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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

Office of Air Quality Planning and Standards (OAQPS)

Research Triangle Park, North Carolina 27711

December 14, 2012

MEMORANDUM

SUBJECT: Updated comparison of 24-hour PM2.5 design values and visibility index design

values

FROM: James Kelly, Mark Schmidt, and Neil Frank, Air Quality Assessment Division,

OAQPS/OAR /s/

TO: PM NAAQS Review Docket (EPA-HQ-OAR-2007-0492)

This memorandum updates the original area-by-area evaluations in Kelly et al. (2012a),

“Technical Analysis to Support Surrogacy Policy for Proposed Secondary PM2.5 NAAQS Under 

NSR/PSD Program” that compared 24-hour PM2.5 design values and visibility index design 

values.

Two updates to the original comparison of 24-hour PM2.5 design values and visibility 

index design values are presented. First, design values based on 2008-2010 data are recalculated 

to reflect: (1) use of a revised multiplier of 1.6 for purposes of estimating organic mass (OM) 

concentration from measured organic carbon (OC) concentration in the calculation of visibility 

index design values, as compared to a value of 1.4 used in Kelly et al. (2012a)1

; and (2) 

consistency with the data completeness criteria of 40 CFR Part 50, Appendix N and with 

removal of data approved by EPA as exceptional events for the existing 24-hour PM2.5 standard

and the PM2.5 visibility index standard proposed by EPA (77 FR 38890, June 29, 2012).

 1 With regard to the multiplier, there has been considerable debate within the research community about the 

appropriate multiplier for converting measured OC to OM in urban environments. As discussed in Appendix F of 

the Policy Assessment (U.S. EPA, 2011a) for the review of the national ambient air quality standards (NAAQS) for 

particulate matter (PM), the EPA used the SANDWICH mass closure approach in the Urban Focused Visibility 

Assessment (U.S. EPA, 2010b) for purposes of calculating maximum daylight hourly PM2.5 light extinction and 

evaluated which multiplier would produce 24-hour results most similar to the SANDWICH approach using 24-hour 

PM2.5 organic carbon derived from the new Chemical Speciation Network (CSN) carbon monitoring protocol 

established in 2007. Analyses presented in Appendix F of the Policy Assessment indicate that a multiplier of 1.6 is 

most appropriate for purposes of comparing the hourly PM2.5 light extinction with calculated 24-hour extinction (see 

Appendix F, section F.6 for a full explanation). The EPA also considers this higher multiplier to be a better approach 

for urban CSN monitoring sites where the new measurements of organic carbon tend to be lower than those 

produced by the older NIOSH-type monitoring protocol (Malm, 2011). A multiplier of 1.6 is now used to calculate 

OM from PM2.5 OC derived from the new measurement protocol at CSN sites while 1.4 is used for PM2.5 OC from

the older NIOSH-type monitoring.
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Second, design values for the existing 24-hour PM2.5 NAAQS and the proposed PM2.5

visibility index standard are calculated using more recently available 2009-2011 data from CSN 

sites that have speciated PM2.5 measurements collocated with Federal Reference Method (FRM) 

measurements of total PM2.5. As in Kelly et al. (2012a), we then evaluate the relationship 

between these updated design values for the existing 24-hour PM2.5 standard and the proposed 

visibility index standard along with violation status under each standard. 

To provide context for the discussion below, a reproduction of Figure 6 from Kelly et al. 

(2012a) is provided here in Figure 1. This figure depicts data from 102 sites that meet the 

current and/or proposed data completeness criteria of 40 CFR Part 50, Appendix N for the 24-

hour PM2.5 standard and the proposed visibility index standard at a level of 30 dv. As defined in 

Kelly et al. (2012a), the data markers are color-coded according to the U.S. regions and are 

shape-coded according to location in the eastern or western U.S. The four quadrants demarcated 

by solid grey lines identify zones of exceedance for the 24-hour PM2.5 standard and the proposed 

visibility index standard at a level of 30 dv. Note that the quadrants in Figure 1 are based on 

design values of 35.5 μg/m3 and 30.5 dv (rather than 35 μg/m3 and 30 dv) to reflect rounding 

conventions. 

Updated design values for the 2008-2010 period are presented in Figure 2 based on the 

methodology updates described above (i.e., use of an updated OC-to-OM multiplier in 

calculating visibility index design values, ensuring consistency with the data completeness 

criteria of 40 CFR Part 50, Appendix N, and exclusion of data approved by EPA as exceptional 

events for the existing 24-hour PM2.5 standard and the proposed visibility index standard). Based 

on the updated data selection criteria, data from 5 additional monitoring sites are considered 

valid here compared with Kelly et al. (2012a). For example, the CSN site in Fairbanks, Alaska is 

considered valid in this analysis but was not included previously. 

The updated 2008-2010 design values show a similar relationship between visibility 

index and 24-hour PM2.5 design values to that shown in Kelly et al. (2012a) in terms of which 

quadrant each data point falls into (see Figures 1 and 2, below). The change in OC-to-OM 

multiplier had a minor impact on visibility index design values in part because only a subset of 

the OC measurements were based on CSN’s newer monitoring protocol and required use of the 

updated multiplier. In addition, the updates to ensure consistency with data completeness criteria 

and treatment of exceptional events had a minor impact on both the visibility index and 24-hour 

PM2.5 design values. As a result, the updated 2008-2010 analysis still shows that no design value 

falls into the upper-left quadrant where the visibility index of 30 dv is exceeded and the 24-hour 

PM2.5 design is less than 35 μg/m3

.

