UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON D.C. 20460 

OFFICE OF THE ADMINISTRATOR 

 SCIENCE ADVISORY BOARD 

May 21, 2009 

EPA-CASAC-09-010 

The Honorable Lisa P. Jackson 

Administrator 

U.S. Environmental Protection Agency 

1200 Pennsylvania Ave., NW 

Washington, D.C. 20460 

Subject: Consultation on EPA’s Particulate Matter National Ambient Air Quality 

Standards: Scope and Methods Plan for Urban Visibility Impact Assessment

Dear Administrator Jackson: 

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

National Ambient Air Quality Standards (NAAQS) Review Panel met on April 2, 2009 to 

provide a consultation on EPA’s Draft Particulate Matter National Ambient Air Quality 

Standards: Scope and Methods Plan for Urban Visibility Impact Assessment (February 2009). 

The CASAC uses a consultation as a mechanism for technical experts to provide comments on 

an EPA draft plan for developing technical assessments as part of CASAC’s responsibilities in 

the NAAQS review process. In this letter, CASAC offers some general thoughts on the visibility 

plan. More specific written comments by the individual members are enclosed. 

The Plan could be improved by providing more background and justification regarding 

the importance of visibility as a health endpoint. CASAC also suggests that EPA consider ways 

of developing and implementing a visibility monitoring approach based on direct measurement 

of visibility using low cost instruments such as a size selective integrating nephelometer in 

combination with an aethalometer. EPA might also consider ways to analyze and present 

network design and operation plans to use direct measurement of visibility to characterize PM 

effects for both the primary and secondary PM NAAQS. Consistent with our comments on the 

Integrated Science Assessment for Particulate Matter (First External Review Draft, December 

2008), we would also recommend that EPA identify and briefly characterize additional metrics, 

other than visibility, for ecosystem and environmental impacts of PM that could be the 
foundation for future assessments even if there are insufficient data to support the current 

assessment as a basis for a new secondary NAAQS. 

As this is a consultation, we do not expect a formal response from the Agency. We thank 

the Agency for the opportunity to provide advice early in the PM NAAQS review process and 

look forward to the First Draft PM NAAQS Urban Visibility Impact Assessment in October 

2009. 

Sincerely, 

/Signed/ 

Dr. Jonathan M. Samet, Chair 

Clean Air Scientific Advisory Committee 

Enclosures: Enclosure A: CASAC Particulate Matter Review Panel Roster 

 Enclosure B: Compendium of Individual Comments 

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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 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 

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. Douglas Crawford-Brown, Professor Emeritus, Department of Environmental Sciences and 

Engineering, University of North Carolina at Chapel Hill, Chapel Hill, NC 

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5

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 

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 
Enclosure B 

Compendium of Individual Comments 

CASAC Particulate Matter Review Panel on 

PM NAAQS: Scope and Methods 

Plan for Urban Visibility Impact Assessment (Feb. 2009) 

Comments from Dr. Lowell Ashbaugh ____________________________________________ 2

Comments from Dr. David A. Grantz _____________________________________________ 4

Comments from Dr. William Malm_______________________________________________ 8

Comments from Mr. Rich Poirot________________________________________________ 16

Comments from Dr. Ted Russell ________________________________________________ 21

1
Comments from Dr. Lowell Ashbaugh 

Ashbaugh Review of Scope and Methods Plan for Urban Visibility Impact Assessment 

Chapter 1 – Introduction 

Charge questions

1. What are the Panel members’ views on the general structure and overall design of the 

planned analyses? 

2. Is the plan clear and transparent in its description of the planned approaches? Are the 

various assumptions and judgments that must be made in carrying out the planned 

assessments clear and transparent? 

3. Given the goals set forth for the planned analyses, has the plan appropriately drawn from 

the existing scientific and technical information in developing the overall approach? Are 

there relevant features that should be added or modified in the planned approach? 

4. In addition to the sub-daily PM2.5 alternative standard considered in the last PM NAAQS 

review and summarized in this chapter, an alternative standard structure is being 

considered in this review. This alternative structure would use daylight hourly PM light 

extinction, which can be measured either by a combination of instruments (nephelometer 

– PM light scattering and aethalometer – PM light absorption) or calculated from PM 

speciation and concurrent relative humidity data using a linear algorithm. 

a. What are the Panel members’ views regarding this alternative structure and its 

utility in the context of this PM NAAQS review? 

b. What are the Panel members’ views regarding advantages and disadvantages of 

this alternate structure compared to the sub-daily PM mass concentration 

approach? 

The general structure and overall design of the planned approaches to the “standard” assessment 

of the public preferences for and value of urban visibility are good. I particularly like the idea of 

assessing whether these preferences vary across the country in cities with differing backdrops. 

However, it’s not clear what the EPA is planning to do regarding the alternative secondary 

standard. The discussion of the “standard” assessments in sections 1.3.1 and 1.3.2 uses 

declarative sentences that start out “we are planning to ...”, but the alternative standard 

discussion in section 1.3.3 is much less clear. The foundation for the alternative standard is 

presented well but the section does not contain a clear plan. It contains phrases such as “could 

include” or “would then be specified.” 

The plan for the “standard” assessment draws appropriately from the existing scientific and 

technical information, building upon prior studies of visibility impact in remote areas to 

investigate urban visibility impact. The discussion of the alternative standard also draws upon 

earlier work to lay out the foundation, but does not go further to set forth a plan. 

Overall, I like the concept presented in the alternative secondary standard structure. PM mass 

measurements for a sub-daily 4-hour period, and especially speciation of such measurements, 

pose difficult technical challenges. The amount of PM collected in such a short time is hard to 

2
measure gravimetrically and even harder to analyze for speciated components. I am concerned 

about the use of “known relationships between PM mass and speciated components” to relate 

PM mass to PM light extinction. This concept assumes the speciation fractions during the 

daylight hours (or the 4-hour period) are the same as those measured during a 24-hour period. 

This is clearly not the case with nitrate particles, and is likely not the case for other species, 

either. Further investigation or review of existing information is needed here. 

