January 20, 2010

MEMORANDUM

SUBJECT: CASAC Review of Particulate Matter Urban-Focused Visibility Assessment –

Second External Review Draft

FROM: Lydia N. Wegman, Director /s/

Health and Environmental Impacts Division (C504-02) 

Office of Air Quality Planning and Standards

United States Environmental Protection Agency

TO: Holly Stallworth

Designated Federal Officer 

Clean Air Scientific Advisory Committee 

EPA Science Advisory Board Staff Office

Attached is the second draft assessment document: Particulate Matter Urban-Focused 

Visibility Assessment – Second External Review Draft (Visibility Assessment, January 2010), 

prepared by the Environmental Protection Agency’s (EPA) Office of Air Quality Planning and 

Standards (OAQPS) staff as part of EPA’s ongoing review of the secondary (welfare-based) 

national ambient air quality standards (NAAQS) for particulate matter (PM). This document will 

be one of the documents to be reviewed by the Clean Air Scientific Advisory Committee 

(CASAC) PM NAAQS Review Panel (the CASAC PM Panel) at a public meeting to be held in 

Chapel Hill, NC on March 10-11, 2010. I am requesting that you forward this draft document to 

the CASAC PM Panel to prepare for the March meeting.

We are also preparing two other draft documents associated with the PM NAAQS review 

for review by the CASAC PM Panel. I expect to send the Quantitative Health Risk Assessment 

for Particulate Matter – Second External Review Draft to you by the end of January for review 

at the March 10-11, 2010 meeting. The other draft document, Policy Assessment for the Review 

of the Particulate Matter National Ambient Air Quality Standards: First Draft, will take 

somewhat longer to prepare than had been planned, such that I expect to send that document to 

you by the end of February. This schedule provides that the CASAC PM Panel will have had the 

opportunity to have an initial look at the first draft Policy Assessment document prior to their 

meeting to review the other two documents, although we believe it is appropriate for the CASAC 

PM Panel to have additional time to review this document. It is my understanding that you are 

now scheduling a public teleconference, targeted for the end of March, for the Panel to review 

the first draft Policy Assessment.
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The first draft of the Visibility Assessment was reviewed by CASAC at an October 5-6, 

2009 meeting in Chapel Hill, NC. Comments from the CASAC PM Panel (Samet, 2009) as well 

as public comments were considered in developing this second draft assessment document being 

released at this time. For example, based on CASAC comments, OAQPS staff has expanded the 

analyses to include consideration of a 98th percentile form of a light extinction-based alternative 

standard. Additional analyses have also been done based on lowering the relative humidity limit 

from 95% to 90% and using this limit as a screen instead of a cap to more clearly exclude 

weather events like fog or precipitation from the analyses. In addition, a logit analysis of the 

results of the urban preference studies has been conducted for four cities to provide statistical 

support for the conclusions based on those results, as requested by CASAC. A technical 

memorandum is now being prepared that details the further application of the logit analysis to 

the four city preference study results, the insights gained from its use, and the strengths and 

limitations associated with using this statistical approach in this context. We expect to send this 

memorandum to the CASAC PM Panel by the end of January as supplemental material to be 

reviewed in conjunction with the review of the second draft Visibility Assessment. We will take 

comments from the CASAC PM Panel and the public on the second draft assessment document 

and related materials into consideration in finalizing the Visibility Assessment.

Document Availability

This second draft Visibility Assessment is being made available to the CASAC PM Panel 

in the form of an attached electronic file. The document is also available from the EPA website 

at http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_2007_pd.html. Printed copies of this

document will be sent to CASAC PM Panel members via Federal Express. We will mail a 

printed copy of the main body of the document only. The appendices can be accessed 

electronically as noted above and printed copies of the appendices will be made available to 

Panel members upon request.

Charge to the CASAC PM Review Panel

We ask the CASAC PM Panel to focus on the charge questions listed below in their 

review of the Particulate Matter Urban-Focused Visibility Assessment – Second External Review 

Draft (Visibility Assessment, January 2010). 

