research Characterization Study) and the IMPROVE program 

(Interagency Monitoring of Protected Visual Environments) have samplers in urban areas, 

including some of the 15 cities examined here. The IMPROVE network routinely measures 

PM10-2.5 mass, and SEARCH measures PM10-2.5 using dichotomous virtual impactors for several 

species. Data from both networks might be useful for comparison with the results of the 

modeling method used here. 

The approach taken to assess urban visibility is sound. Given the relative lack of measurements 

(except for the few sites that have IMPROVE samplers) the approach to obtaining estimates of 

hourly daytime visibility is a good one. The method is clearly articulated. 

The approach used to estimate PRB is also sound. CASAC believes it may serve as a useful 

check on the modeled results to compare appropriate IMPROVE visibility measurements with 

the PRB results. 

It appears that EPA generalized each “season” so that each location was defined to have the same 

number of daylight hours. It is not clear what benefit this provides, and the suggestion (footnote 

23) that this may be eliminated in the final version of this assessment seems appropriate. 

CASAC questions the use of a ratio of PM10-2.5 to PM2.5 to allocate hourly measurements. Both 

PM10 and PM2.5 change rapidly, but PM10 does so especially. Whenever possible, it would be 

useful to use a ratio based on daily values to perform the allocation instead of the longer time 

average (monthly or annually) that was used here. 

15) We consider the results generated by these analyses to be reasonable based on PM 

composition and relative humidity data. Does the Panel agree? Are there other tests of 

reasonableness that could be applied? 

The results presented in this assessment are reasonable given the data that are available, the 

methods employed in the Assessment to estimate hourly light extinction, and the current state of 

understanding of the effects of PM composition and humidity on extinction. If time allows, 

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CASAC recommends that EPA explore the use of independent data as consistency checks for the 

hourly estimates of species composition and/or light extinction. Examples of hourly PM 

composition data include the SEARCH sites in Atlanta and Birmingham and the Pittsburgh, St. 

Louis and Fresno Supersites. Transmissometer data should be available for Phoenix, and airport 

ASOS data may provide useful independent light scattering measurements for some urban areas 

if it can be obtained in its raw, uncensored form. These data should be used to help identify 

biases and to provide insight regarding the precision of estimated extinction. 

16)In addition to a qualitative discussion of possible sources of uncertainty and variability, 

are there quantitative methods for addressing uncertainty and variability associated 

with these assessments that the Panel would recommend? 

Although this chapter does present information that pertains to both uncertainty and variability, it 

could benefit from more structure. For example, this Assessment could be consistent with the 

Risk and Exposure Assessment (REA) in referencing a framework for dealing with variability 

and uncertainty, such as the WHO (2008) framework, and explaining as to which tiers of 

assessment are applied here. Tier 0 is a point estimate based on default values. Tier 1 includes 

qualitative (but structured) assessment and comparison of sources of uncertainty, including bias 

and imprecision. Tier 2 includes sensitivity analysis. Tier 3 includes quantitative analysis of 

uncertainty using probability distributions. For this Assessment, at a minimum a structured Tier 

1 approach, supplemented with some Tier 2 applications, would be appropriate. 

For models that are calibrated based on empirical data, a statistical estimate of the goodness-offit of the model and the standard error of the estimate should be made. The latter can be used as 

part of a Tier 3 approach to uncertainty analysis under the WHO framework. 

17) A number of appendices are provided at the end of this document. Does the Panel 

agree that this information is useful to retain? Does the Panel agree with the level of 

detail provided in the body of the report and its organization and distribution 

throughout the document? 

It is useful to retain the appendices, which are needed to support the discussions in the main 

document. Given the similarities and differences between the currently proposed metrics for a 

secondary standard, and those considered for a sub-daily fine mass-based urban visibility 

standard in the last round of PM NAAQS review, we conclude that the level of detail presented 

in the document is appropriate to adequately inform the Administrator as to the basis for 

recommending a new secondary standard. Hopefully these appendices can be revised to reflect 

the changes that would result from considering some of the alternative options CASAC has 

suggested. 

18) Does the Panel agree with the approaches used to simulate just meeting air quality 

conditions for the current and alternative PM standards? In particular, is use of the 

proportional rollback approach appropriate in the context of the urban PM visibility 

assessment? 

It seems unlikely that all PM species would be rolled back by the same proportions in efforts to 

just attain existing or new primary or secondary PM standards. However, there seems to be no 

logical basis for suggesting specific alternatives to the proportional rollback model for use in this 

application. Table 4-7 is an informative display of relevant information which might be 

expanded for the final visibility risk assessment or in the policy assessment to include other 

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primary PM standard combinations, such as 13/30. CASAC has suggested variations in several 

elements of the secondary PM standard, and it would be informative to see how those revisions – 

in possible combinations with higher percentiles - might change the relative mixtures of primary 

PM and secondary light extinction levels across the urban study areas. 

The fact that the proposed indicators (and averaging times, levels and forms) for the primary and 

secondary standards are entirely independent from each other, makes it inherently more difficult 

to assess particular combinations of primary and secondary NAAQS metrics that are likely under 

future rollback scenarios. At the same time, the totally independent nature of the primary and 

secondary indicators and of the health and welfare objectives they address makes it less critical 

to know the exact future combinations that might result from hypothetical rollback scenarios. 

The standards can be set independently, using metrics that best reflect the separate health and 

welfare objectives, regardless of the progress toward one that may (or may not) be made with 

progress toward the other. 

19) To what extent does the Panel consider the presentation of "what if" scenarios for 

retention of the current secondary PM2.5 NAAQS and consideration of alternative, 

more protective secondary NAAQS in sections 4.2 and 4.3 to be clearly written with an 

appropriate level of detail? Do the correlation analyses presented in Appendix D 

provide sufficient insight into the suitability of alternative indicators based on sub-24 

hour averaging periods for PM2.5? Are there additional alternative standard scenarios 

that should be evaluated? 

The presentation of “what if” scenarios for the current standards and alternative new secondary 

standards is clearly written, with appropriate details conveyed in sections 4.2 and 4.3 and 

associated appendices. As indicated above, CASAC recommends evaluating several revisions to 

the proposed secondary standard metrics that will generally tend to dry out the aerosol, 

somewhat reduce the current East/West differences and result in lower extinction estimates. It 

would be informative to see the results of some of these “softer metrics” compared to the 

alternative primary PM standards. Since those changes would essentially screen out many of the 

worst visibility hours from the regulatory metric, it would also be appropriate to consider 

combining them with a higher percentile forms – up to and including the 98th percentile. 

The correlation analyses presented in Appendix D help show similarities and differences among 

various sub-daily extinction and fine mass indicators across the different urban areas. The 

correlations between 4-hour afternoon light extinction and PM2.5 are surprisingly poor for several 

sites (St. Louis and Philadelphia). Presumably the afternoon correlations for these and many 

other sites will improve if the 95% RH cap is replaced with a 90% RH screen, as recommended. 

If not, it would be useful to take a closer look at the estimation methods and input data. 

Enclosure C 

Compendium of Preliminary Pre-Meeting Comments 

CASAC Particulate Matter Review Panel 

Particulate Matter Urban-Focused Visibility Assessment (September 2009) 

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Ashbaugh Comments (Dr. Lowell Ashbaugh) ......................................................................... 18

Cowling Comments (Dr. Ellis Cowling).................................................................................... 20

Frey Comments (Dr. H. Christopher Frey).............................................................................. 28

Grantz Comments (Dr. David Grantz)..................................................................................... 31

Helble Comments (Dr. Joseph Helble)...................................................................................... 32

Hopke Comments (Dr. P. K. Hopke)......................................................................................... 34

Malm Comments (Dr. William Malm)...................................................................................... 36

Poirot Comments (Mr. Rich Poirot).......................................................................................... 41

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Ashbaugh Comments (Dr. Lowell Ashbaugh) 

Comments on 1st Draft Urban-Focused Visibility Assessment for PM, September 2009, 

Lowell Ashbaugh

Charge Questions 12-14: 

Chapter 3 – Estimation of Current PM Concentrations and Light Extinction 

12) Are the goals articulated in the first paragraph of this chapter achieved in the remainder 

of the assessment? If not, does the Panel have suggestions for additional assessments that 

should be done? 

13) Are the methods and approaches taken in these assessments, including those for monitor 

site selection, incomplete data adjustments, and the use of the CMAQ model to augment 

speciation data, appropriate and is the rationale for their selection clearly articulated? 

14) Is the approach used to estimate PRB as described in chapter 3 and Appendix C 

appropriate? 

The goals articulated in the first paragraph of Chapter 3 are largely achieved in the document. 

There is one addition that may be appropriate - the IMPROVE program has samplers in several 

urban areas, including some of the 15 cities examined here. The IMPROVE data could be used 

as a check on the results of the method used here. It’s also not exactly correct that there is no 

systematic monitoring network in place for PM10-2.5. The IMPROVE network routinely measures 

PM10-2.5, including at selected urban areas, so the data may be useful for this analysis. 

The approach taken to assess urban visibility is sound. Given the relative lack of measurements 

(except as noted above for IMPROVE) this approach to obtain estimates of hourly daytime 

visibility is a good one. The method is clearly articulated. 

The approach used to estimate PRB is also sound. It might be good to compare the “best” 

IMPROVE visibility measurements with the PRB results, though. Whenever possible, it serves 

as a useful check on the modeled results to compare them to appropriate measurements. 

I’m uncomfortable with the use of a ratio of PM10-2.5 to PM2.5 to allocate hourly measurements. 

Whenever possible, it would be useful to use a daily ratio to perform the allocation instead of a 

longer time average (monthly or annually). Both PM10 and PM2.5 change rapidly, but PM10 does 

so especially. 

Table 3.2 describes Fresno as being in southern California. Although as a “real” northern 

Californian, I don’t consider Fresno to be in northern California, I can’t say it’s regarded as 

being in southern California, either. That distinction normally applies to that part of California 

south of the Tehachapi Mountains. Fresno is representative of the Central Valley, but not 

southern California. 

The sentence that ends on page 3-11, lines 11-12, seems to be missing something but I can’t 

figure out what it needs to be coherent. 

The x-axis labels for Figure 3-4 (both figures) are incorrect. They should run from 01:00:00 am 

to 12:00:00 pm. 

Page 3-18, line 25, should read “principal”, not “principle”. In the discussion of nitrate sampling 

on the same page, there should be a note that the fate of ammonium collected on a Teflon filter is 

not known. In particular, it’s known that ammonium nitrate can evaporate from a Teflon filter 

during collection. It’s suspected the ammonium will be lost along with the nitrate, but this has 

not been satisfactorily investigated. The same thing is true of ammonium collected on a nylon 

filter, but in that case it’s well documented that the nitrate is retained. 

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I’m not sure what the discussion of sulfate on page 3-19 is intending to say. What does it mean 

that the continuous instruments are more like the FRM than the CSN in regards to sulfate? And 

why is it similar to nitrate, when nitrate is volatile but sulfate is not? 

On page 3-24, PM10-2.5 was obtained by subtracting PM2.5 from PM10. But in setting negative 

values to zero, you bias the results. There is uncertainty inherent in all PM10 and PM2.5

measurements. If the measurements are close, there is a probability that the difference will be 

negative. This possibility should not be ignored. 

The sentence on page 3-27, lines 8-10, could be more clearly stated. The phrase “Except for the 

two Texas and the non-California western urban areas...” is confusing. I believe it would be 

clearer to say “All urban areas east of the Mississippi, and California and Texas urban areas...” 

but I defer to the authors to make the sentence easier to understand. 