3
Comments from Dr. David A. Grantz 

This plan provides a good distillation of the newly released Integrated Science Assessment for 

PM, First External Review Draft (December 2008). It therefore reflects both the strengths and 

weaknesses of that document. The following represent suggestions for revision of the Plan, not in 

order of importance nor priority, and focusing on Section 1 (Introduction) but not limited to that 

section. 

1. Better define the meaning and mode of application of the “policy relevant” filter to 

be applied in all aspects of this review. 

The recurring restatement of the “policy relevant” caveat (beginning with three mentions on page 

1-1) is an indicator of a problem throughout this document (and the ISA upon which it is based). 

Once the focus on visibility as the (only) surrogate for welfare impacts of PM was established, 

other information seems to have become not “policy relevant”. The previous Criteria Document 

for PM (US EPA 2006), and related agency actions, did not find sufficient data on non-visibility 

parameters to inform a secondary standard (page 1-4, line 8-13). This might have initiated 

focused research in the intervening period but did not. 

The current Plan, in its approach to these other impacts, does not ascribe to them sufficient 

gravity to encourage the research needed for the next PM assessment cycle. This contrasts with 

similar material related to human health endpoints of PM10-2.5, some of which are appropriately 

to be addressed in the current review cycle (see 2009 Health Plan, Section 3.6, first paragraph). 

It would be useful near the beginning of the discussion to insert a comprehensive definition of 

“policy relevant”, its mode of application in the current review process, and a brief consideration 

of the likely consequences of doing so. This filter is an important behind the scenes driver of the 

review, is a new addition to the process, and should receive some discussion. 

2. Combine and expand the discussion of “other” welfare effects of PM (not related to 

visibility), pointing to data and approaches that while outside the current scope of 

work may be useful in the future. 

The minimal discussion of non-visibility impacts in the text (page 1-4 and pages 1-10 to 1-11), is 

augmented by a slightly more complete presentation in Appendix A. I suggest that these be 

combined, and placed within the text. This section should be expanded, to more closely parallel 

the visibility sections, to call attention to data that are feasible to obtain but currently lacking, 

that might usefully inform the secondary standard setting process. These data could address a) 

ecosystem impacts of locally relevant particles (analogous to the local approach taken with 

respect to humidity/PM relationships to visibility), b) effects on climate (considerable data now 

reaches the stage of likely causality, and more is known about climate forcing and PM than is 

indicated (pages A7-A9), c) fouling of leaf surfaces and possible interference with gas exchange 

4
and radiation interception, particularly in urban locations, d) interception and scattering of 

incident radiation at the vegetated landscape scale. 

The text (1-12, line 16), refers to an “initial qualitative approach” to non-visibility impacts, to be 

found in Appendix A. However, there is no approach, nor goal, nor suggestion of needed data, to 

be found in Appendix A. In combining the Appendix with the main text, such approaches and 

goals should be added. 

In the summary of ecosystem affects (page 1-4, line 3-5), the review of the previous CD for PM 

is missing the previous emphasis on transport. While the previous review concluded that 

chemical composition dominates ecosystem effects of PM, the transport to and within 

ecosystems depends strongly on particle size and distance between source and receptor. This 

should be included in the discussion. 

3. The focus on visibility, an aesthetic parameter, should be linked more closely to 

other potential impacts of PM that are more representative of damage, otherwise 

reliance on visibility introduces a number of unintended consequences.

The exclusive emphasis on visibility may have unintended consequences in the future. Proposed 

solutions to the temporal mismatch (page 1-19, lines 20-29) between health related and visibility 

related averaging periods, will require development of FRM hourly monitors and lead to the 

discounting of nighttime PM values. The suggested focus on daylight hours is primarily to bring 

cohesion to the visibility (rather than PM) data in the face of varying RH. This leaves the dark 

period as a potential window of opportunity for dumping of PM-rich emissions, to avoid a more 

restrictive potential secondary standard. The skewing of the averaging period by use of visibility 

as a surrogate for welfare effects should be resisted as much as possible. An approach is to use 

calculated visibility, once fully validated (see #3, below), in daylight and nighttime hours, or 

over 24 hour averaging periods e.g. page 1-20, lines 1-7). 

The acceptance of a secondary standard by regulated communities will depend on its being 

relatively simple and intelligible to these communities and to the public. The proposed regulation 

of calculated visibility, rather than of PM concentration, is problematic in this regard. If visibility 

is to be regulated, then appropriate FRMs and appropriate PRB levels must be established. As 

PRB levels likely vary across the country, it must be established that regionally valid PRBs for 

visibility reflect similar concentrations of PM. The alternative (page 1-20, line 8-22), to set a 

uniform national visibility standard may prove untenable because PRB visibility is expected to 

differ regionally. This situation is further complicated by seasonal trends in RH, which will lead 

to seasonality of NAAQS for PM. It appears unlikely that regulated communities will embrace a 

standard that implies regionally differing levels of PM, to achieve differing levels of visibility. 

Given the likely regional variability in PRB for visibility, the use of locally relevant scenes for 

valuation should be given greater consideration (page 1-19, lines 5-12), despite the complexity of 

this approach. Fewer total scenes might be evaluated, but with greater precision and local 

5
relevance, than is suggested in the plan. This might allow scope for locally relevant “valuation” 

(page 1-19, line 10) which could be a powerful metric. 

4. Define UVA and its relationship to the REA and ISA, at the point of its first 

introduction. 

It is not clear from the text what the relationship between the urban visibility assessment (UVA) 

and Risk and Exposure Assessment (REA) is. This may only be semantic and can be easily 

addressed with a few additional words on page 1-1, near line 21. 

5. Greater attention should be paid to linking reconstructed (calculated) light 

extinction to measurements of visibility. 