Chapter 1 of this draft document discusses the current scope of the visibility assessment, 

including modifications based on CASAC consultation and information provided through public 

comment. Chapter 2 presents the results of the reanalysis of the urban visibility preference 

studies. Chapters 3 and 4 describe the methods used to evaluate recent PM2.5 air quality and light 

extinction levels, with PM2.5 air quality concentrations simulated to just meet alternative PM 

light extinction standards as well as current and alternative PM2.5 standards, including key results

and observations from the evaluations. Throughout these chapters, associated issues and 

uncertainties are discussed. 

1) In general, what are the Panel’s views on the level of detail provided in the body of the 

report and associated appendices? Does the Panel agree that all of this information is 

useful to retain or is there material that the Panel would recommend deleting? Does the 
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Panel have any suggestions regarding the organization and distribution of information 

throughout the document and in the appendices? 

2) In the Panel’s view, to what extent does the logit analysis presented in Chapter 2 of the 

second draft Visibility Assessment and further expanded and described in the 

supplemental memorandum add value to the urban visibility preference study analysis 

and provide additional support for combining and comparing the results from the four 

cities, as shown in Figure 2-16? What are the Panel’s views on the clarity and adequacy 

of the descriptions associated with such a combined assessment and on the conclusions 

that can be drawn from the assessments? Please provide comments on any additional 

insights that might be drawn from these analyses or on any additional caveats that should 

be considered.

3) What are the Panel’s views on the extent to which the analysis of the frequency of cooccurrences of hourly relative humidity values below and above 90 percent with other 

meteorological events such as rain or fog (Chapter 3, section 3.3.5; Table 3-6) provides

scientific support for consideration of how to address relative humidity in defining the 

form of a standard based on a PM light extinction indicator?

4) In response to CASAC recommendations, descriptions of current conditions and results 

of just meeting NAAQS scenarios that considered all daylight hours were added to those 

based on maximum daily 1-hour indicators. The 98th percentile form was also included 

along with the 90th and 95th percentiles. Tile plots of hourly PM light extinction (Figure 

3-12) and composition bar graphs of the top 10% of days for maximum daily 1-hour and 

aggregated individual daylight hours (Figure 3-13) were shown in part to help illuminate 

the similarities and differences between these various indicators with respect to current 

conditions. Similarly, additional figures and table entries were generated to illustrate the 

characteristics of various PM light extinction NAAQS scenario forms (Tables 4-2, 4-3, 4-

5, and 4-7; Figures 4-1 through 4-3).

What are the Panel’s views regarding EPA staff interpretations of these displays included 

in the text? Are there supplemental or alternative interpretations the Panel would 

suggest? Are there additional approaches the Panel would suggest regarding ways to

summarize, display, or assess the results of these analyses, including similarities and 

differences between the various scenarios?

We look forward to discussing these issues with the CASAC PM Panel at our upcoming 

meeting. Should you have any questions regarding this draft document, please contact Ms. Vicki 

Sandiford (919-541-2629; email sandiford.vicki@epa.gov).

Attachments

cc: Vanessa Vu, SAB, OA
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Karen Martin, OAQPS/HEID

Neal Fann, OAQPS/HEID

Beth Hassett-Sipple, OAQPS/HEID

Bryan Hubbell, OAQPS/HEID

Meredith Lassiter, OAQPS/HEID

Phil Lorang, OAQPS/AQAD

Zach Pekar, OAQPS/HEID

Marc Pitchford, OAQPS/HEID

Pradeep Rajan, OAQPS/HEID

Mary Ross, ORD/NCEA-RTP

Vicki Sandiford, OAQPS/HEID

Lindsay Stanek, ORD/NCEA-RTP

Susan Stone, OAQPS/HEID

 John Vandenberg, ORD/NCEA-RTP