Table 3-6 contains a summary line labeled “Average” that is truly an average. But it should be a 

weighted average, as each urban area has a different number of days with estimates. A weighted 

average would have 87%, 64%, and 35% instead of 86%, 61%, and 33% for the three CPLs. 

On page 3-36, lines 23-25, it could be noted that the extreme haze hours at Fresno are 

exclusively late fall and winter. Extreme days in Los Angeles span the year. The causes of haze 

at these two sites are quite distinct; it should not be suggested that they are similar. 

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Cowling Comments (Dr. Ellis Cowling) 

Individual Comments on the Separate Risk-Assessment Document for PM 

Welfare Effects on Urban-Focused Visibility

Before offering my Individual Comments in response to the 19 Charge Question on the RiskAssessment Document for PM Welfare Effects on Urban-Focused Visibility, permit me to 

indicate how pleased I was to read the following statement in Lydia Wegman’s transmittal note 

of September 9:

“In addition, on or about September 15, 2009, we plan to release a very preliminary 

draft of another document, Policy Assessment for the Review of the Particulate Matter 

National Ambient Air Quality Standards: Preliminary Draft (Policy Assessment), to 

facilitate discussion with the CASAC PM Panel on our ongoing efforts to prepare an 

external review draft Policy Assessment.” 

Chapter 1 – Scope of Visibility Assessment

Charge Question 1:

After careful consideration of the evidence provided in the second draft ISA, and in 

particular the significant body of work that has been conducted by the Regional 

Planning Organizations under the Regional Haze Rule, (i.e. information on urban 

and rural PM concentrations and compositions), we have decided to continue to 

focus this assessment on the PM induced visibility impairment that is occurring in 

urban areas. What is the Panel’s view on this approach? Is the rationale supporting 

the selected approach clear and appropriate? 

I consider this approach to be very appropriate for the several reasons outlined on page 1-6 of 

Chapter 1 including: 1) Generally higher ambient air concentrations of PM in urban than in rural 

areas, 2) Availability of more science-based assessments in urban than in rural areas, and 3) The 

larger numbers of persons in urban than in rural areas whose enjoyment of decrease visibility 

impairment would be affected. The approach taken – use of light extinction as the air-quality 

indicator of concern and the decision to minimize the influence of compounding effects of 

relative humidity by making compliance measurements in afternoon hours when variation in 

relative humidity usually are less than in other hours of the day is clearly explained and seems 

well justified. 

Charge Question 2:

After further considering the nature of urban versus more remote area PM, and in 

light of discussions with CASAC at the April 2, 2009 meeting, we have decided not 

to develop an urban optimized algorithm at this time, but instead to rely on the 

original IMPROVE algorithm to relate urban PM to local haze (PM light 

extinction). Is the Panel generally supportive of this approach? Is the rationale 

supporting this decision appropriate and clearly presented? 

The decision to rely on the original IMPROVE algorithm to relate urban PM concentrations to 

local haze and light extinction seems well justified and appropriate (as the CASAC PM Panel 

20
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indicated earlier). The rationale for this decision is explained clearly in the text of this UrbanFocused Visibility Assessment document. 

Charge Question 3:

In a change from the planned approach presented in the Scope and Methods Plan, 

we have decided to conduct a reanalysis of the urban visibility preference studies 

available at the time of the 2006 PM NAAQS review, rather than conducting new 

public preference studies since it is highly unlikely that the results of new studies 

could be completed in time to inform this review. This reanalysis was designed to 

explore the similarities and differences (comparability) between the current studies 

and to assess what information could be drawn from these results to inform the 

selection of visual air quality (VAQ) candidate protection levels (CPLs) to be used in 

subsequent impact assessments. This reanalysis also includes a recent study by 

Smith and Howell (2009) for Washington, D.C. which was presented to the CASAC 

during the public comment phase of the April 2, 2009 meeting and later provided to 

EPA staff. Does the Panel agree that the information provided by this reanalysis is 

useful to inform the selection of CPLs? Does the Panel agree that inclusion of the 

Smith and Howell (2009) is appropriate in both the ISA and Visibility Assessment? 

To what extent does the Panel consider that the reanalysis of the urban visibility 

preference studies is clearly and appropriately characterized? 

Yes, I agree with the decisions to reanalyze the urban visibility data and information available at 

the time of the 2006 PM NAAQS review and to take advantage of the results obtained in the 

Smith and Howell 2009 study in Washington DC rather than to conduct new public preferences 

studies. I also agree that the information provided by the reanalysis approach is useful to inform 

the selection of candidate protection levels (CPLs). But I suggest that the term “candidate 

protection PM concentrations” (CPPMCs) be used instead of CPLs in order to be clear about the 

distinction between the air concentrations of PM actually MEASURED at a monitoring site and 

the air concentrations that might be ALLOWED under a proposed new PM secondary standard. 

Charge Question 4:

We have chosen to use the range that represents the 50th acceptability criteria across the 

four cites studied (i.e., the VAQ level that best divides the photographs shown into two 

groups: those with a VAQ rated as acceptable by the majority of the participants, and 

those rated not acceptable by the majority of participants) as CPLs to characterize the 

nature of the impact on urban VAQ associated with current PM levels. Please comment 

on the clarity and appropriateness of the rationale supporting this decision. Does the 

Panel have suggestions for alternative ranges to consider? 

This Charge Question mentions “the photographs shown” – which I assumed meant “the 

photographs shown in this Urban Focused Visibility Assessment document,” but when I couldn’t 

find any of the various sets of photographs used by the participants in the various visibility 

preference studies discussed in this document, I came to realize that the term “photographs 

shown” must have been intended to refer to “the photographs shown to the participants.” Thus 

without being able to see the photographs in question, I have no personal basis for evaluating the 

21
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proposal to “use the range that represents the 50th acceptability criteria across the four cities” as 

opposed to any “other alternative ranges” that might be considered.” 

Please also note the ambiguity in use of the term “current PM levels” in the statement of Charge 

Question 4 -- when the purpose of this Charge Question is to ask for CASAC guidance about 

methods to identify “candidate protection levels.” As in my response to Charge Question 3, it is 

very important to distinguish between air concentrations of PM that are actually MEASURED 

from those that might be ALLOWED under a proposed new PM secondary standard. Frequent 

use of the same term -- “level” -- to describe both what is MEASURED and what is ALLOWED 

is why the alternative terms “measured air concentration” and “allowable air concentration” 

would avoid confusion in the minds of both CASAC Panelists and some air-quality monitoring 

personnel. 

On the other hand, when I carefully read through the detailed descriptions of how the urban 

visibility preference studies were conducted in the four cities, I was very impressed with the 

participant-to-participant consistency and apparent scientific rigor of this approach – including 

especially the displays of participant preferences shown in Figures 2.2, 2.3, 2.5, 2.9, 2.11, and 

2.12 and the thorough discussion on pages 2-24 and 2-25 of alternatives hypotheses to explain 

the differences in numbers of deciviews found “acceptable” to 50% of the participants in the four 

cities. Thus, I am convinced that the 50% acceptability criterion is a sound basis for establishing 

reliable candidate protection levels (CPLs). 

Charge Question 5:

A new indicator relating ambient PM to urban VAQ (i.e., PM light extinction) has been 

evaluated to improve our characterization of the relationship between ambient PM and 

visibility impairment. Is the Panel generally supportive of this approach? To what 

extent have we provided an adequate justification for the new indicator used? 

Yes, as indicated in my response to Charge Question 2, I believe light extinction is a very 

appropriate means by which to relate “ambient PM” (concentrations) to urban Visual Air Quality 

(VAQ). I find the justification provided in this Urban-Focused Visibility Assessment document 

very adequate as a means by which to relate ambient PM concentration to urban VAQ. 

Charge Question 6:

An averaging time of one hour as a practical minimum time period was used in this 

assessment in recognition that, while the visibility impacts are nearly instantaneous, 

the urban VAQ does not generally change significantly from minute to minute, but 

does vary from hour to hour. To what extent does the Panel support this approach? 

Does the Panel consider the rationale supporting this approach to be clearly and 

appropriately presented?

The justification provided on page 2-26 for the selection of one hour as a suitable “averaging 

time” for candidate visibility protection secondary standards seems very adequate to me. 

Charge Question 7:

We have chosen to use the 90th and 95th percentile forms in our assessment of 

alternative secondary (welfare-based) standards. Please comment on the use of these 

alternative forms. 

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I agree with the decision discussed on the bottom of page 2-26 that using the 90th and 95th

percentile forms in this Urban Focused Visibility Assessment document is appropriate. 

Chapter 2 – Urban Visibility Preference Studies

Charge Question 8:

To what extent does the Panel support the graphical displays presented in this 

Chapter? As currently presented, do these figures summarize the assessment 

results? We have combined data from multiple studies for two locations – British 

Columbia and Washington D.C. - Abt Associates, 2001; Smith and Howell, 2009 

test 1) as presented in Figure 2-14. Does the Panel agree with developing a 

composite dataset for each of these two urban areas? 

As indicated in my response to Charge Question 4 (above), I am very impressed with the 

reliability of the graphical display method described in Chapter 2. Yes, the individual figures for 

each of the four cities appear to me to summarize the assessment results for each city very nicely. 

Also, the combined display of data for all four cities shown in Figure 2-14 provides an integrated 

overview of the results in all four cities. Thus, I simply do not understand what is meant by the 

last part of this Charge Question 8 – “Does the Panel agree with developing a composite 

dataset for each of these two urban areas?”

Charge Question 9: 

Despite significant differences in study characteristics (e.g., size, location), to what 

extent does the Panel support combining and comparing the results from the four cities, 

as shown in Figure 2-14? What is the Panel’s view on the clarity and adequacy of the 

descriptions of the uncertainties and limitations associated with such a combined 

assessment and the conclusions that can be drawn from the assessments? Please provide 

comments on additional insights, uncertainties, or caveats that should be considered. 

As indicated in my response to Charge Question 8, I think the combined analysis of results from 

all four cities shown in Figure 2-14 demonstrates the general efficacy of the methods used Urban 

Visibility Preference Studies. The variation in number of deciviews that was found “acceptable” 

in the four cities suggests that a uniform national standard for all urban areas across the whole 

nation is probably neither practicable or scientifically well-justified at present. It seems to me 

that some degree of regional and/or individual urban-area flexibility in implementation should be 

explored. Perhaps based on consensus judgments of the citizens in different urban centers within 

individual states. Or perhaps recommending only that flexible “targets” rather than fixed 

“national standards” should be established that would be designed to facilitate and encourage 

improvement in urban visibility progressively over time – for example, by state and/or federal 

requirements and/or incentives that would encourage individual urban areas to design visibilityimprovement programs that will decrease the number of days per year that visibility is judged 

“unacceptable.” 

Charge Question 10:

We have used the combined results presented in this chapter to develop a range of 

CPLs that are used in subsequent steps of the assessment. To what extent does the 

Panel support the range of CPLs used and the justification provided for selecting this 

range? Does the Panel recommend consideration of any alternative approaches or 

criteria for selecting CPLs? 

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The complexity of the analysis described in Chapters 2 and 3 and the variability by region across 

our country, and the relative lack of consistent data relating directly to light extinction and the 

limitations of having to depend on modeling estimates all combine to challenge my own meager 

understanding of some if not all of these complicating factors. Maybe as much because of- and 

also in spite of- the bewilderment that I experienced in reading Chapter 3, I believe that EPA 

staff have developed a range of CPLs that appear very reasonable to me. At the same time, I 

look forward eagerly to hearing the considered professional judgments of two of my fellow 

CASAC PM Panel members whose experience and capacity for making informed policy-relevant 

judgments under conditions of substantial measurement uncertainty is substantially greater than 

my own – Bill Malm and Rich Poirot. 

