January 20 , 20 1 0

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 Ai r Quality Planning and Standards United States Environmental Protection Agency

TO: Holly Stallworth Designated Federal Officer Clean Air Scientific Advisory Committee
EPA Scie nce Advisory Board Staff Office

Attached is the second draft assessment docum ent : Particulate Matter Urban

- Focused Visibility Assessment

– Second External Review Draft ( Visibility Assessment, January 20 10 ), 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)
nation al ambient air quality standard s (NAAQS) for particulate matter (PM ). Th i s document will
be one of the documents to be reviewed by the Clean Air Scientific Advisory Committee (CASAC) PM NAAQS Re view Panel (the CASAC PM Panel) at a public meeting to be held in Chapel Hill, NC on March 10

- 11 , 20 1 0

. I am requesting that you forward th is draft document to the CASAC PM Panel to prepare for the March meeting.

We are also preparing two other draft doc uments 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, 20 10 me eting. 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 ap propriate 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.2 The fi rst 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 th is 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 98 th percentile form of a light extinction

- based alternative
standard. A dditional a nalyses have also been done based on lower ing 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 preferenc e 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 detail s the fu r ther application of the logit analysis to
the f our 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 supple mental 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 th e second draft assessment document
and related materials into consideration in final izing the V isibility A ssessment

.

Document Availability

Th is second d raft 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 th is
document will be sent to CASAC PM Pa nel members via Federal Express. W e will mail a printed copy of the ma in body of the document only. The appendices can be accessed
electronic ally 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 o n the charge questions listed below in their
review of the Particulate Matter Urban

- Focused Visibility Assessment

– Second External Review Draft (Visibility Assessment, January 20 1 0 )

.

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 PM 2.5 air quality and light
extinction levels

, with PM 2.5 air quality concentrations simulated to just meet alternative PM light extinction standards as well as current and alternative PM 2.5 standards , including key results
and obs ervations 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 ag ree that all of this information is
useful to retain or is there material that the Panel would recommend deleting ? Does the3 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 combin ing and compar ing the results from the four
cities , as shown in Figure 2

- 1 6 ? What are the Panel ’s view s on the clarity and adequacy
of the descri ptions associated with such a combined assessment and on the conclusi ons 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 co

occurrences 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) provide s scientific support for consideration of how to address rela tive humidity in defining the form of a standard based on a PM light extinction indicator?

4) In response to CASAC recommendation s , descriptions of current conditions and results of just meeting NAAQS scenarios that considered all daylight hour s were added to those
based on maxi mum daily 1

- hour indicators. T he 98 th percentile form was also included along with the 90 th and 95 th percentile s

. Tile plots of hourly PM light extinction (Figure

3

- 12) and composition bar graphs of the top 10% of days for maximum d aily 1

- hour a nd aggregated individual daylight hour s (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 similarit ies and
differe nces 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 th is draft document , please contact Ms. Vicki Sandiford (919

- 541

- 2629 ; email sandiford.vicki@epa.gov )

.

Attachment s

cc: Vanessa Vu, SAB, OA4 Karen Martin, OAQPS/HEID Neal Fann, OAQPS/HEID
Beth Hassett

- Sipple, OAQPS/HEID Bryan Hubbell, OAQPS/HEID Meredith Lassiter, OAQPS/HEID Phil Lorang, OAQ PS/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

- R TP Susan Stone, OAQPS/HEID John Vandenberg, ORD/NCEA

- RTP