Attachment A: Final Version – For AAMMS Distribution – February 16, 2010. 

Charge Questions for the 

Clean Air Scientific Advisory Committee’s (CASAC) 

Ambient Air Monitoring and Methods Subcommittee (Subcommittee) 

Advisory on Light Extinction Monitoring 

to Support a Possible Alternative Secondary PM NAAQS 

February 24th and 25th, 2010 

Purpose of the Advisory 

If EPA revises the secondary PM NAAQS to be based on a light extinction indicator, 

EPA will face the following tasks as part of setting the standard and developing 

monitoring requirements. EPA is seeking CASAC advice in order to better plan its 

approaches to these tasks. 

1. Establish a specific Federal Reference Method (FRM) for measuring light 

extinction, or establish specifications and procedures for approval of specific 

methods/instruments to be Federal Reference Methods. 

2. Establish specifications and procedures for approval of Federal Equivalent 

Methods (FEM). 

3. Provide network design and probe and siting criteria to State, local, and tribal1 

monitoring agencies. 

Charge Questions 

Questions regarding a PM Light Extinction Measurement Goal and Method 

The accompanying white paper proposes an overall PM light extinction 

measurement goal. This goal would provide for measuring daylight hourly PM light 

extinction at a wavelength of 550nm with an aerosol size fractionation of PM10 under 

ambient relative humidity conditions with overall accuracy and precision < 10% in a 

range of condition from 10 Mm-1 to 1000 Mm-1 for relative humidity conditions < 

90%. EPA staff believe that such a goal would be reasonable starting point for 

establishing performance specifications to support light extinction measurements for 

a PM visibility standard. 

1

 Note: Minimum monitoring requirements do not specifically apply to Tribal monitoring programs; 

however, when Tribal monitoring programs intend to use data for comparison to the NAAQS, approved 

methods and other criteria do apply. 

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Final Version – For AAMMS Distribution – February 16, 2010. 

1. Does the Subcommittee agree with the goal identified? Please comment on 

each of the specifications for the goal, the adequacy of each specification, and 

whether each specification is attainable. If applicable, please explain other 

useful options for the specifications and a rationale for why a different 

specification should be considered. 

a. Wavelength of 550 nm 

b. Aerosol size fractionation at PM10 

c. Operation at ambient relative humidity 

d. Overall accuracy and precision < 10% 

e. Range of conditions from 10 Mm-1 to 1000 Mm-1 

f. Valid measurements (with all other appropriate checks) when sampled at 

< 90% relative humidity 

2. Based on the method selected there may be additional specifications that should 

be considered for a PM light extinction measurement goal. Please comment on 

inclusion of the following additional performance specifications: 

a. Measurement averaging times 

b. Instrument specific parameters such as angular integration for 

nephelometers? 

c. Calibration with a gas that has known Rayleigh scattering properties. 

If applicable, please explain the parameter(s), whether the parameter applies to 

one or more types of instruments, the purpose of the parameter(s) and an 

appropriate goal to support a PM light extinction measurement. 

3. As summarized in the white paper, EPA staff believe that currently available 

nephelometer light scattering and filter transmission light absorption 

measurement instruments are suitable to meet the light extinction goals. 

a. To what extent does the Subcommittee support the staff’s position that 

currently available nephelometer light scattering and filter transmission 

light absorption measurement instruments are suitable to meet the light 

extinction goals? 

b. What are the Subcommittees thoughts on alternative instrumental 

approaches that should be considered to meet the light extinction goals? 

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Final Version – For AAMMS Distribution – February 16, 2010. 

4. Considering the potential need to deploy nephelometer light scattering and filter 

transmission light absorption instruments in routine monitoring applications, EPA 

solicits the Subcommittee’s input on: 

a. Suggestions for improvement to the commercial versions of these 

technologies for optimization in future routine monitoring applications for 

light extinction. Note: please offer any suggestion for improvement either 

generically for all types of instruments or for specific makes and models. 

A good starting point for existing makes and models might include both 

light scattering nephelometers correlated to PM mass already used in 

routine monitoring programs as well as filter-based absorption methods 

used in support of characterizing black carbon PM. 

b. If applicable, what are the Subcommittees suggestions for improvement of 

alternative instrumental approaches for use in future routine monitoring 

applications? 