Charge Question 11:

Overall, we consider this assessment useful for providing information for the design 

of future urban visibility preference studies. Does the Panel support this conclusion 

and does the Panel have any recommendations for changes that could be made in 

the discussions of this information to enhance its usefulness for this purpose? 

I agree that the assessment provided in this document is very useful for the design of future 

urban visibility preference studies and hope that EPA and the various air-quality-management 

units of several states will join with interested stakeholders to make persistent efforts to ensure 

that many more such studies are undertaken in the future. 

Chapter 3 – Estimation of Current PM Concentrations and Light Extinction

Charge Question 12:

Are the goals articulated in the first paragraph of this chapter achieved in the 

remainder of the assessment? If not, does the Panel have suggestions for additional 

assessments that should be done? 

Yes, I believe that the goals set out in the first paragraph of Chapter 3 were achieved very 

substantially. But once again, for many of the same reasons stated above in my response to 

Charge Question 10, I look forward to hearing the much better informed professional judgments 

of my colleagues on this PM NAAQS review Panel – especially Bill Malm and Rich Poirot. 

Charge Question 13: 

Are the methods and approaches taken in these assessments, including those for 

monitor site selection, incomplete data adjustments, and the use of the CMAQ model to 

augment speciation data, appropriate and is the rationale for their selection clearly 

articulated? 

Yes, I agree that the methods and approaches taken in these assessments including those for 

monitor site selection, incomplete data adjustments, and the use of the CMAQ model to augment 

speciation data are appropriate and that the rationale for their selection was described very 

adequately. 

Charge Question 14:

Is the approach used to estimate PRB as described in chapter 3 and Appendix C 

appropriate? 

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Yes, I believe that the approach used in estimating policy-relevant background as described on 

page 3-37 and in Appendix C was appropriate. 

Charge Question 15:

We consider the results generated by these analyses to be reasonable based on PM 

composition and relative humidity data. Does the Panel agree? Are there other tests of 

reasonableness that could be applied? 

Once again, as indicated in my response to Charge Questions 10 and 12 (above), I believe that 

EPA staff have done a very good job of dealing with the issues of variation in PM composition 

and the complicating influence of variation in relative humidity. 

Charge Question 16: 

In addition to a qualitative discussion of possible sources of uncertainty and variability, 

are there quantitative methods for addressing uncertainty and variability associated 

with these assessments that the Panel would recommend? 

My experience in dealing with the issues of uncertainty and variability in both PM monitoring 

and visibility measurements is not sufficient for me to provide useful suggestions about 

quantitative methods for dealing with the issues of uncertainty and variability in these 

assessments. 

Charge Question 17: 

A number of appendices are provided at the end of this document. Does the Panel 

agree that this information is useful to retain? Does the Panel agree with the level of 

detail provided in the body of the report and its organization and distribution 

throughout the document? 

My professional judgment is that the information in the appendixes is useful and should be 

retained. I also agree that the amount of detail in the body of this report and its organization and 

distribution throughout the document constitute a very good job well done – especially 

considering the complexity and limitations of available scientific data and information. 

25
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Chapter 4 – Total Light Extinction Under “What If” Conditions of Just Meeting Specific

Alternative Secondary NAAQS

Charge Question 18:

Does the Panel agree with the approaches used to simulate just meeting air quality 

conditions for the current and alternative PM standards? In particular, is use of the 

proportional rollback approach appropriate in the context of the urban PM visibility 

assessment? 

Yes, I agree with the approaches used to simulate just meeting air quality conditions for the 

alternative light-extinction based PM standards that are discussed in Chapter 4. I also believe 

that it is almost always sensible and useful to consider a proportional rollback approach in the 

context of any air-pollution management systems – including urban visibility assessments. 

But I am puzzled about how to consider these approaches in the context of the current PM 

secondary standard which is based on mass of PM rather than on light extinction. Perhaps my 

confusion about this matter will be decreased during our up-coming CASAC PM NAAQS 

meeting on October 5 and 6, and then I can reconsider this Charge Question and my response to 

it when I better understand the transition that will occur if and when we change from the current 

identical mass-based primary and secondary standards for PM to one or more alternative 

secondary standards that will be based on light-extinction rather than on mass of PM.. 

Charge Question 19:

To what extent does the Panel consider the presentation of "what if" scenarios for 

retention of the current secondary PM2.5 NAAQS and consideration of alternative, 

more protective secondary NAAQS in sections 4.2 and 4.3 to be clearly written with 

an appropriate level of detail? Do the correlation analyses presented in Appendix D 

provide sufficient insight into the suitability of alternative indicators based on sub24 hour averaging periods for PM2.5? Are there additional alternative standard 

scenarios that should be evaluated? 

As indicated in my response to Charge Question 18, I am puzzled about how to consider the 

presentation of “What if” scenarios in the context of the current PM secondary standard which is 

based on mass of PM rather than on light extinction. Perhaps my confusion about this matter 

will be decreased during our up-coming CASAC PM NAAQS meeting on October 5 and 6, and 

then I can reconsider this Charge Question and my response to it when I better understand the 

transition that will occur if and when we change from the current identical mass-based primary 

and secondary standards for PM to one or more alternative secondary standards that will be 

based on light-extinction rather than on mass of PM. 

Permit me also to call attention to my response to Charge Question 9 (above) in which I have 

raised the issue of having flexible “targets” rather than fixed “national standards” for 

improvement of urban visibility: 

“It seems to me that some degree of regional and/or individual urban-area flexibility in 

implementation should be explored. Perhaps based on consensus judgments of the 

citizens in different urban centers within individual states. Or perhaps recommending 

only that flexible “targets” rather than fixed “national standards” should be established 

that would be designed to facilitate improvement in urban visibility progressively over 

time – for example, by state and/or federal requirements and/or incentives that would 

26
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encourage individual urban areas to design visibility-improvement programs that will 

decrease the number of days per year that visibility is judged “unacceptable.” 

27
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Frey Comments (Dr. H. Christopher Frey) 

I was asked to respond to Charge Questions 15, 16, and 17. 

Charge Question 15: We consider the results generated by these analyses to be reasonable 

based on PM composition and relative humidity data. Does the Panel agree? Are there other 

tests of reasonableness that could be applied?

Response: EPA has undertaken a reasonable review of available data and development of 

databases from which light extinction has been estimated. The choice of the estimation equation 

is explained. The data and equation are applied to case study examples for numerous urban 

areas. Thus, the assessment takes into account variability in: (a) the daily mass concentration of 

PM2.5 and coarse PM (PM10-2.5), (b) speciation; and (c) relative humidity. The document 

discusses the geographic variability in speciation and provides plausible explanations for interregional variability. The capabilities and limitations of the existing monitoring networks are 

appropriately identified. The hierarchical approach for use of FRM data, FEM data, and 

estimates from chemical transport models is reasonable. The development of diurnal patterns of 

PM2.5 components is reasonable. The summaries of data are useful and are interpreted 

reasonably. For example, the distribution of hourly estimated light extinction and the discussion 

of the patterns in these estimates is very useful. The discussion of the implications of 

observations of relative humidity of greater than 95% is useful and appropriately developed. As 

pointed out, such high humidity may often be associated with precipitation or fog events in 

which there is not an expectation of high visibility. 

In terms of the data regarding PM sizes and their components, EPA has considered the 

appropriate measurement and modeling data sets. With respect to the discussion of humidity 

levels, perhaps there could be comparison to precipitation measurement data to quantify or at 

least provide qualitative insight into the proportion of relative humidity data above 95 percent 

that are associated with precipitation events. This would further strengthen what may already be 

an adequate argument for capping RH at 95 percent when inputting data to the IMPROVE 

equation. 

Charge Question 16: In addition to a qualitative discussion of possible sources of uncertainty 

and variability, are there quantitative methods for addressing uncertainty and variability 

associated with these assessments that the Panel would recommend?

Response: Although this chapter does present information that pertains to both uncertainty and 

variability, it could benefit from more structure. For example, this Assessment could be 

consistent with the Risk and Exposure Assessment (REA) in referencing a framework for dealing 

with variability and uncertainty, such as the WHO (2008) framework, and explaining as to which 

tiers of assessment are applied here. Tier 0 is a point estimate based on default values. Tier 1 

includes qualitative (but structured) assessment and comparison of sources of uncertainty. Tier 2 

includes sensitivity analysis. Tier 3 includes quantitative analysis of uncertainty using 

probability distributions. For this Assessment, at a minimum a structured Tier 1 approach, 

supplemented with some Tier 2 applications, would be appropriate. 

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A good example of how to implement the Tier 1 approach is illustrated in Table 3-13 (p. 79) of 

Risk Assessment to Support the Review of the PM Primary National Ambient Air Quality 

Standards, External Review Draft (September 2009). A similar tab le in this Assessment would 

be a useful way to summarize and help synthesize the implications of the various sources of 

uncertainty. Such a table would contain an identifier and description of the source of 

uncertainty, a statement of the direction (high, low, both, or none) of the response of the 

assessment endpoint to uncertainty in the input, an assessment of the knowledge base, and a 

commentary that provides rationale for the assessment. In addition, there should be some 

discussion of relationships (if any) among pairwise combinations of sources of uncertainty. The 

qualitative discussion should conclude with an assessment of which sources of uncertainty are 

the most significant, and whether or how such sources of uncertainty would bias the answer or 

lead to imprecision in the answer. 

In some cases, a Tier 2 sensitivity analysis would be useful. It would be helpful to present a 

simple evaluation of how the estimated light extinction from the IMPROVE equation varies 

based on ranges of values for each input that are representative of observed values of PM 

component and mass concentration and relative humidity. For example, the results in Figures 3-

12 to 3-19 generally (with some exceptions, of course) imply that organic carbon, sulfate, and 

nitrate are often the three most important components that affect light extinction. This result is 

not entirely obvious just by inspection of the Equation on page 3-16 because the sensitivity of the 

estimated light extinction depends on two factors: (a) on the functional form and the coefficient 

for each term; and (b) the typical values (and variability) for the inputs. 

Furthermore, there is considerable discussion in the text regarding some of the uncertainties 

associated with apportioning PM2.5 to the mass of components. However, there seems to be no 

analysis of whether or by how much these potential errors would make a difference when 

estimating light extinction. Can statements be made as to how much these factors might 

introduce errors in terms of either relative (%) or absolute (ug/m3

) bias, imprecision, or both? 

Such statements could be translated into a sensitivity analysis for a few representative cases to 

demonstrate by how much would the estimate of light extinction vary. 

The difficulty in estimating the coarse PM concentration, particularly when PM10 and PM2.5

monitors were not sited concurrently, might lead to more uncertainty in light extinction estimates 

for some regions than others. For example, Phoenix appears to have a larger contribution of 

coarse PM to light extinction than many other areas, although on a given day coarse PM appears 

to be the dominant contributor in other locations such as Los Angeles and St. Louis. Since 

coarse PM is expected to have more geographic variability than fine PM because of higher 

deposition rates and because of the mode of its formation, it would be important to at least 

emphasize as to whether the coarse PM data for these three cities are based on collocated 

monitors. If not, then some additional qualitative discussion, and perhaps sensitivity analysis, 

could provide insight regarding the robustness of the estimates of light extinction. 

Charge Question 17: A number of appendices are provided at the end of this document. Does 

the Panel agree that this information is useful to retain? Does the Panel agree with the level of 

detail provided in the body of the report and its organization and distribution throughout the 

document?

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Response: The level of detail in the body of the report is appropriate. It is appropriate that 

additional details are in the appendices. 

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Grantz Comments (Dr. David Grantz) 

Grantz Individual Comments: 

Charge Question 1.