The generality of the relationship between reconstructed visibility parameters, (calculated as 

functions of PM2.5 and RH), is documented by reference to a strong correlation of such 

parameters with PM2.5 (page 1-7, lines 5-12). The reasoning is somewhat circular. Better to focus 

on relationships between measured visibility parameters and PM concentrations to demonstrate 

these relationships, and on relationships between measured and reconstructed visibility to 

support the methodology. To this end, an appropriate goal of this review cycle (page 2-7) is the 

proposed development and validation of an urban optimized IMPROVE algorithm. 

The discussion of needed visibility parameters, use of models to obtain them, and suitable 

averaging periods, is spread over much of the existing text. This should be consolidated into the 

text preceding introduction of the urban optimized IMPROVE algorithm, possibly prior to 

discussion of appropriate PRB for visibility (section 2.1.1). This would allow determination of 

what is needed for each purpose, and then how best to achieve it. 

6. Maintain effort to provide quantitative valuation of welfare effects, including in this 

case visibility impairment. 

The welfare effects and associated secondary standards have not been as influential as primary 

health standards for a variety of reasons. Valuation can be an important tool for welfare impacts. 

It is a mistake for the current plan to dismiss this goal as too complex (page 3-6, line 11-12), 

even if only incremental progress can be made in the current review cycle. 

7. Minor error. 

6
It is stated that both average radius and number of cloud droplets declines with increasing PM 

derived CCN (page A-9, lines 9-10). Droplet number is likely to increase with decreasing radius 

under these conditions. 

8. The summary of previous rulemaking should be revised to include the recent court 

decision of 24 February 2009. 

The discussion (page 1-5, lines 1-10) is incomplete due to court action after preparation of this 

draft Plan. This should be addressed for completeness, as the current treatment is now somewhat 

misleading. 

9. Revise text to condense and focus arguments. 

The lengthy discussion on visibility impairment and its relationship with fine PM (page 1-6, line 

16 to page 1-8, line 4) is somewhat diffuse and does not lead to a clear conclusion. The objective 

of the section should be laid out initially, and the key conclusion summarized at the end. This 

might be facilitated by bringing it all into a single subsection under 1.1.1. 

10. Elaborate upon key elements that are mentioned in passing. 

Important other protections of visibility contained in the Regional Haze Rule and elsewhere 

receive passing mention (page 1-11, lines 13-20). These are key to understanding the goals of 

this Plan and should be expanded upon. 

Certain states have enacted visibility rules (page 1-8, line 18). As above, it would be helpful to 

know which states, what rules, and some indication of successes and shortcomings of the 

approaches taken. 

7
Comments from Dr. William Malm 

Chapter 1 – Introduction 

The introductory section presents an overview of the goals and planned approaches for 

conducting urban visibility assessments. 

1. What are the Panel members’ views on the general structure and overall design of the 

planned analyses? 

Very energetic for the assumed time frame to complete the assessment. 

2. Is the plan clear and transparent in its description of the planned approaches? Are the 

various assumptions and judgments that must be made in carrying out the planned 

assessments clear and transparent? 

The plan is transparent in it description; however, the assumptions and associated uncertainties 

are yet to be determined. 

3. Given the goals set forth for the planned analyses, has the plan appropriately drawn from the 

existing scientific and technical information in developing the overall approach? Are there 

relevant features that should be added or modified in the planned approach? 

I suggest that the approach is overly complex and may go beyond the information that is 

currently available, or at best easily available. 

4. In addition to the sub-daily PM2.5 alternative standard considered in the last PM NAAQS 

review and summarized in this chapter, an alternative standard structure is being considered in 

this review. This alternative structure would use daylight hourly PM light extinction, which can 

be measured either by a combination of instruments (nephelometer – PM light scattering and 

aethalometer – PM light absorption) or calculated from PM speciation and concurrent relative 

humidity data using a linear algorithm. 

a. What are the Panel members’ views regarding this alternative structure and its 

utility in the context of this PM NAAQS review? 

I think this is a great idea; set a standard based on extinction derived from direct 

measurements of scattering and absorption. If the standard is violated, there are a 

number of ways to estimate which of the major species is primarily responsible. For 

8
instance, it might be a nearby (or distant) wildfire whose effect might be assessed in a 

variety of ways including measurements, models, and just plain old human observations. 

b. What are the Panel members’ views regarding advantages and disadvantages of this 

alternate structure compared to the sub-daily PM mass concentration approach? 

A standard based on direct measurement of extinction is much superior. 

5. The public’s preferences for urban visual air quality (VAQ) levels were assessed in the last 

review in part by considering results from past western U.S. urban preference studies that used 

scenes with distant mountain backdrops. 

a. What are the Panel members’ views on the usefulness of the planned approach to 

conduct a focus group study in different regions of the country with different scene types 

to enhance our understanding of the applicability of this earlier body of work to U.S. 

urban areas in general? 

This is an excellent idea, especially if one can pick representative scenes with realistic 

distributions of haze. 

b. What are the Panel members’ views regarding the scope and approach envisioned for 

this effort, given the time available in this review? 

A lot to get done within the time frame specified. 

Chapter 2 – Assessment of Urban Visibility Conditions 

1. Visibility impairment is caused by both PM2.5 and PM10-2.5, though the latter is less 

effective on a per unit concentration basis and there is less available PM10-2.5 data available in 

urban areas with which to conduct an assessment. PM in the atmosphere includes liquid water 

which contributes to light extinction but is removed when filter samples are desiccated prior to 

mass and composition analysis. 

c. What are the Panel members’ views regarding to what degree and using what 

approaches EPA should assess the role of PM10-2.5 in urban light extinction? 

Could set the standard on only visibility impairment associated with PM2.5 and fit 

nephelometers with 2.5 inlets. Could use adjacent IMPROVE monitors to estimate 

coarse PM. Could design nephelometers to cycle between PM2.5 and PM10 and directly 

measure coarse PM scattering; there would be little extra cost to do this. 

Could assume a constant seasonal background for coarse PM, although the background 

could vary significantly between urban areas. I would favor a direct measurement with 

nephelometers. 

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d. What are the Panel members’ views regarding the advantages and disadvantages of 

using direct measurements of ambient PM light extinction (e.g., nephelometerplus 

aethalometer, or transmissometer) compared to using a linear algorithm that includes 

particle composition and concurrent relative humidity to calculate (reconstitute) PM light 

extinction? 