Questions Regarding the Establishment of Specifications and Procedures for 

Approval of Federal Reference Methods (FRM’s) and Federal Equivalent Methods 

(FEM’s). 

For all established NAAQS, EPA has an FRM and FEM approval process that includes 

criteria and procedures so that designated methods produce data that are appropriate 

for comparison to the applicable air standard. This process has a number of 

advantages such as ensuring methods meet performance criteria and expediting 

procurements (e.g., during procurement, a monitoring agency can state that a method 

must be FRM or FEM rather than have to state each of the performance criteria and rely 

on the vendor to affirm that they meet the criteria), but also some disadvantages such 

as potentially limiting the entry of new instruments to the market due to the often high 

cost of field testing. 

EPA is interested in exploring how approval of methods as FRM’s and FEM’s can be 

optimized. During a review of its Clean Air Research Program2

 the Board of Scientific 

Counselors recommended “...that ORD revise the procedures for designation of an 

approved instrument method, which will accommodate and provide incentives for the 

development and introduction of new measurement technologies for air quality 

2

 Review of the Office of Research and Development’s Clean Air Research Program at the U.S. 

Environmental Protection Agency, Final Report, August 28, 2009; Revised, September 23, 2009. 

http://www.epa.gov/osp/bosc/pdf/air0910rpt.pdf 

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Final Version – For AAMMS Distribution – February 16, 2010. 

monitoring” (pages 5, 13, and 17). The timing of this recent BOSC review fits nicely 

with EPA’s need to develop criteria and procedures to approve FRM’s and/or FEM’s for 

a potential light extinction measurement; therefore. EPA is seeking advice now as we 

formulate our plans for how light extinction methods may ultimately be approved by 

EPA. 

If a traditional approach to designation of light extinction measurements is taken, EPA 

will need to define how FRM’s are to be approved so that a reference method is 

available for approval of potentially subsequent FEM’s and/or deployment in routine 

monitoring networks. Considering the need to establish FRM’s and performance criteria 

for FEM’s to meet the light extinction measurement goal and also considering the 

recommendation above from the BOSC review, please address the following questions: 

5. Identify the advantages and disadvantages of the following potential options for 

approval of a light extinction method as a FRM. Please provide specific advice 

on how to best address scientific questions on interferences, precision, accuracy, 

and operability; degree of data needed to support decisions; who could perform 

the work; what kind of peer review would be appropriate, and whether the 

approach would potentially lead to more innovation in the measurements system 

or not. Note: if an option could lead to more or less innovation, depending on 

other factors, please explain. 

a. Translate the measurement goal to a performance standard(s) plus 

procedures for demonstrating that the performance standard is met, 

without specifying any particular measurement principle. What aspects of 

performance should the standards cover? 

b. Specify the measurement principle(s), calibration procedure(s), and 

operational performance requirements and demonstration procedures? 

What aspects of performance should the standards cover? 

c. Specify a particular instrument model or models as the Federal Reference 

Method, and rely on the equivalent method process to allow for approval 

of other models. What side-by-side performance testing requirements 

would be appropriate under this approach? 

d. Provide the specification for the measurement principle(s), calibration 

procedure(s), and operational performance requirements and 

demonstration procedures as in b. above; but also specify one or more 

specific makes and models that would serve as already approved 

reference methods. Note this would be similar in practice to the 

Australian/New Zealand StandardTM, Methods for sampling and analysis of 

ambient air, Method 12.1: Determination of light scattering – Integrating 

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Final Version – For AAMMS Distribution – February 16, 2010. 

nephelometer method. In that method, a generic approach for the method 

is provided with an appendix that describes the calibration and response 

of specific integrating nephelometers. 

6. Which aspects of a light extinction measurement could be adequately assessed 

in a laboratory and which require field studies (perhaps across multiple air 

sheds). For example, are laboratory challenges for a calibration gas and other 

similar test sufficient to test an instrument, or are experimental studies needed to 

ascertain the sensitivity of (or effects of humidity on) the instruments and are field 

challenges required to evaluate different real world aspects of the performance 

standard (e.g., aerosols varying geographically and interferences)? If a 

combination of both, please explain which aspects of an instrument are best 

suited for laboratory challenges and which in the field. 

7. Would some aspects of performance be better addressed through a design 

standard, e.g., for the flow rate and the geometry of the PM10 inlet, rather than a 

performance specification and demonstration requirement? 