The rationale for pursuing a Secondary PM Standard based on daylight visual impairment in 

urban areas is thoroughly and clearly laid out in this document. The approaches, the reliance on 

computed visibility, and the concept of preference studies are appropriate. The authors have 

prepared a very complete, readable and informative document. Given the momentum in this 

direction, it is clear that this round of PM assessment is likely to proceed along these lines. 

However, I am not convinced that this is the best path. 

I do not find the arguments compelling that a daytime only standard is appropriate. Is night-time 

dumping of PM acceptable with respect to welfare? 

I am not convinced that an urban standard is appropriate. Is it ok to dump PM to the atmosphere 

if it will diffuse to rural areas relatively quickly? 

I am particularly concerned that visibility should be taken to represent welfare effects. It does 

have the distinct advantage of being amenable to calculation from existing mass measurements 

of PM, in contrast to the potentially more important impacts on materials, ecosystems, and 

climate. However, the current near-total focus on urban visibility may come to constrict our 

ability to identify or assess other endpoints, including by redirecting research funds to the now 

policy relevant visibility issue. 

Charge Question 9.

I am not particularly comfortable with the combined analysis of the preference studies from the 

four cities. The authors have presented the uncertainties and analytical issues fairly well. 

However, despite the consideration of four different hypotheses to explain the different VAQ 

preferences in east and west, I think the actual reason was missed. It is a subset of Hypothesis 4, 

but more closely related to individual experience after some time in a location. What differs is 

personal expectations, not differences in background, lighting etc. It is not reasonable to expect 

that people in eastern cities who rarely see 20 miles would express a preference for 40 miles, 

whereas in the west this visual range is not unusual and might be expected. As a result, the data 

are inherently not-combinable. They define different things. Some manipulation of ambient RH 

could bring this all back to mass, and make the data compatible, but this would not really be a 

visibility standard. 

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Helble Comments (Dr. Joseph Helble) 

Question 2: The decision to use the IMPROVE algorithm to relate urban PM to local haze is 

reasonable. Given the uncertainties in the underlying data, particularly PM composition, needed 

to model visibility, the rationale for utilizing the existing algorithm is appropriate and clearly 

articulated by the Agency. 

Questions 5 – 6: 

5) A new indicator relating ambient PM to urban VAQ (i.e., PM light extinction) has been evaluated 

to improve our characterization of the relationship between ambient PM and visibility impairment. Is 

the Panel generally supportive of this approach? To what extent have we provided an adequate 

justification for the new indicator used? 

The use of visual air quality as an indicator of urban VAQ is reasonable, and adequate justification is 

provided. Some discussion of the shortcomings of this approach (resulting from this measurement 

being a local estimate of a quantity that is integrated over km length scales) would have been helpful, 

but it is acknowledged that point PM mass measurements are similarly constrained. 

6) An averaging time of one hour as a practical minimum time period was used in this assessment in 

recognition that, while the visibility impacts are nearly instantaneous, the urban VAQ does not 

generally change significantly from minute to minute, but does vary from hour to hour. To what 

extent does the Panel support this approach? Does the Panel consider the rationale supporting this 

approach to be clearly and appropriately presented? 

One hour averaging is a reasonable compromise. 

Chapter 4 – Total Light Extinction Under “What If” Conditions 

Questions 18 and 19: Proportional rollback, what-if scenarios 

The approaches EPA has used are reasonable. Given the uncertainty in calculations of light 

extinction by PM, proportional rollback is reasonable. 

Regarding Sections 4.2 and 4.3 – the information is clearly presented, although Figure 4.1 is 

difficult to read in print. The on-line version, with the figure expanded at least 2x, is necessary 

for seeing detail. 

Figure 4-1: Comparison of the different scenarios would benefit from a single figure comparing 

these for a single city. 

Chapter 3 – Estimate of Current PM Concentrations and Light Extinction 

Questions 12 and 13: Goals, Methods, Approaches: 

The goal of Chapter 3 was to examine urban study area light extinction and develop an improved 

understanding of the underlying causes and patterns. The stated goal of Chapter 4 was to 

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examine “what if” scenarios. The goal of Appendix D, to examine potential alternative 

indicators for light extinction. 

Chapters 3 and 4 meet the goals articulated in the first two sentences above. Chapter 3 in 

particular provides a thorough assessment of the limitations of existing data and the uncertainties 

related to an assessment of visibility (light extinction). 

Appendix D presents data on the relationship between various PM measures and light extinction. 

The data are presented clearly, but there is no discussion of the findings. Interpretation would 

have been helpful. 

It appears that EPA generalized each “season” so that each location was defined to have the same 

number of daylight hours. It is not clear what benefit this provides, and the suggestion (footnote 

23) that this may be eliminated in the final version of this assessment seems appropriate. 

The sentence beginning on the bottom of page 3-25 (line 28): “Persons...” seems speculative. 

Are there studies to support this? I suspect that those in foggy locations would value clear 

visibility, even if they do not expect it. 

Line 31, page 3-36: not clear what is meant by “..carbonaceous-caused extreme hours...” 

Presumably this refers to periods of extremely high light extinction caused by a higher than 

normal concentration of carbon-containing PM. 

Typo - Appendix D page D-1 refers to CMAQ profiles in section 3.2.4. This report does not 

contain a section 3.2.4. 

Question 14 – PRB

The approach used to estimate PRB is reasonable. 

Other comments 

1. delete “that will be developed” from line 8, page 1-2 

2. line 13, p. 1-7, rational should be rationale 

3. line 32 p 1-10, should read urban areas (plural) 

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Hopke Comments (Dr. P. K. Hopke) 

Comments by P.K. Hopke on the Particulate Matter Urban-Focussed Visibility Assessment 

This assessment generally makes appropriate use of the available ambient aerosol data. There 

are some issues that need to be addressed to improve the input values, but the approach taken in 

this assessment are reasonable and appropriate to the task at hand. 

On page 3-4, line 9, It says “most accurate method” There is no basis for detemining 

“accuracy” with respect to any ambient mass monitor system. Precision can be assessed, but not 

accuracy. 

There seems to be a misconception that the FRM measures ambient mass concentrations related 

to light scattering. This is clearly incorrect. We know that the FRM loses significant quantities 

of both nitrate and semivolatile organic matter so that it will generally underestimate the mass 

concentration that would be present to degrade visibility. Thus, the methodology that adjusts the 

continuous measurement of PM mass to match the FRM are misguided. The unmodified FDMS 

measurements are probably the best estimates we have of the actual airborne PM mass (although 

we really have no good way to verify this hypothesis). It will generally produce values greater 

than the FRM so that scaling the continuous measurements down to the FRM value will result in 

overestimating the effect of mass on light extinction. 

They have probably done as well as they can with the PM2.5-10 values since EPA has been 

unwilling to develop an effective monitoring system for this size class. The results presented 

here continue to demonstrate the problem with the difference method and it is unfortunate that 

they have not built impactor based systems to actually measure this size fraction mass. 

The choice of data for mass and composition are reasonable. In terms of uncertainty and 

variability, they have again done a reasonable job. 

They should retain the appendices. 

5) A new indicator relating ambient PM to urban VAQ (i.e., PM light extinction) has been evaluated 

to improve our characterization of the relationship between ambient PM and visibility impairment. Is 

the Panel generally supportive of this approach? To what extent have we provided an adequate 

justification for the new indicator used? 

Yes, the public perception of poor air quality when visibility is diminished. Light extinction is an 

easily measured quantity that can be related in a reasonable way to observed visibility so that it 

provides an objective measure of VAQ. Thus, the framework for adopting an extinction 

measurement as the basis for a new indicator have been appropriately presented. 

6) An averaging time of one hour as a practical minimum time period was used in this assessment in 

recognition that, while the visibility impacts are nearly instantaneous, the urban VAQ does not 

generally change significantly from minute to minute, but does vary from hour to hour. To what 

extent does the Panel support this approach? Does the Panel consider the rationale supporting this 

34
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approach to be clearly and appropriately presented? 

At this time, the idea of an hourly averaging period is reasonable and the presentation in the 

document is credible. In the absence of very much data, it is hard to fully assess the temporal 

variability, but it is likely that in most circumstances, such an averaging period would be reasonable 

and obviously then matches the time frame for which the commonly measured pollutant variables are 

measured. 

15) We consider the results generated by these analyses to be reasonable based on PM composition 

and relative humidity data. Does the Panel agree? Are there other tests of reasonableness that could 

be applied? 

The results presented in this assessment are reasonable given the data that are available and the 

current state of understanding of the effect of composition on extinction. In the absence of measured 

hourly compositional data, it is hard to envision other tests of reasonableness. 

16) In addition to a qualitative discussion of possible sources of uncertainty and variability, are there 

quantitative methods for addressing uncertainty and variability associated with these assessments that 

the Panel would recommend? 

Although this chapter does present information that pertains to both uncertainty and variability, it 

could benefit from more structure. For example, this Assessment could be consistent with the 

Risk and Exposure Assessment (REA) in referencing a framework for dealing with variability 

and uncertainty, such as the WHO (2008) framework, and explaining as to which tiers of 

assessment are applied here. Tier 0 is a point estimate based on default values. Tier 1 includes 

qualitative (but structured) assessment and comparison of sources of uncertainty. Tier 2 includes 

sensitivity analysis. Tier 3 includes quantitative analysis of uncertainty using probability 

distributions. For this Assessment, at a minimum a structured Tier 1 approach, supplemented 

with some Tier 2 applications, would be appropriate. 

17) A number of appendices are provided at the end of this document. Does the Panel agree that this 

information is useful to retain? Does the Panel agree with the level of detail provided in the body of 

the report and its organization and distribution throughout the document? 

It is useful to retain the appendices. Without them, it would be difficult to follow the arguments 

presented in the document in detail. Given that there were questions regarding the setting of an 

urban visibility standard in the last round of standard review, we feel that the level of detail 

presented in the document is appropriate to adequate inform the Administrator as to the basis for 

recommending a new secondary standard. 

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Malm Comments (Dr. William Malm) 

 1) After careful consideration of the evidence provided in the second draft ISA, and in particular 

the significant body of work that has been conducted by the Regional Planning Organizations 

under the Regional Haze Rule, (i.e., information on urban and rural PM concentrations and 

compositions), we have decided to continue to focus this assessment on the PM induced 

visibility impairment that is occurring in urban areas. What is the Panel's view on this approach? 

Is the rationale supporting the selected approach clear and appropriate? 

I believe the approach is appropriate but the “assessment” document could be more clearly 

presented. The rationale given for the selected approach is appropriate. Some issues 

concerning clarity are outlined below. 

On page 3-18 it’s stated that the mass scattering multipliers are different for each of the species 

in the IMPROVE algorithm – they are the same for sulfates and nitrates. The discussion under 

nitrates on the same page is somewhat misleading. The FRM sampler provides an estimate of 

gravimetric mass that can be severely underestimated because of nitrate volatilization; where 

nitrates are a significant fraction of PM2.5, the underestimation may be greater than 50%. Some 

range of uncertainty of error associated with the FRM sampler should be discussed. How can 

nitrate not be fully neutralized? (Not like sulfate.) Also, nitrate may be in the form of calcium or 

sodium nitrate, since in some cases measured PM2.5 nitrate is the fine tail of the coarse mode. 

Under “sulfate” on page 3-19 the statement is made “...continuous PM2.5 instruments can be 

assumed to be more like FRM...” What is the justification for this statement? I don’t believe it 

to be true! 