Updated design values for the 2009-2011 period are presented in Figure 3 for 100 CSN 

monitoring sites with collocated FRM measurements. Results based on the more recent 2009-

2011 data reveal a similar relationship between visibility index and 24-hour PM2.5 design values 

as those based on 2008-2010 data. As in Figures 1 and 2, no design value falls into the upperleft quadrant where the visibility index of 30 dv is exceeded and the 24-hour PM2.5 design is less 

than 35 μg/m3. Therefore results based on the 2009-2011 data corroborate the findings based on

2008-2010 data as described in Kelly et al. (2012a) and here.
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In summary, we made two updates to the original area-by-area evaluations by Kelly et al. 

(2012a) of the relationship between 24-hour PM2.5 design values and visibility index design 

values. Consistent with the results of Kelly et al. (2012a), the updated analyses indicate that no 

area had a visibility index design value that exceeded 30 dv while meeting the existing 24-hour 

PM2.5 standard level of 35 μg/m3. Furthermore, in light of the empirical relationship between the 

data points depicted in Figures 2 and 3, below, it appears that the updated analyses corroborate 

the conclusion of Kelly et al. (2012a) that a visibility index standard, as proposed, at a level of 30 

dv would likely be attained at sites that violate both the existing 24-hour PM2.5 standard and a 

visibility index standard at a level 30 dv if PM2.5 concentrations were reduced such that the 24-

hour PM2.5 standard level was attained. 

References

Kelly J; Schmidt M; Frank N; Timin B; Solomon D; Rao V (2012a). Technical Analyses to 

Support Surrogacy Policy for Proposed Secondary PM2.5 NAAQS under NSR/PSD 

Program. Memorandum to EPA Docket No. EPA-HQ-OAR-2007-0492 through Richard 

Wayland, Director, Air Quality Assessment Division, U.S. EPA Office of Air Quality 

Planning and Standards. June 14, 2012. Available: 

http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_2007_td.html. 

Malm WC; Schichtel BA; Pitchford ML (2011). Uncertainties in PM2.5 gravimetric and 

speciation measurements and what we can learn from them. J Air Waste Manag Assoc, 

61(11): 1131-49.

U.S. EPA (2010b). Particulate Matter Urban-Focused Visibility Assessment – Final Report. 

Office of Air Quality Planning and Standards, U.S. Environmental Protection Agency, 

Research Triangle Park, NC. EPA-452/R-10-004. July 2010. Available: 

http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_2007_risk.html.

U.S. EPA (2011a). Policy Assessment for the Review of the Particulate Matter National Ambient 

Air Quality Standards. Office of Air Quality Planning and Standards, U.S. Environmental 

Protection Agency, Research Triangle Park, NC. EPA 452/R-11-003. April 2011. 

Available: http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_2007_pa.html. 
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Figure 1. Design values for 24-hour PM2.5 standard and proposed visibility index standard

based on 24-hour average PM2.5 measurements from 2008 to 2010 for sites that meet the 

current and/or proposed data completeness criteria of 40 CFR Part 50, Appendix N, for the 

24-hour PM2.5 standard and the proposed visibility index standard at a level of 30 dv

(presented as Figure 6 in Kelly, et al., 2012a)2

 2 Quadrants in the figure are based on design values of 35.5 μg/m3 and 30.5 dv (rather than 35 μg/m3 and 30 dv, 

respectively) to reflect rounding conventions.

Definition of U.S. regions 

considered in this analysis
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Figure 2. Updated3 design values for 24-hour PM2.5 NAAQS and secondary visibility index 

NAAQS based on 24-hour average PM2.5 measurements from 2008 to 2010 for sites that 

meet the current and/or proposed data completeness criteria of 40 CFR Part 50, Appendix 

N, for the 24-hour PM2.5 NAAQS and the 30-dv visibility index NAAQS4

 3 Updated Figure 6 from Kelly et al., 2012a, based on 2008-2010 data. In this update, OM is calculated as 1.6 * 

PM2.5 OC when OC is measured with the CSN’s revised monitoring protocol for carbon. Also, speciated PM2.5 

component measurements as well as PM2.5 mass were excluded on days when PM2.5 mass concentrations were 

approved by EPA as an exceptional event.

4 Quadrants in the figure are based on design values of 35.5 μg/m3 and 30.5 dv (rather than 35 μg/m3 and 30 dv, 

respectively) to reflect rounding conventions.
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Figure 3. Design values for 24-hour PM2.5 NAAQS and secondary visibility index NAAQS 

based on 24-hour average PM2.5 measurements from 2009 to 2011 for sites that meet the 

current and/or proposed data completeness criteria of 40 CFR Part 50, Appendix N, for the 

24-hour PM2.5 NAAQS and the 30-dv visibility index NAAQS5

 

5 Quadrants in the figure are based on design values of 35.5 μg/m3 and 30.5 dv (rather than 35 μg/m3 and 30 dv, 

respectively) to reflect rounding conventions.