I think a direct measurement is the obvious and best way to go; there is less uncertainty 

as to whether there is a standard violation and would be cost effective in that only two 

easily calibrated and operated instruments are used. If there is a standard violation, all 

data analysis tools can be brought to bear in assessing why the standard was violated as 

opposed to relying on a subset of those tools to measure the variable of concern. 

e. What are the Panel members’ views regarding the importance of refining the 

IMPROVE algorithm so that it is optimized for urban areas in order to estimate PM light 

extinction for use in this assessment? 

Assume some much simpler algorithm to estimate extinction from PM. There will always 

be a large degree of uncertainty or variability in directly linking bext to individual 

species, and the data requirements to do this as “right” as possible are significant (realtime particle number size distribution along with continuously speciated data would be 

desirable). 

2. This plan anticipates using the same approach to estimate policy-relevant background PM 

levels as in the health risk assessment. What are the Panel members’ views on this planned 

approach and its use in the context of the urban visibility conditions assessment? 

Given all the uncertainties in emission inventories (like NH3, which is key to understanding 

transport of secondary aerosols), deposition rates (given the dependence on surface 

characteristics), gross underestimations for SOA production and so forth, why does one think a 

model can develop reliable estimates of PRB? At least CTM estimates and background data 

should be combined to come up with a best estimate with uncertainty bounds (+/- factors of ? 

probably 2 or more). These issues are even more important if it is anticipated that CTM results 

can be meaningfully used for hourly concentrations of relevant species. 

3. The planned approach includes consideration of high time resolution PM mass and component 

concentrations as well as PM light extinction data, together with relative humidity values, to 

assess recent air quality as well as air quality simulated to just meet current and alternative 

standards under consideration. However, such high time resolution data are not broadly 

available. 

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a. What are the Panel members’ views on this planned approach or on other approaches 

or data sources that might be explored? 

All data and past modeling efforts should be brought to bear to make this assessment. 

The approach should be simple and transparent, with defined, broad objectives so that 

large uncertainties are recognized and accepted. The goal should be to develop 

methodologies that are unbiased, in that on the average you get it right even if there are 

large and significant errors on short time scales. 

b. What are the Panel members’ views regarding useful approaches for characterizing the 

uncertainties associated with the urban visibility conditions assessment? 

I didn’t see where there was a detailed plan on assessing uncertainties. 

Chapter 3 – Quantitative Visual Air Quality Impact Assessment 

1. EPA plans to conduct public preference studies to supplement the information from past 

studies concerning VAQ levels that impact public welfare. The planned approach is to conduct 

an investigative focus group in one location to develop the approach that will then be used in 

group interviews in four urban areas. The proposed approach would ask participants to view an 

urban-specific iconic scene and a generic scene in each study location, all selected to be sensitive 

to changes in PM light extinction using WinHaze superimposed haze levels. 

a. What are the Panel members’ views regarding the appropriateness of EPA’s preference 

study approach and its adequacy to accomplish the goals outlined in the plan? 

I think the approach is appropriate and adequate. 

b. What are the Panel members’ views with respect to alternative approaches that could 

be considered in this review to acquire this and/or additional relevant information? 

2. EPA is not planning to include monetary valuation study questions (e.g. willingness to pay, 

conjoint analysis) in the studies conducted for this review. 

a. What are the Panel members’ views regarding the usefulness of adding valuation 

questions to the proposed four urban-areas group interview surveys and the adequacy of 

such limited information to inform estimate of the value of improved visibility in the 

broader nationwide context? 

Because this is a welfare-based standard, I suggest that adding a monetary value to a 

visibility level is not necessary and even inappropriate. 

b. What are the Panel members’ views with respect to possible approaches for estimating 

monetary values associated with improved visibility that should be considered? 

11
Page 1.4: It would be helpful here to have a discussion, a listing of what the standards are, 

annual versus 24 hr, primary versus secondary. 

Page 1.5: The statement, “Visibility which can be defined as the degree to which the atmosphere 

is transparent to visible light...,” is patently wrong. The discussion in the following sentences of 

the document is also incongruent with this statement. “Transparent” (of transmittance) is only a 

small (part significant) part of visibility impairment. 

Page 1.6: The statement, “Visibility is often described in terms of visual range, extinction, or 

haziness,” is correct. However, a sentence should be added to state that none of these metrics 

really captures “visibility”. Probably the best metric would be contrast transmittance weighted in 

proportion to the wavelength sensitivity of the human eye. Even this metric would not take into 

account color shifts and human perceptual responses. One has to take some approach to 

characterizing visibility, but the EPA should show that it recognizes all the associated 

limitations. 

The statement, “Direct relationships exist...,” is not true. There are approximate relationships 

between species and extinction. Apportionment of particle chemistry (species) to extinction is an 

ill-defined problem and in principle cannot be done (White, 1986). Briefly discuss the problem 

of internal mixtures. (This discussion is lacking in the PM assessment write up as well) 

“...reasonable estimates...” What does reasonable mean? Each person has his own 

interpretation. Estimating extinction from PM measurements can easily be in error by a factor of 

2 or more; is that reasonable? State the uncertainties that might be expected. 

Wording of the paragraph, “Due to regional...,” is cumbersome. Consider rewriting. 

Page 1.7, first paragraph: The whole discussion of correlation doesn’t seem to be relevant. One 

can have a high degree of correlation between variables that are different by orders of magnitude. 

So if you have a correlation between PM and extinction, it doesn’t mean you know anything 

about the level of extinction based on the PM value. 

Second paragraph again has this meaningless discussion about correlations. The Schmidt et al. 

(2005) reference keeps coming up; was this peer reviewed? 

Page 1.8: I think the rationale that a 12-4 visibility standard has greater importance than a 24-hr 

standard is spurious. Most people drive to and from work early in the morning and later in the 

afternoon. It is this time period when most people have an opportunity to see and appreciate 

their surrounding landscape, not when they are sitting in their offices in the middle of the day. 