Page 3-19: Some reference is made to the SANDWICH approach; this approach should be 

explained at some minimal level. Figure 3.5 outlines the sequence of steps to estimate hourly 

PM2.5 components and total light extinction from modeling results. Some effort should be made 

to assess the potential uncertainty in the estimated extinction after all the normalizations and 

adjustments. The approach may very well be reasonable and the best that can be done under the 

circumstances, but the renormalizations will tend to overestimate the contribution of some 

species to extinction while underestimating others. A careful examination of potential biases 

should be explored. There are some temporally continuous speciated datasets “out there” that 

can be used to compare to the approach outlined in Figure 3.5: the SEARCH dataset (at least 

Birmingham and Atlanta), all the supersites, certainly Pittsburgh and Fresno. 

Page 3.24: It is stated that the contribution of coarse mass to extinction is low compared to 

PM2.5. How low? Might want to include a table showing PM2.5 extinction compared to PM10 - 

PM2.5. 

Page 3.25: When did you start counting the number of minutes the sun was up for a full hour of 

sunlight, when the sun was just visible or when the whole disk was above the horizon? It wasn’t 

entirely clear as to how the 90th or 95th percentile extinctions values were used to pick design 

values. I assume that the highest 1-hour extinction value for each day was selected and then the 

90th or 95th percentile was selected for that distribution. It seems that RH was capped at 95% 

and then these values went into the average. If the RH was 99%, it was set to 95% and then the 

scattering was calculated based on the commensurate modeled concentrations but with 95% RH. 

What was done needs to be clearly stated. As written on page 4.3, it seems that this one value 

for each of 3 years was averaged to get a resulting design value. However, I assume that all 

values above the 90th or 95th percentile were averaged, and these averages over years were 

36
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further averaged to get the final design value. Need to clarify! How were the PRB values 

applied? Yearly averages and then subtracted, or averaged over 3 years and then subtracted? 

Again, it should be made explicitly clear. Should discuss the uncertainty or biases associated 

with a linear rollback. All species will not decrease at the same rate. Might do some sensitivity 

analysis to see what it means to have rollback that affects mostly sulfate versus nitrates versus 

organics. The resulting extinction could be quite different, depending on the scheme one uses. 

2) After further considering the nature of urban versus more remote area PM, and in light of 

discussions with CASAC at the April 2,2009, meeting, we have decided not to develop an urban 

optimized algorithm at this time, but instead to rely on the original IMPROVE algorithm to relate 

urban PM to local haze (PM light extinction). ls the Panel generally supportive of this approach? 

Is the rationale supporting this decision appropriate and clearly presented? 

I support this approach. In lieu of the uncertainty of Roc factors for urban environments, other 

biases in estimating species concentrations in urban areas, and lack of optical measurements in 

urban areas, the use of an IMPROVE-type extinction algorithm that adjusts mass scattering 

efficiencies on the basis of mass concentrations would be inappropriate. However, the simpler 

original IMPROVE equation should be modified to include seasalt, and some thought should be 

given to what the appropriate Roc factor might be. Certainly 1.8, as used in the IMPROVE 

equation, may be too high. Something around 1.2–1.4 may be more appropriate. 

3) In a change from the planned approach presented in the Scope and Methods Plan, we have 

decided to conduct a reanalysis of the urban visibility preference studies available at the time of 

the 2006 PM NMQS review, rather than conducting new public preference studies since it is 

highly unlikely that the results of new studies could be completed in time to inform this review. 

This reanalysis was designed to explore the similarities and differences (comparability) between 

the current studies and to assess what information could be drawn from these results to inform 

the selection of visual air quality (VAQ) candidate protection levels (CPLs) to be used in 

subsequent impact assessments. This reanalysis also includes a recent study by Smith and 

Howell (2009) for Washington, D.C., which was presented to the CASAC during the public 

comment phase of the April 2, 2009 meeting and later provided to EPA staff. Does the Panel 

agree that the information provided by this reanalysis is useful to inform the selection of CPLs? 

Does the Panel agree that inclusion of the Smith and Howell (2009) is appropriate in both the 

ISA and Visibility Assessment? To what extent does the Panel consider that the reanalysis of the 

urban visibility preference studies is clearly and appropriately characterized? 

I support the reanalysis effort, the selection of CPLs, and the inclusion of the Smith and Howell 

data. The analysis could be done in a more elegant and statistically defensible way. A logistic 

distribution could be fitted to the data and percentile values extracted from the curve fit, along 

with associated confidence intervals. Then one would have some idea as to whether the values 

derived from various studies are indeed statistically different from each other. 

4) We have chosen to use the range that represents the 50th acceptability criteria across the four 

cites studied (i.e., the VAQ level that best divides the photographs shown into two groups: those 

with a VAQ rated as acceptable by the majority of the participants, and those rated not 

acceptable by the majority of participants) as CPLs to characterize the nature of the impact on 

urban VAQ associated with current PM levels. Please comment on the clarity and 

37
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appropriateness of the rationale supporting this decision. Does the Panel have suggestions for 

alternative ranges to consider? 

The presentation is clear. I do not have strong feelings about what percentile should be selected. 

I guess I would choose, as has been done in various perception threshold studies, something like 

the 90th percentile. Here again, it would be nice to have a logistic model developed that would 

allow a selection of any percentile with a defined uncertainty. Selection of percentile levels by 

“eye” as was done in this assessment is certainly less elegant but not necessarily inappropriate. 

5) A new indicator relating ambient PM to urban VAQ (i.e., PM light extinction) has been 

evaluated to improve our characterization of the relationship between ambient PM and visibility 

impairment. Is the Panel generally supportive of this approach? To what extent have we provided 

an adequate justification for the new indicator used? 

Justification for using extinction has been provided, and because it is more representative of 

“visibility”, it certainly is more appropriate than a mass design level. 

6) An averaging time of one hour as a practical minimum time period was used in this 

assessment in recognition that, while the visibility impacts are nearly instantaneous, the urban 

VAQ does not generally change significantly from minute to minute, but does vary from hour to 

hour. To what extent does the Panel support this approach? Does the Panel consider the rationale 

supporting this approach to be clearly and appropriately presented? 

It has been demonstrated that significant changes in mass concentrations can take place on 

temporal increments of minutes. However, averaging these changes over distances of many 

kilometers may average out to small changes in “visibility” or path average extinction. Some 

quantitative justification should be given as to why an hour is the smallest time increment that 

should be considered. What are the standard deviations of hourly averages of various aerosol 

species? These data again are available at some urban areas and certainly for the “super” 

sites.7) We have chosen to use the 90th and 95th percentile forms in our assessment of 

alternative secondary (welfare-based) standards. Please comment on the use of these alternative 

forms. 

It is my understanding that it is the 90th or 95th percentile of the max hour on each day is what 

has been considered. I would suggest that this approach be compared to just selecting the 90th

or 95th percentile extinction for all days (only daylight hours) without concern for daily 

maximums. What are the advantages or disadvantages of one approach over the other? What 

are the potential effects of these choices on emission control strategies? Will they be any 

different? At first glance at Figures 3.12–3.19, it seems that the relative mix of aerosol 

contribution to extinction is the same on all 90th percentile max extinction days, so maybe it 

doesn’t make much difference which strategy is taken for calculating the design extinction 

values. However, I would suggest exploring a number of alternative strategies and seeing how 

the positives and negatives compare and contrast for each approach. 

8) To what extent does the Panel support the graphical displays presented in this chapter? As 

currently presented, do these figures clearly summarize the assessment results? We have 

combined data from multiple studies for two locations -British Columbia and Washington, D.C. -

(Abt Associates, 2001; Smith and Howell, 2009 test 1) as presented in Figure 2-14. Does the 

Panel agree with developing a composite dataset for each of these two urban areas? 

38
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I think you can combine the dataset for a “composite” analysis. However, I think you should 

develop a logistic model as discussed above. 

9) Despite significant differences in study characteristics (e.g., size, location), to what extent 

does the Panel support combining and comparing the results from the four cities, as shown in 

Figure 2.14? What is the Panel's view on the clarity and adequacy of the descriptions of the 

uncertainties and limitations associated with such a combined assessment and the conclusions 

that can be drawn from the assessments? Please provide comments on additional insights, 

uncertainties, or caveats that should be considered. 

See comments above and comments under question 3. 

10) We have used the combined results presented in this chapter to develop a range of CPLs that 

are used in subsequent steps of the assessment. To what extent does the Panel support the range 

of CPLs used and the justification provided for selecting this range? Does the Panel recommend 

consideration of any alternative approaches or criteria for selecting CPLs? 

See comments above. Another issue not discussed above is the way the extinction is calculated 

as a function of RH. I would suggest an upper limit of 90% RH. The uncertainties of RH 

measurements above 90% are large, and furthermore, with the RH as high as 95%, one could 

very well have “wisps” of clouds in a sight path, which isn’t accounted for in the extinction 

algorithm. Also, when the RH is above the threshold value for an hour, that hour (or increment 

of time used to do the extinction calculation) should be excluded from any statistic used to 

calculate a design value. I believe the current estimates are based on setting the RH to a capped 

value and including an estimate for extinction at the capped RH value in the average or statistic. 

If the RH is 100% and it is raining or foggy, aerosols are typically not a significant contributor 

to visibility reduction. 

11) Overall, we consider this assessment useful for providing information for the design of future 

urban visibility preference studies. Does the Panel support this conclusion and does the Panel 

have any recommendations for changes that could be made in the discussions of this information 

to enhance its usefulness for this purpose? 

I think that some discussion should be given to the potential role of aerosols on sky color and 

visibility of clouds, which are both integral to good visibility in an urban setting. 

12) Are the goals articulated in the first paragraph of this chapter achieved in the remainder of 

the assessment? If not, does the Panel have suggestions for additional assessments that should be 

done? 

Goals are well articulated. See comments under question 1. Certainly, model results of 

speciated aerosol concentrations should be compared to available monitoring data, such as the 

SEARCH and supersite datasets. 

13) Are the methods and approaches taken in these assessments, including those for monitor site 

selection, incomplete data adjustments, and the use of the CMAQ model to augment speciation 

data, appropriate and is the rationale for their selection clearly articulated? 

See comments above. If you are going to include a discussion of monitoring site selection, then 

it seems that some discussion of requirements for monitor design should be included. For 

39
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instance, how will an ambient scattering measurement be made (as opposed to heating the 

aerosol before the scattering measurement is made), will the nephelometer have a size-selective 

inlet, how will coarse particle scattering be measured or estimated, how will ambient absorption 

be measured, and so forth?. I would suggest that these issues are as or more important than site 

selection. Either site selection issues should be left out of this document, or general monitoring 

equipment design criteria should be included. 

14) Is the approach used to estimate PRB as described in chapter 3 and Appendix C appropriate? 

Yes – see comments above as to how averages were calculated. 

15) We consider the results generated by these analyses to be reasonable based on PM 

composition and relative humidity data. Does the Panel agree? Are there other tests of 

reasonableness that could be applied? 

May or may not be reasonable; compare modeled results to available datasets. 

16) In addition to a qualitative discussion of possible sources of uncertainty and variability, are 

there quantitative methods for addressing uncertainty and variability associated with these 

assessments that the Panel would recommend? 

Compare to available datasets, and then a real handle on uncertainty can be achieved. 

17) A number of appendices are provided at the end of this document. Does the Panel agree that 

this information is useful to retain? Does the Panel agree with the level of detail provided in the 

body of the report and its organization and distribution throughout the document? 

The information should be retained. 

18) Does the Panel agree with the approaches used to simulate just meeting air quality conditions 

for the current and alternative PM standards? In particular, is use of the proportional rollback 

approach appropriate in the context of the urban PM visibility assessment? 

See comments above. 

19) To what extent does the Panel consider the presentation of "what if” scenarios for retention 

of the current secondary PM2.5 NAAQS and consideration of alternative, more protective 

secondary NAAQS in sections 4.2 and 4.3 to be clearly written with an appropriate level of 

detail? Do the correlation analyses presented in Appendix D provide sufficient insight into the 

suitability of alternative indicators based on sub-24 hour averaging periods for PM2.5? Are there 

additional alternative standard scenarios that should be evaluated? 

See discussion above under question 10. Some alternative approaches should be considered. 

Picking a 90th or 95th percentile, using all days, as opposed to 1-hour maximum extinction on 

each day, should be considered. Set the upper RH value to 90%, and any hour with RH above 

that value should not be included in the design value.

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Poirot Comments (Mr. Rich Poirot) 

General Comments on the September, 2009 PM Visibility Risk Assessment: 

Compliments to the authors – this is an excellent assessment in all respects. Its thoughtfully 

conceived, well organized and very clearly written. There is also a substantial amount of 

underlying number crunching and analysis for which I can appreciate many of the difficulties, 

and am most impressed by how much work you did in such a short time. The complex analyses 

also required a number of important “decision points” where options were considered and 

specific choices were made. In all cases, these appear to be logically reasoned and clearly 

described, such that the reader can follow every step of the analysis, with additional details 

provided in the appendix. I agree with most, but not all of these choices (though I don’t disagree 

strongly with any). This is an outstanding first draft and should make a very sound basis for 

recommending a range of secondary standards to the Administrator. 

While I concur that a sub-daily PM light extinction indicator could be a very appropriate and 

effective regulatory metric, I also thought a sub-daily PM2.5 mass indicator - proposed in the last 

PM NAAQS review cycle - could have been effective as well, and that the difference between 

the two approaches is primarily due to aerosol water content. I think each approach has 

advantages and disadvantages over the other, and that there is a lot of potentially productive 

“middle ground” between the “all dry” and “all wet” approaches. For example, the Denver 

standard uses a PM light extinction indicator but as a daily maximum 4-hour average (rather than 

1-hour maximum), constrained to the 8-hour period between 8 AM and 4 PM (not all daylight 

hours) and limited to hours when RH is less than 70% (not <95%). This is essentially a dryer 

(and for a given level and percentile form, less stringent) version of what staff is currently 

proposing, but it could still be an effective basis for a secondary PM NAAQS (accompanied by a 

higher percentile). 

Some specific alternatives that I would like to see considered include: 

• Use a RH limit as a screen (don’t include hours that exceed it) rather than a cap 

(converting values above the cap to the cap value). One of the important effects of the 

screen is to eliminate times with precipitation or fog. A cap doesn’t do this. 

• Lower the RH screen from 95% to 90%. For similar reasons as above, and also because 

of the extreme steepness of the f(RH) curve in this area, where a small error in the RH 

measurement, or in its temporal or spatial representativeness, could make a huge 

difference in the results. This lower screen will miss some of the worst visibility 

conditions, but there will be better confidence that the visibility levels in the remaining 

hours are affected by pollution and not the weather. 

• Consider alternatives to the single worst daylight hour in a day as the regulatory metric. 

Possibilities include use of the second worst hour of the day, use of 2 to 4-hour averages, 

narrow the “daylight window” to exclude the first and last daylight hours (or keep a 

constant 8-hour window as they do in Denver), calculate the percentile based on all hours 

in the year (or in a season) rather than first picking the worst hour of the day. (In addition 

to a bit more “drying”), reasons for this suggestion are to reduce the tendency of this 

metric to be dominated by the first few hours after sunrise. Generally I think a day where 

its hazy all day is more objectionable than one where its hazy at sunrise (which probably 

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makes it “prettier”). A move away from the single worst hour would also help screen out 

effects of instrumental measurement errors, for an indicator which requires combining 

information from 3 separate instruments. 

• Note that many of the above suggested revisions or sensitivity analyses all tend to 

essentially cut off many of the highest humidity and poorest visibility hours, and so they 

might logically be combined with a higher percentile form (like the 98th). 

• Consider moving quickly to initiate a small pilot urban visibility monitoring network to 

identify and work out the bugs in the proposed methods. While there has been 

considerable experience in the visibility research community with the operation of 

ambient nephelometers and aethalometers – in some cases equipped with switching heads 

that alternatively sample different particle sizes – such combinations have not really been 

tried in routine network operations, and it would be critical to get some early experience 

in this area. 

September 2009 PM Visibility Risk Assessment, Charge Questions: 

1) After careful consideration of the evidence provided in the second draft ISA, and in 

particular the significant body of work that has been conducted by the Regional Planning 

Organizations under the Regional Haze Rule, (i.e. information on urban and rural PM 

concentrations and compositions), we have decided to continue to focus this assessment 

on the PM induced visibility impairment that is occurring in urban areas. What is the 

Panel’s view on this approach? Is the rationale supporting the selected approach clear 

and appropriate? 

Yes, focusing a secondary PM NAAQS in a way that is complementary to the existing Regional 

Haze Rule (RHR, which applies in Class I National Parks and Wilderness Areas), is appropriate, 

efficient and the rationale is logical and clearly stated. A somewhat semantic point here is that 

the physical areas occupied by “Class I” Federal lands and by “urban” areas each cover relatively 

small fractions of US land area, and that a secondary standard “complementary to the RHR” 

should apply to all “non-Class 1 areas”, for which “urban visibility” is a effective descriptive 

term that should be intended to represent larger geographical areas and larger ranges of urban, 

suburban and rural non-class 1 area population densities. In addition, there is no need for the 

RHR and secondary (or primary) PM NAAQS to be viewed as mutually exclusive regulatory 

mechanisms. Steps taken to meet a secondary (or primary) PM (or SOx or NOx) NAAQS 

applied to “urban” areas may well result in improvements in regional haze in remote class 1 

areas – and vice versa. 

2) After further considering the nature of urban versus more remote area PM, and in 

light of discussions with CASAC at the April 2, 2009 meeting, we have decided not to 

develop an urban optimized algorithm at this time, but instead to rely on the original 

IMPROVE algorithm to relate urban PM to local haze (PM light extinction). Is the Panel 

generally supportive of this approach? Is the rationale supporting this decision 

appropriate and clearly presented? 

Yes, this is a very reasonable approach at the present time. Its likely that improvements may be 

made to this equation in the future – especially as the new urban extinction data become 

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available, but its highly unlikely that alternative equations would result in major revisions to 

results obtained from the IMPROVE equation. As a practical matter, the equation becomes 

irrelevant (except for purposes of source attribution) as the standard is implemented, since the 

optical indicator essentially aggregates the cumulative effects of all the contributing pollutant 

species without requiring any information on the individual species concentrations or effects. 

3) In a change from the planned approach presented in the Scope and Methods Plan, we 

have decided to conduct a reanalysis of the urban visibility preference studies available at 

the time of the 2006 PM NAAQS review, rather than conducting new public preference 

studies since it is highly unlikely that the results of new studies could be completed in time to 

inform this review. This reanalysis was designed to explore the similarities and differences 

(comparability) between the current studies and to assess what information could be drawn 

from these results to inform the selection of visual air quality (VAQ) candidate protection 

levels (CPLs) to be used in subsequent impact assessments. This reanalysis also includes a 

recent study by Smith and Howell (2009) for Washington, D.C. which was presented to the 

CASAC during the public comment phase of the April 2, 2009 meeting and later provided to 

EPA staff. Does the Panel agree that the information provided by this reanalysis is useful to 

inform the selection of CPLs? Does the Panel agree that inclusion of the Smith and Howell 

(2009) is appropriate in both the ISA and Visibility Assessment? To what extent does the 

Panel consider that the reanalysis of the urban visibility preference studies is clearly and 

appropriately characterized? 

While the Agency is encouraged to allocate sufficient funds and resources to support the planned 

new urban visibility preference studies, the decision to conduct a careful reanalysis of currently 

available urban visibility preference studies – including the recent Smith and Howell (2009) data 

- was the most reasonable approach to developing a range of candidate protection levels (CPLs) 

for use in the current risk assessment. There would not have been time to complete the new 

studies quickly enough to support the current assessment, and the results of those new studies 

will be more useful if they are carefully planned and executed, and not rushed according to an 

accelerated NAAQS review schedule. The reanalysis shows a relatively strong degree of 

convergence in the identification of unacceptable levels of visual air quality across the different 

study areas, and provides a sufficient basis for use of the proposed CPLs in the current risk 

assessment, and for considering alternative levels of secondary PM standards based on PM light 

extinction. 

4) We have chosen to use the range that represents the 50th acceptability criteria across the 

four cites studied (i.e., the VAQ level that best divides the photographs shown into two 

groups: those with a VAQ rated as acceptable by the majority of the participants, and those 

rated not acceptable by the majority of participants) as CPLs to characterize the nature of 

the impact on urban VAQ associated with current PM levels. Please comment on the clarity 

and appropriateness of the rationale supporting this decision. Does the Panel have 

suggestions for alternative ranges to consider? 

Use of a range representing the 50th percent acceptability criteria for VAQ levels across the four 

study areas is an appropriate approach, for which the justification is logical and clearly stated. It is 

likely that these “acceptable” and “unacceptable” VAQ levels will be further refined in the future, as 

results of additional studies become available. But for use in the current NAAQS review, the 

identified range of VAQ levels provides a sound and defensible basis for establishment of a separate 

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secondary standard based on achieving and maintaining acceptable levels of VAQ in urban areas. 

5) A new indicator relating ambient PM to urban VAQ (i.e., PM light extinction) has 

been evaluated to improve our characterization of the relationship between ambient PM 

and visibility impairment. Is the Panel generally supportive of this approach? To what 

extent have we provided an adequate justification for the new indicator used? 

Yes, the proposed new indicator is logically justified and clearly described. The PM light 

extinction indicator will be directly and instantaneously responsive to changes in the 

concentrations of the PM components which affect it, and would have the eloquent advantage 

over all other currently or previously considered NAAQS indicators in that the indicator, in this 

case would be the effect. 

6) An averaging time of one hour as a practical minimum time period was used in this 

assessment in recognition that, while the visibility impacts are nearly instantaneous, the 

urban VAQ does not generally change significantly from minute to minute, but does vary 

from hour to hour. To what extent does the Panel support this approach? Does the Panel 

consider the rationale supporting this approach to be clearly and appropriately 

presented? 

I have no major objections to the use of a one-hour averaging time, and think logical arguments 

for it have been clearly presented here. I don’t completely agree, however, that since visibility 

effects are perceived instantaneously, a one-hour averaging time is necessarily the best or most 

effective averaging time for a regulatory metric. Use of somewhat longer averaging times – such 

as the 4-hour intervals, (selected after careful consideration in both Phoenix and Denver) would 

tend to help screen out effects of short-term outliers, unusual weather, instrumental noise or 

malfunction, and would also tend to place more emphasis on causes of poor visibility which are 

more persistent than ephemeral. 

I’m also not sure I agree that the use of the single worst (daylight) hour in a day (prior to 

calculating a percentile) is necessarily better than applying that same (or another) percentile to all 

the hours of the year (or of the season). Is the 95th percentile of the worst hour of the day a better 

metric than the 98th percentile of of the second worst hour, etc.? Time permitting, it might be 

informative to conduct a sensitivity analysis, exploring effects of some of these alternative 

combinations. 

7) We have chosen to use the 90th and 95th percentile forms in our assessment of 

alternative secondary (welfare-based) standards. Please comment on the use of these 

alternative forms. 