Second paragraph: There is one reference to VAQ and public awareness. I think that a proper 

literature review and summary of all the public awareness/well being should be carried out. The 

3 or 4 urban visibility studies, I would guess, are only a subset of one kind of study that has been 

carried out. 

12
Page 1.9: What are the “nonpollution” effects that could affect visibility? 

Page 1.14: In some place, maybe in a discussion of Figure 1.1, there needs to be a discussion of 

just what is meant by urban visibility: what views, what times, etc., the whys and wherefores of 

why urban visibility is important, and under what circumstances. Some relevant discussion 

appears in Hyslop (2009). 

Page 1.19, second paragraph: “...similarly sensitive to small increments....” I agree that the 

scenes should be similarly sensitive but not necessarily to small increments. They should be 

similar but only to realistic, expected increments and for some representative scene. As written 

it seems that the scene may be picked to be overly sensitive and not representative of a typical 

urban scene. 

Last paragraph: Add some discussion as to what nighttime visibility is of importance. Some 

think that seeing the lights of an urban setting is what is being addressed when discussing 

nighttime visibility, when I think that what may be of concern is the visibility of the night 

skyscape. 

Page 2.1: It is stated that “We will estimate PM light extinction from measured or estimated PM 

mass, composition, and RH, using a refined, urban-optimized linear algorithm” First, the 

equation will not be linear in that the f(RH) term is very nonlinear with RH. Second, because of 

all the limitations of apportioning extinction to mass composition in a defensible way, I suggest a 

very simplified version of the IMPROVE equation. Even the IMPROVE equation, which has 

been optimized for nonurban environments, can for any given 24-hr period, much less any 

shorter time increments, be in error by factors as much as 2 or more. Significant errors are also 

associated with the multiplier that is used to estimate POM from OC, as well as sampling bias 

associated with OC artifacts filter artifacts or with loss of SVOC particles that are dependent on 

sampling face velocities. All these uncertainties suggest that one should not try to create a false 

sense of understanding and accuracy by having an unduly complicated algorithm to estimate 

extinction from mass measurements, speciated or not. 

Should the figure reference be 1.2 instead of 2.2? 

Page 2.2: How will PRB be used in visibility standard setting – why does one need to know 

PRB? 

The first paragraph under 2.1.1 needs either more background or a more clear presentation. “In 

the last staff paper....a possible secondary PM standard were (or was – or more possible 

secondary standards) compared to % tilepoints in the estimated distribution of 24 hr average PM 

PRB.” Now, what am I suppose to take away from that statement? What is a policy relevant 

background? “...standards were compared to...” – to what end, for what purpose? 

13
I am amazed at the statements concerning using CTM to estimate backgrounds because “This 

approach can provide more spatial and temporal resolution for estimating PRB compared to the 

use of measurements given the sparse nature of remote measurement sites and the concern the 

remote sites are affected by non local anthropogenic sources.” I agree that one can derive more 

spatial and temporal resolution using a CTM but, again, to what end? Given all the uncertainties 

in emission inventories (like NH3, which is key to understanding transport of secondary 

aerosols), deposition rates (given the dependence on surface characteristics), gross 

underestimations for SOA production, and so forth, why does one think a model can develop 

reliable estimates of PRB, whatever that is? At least CTM estimates and background data should 

be combined to come up with a best estimate with uncertainty bounds (+/- factors of ? probably 2 

or more). 

Why not just make best estimates of PRB for broad areas of the country and go with that? 

Again, I believe all this modeling is giving a false sense of understanding. Make a best estimate 

and go with it, incorporating all current knowledge, which can include CTM runs. 

Page 2.4: Here the discussion goes to PRB on an hourly basis. The model performance only 

deteriorates as one goes to shorter time frames. It would be interesting to estimate model 

uncertainty with the uncertainty in using a very simplified PM to bext algorithm. I would bet that 

the model uncertainty is far greater than any uncertainty in estimating bext from PM. 

Under recent conditions I would suggest relying on available nephelometer and TEOM (or some 

other estimate of hourly PM) measurements to come up with a range of hourly bext values rather 

than estimate them from 24-hr-average PM filter-based measurements. Knowing the hourly 

distribution of bext is really what is needed for the assessment. It will not make much difference 

which species are responsible for the extinction. Of course, for any control strategy, bext will 

have to be linked back to the relevant species. 

Page 2.6: Where is Table B-1? Is Table B-2 the relevant table? 

Page 2.9, section 2.5: In am not certain there is sufficient available data to characterize 

uncertainties at the level suggested. 

Page 3.4: “...that have long sight paths....” Better stated as sensitive view sheds. “...selected 

to have sensitive scenic elements...” might be changed to also state that not only will they be 

sensitive, but typical and not unusual. 

Page 3.5: I don’t think the strategy for selecting extinction ranges under the section assessment 

scenarios makes sense. Why would the maximum hourly value be restricted to something less 

than the current 24-hr-average NAAQS level? If one is using hourly values, then it should be 

restricted to the max hourly value that would typically make up the 24-hr-average value. 

Depending on the distribution of hourly numbers that make up the 24-hr average, one would 

have significantly greater values than 35 μg/m3

, maybe as high as 100 μg/m3 or more! 

14
REFERENCES 

Hyslop, N. P. 2009. Impaired visibility: the air pollution people see. Atmospheric 

Environment, 43, 182-195. 

White, W. H. 1986. On the theoretical and empirical basis for apportioning extinction by 

aerosols: a critical review. Atmospheric Environment, 20, 1659-1672. 

15
Comments from Mr. Rich Poirot 

1. Visibility impairment is caused by both PM2.5 and PM10-2.5, though the latter is less 

effective on a per unit concentration basis and there is less available PM10-2.5 data 

available in urban areas with which to conduct an assessment. PM in the atmosphere 

includes liquid water which contributes to light extinction but is removed when filter 

samples are desiccated prior to mass and composition analysis. 