The use here of the 90th and 95th percentiles is reasonable for purposes of this first draft 

assessment, and helps illustrate the important effects that selection of the form can have on the 

frequency and magnitude by which certain levels are exceeded (or not). That being said, I don’t 

think either of these (or any other percentile) can be uniquely well-justified compared to other 

forms. In the last PM NAAQS review, EPA staff had initially suggested the 90th percentile, as 

being “consistent” with the regional haze Rule (which focuses on the worst 20 % of days, for 

which the average is roughly equal to the 92nd percentile. However, the flaw in this “logic” is 

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that the RHR specifically focuses on and requires improvements in these worst days, while using 

a similar percentile as the form for an urban visibility standard would essentially discard these 

worst days as unimportant. The 90th percentile also seems like an illogical metric to combine 

with the worst hour of the day (the whole day is unacceptable if even a single hour exceeds the 

threshold, but this only becomes unacceptable if it happens more than 37 days a year). 

Off hand, I don’t think its necessarily logical that a welfare standard should be set at a lower 

(less stringent) percentile than a (“more important”) health standard. By this logic, secondary 

standards should always be set more leniently (or equal to) the “more important” primary 

standards (as they, ah, have been...). If adverse effects on either health or welfare are 

experienced during single or a few high short-term exposures, then the percentile forms of both 

kinds of standards should be set accordingly. From that perspective, the 98th percentile might 

also be considered as appropriate for protecting visibility, and should not be considered 

unreasonable without better justification. A (much) more leisurely pace toward attaining a “less 

important” secondary standard is accommodated by less urgent implementation requirements. 

At the same time, since there is no compelling basis for selecting a specific (relatively high) 

percentile, this might currently be considered as an area where there is “flexibility” to balance 

alternative percentiles with other elements of the standard to find the best combinations of 

indicator, averaging time level and form which would lead to maximum improvements over 

different geographical regions but which would also be feasible to attain over reasonable time 

periods (or accompanied by guidance outlining schedules of reasonable progress in the 

implementation phase). 

 8) To what extent does the Panel support the graphical displays presented in this 

chapter? As currently presented, do these figures clearly summarize the assessment 

results? We have combined data from multiple studies for two locations - British 

Columbia and Washington, D.C. - (Abt Associates, 2001; Smith and Howell, 2009 test 1) 

as presented in Figure 2-14. Does the Panel agree with developing a composite dataset 

for each of these two urban areas? 

The graphic displays (such as those in Figures 2-2, 2-3, 2-5, 2-7, and 2-9 through 2-14) which 

compare visibility levels in deciviews with percent respondents rating the VAQs acceptable are 

an excellent way of showing results from the different studies in common units, showing the 

range or distributions among different respondents and across the different study areas, and for 

showing the convergence of results for acceptable/unacceptable visibility in the range of 20 to 30 

dv. The use of composite data sets based on combined study results for the British Columbia 

studies and for the Washington DC studies is also a logical approach, and seems appropriate for 

combining these study areas with results from the Phoenix and Denver studies. 

9) Despite significant differences in study characteristics (e.g., size, location), to what 

extent does the Panel support combining and comparing the results from the four cities, 

as shown in Figure 2-14? What is the Panel’s view on the clarity and adequacy of the 

descriptions of the uncertainties and limitations associated with such a combined 

assessment and the conclusions that can be drawn from the assessments? Please provide 

comments on additional insights, uncertainties, or caveats that should be considered. 

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I think combining results from the different study areas is an appropriate approach that takes 

maximum advantage of all the currently available data. To a certain extent, the relative 

convergence of results from the different study approaches and study areas strongly supports the 

use of these data collectively for use in identifying a relatively narrow range of acceptable VAQs 

across many locations and respondents. Although a formal uncertainty analysis was not 

conducted (nor do I believe it would have been very informative here), I think the descriptive 

information on uncertainties and limitations of this approach conveys that information clearly. It 

should also be noted that the uncertainties associated with this approach for selecting a level or 

range of acceptable VAQ are likely to be relatively small compared to uncertainties associated 

with other aspects of a secondary PM standard such as the averaging time, representing a day by 

its worst 1 (or more) VAQ hour(s), and the frequencies (percentiles) for which unacceptable 

VAQ conditions are acceptable. 

While the focus here has been on discerning a decision point or range of VAQ conditions 

considered acceptable vs. unacceptable, it should also be recognized that there are welfare 

benefits associated with improvements across all parts of the visibility spectrum, including for 

example a shift of days with “good” to “excellent” VAQ. As a practical matter, efforts made to 

improve visibility conditions in any part of the distribution are likely to shift the entire 

distribution, with benefits that will be understated by a single 50% acceptability approach. 

10) We have used the combined results presented in this chapter to develop a range of 

CPLs that are used in subsequent steps of the assessment. To what extent does the Panel 

support the range of CPLs used and the justification provided for selecting this range? 

Does the Panel recommend consideration of any alternative approaches or criteria for 

selecting CPLs? 

As indicated above, the chapter presents a clear justification for the range of CPLs used in 

subsequent steps of this assessment. Its likely that results of future urban visibility preference 

studies can help refine (and may well lower) this range in the future, but the proposed range is 

logically derived from currently available research results and appropriate for use in evaluating 

and proposing a range of PM light extinction levels that could form the basis of a secondary 

standard to protect urban visibility in the current NAAQS review cycle. While other approaches 

for selecting alternative CPLs might be considered, I think the current range is reasonable, and 

that a lot of additional work in this area is not currently justified, considering the relatively large 

influence that other elements of a standard (such as averaging time, percentile form, RH screens, 

etc.) will have on the ultimate effectiveness of the standard. 

11) Overall, we consider this assessment useful for providing information for the design 

of future urban visibility preference studies. Does the Panel support this conclusion and 

does the Panel have any recommendations for changes that could be made in the 

discussions of this information to enhance its usefulness for this purpose? 

Yes, I would agree that this assessment provides a very useful basis for identifying designs for 

and priority information needs from future urban visibility studies. As indicated above (and 

supported by the discussions of issues in this assessment), such studies should be designed to 

provide information that will help refine other elements of a PM light extinction standard in 

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addition to identifying the level(s) of VAQ considered unacceptable across a range of different 

urban locations and viewing conditions. For example: what frequency of poor visibility 

conditions is acceptable; should this frequency be applied to the single worst hour of a day or 

should days with prolonged periods of poor visibility be considered more adverse than days with 

poor visibility for only an hour or two; what are preferences for shifts in other percentiles of the 

visibility frequency distribution – such as an increase in the frequency of very clear days, etc? 

As always, there is a need to carefully balance the advocacy of future research needs with the 

confidence in using the best currently information to make sound decisions to better protect 

human health, welfare and environment. In this case, currently available information is more 

than adequate to establish a basic secondary standard based on PM light extinction, even while 

future research is clearly needed to refine such a standard in subsequent NAAQS review cycles. 

It should also be noted that the more detailed information on hourly light scattering and 

absorption that will result from implementing a standard with a PM extinction indicator, will 

itself provide an invaluable information resource that will help support future urban visibility 

preference studies and refine light extinction equations for different mixes of urban aerosols. 

These data would also be available for near-real time reporting and public communication, 

would be useful for air quality forecasts and (in combination with other continuous 

measurements) will provide added information on highly time-resolved fine & coarse particle 

composition, including black carbon, of value for various health effects studies as well). 

12) Are the goals articulated in the first paragraph of this chapter achieved in the 

remainder of the assessment? If not, does the Panel have suggestions for additional 

assessments that should be done? 

Yes, this represents a very thorough and clearly presented analysis, and is extremely informative 

for a first draft. As indicated elsewhere, I think the RH limit should be used as a screen (discard 

hours above it) rather than a cap (set hours above it equal to it) and should be set at 90% rather 

than 95%. Other possible metrics that might be explored – if time and resources permit - in a 

second draft assessment might include: use of 2nd highest daylight hour, or the max 2 to 4-hour 

average (combined with higher percentiles), calculating percentiles based on all daylight hours in 

a year or a season, shortening the daylight window to exclude the (typically most humid) hour(s) 

just after sunrise and just before sunset, etc. 

13) Are the methods and approaches taken in these assessments, including those for 

monitor site selection, incomplete data adjustments, and the use of the CMAQ model to 

augment speciation data, appropriate and is the rationale for their selection clearly 

articulated? 

All of the above are reasonable and were clearly explained. Results from a few of the selected 

cities don’t seem quite right to me, but this tends to make me suspicious of the data, rather than 

the methods, as the latter seem logical to me. For example, the coarse mass from St. Louis 

doesn’t make sense, and I’m surprised by (& suspicious of) the poor correlations between 4-hr 

afternoon Bext and fine mass in Table D-1 and Figure D-2 for St. Louis (R2

 = 0.27) and 

Philadelphia (R2

 = 0.36) – although I’m sure these will improve if an RH screen (not a cap) were 

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applied to the Bext estimates. 

 

14) Is the approach used to estimate PRB as described in chapter 3 and Appendix C 

appropriate? 

The PBR calculations look fine, and I’ve noticed that for most species they seem to agree 

reasonably well with the carefully derived estimates of “natural background” at nearby 

IMPROVE sites for the Regional Haze Rule. For other species (nitrate and fine soil) the PBR 

estimates seem (illogically) much lower than the IMPROVE natural background estimates. I 

don’t think this is actually very important, but it might be informative to compare these (small 

numbers in both cases) if time allows. 

15) We consider the results generated by these analyses to be reasonable based on PM 

composition and relative humidity data. Does the Panel agree? Are there other tests of 

reasonableness that could be applied? 

Generally they look reasonable to me. Possibly you could compare estimated hourly extinction 

estimates for Phoenix with the transmissometer data there. For Atlanta, the hourly species 

estimates might be compared with the hourly SEARCH data there if the time periods overlap. 

Similar comparisons might be made with the hourly species composition from the Supersite data 

from Fresno, St. Louis and Pittsburgh. For many locations, the airport ASOS data are actually 

not bad, especially if the raw data can be accessed prior to binning and truncation. Even with the 

censored data, some comparison might be made for hours when very poor visibility (< 10 km) 

was reported (or predicted), again with RH screening. See also #s 16 & 17 below. 

16) In addition to a qualitative discussion of possible sources of uncertainty and 

variability, are there quantitative methods for addressing uncertainty and variability 

associated with these assessments that the Panel would recommend? 

The qualitative discussions are helpful. I think some quantitative comparisons between 

measurements and measurements such as those suggested in #15 above could help convey some 

of the uncertainties. I consider the comparisons between 4-hr afternoon Bext and fine mass in 

Table D-1 and Figure D-2 as good illustrations of causality, the similarities and differences and 

variability between a dry and wet (ambient) indicator – during the generally drier daylight hours, 

and also as a form of QA that (for me) adds confidence in the estimates. If the Bext in those 

comparisons were further dried by use of 90% RH screen, the fit and our resulting confidence 

should both improve. 

As indicated above, I think the graphic displays (such as those in Figures 2-2, 2-3, 2-5, 2-7, and 

2-9 through 2-14) which compare visibility levels in deciviews with percent respondents rating 

the VAQs acceptable are an excellent way of showing the variability in results among different 

respondents and across the different study areas, as well as showing the convergence of results 

for acceptable/unacceptable visibility in the range of 20 to 30 dv. This provides a quantitative 

sense of the range of the uncertainty that selection of any single VAQ level will have in 

representing responses of all viewers in all areas. 

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Elsewhere in these comments I suggest alternative metrics for some of the parameters used here. 

If time allows, quantitative comparisons of results from these “sensitivity runs” might also give 

some indication of variability and uncertainties associated with selecting any approach among 

several that are approximately equally justifiable. 