 

c. What are the Panel members’ views regarding to what degree and using what 

approaches EPA should assess the role of PM10-2.5 in urban light extinction? 

Coarse mass typically accounts for less than 10% of extinction in the eastern US, but is likely to 

be an important contributor to visibility impairment in arid southwestern urban areas. Phoenix 

should be a fairly extreme example, where IMPROVE data indicate coarse mass contributes 

about 25% of extinction on average, about 15% on the haziest 20% days and exceeds the effects 

of fine mass about 2% of the time. I don’t see many reasons to exclude coarse mass in an 

assessment of urban extinction (and for that matter as a potential component of a secondary 

NAAQS regulatory metric, which might conceivably be composed of something like fine mass 

plus some small fraction of coarse mass) if data are available. If coarse data aren’t available, a 

reasonable approximation might be made using fine crustal elements, or PM10 data from nearby 

sites. If an optical regulatory metric is being considered – such as measured by nephelometer, 

nephelometer plus aethalometer, or transmissometer – coarse particles will contribute to the 

measured effects, and may in some cases offer potential control options. 

Disadvantages of including coarse mass include that it adds complexity and monitoring cost 

without providing a substantially better indicator in most locations. Coarse mass measurements 

are also likely to be less spatially representative than fine mass, and highest coarse mass 

concentrations may often be associated with natural windblown dust events. Light scattering by 

coarse mass is also not efficiently quantified by nephelometry, and complex data adjustments 

may be needed if an optical indicator is being considered for a secondary standard. 

d. What are the Panel members’ views regarding the advantages and 

disadvantages of using direct measurements of ambient PM light extinction (e.g., 

nephelometer plus aethalometer, or transmissometer) compared to using a linear 

algorithm that includes particle composition and concurrent relative humidity to 

calculate (reconstitute) PM light extinction? 

16
For conducting an assessment of visibility effects, preferences, valuation, etc., the optical 

measurement data are just not likely to be available in non-class-1 areas (except cities like 

Denver and Phoenix where they are used to assess compliance with visibility standards). 

Conceivably, and with much work (which would be interesting and productive), airport ASOS 

measurement data, with appropriate screens and/or adjustments for RH effects, might be used as 

the basis of an assessment of current effects, but the required effort would be substantial. 

Whereas, speciated PM2.5 data from CSN (or urban IMPROVE) sites, with collocated or nearby 

PM10 in most cases would allow calculations of (every 3rd day, 24-hour) reconstructed extinction 

– similar to what’s done for the Regional Haze Rule – with existing data. Conceivably, the 24-

hour filter data might be combined with lower daytime-only RH data to produce extinction 

estimates more in line with the times when people are awake and outdoors experiencing visual 

air quality effects. As a rough approximation, the speciated filter data might be disaggregated to 

hourly values based on continuous mass measurements – either assuming the speciation remains 

constant throughout the day (which we know is wrong) or using some knowledge of the typical 

diurnal cycles of sulfate, nitrates EC, etc. Alternatively, you could make pretty reasonable hourly 

extinction estimates just using continuous fine mass and a generic (or regionally adjusted) 

aerosol extinction efficiency. A generic (or regional) aerosol f(RH) function would further 

improve these estimates. 

In considering future regulatory metrics, using optical measurements would have the major 

advantages that the perceptible effect would be the indicator, and could reflect the much shorter 

(than 24 hour) averaging times and daylight conditions over which effects are perceived. The 

combination of nephelometer plus aethalometer would provide valuable continuous information 

on separate light scattering (of the actual ambient aerosol as people see and breathe it – not some 

hypothetical construct with some of the water and other volatile species artificially removed) and 

light adsorption (which, with a dual wavelength aethalometer, would add other visibility and 

health-relevant information on source influences like diesel exhaust and wood smoke). 

Disadvantages of optical indicators include the costs and logistical problems of adding 

substantial new instrumentation, difficult (in some cases impossible) siting criteria for long-path 

transmissometers, inefficient measurement of coarse particle scattering by nephelometers, and 

need for (much) better standard operating and data processing procedures for aethalometers. 

Also with optical indicators, you know if and when an exceedance occurs, but not necessarily 

much about what caused it, for which much supplemental information may be needed. Whereas 

with a continuous mass -related indicator, you have a better sense of pollutant causality, but less 

certainty about the actual perceived effect at any given time. A species reconstructed extinction 

indicator provides a good link to pollutant causes, but has poor and intermittent time resolution 

e. What are the Panel members’ views regarding the importance of refining the 

IMPROVE algorithm so that it is optimized for urban areas in order to estimate 

PM light extinction for use in this assessment? 

17
It seems logical that some revisions could be developed for the IMPROVE algorithm so that it is 

optimized for urban areas. However, I doubt that such revisions will end up being very 

substantial, and I’m not sure sufficient data exist to do this empirically for more than a few 

locations, in which case it will be hard to know whether the change is generic urban or cityspecific. Possibly, some quick revisions could be based solely on the slightly different nature of 

the IMPROVE and CSN speciation data (for example to accommodate differences in the carbon 

measurement methods). We might also expect urban OC to be generally less aged (less 

oxygenated and smaller sizes) than rural counterparts. Possibly urban coarse particles will be 

“darker” and have higher extinction efficiencies than rural counterparts. If there are sufficient 

data, possible revisions to the equation should be considered, but if such data aren’t available, 

this shouldn’t be a major concern. 

2. This plan anticipates using the same approach to estimate policy-relevant background 

PM levels as in the health risk assessment. What are the Panel members’ views on this 

planned approach and its use in the context of the urban visibility conditions 

assessment? 

I don’t have a strong opinion on this. I assume the PRB estimates will be species-specific, which 

may be important, given that the most efficient light scattering and absorbing species (sulfates, 

nitrates, and EC) are predominantly anthropogenic. It might be interesting to compare the 

modeled PRB concentrations at IMPROVE sites with the haze reg estimates of “natural 

background”. Theoretically, the difference between the two is from non-North American 

anthropogenic sources. 