17) A number of appendices are provided at the end of this document. Does the Panel 

agree that this information is useful to retain? Does the Panel agree with the level of 

detail provided in the body of the report and its organization and distribution throughout 

the document. 

I find the information provided in the appendices very informative – it really helps to “see” and 

gain confidence in the complex data and relationships which underlie the resulting estimates. I 

think the level of detail, the organization of information and its distribution within the 

assessment document and appendices is excellent. This is very well conceived, organized, 

executed, and written – a pleasure to read! 

As indicated in previous comments, I’m suspicious of some of the poor correlations in Appendix 

D and hopeful that the recommended 90% RH screen will improve those results. Also, It would 

help if some justification were provided for the use of the LOESS regression in Appendix D. I 

note that most of the regression lines are nearly straight, while a few curve up and others curve 

down, but I’m not aware of the theoretical justifications for these differing non-linear 

relationships, and would also like to have a better sense of how a LOESS R2

 of 0.8 compares to 

that from an ordinary least squares approach. Also if you used a linear regression, you could 

also report the slopes and intercepts (if any) in Table D-1, which would convey added 

information of interest. 

18) Does the Panel agree with the approaches used to simulate just meeting air quality 

conditions for the current and alternative PM standards? In particular, is use of the 

proportional rollback approach appropriate in the context of the urban PM visibility 

assessment? 

I think this approach is reasonable and have no alternatives to suggest. In reality I don’t think we 

would expect all species to be rolled back by the same proportion, but don’t think there’s much 

basis to assume otherwise. Table 4-7 is an especially informative display of relevant information! 

I assume that for the final policy assessment it would be possible to add results for other PM 

combos like 13/30, etc. 

You show the (90th and 95th percentile) Bext levels associated with just meeting various PM2.5

standards. I wonder if (at least for one or two urban areas) it might be possible to provide an 

indication of what the distributions of hourly Bex levels would be under current conditions and 

associated with just meeting the alternate PM standards – (and might these also be compared 

with the distributions associated with just meeting the alternative Bex standards)? Then, if there 

are Winhaze (or sufficient actual haze cam photos) images available, maybe some percentiles 

along these distributions be graphically illustrated. This might be an effective way of 

communicating the additional benefits that the optical standard might provide – especially at the 

clearest and haziest ends of the distribution. Unfortunately, the available Phoenix and Dallas 

WinHaze photos would likely be poor choices for such illustrations since they are relatively 

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clean to start with, although you could increase extinction there up to the levels allowed by the 

current standards. 

19) To what extent does the Panel consider the presentation of "what if" scenarios for 

retention of the current secondary PM2.5 NAAQS and consideration of alternative, more 

protective secondary NAAQS in sections 4.2 and 4.3 to be clearly written with an 

appropriate level of detail? Do the correlation analyses presented in Appendix D provide 

sufficient insight into the suitability of alternative indicators based on sub-24 hour 

averaging periods for PM2.5? Are there additional alternative standard scenarios that 

should be evaluated? 

As indicated above, I suggest using a 90% RH screen and several other “sensitivity runs” that 

will generally tend to dry out the aerosol, somewhat reduce the current East/West differences and 

result in lower extinction estimates. It would be informative to see the results of some of these 

“softer metrics” compared to the alternative primary PM standards. Since those changes would 

essentially screen out many of the worst visibility hours from a regulatory metric, I think it 

would also be appropriate to consider combining them with a higher percentile form – such as 

98%tile, which I think can be as logically justified (or more so) as (than) the 90th or 95th. 

A related point - a bit off-topic here, but maybe appropriate for the policy assessment (which I 

haven’t read yet) - is that since secondary standards have no required time fuse, it should not 

necessarily viewed problematic if some (eastern) areas have much higher “residual secondary 

non-attainment” (hours above the Bext standard after primary standards are attained) than other 

(western) areas, as this would reflect actual visibility conditions and EPA guidance might 

suggest the concept of a staggered time schedule where dirtier areas have more time and vice 

versa such that all areas might be expected to show similar “rates of progress”, as in the Regional 

Haze rule. Such a “progress-based” secondary standard would allow better coordination with the 

Regional Haze Rule, and would better accommodate different visibility, pollutant, humidity 

and/or scenic charistics in different regions. 

Specific Comments on September 2009 PM Visibility Risk Assessment 

p. 1-7, lines 16-20: I don’t fully agree with the “logic” that since visibility is worse when RH is 

highest, then greater protection is needed during the times when humidity is highest. 

p. 1-10, line 28: Add “s” to “characterization” or change “were” to “was”. 

p. 2-14, lines 16-18: Might it be possible to consider compliance metrics which – like in Phoenix – 

are not based on a single absolute threshold, but rather based on a required shift in the distribution. 

Arguably, there are important welfare benefits in shifting from moderate to good and from good to 

excellent visibility, this kind of approach would be more consistent with the Regional Haze Rule, and 

it would be more accommodating of regional differences in particle composition and humidity. 

p. 2-26, lines 17-36: Considering the extensive preceding assessments to derive an appropriate range 

of levels for a visibility standard, only a short paragraph is devoted to identifying a proposed 

averaging time and form. I don’t think either the 1-hour average or the 90 or 95th percentiles are 

uniquely well justified here, and in fact seem somewhat inconsistent with each other (“its an 

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intolerable day if visibility is impaired even for an hour, but I don’t mind until it happens more than 

37 days a year”). One effect of the 1-hour averaging time (considered over all daylight hours) is to 

assure that the day’s maximum (or near max) humidity is encountered, often also at the period of 

lowest wind speed, lowest mixing height – and often at a period when its most difficult to discern 

between morning fog and pollution effects. In many mountain/valley settings, winter visibility may 

be poor for only an hour or 2 just after sunrise, but much clearer throughout the rest of the day. But 

unlike a short-term health effect, one hour of bad visibility does not necessarily ruin your whole day, 

and a day when visibility is impaired all day long should logically be considered more adverse than a 

day with one bad hour. So I see no logic in calling the entire day impaired if it has one bad hour, and 

a percentile form – like 90th, 95th or 98th – might more logically be applied to all the daylight hours, 

rather than to the single maximum hour. 

It might also be noted that both the Denver and Phoenix standards and what EPA staff proposed last 

time are all based on 4-hour averaging times. Yes, visibility is perceived instantaneously, but 

arguably impairment that persists over longer periods is more objectionable than short-term 

aberrations, when it may be most difficult to discern the difference between pollution and natural 

influences. A somewhat longer averaging time would also help minimize effects of measurement 

errors, and would have the effect of reducing, somewhat, the influence of RH on the regulatory 

metric. 

p. 3-8, Figure 3-3: You could add the dates (2000-2004) to the figure caption. 

P 3-8, line 12: Add “and high relative humidity” to this list. You might also add “This is also the 

time of day when ground fog is most prevalent, and when its most difficult to discern the difference 

between natural and manmade causes of impairment.” 

p. 3-10, Table 3-2: Its seems curious that for all cities where the site-specific value is not the same as 

the design value, the site-specific values for both annual and 24-hour are always greater than the sitespecific. 

p. 3-16, line 14: I assume that the “old” IMPROVE conversion factor of 1.4 x OC is used here, 

right? Was any adjustment used to account for differences between IMPROVE and CSN OC and EC 

data? If so, specify; if not, some explanation is warranted. 

p. 3-17, Figure 3-4: The time intervals (X-scales) are incorrect. Also, I’m surprised to see the winter 

Detroit EC & OC are so much higher for evening rush hour than for morning rush hour. I don’t think 

that’s a typical pattern – or at least would be different in areas where winter inversions are common. 

p. 3-21, lines 1-10: It would be interesting to see how the SANDWICH organic matter compares to 

1.4 (or 1.8) x OC. 

p. 3-25, lines 22-28: Now I’m confused – I thought you were going to exclude hours with RH>95%. 

Now it sounds like you will “cap” them by setting the f(RH) for hours > 95% to the f(RH) at 95% - 

which is something like 7.4. I don’t like this at all, as one of the good reasons to exclude such hours 

is to exclude periods of fog or precipitation. Its like you’re saying, lets pretend its not raining but 

that its just really humid. As indicated elsewhere, I think it would have been useful to conduct this 

exercise with alternative constraining metrics – such as a 70% (Denver), 80%, and/or 90% (Phoenix) 

RH cutoff, a 4-hour average, a shorter and consistent (8 AM to 4 PM) daytime window, and/or 

percentiles based on all the daylight hours rather than just the worst hour in a day. 

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p. 3-28, Figure 3-7b: The PM10-2.5 data for St. Louis don’t seem like they could possibly be very 

representative. Also, I’m reminded of seeing some of the coarse particle composition data from Bob 

Vanderpool and noting the very high coarse EC content in cities with steel mills like Birmingham – 

where using the IMPROVE coarse scattering efficiency may understate effects. 

p. 3-30: It looks to me like a metric based on all the daylight hours would be more stable and less 

variable across the different sites than the daily max hour. 

p. 3-35: This figure helps illustrate why a RH cutoff (not a cap) at 90% RH (or lower) would result in 

a temporally and spatially more stable metric than capping at 95%. I wonder how this figure would 

look if you limited it to only the daily 1-hr max Bext values? 

p. 3-37: Was an hourly f(RH) used in the PRB estimates? 

p. 4-1, lines 11-12: I disagree with the recommendation that (only) a PM10 inlet be used. Neither the 

nephelometer or aethelometer respond as efficiently to coarse particles as to fine ones, and the 

measurements with a 10 micron head would not truly represent the total PM scattering or absorption. 

At a minimum, I would consider a “switching” inlet which alternately samples at PM10 and PM2.5 (or 

PM1). If that doesn’t work, I would use a 2.5 or 1 micron inlet only and rename the indicator “fine 

particle light extinction”. Coarse particles contribute much less in most urban areas, Phoenix already 

has a visibility standard, and arguably coarse particles could be excluded with the argument that 

concentrations are much less spatially uniform than fines and so a point measurement is unlikely to 

be representative of areal conditions (see your St. Louis data to illustrate this point). A possible 

alternative would be to “allow” use of fine cut neph +aeth at locations where PM coarse extinction is 

estimated to be less than 10% on hazy days and require transmissometers elsewhere. 

An additional question here is that since it may take some time to implement a new network to 

measure PM light extinction, are there any ways to estimate PM light extinction from existing 

measurements? I would think reasonable estimates could be derived from a combination of 

continuous PM10, PM2.5, RH and a “generic aerosol f(RH)” growth function. This f(RH) function 

might be reduced or eliminated for periods when it could be otherwise demonstrates that nonhygroscopic aerosols, such as wood smoke or soil dust were prevalent. Another alternative approach 

would be for EPA to work collaboratively with NOAA to improve the quality and accessibility of 

airport ASOS visibility data. This would reduce costs for a new “urban” visibility measurement 

program, as there are many hundreds of such sensors in operation throughout the country, and since 

the Bext threshold for the ASOS sensors (in its original form before it gets censored and binned) is 

about 50 Mm-1, well below even the cleanest level of 74 Mm-1 (20 dv), currently being considered as 

a level for a secondary PM NAAQS. This approach would have the added benefit of improving 

aviation safety. 

p. 4-5, Table 4-3: I wonder if it would be possible to calculate the 24-hr & annual PM2.5

concentrations & changes that would be associated with meeting these different optical limits. 

p. 4-7, Table 4-4: Would it be possible to calculate and show the Bext levels & changes that would be 

associated with these PM limits? 

p. C2, Table C-1: It might be informative to compare the IMPROVE “natural background” 

calculations to these modeled estimates. For some species (SO4, OC, EC) I think the estimates are 

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53

similar but for nitrate and soil, the IMPROVE natural background levels (which should be lower) are 

substantially higher.