3. The planned approach includes consideration of high time resolution PM mass and 

component concentrations as well as PM light extinction data, together with relative 

humidity values, to assess recent air quality as well as air quality simulated to just meet 

current and alternative standards under consideration. However, such high time 

resolution data are not broadly available. 

a. What are the Panel members’ views on this planned approach or on other 

approaches or data sources that might be explored? 

As indicated previously, I don’t think its unreasonable to disaggregate the species data to hourly 

values using the continuous mass data – either by assuming uniform temporal composition or by 

weighting the species disaggregation by what we understand about photochemical production of 

sulfates, nitrates and secondary OC, as well as what we know about diurnal and day of week 

patterns in traffic emissions of EC, primary OC, road dust, etc. An alternative reasonable 

approach would be to just use the continuous fine mass data combined with generic dry 

scattering efficiencies and generic aerosol f(RH) growth functions. Possibly these functions 

could be varied on a regional or seasonal basis. 

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b. What are the Panel members’ views regarding useful approaches for 

characterizing the uncertainties associated with the urban visibility 

conditions assessment? 

I think the proposed approaches in the welfare assessment plan for characterizing uncertainty are 

reasonable. Many of the uncertainties associated with relating specific PM mass and/or species 

concentrations to optical effects are associated with the varying water content of the ambient 

aerosol. In terms of conducting assessments or specifying regulatory metrics, those uncertainties 

can be substantially constrained by limiting the time periods for consideration to those when 

water effects are least important. This uncertainty in the “aerosol to optics” relationship can also 

be reduced by use of an optical indicator (in which case we increase the uncertainty in the 

contributions from pollutant mass or species vs. water). This uncertainty can also be constrained 

by limiting the time periods for consideration to those when water effects are least important, at 

which times the effects of different species are also minimally important. Further, the visual air 

quality at any given PM2.5 concentration (of typical composition) can always be described as: as 

least as bad as X , but getting worse, in a very predictable way, as humidity increases. 

In addition to uncertainties in the relationship between aerosol and optical effects, different 

scenes can vary widely in their inherent sensitivities to changes in light extinction, with effects 

being cumulative and greatest over longer sight paths. There are also substantial variations in 

individual perception and preferences, and subsequently in the levels or ranges of visual air 

quality considered to be adverse. Much of this variability can and should be quantified using 

standard survey techniques, but it may be difficult to develop visual survey instruments which 

are representative of the kinds of views characteristic of individual urban areas which are 

“equivalent” to or exchangeable with views in other areas. A generic urban view without 

recognizable local landmarks may be inherently less scenic than the actual views characteristic 

of most individual urban areas. 

One area of uncertainty relating to the regulatory metrics of secondary and primary standards 

that I believe is typically given insufficient attention is the form of the standard. Time ran short 

during the last PM NAAQS review cycle, and there was not time (or a sufficient technical basis) 

to consider and recommend a clear form of the proposed secondary standard. Staff & CASAC 

recommendations were for a sub daily 4 to 8-hour PM2.5 indicator in the range of 20 to 30 ug/m3

. 

But the recommended range was so broad - 90th or 92nd to 98th percentile – that it spanned a huge 

range, from being slightly more protective than the new 24-hour primary standard to being much 

less protective than the new primary standard. I think the lower bound (90th percentile) of that 

recommended form was much too lenient and based on flawed rationale. In any event, I suggest 

the agency put some effort into better understanding the frequencies at which the public is 

willing to accept various degradations of visual air quality. Some consideration might also be 

given to including a temporally varying form as a component of a secondary standard. For 

example, in an analog to the progress-oriented regional haze regulations, maybe there could be a 

19
requirement that the frequency of exceedances of a subdaily optical or PM threshold decrease 

over time. Possibly this concept of “improving visibility” – or a shift in the frequency 

distribution - could be explored by survey instrument, as I don’t think its very likely that people 

have an absolute sense that it’s the 36th or 18th or 7th haziest day a year that breaks the camel’s 

back... 

20
Comments from Dr. Ted Russell 

Though generally pleased with the PM NAAQS Scope and Methods Plan for Urban Visibility 

Impact Assessment (hereafter, SM-Welfare) for how they plan to consider welfare impacts, they 

should go beyond urban visibility to include climate. Indeed, climate concerns might trump 

other concerns, and one could conceive of controls that might actually exacerbate climate 

forcing. As noted in the SM, there are significant uncertainties as to how PM will impact 

climate, but climate impacts have the potential to significantly impact our assessment of potential 

risks, particularly in terms of how specific components impact climate versus other components. 

As such, I am currently concerned that the SM-Welfare may miss a major piece of the necessary 

analysis. On the other hand, if one is just dealing with urban visibility, the current SM does 

provide a good roadmap to developing the information for providing advice on the potential 

revision to the secondary PM NAAQS. On the other hand, without consideration of the potential 

climate impacts, I would worry that the advice may not be fully informed. Dealing with the 

issue more fully here can help lay the foundation for the next review as well. 

Like the SM-Health, I do note a few deficiencies, both in the document as well as the plan, e.g., 

(as noted in the SM-Health) it would have been very nice if the document had a section 

summarizing criticisms by CASAC and others on the prior risk and exposure assessments, and 

how they have responded. This could be done by grouping the types of comments made, and 

how they plan to address them, and where in the current document the planning takes on those 

criticisms, very much like a typical response to review document. This should become standard 

in the process. 

Chapter 1 

While I think that Chapter 1 does a fine job of laying out the issue, and noting that there is a 

relationship between PM levels and light extinction, it is written too much as the same chapter 

for the SM-Health in that it is oriented towards saying that strong relationships have been 

observed between PM and visibility. Unlike health, there is much less reason to address this 

issue by relying on statistics/finding associations. The underlying physics is well known. This 

should be the major focus. For one, it will focus the uncertainty assessment, and it will shift the 

discussion more to how certain are the effects. After laying out that the physics are well known, 

and that this can be addressed by first principle analyses if desired, one can then say that the 

correlations follow the physics. Indeed, after going through the rest of the SM-Welfare, I get the 

feeling that the problem is being made too big, and that the analyses are going to be overly 

complex, in part because we know so much more about the physics. I would reconsider what 

needs to be done here to provide the type of analysis that would lead to a different standard than 

for the primary NAAQS, and recognize that there is going to be much more known about the 

21
physics here, so a certain amount of less detailed analysis can be tolerated because the 

uncertainties will still be quite reasonable. 

On page 1-19, line 27, there is the comment that visibility effects are less well understood at 

night. This is true, but it misses something larger. We still understand the optics, and the 

visibility effects are probably understood quite well enough. What is less well understood is the 

importance/value. Most folks, I suspect, would say that visibility at night is less important, and 

that is what drives how to proceed, not our lack of understanding. 

On page 1-20, it would be best, if it is not too much trouble, to incorporate both composition and 

RH, and this should be relatively straight forward using CMAQ results. On the other hand, if 

this is a very burdensome analysis, (though I would think it actually might be easier), then the 

former approach is fine. 

I am positively disposed to having a PM light-extinction-based standard and using a 

nepholometer/aetholometer pair to provide PM light extinction. This would minimize some 

issues in computing light extinction. However, it also adds an issue about how to simulate 

attainment as some additional uncertainties are added, though I think those are quite reasonable. 

Chapter 2:

The plan to assess urban visibility conditions is reasonable in most cases, though it does get a bit 

unclear, and it reads as though what level of uncertainty is allowable has not been decided, 

leading to some areas where the levels of analysis do not match. 

In regards to the PRB, they suggest using CMAQ results, but if that is too time consuming, they 

might use the prior review’s analyses. Using CMAQ results should not be overly complex, and 

is preferable. They plan to use those results in many other ways, so they should make it so doing 

the PRB-light extinction calculation just part of what is routinely found. There is one problem 

with relying on CMAQ results, and this is found (I think) in the ISA. CMAQ will have 

considerable uncertainty in simulating PMcoarse from soil, and this can complicate finding the 

PRB. Table 3-26 in the ISA does not provide an evaluation of CMAQ PMcoarse results. Note, 

like the SM-Health, I would still be cautious about laying the poor performance of CMAQ in the 

west to grid resolution problems. 

On page 2-4, they start dealing with how to address PM10-2.5. I am not convinced that much 

ado about nothing is being made. How important is the anthropogenic component of PM10-2.5 

to visibility? I did not see this in the ISA (I saw course contribution, but not anthropogenic 

coarse contribution). Next, I think the approach being proffered in lines 28-29 is going to add 

unnecessarily to the uncertainty as it integrates two relatively large uncertainties. The first is the 

uncertainty in the source apportionment. The second is in the coarse-to-fine ratio. Both can be 

large. I would like to see how much anthropogenic PMcoarse adds to VAQ issues in urban 

22
areas. If anthro-PMcoarse is high on days when dust is high, one is much less concerned given 

the optical relationship. 

Section 2.1.3 assumes that the standard will be based on PM-mass, possibly with compositional 

information as well. A deficiency is that it glosses over how compositional information will be 

used as few places will have sub-daily compositional information. Unlike the SM-health 

analysis, compositional information plays a much larger role here. They should also add a 

section on the approach if they use aetholometer/nepholometer pairs to measure light extinction 

directly. In that case, meeting an alternative secondary PM standard will be much less uncertain: 

a whole set of calculations need not be done. 

In response to the specific charge questions associated with Chapter 2: 

1a (or c): Role of PMcoarse: As noted above, you should consider the importance of this 

component to visibility, noting that we are interested in the anthropogenic component, and that 

the anthropogenic component may be large when the natural component is large, so that has 

much less importance. 

1b. As noted above, I am positively disposed to this approach. It does shift some uncertainties, 

but I think it reduces them and simplifies the process. In general, given our knowledge of the 

physics and chemistry of the system, we have a pretty good way of relating emission changes in 

the more important species to visibility changes. 

1c. Revising the IMPROVE algorithm may not be necessary. A first assessment can be to see 

what might be gained, and see if much effort is required. Is a reanalysis going to reduce an 

uncertainty or bias to the degree it will influence the process. I suspect it is relatively 

straightforward, so it may be deemed appropriate even if it only slightly modifies the results. 

2. For the most part, following the same method to estimate PRB is fine, though more 

attention to compositional information and PMcoarse must be included, including model 

evaluation and how high levels of PRB-light extinction correlate to high levels of 

anthropogenic light extinction. 

3a. I applaud the use of more high resolution compositional information. You have identified 

SEARCH as providing such data. There are other high resolution data (e.g., from the Supersites 

and other special studies) that can provide information on how composition changes with time, 

and CMAQ can as well (though this will add other uncertainties). 

3b. The uncertainties section is rather short at present, though correctly notes that there will be 

uncertainties in the light extinction calculation. However, I suspect those will be small. How 

CMAQ results are used, and uncertainties in those results, will be significant and should be 

assessed. How (and if) source apportionment is done on ambient data is also an issue. They 

23
24

need to address the uncertainty in how well visibility improvement can be calculated and how 

well they can predict how PM composition will change with controls. 

Chapter 3.

The central theme in Chapter 3 is the development of how to quantitatively value the impact of 

the perception of urban visual air quality on individuals, citing the lack of information about 

public preferences. 

Appendix A.

Again, climate may be the single biggest welfare (and it is also linked to health) concern from 

PM. In the REA I think that much more emphasis on climatic impacts, positive and negative, 

and compositionally dependencies, should be given than is indicated here. It is actually quite 

possible to do a quantitative analysis, though such an analysis may be fraught with uncertainties. 

However, if the conclusion is that certain species that increase urban light extinction also 

enhance global warming, one might be very tempted to develop a standard that addresses those 

components. For example, if an aethalometer/nephelometer pair is used, one might be tempted 

to weight the light absorption component, or if it is done by component based on speciated PM 

mass, one might provide additional control on light absorbing species. Might one have a 

standard that sets a limit on total extinction, however, doubling (or more) the absorbing 

component?