8. What data and analysis does the Subcommittee believe EPA staff should have 

studies or performed in establishing some kind of FRM (5.a-d) for use in 

regulatory decisions and to help inform the public? 

9. As detailed in the white paper, there are a number of instrumental approaches 

that could be used for making these measurements, including single instruments 

that measure total light extinction or instrument combinations that measure light 

scattering and light absorption separately. Some of the methods have inherent 

limitations that require data adjustments for known biases. While we have 

already solicited advice on a method to meet the light extinction measurement 

goal, we would like to explore this topic further as it relates to options for FRM’s 

and FEM’s and their eventual deployment in routine monitoring networks. 

a. Of the available or soon to be available approaches, are any sufficiently 

limited so that EPA should not further consider them as FRM candidates, 

need not ensure that the FEM provisions provide a path to their approval 

as FEMs, and should not consider them when offering advice to or 

procuring equipment for state, local, and tribal agencies? 

b. Are any of the methods clearly superior in operation and also meet the 

measurement goal, such that they should be adopted as the FRM and 

thus serve as the “gold standard” for approval of FEMs (under one of the 

three FRM approaches listed in question 5(c or d)), and/or for possible 

widespread deployment? 

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Final Version – For AAMMS Distribution – February 16, 2010. 

c. What does EPA staff need to know about the biases of various 

instruments and should the FRM and FEM require methods to adjust for 

these biases to ensure data of known quality? 

d. What weight should EPA give to other factors in establishing a reference 

method for routine PM light extinction monitoring? Please comment on 

each of the following: 

i. resources needed to acquire and fully support routine operation; 

ii. current availability; 

iii.	 record of successful field experience; and 

iv. ability to generate supplemental information (e.g. multiwavelength 

scattering/absorption, albedo, forward/backscattering, scattering 

polarization, etc.)? 

Questions Regarding Network Design and Probe and Siting Criteria 

EPA anticipates that a network design strategy would focus on sites that are well 

suited to characterize visibility impairment on an area-wide basis such as 

neighborhood and larger scales that have the highest levels of PM. Probe and siting 

criteria should include specifications that minimize ground effects and other positive 

and negative interferences (e.g., an HVAC vent), and are consistent with the intent 

of the NAAQS. 

10.To what extent does the Subcommittee concur that it would be appropriate to 

focus a network design strategy on sites that can characterize the maximum 

visibility impairment across an urban area? What other considerations should 

EPA include in setting a network design strategy? 

11.EPA and the State monitoring programs have an extensive historical dataset of 

PM2.5 mass and speciation measurements. In the Visibility Assessment 

Document,3 EPA used existing PM speciation and mass data to evaluate visibility 

impairment at a single site in each of 15 cities. However, the selection of sites 

used in this evaluation was severely constrained by the availability of sites with 

the necessary types of collocated measurement, and in several cases the site 

used was not the site with the highest concentrations of PM in the respective city. 

EPA expects that a review of available data within each city combined with 

3

 Available at http://www.epa.gov/ttn/naaqs/standards/pm/data/20100121UFVAforCASAC.pdf 

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information from networks assessments4 would be appropriate to identify likely 

candidate locations for light extinction measurements. Such measurements are 

likely to be in the area of expected maximum PM concentration that are also at 

neighborhood or urban scale and would complement and be complemented by 

PM mass and speciation measurements. 

a. To what extent does the Subcommittee support collocation of PM mass 

and light extinction measurements to complement each of the 

measurements systems while also achieving the purpose of both the 

primary NAAQS and potential secondary NAAQS? Please offer specifics 

as to the advantages and disadvantages of collocating both types of 

measurements systems in an area-wide location of expected maximum 

concentration. 

b. Considering the intra-urban variability of PM in any city, what additional 

factors (e.g., population, expected poor visibility, scenic views, etc.) should 

be considered to prescribe monitoring locations? Under what 

circumstances would multiple sites be appropriate to characterize the 

maximum area-wide visibility impairment across an urban area? 

12.What aspects of probe and siting criteria should be emphasized to ensure that 

the placement of a PM light extinction instrument is not in a local “heat island” 

which could also be a “dry spot” with respect to relative humidity? 

13.In an urban area the average height of the typical sight path is likely well above 

the inlet height of most current air quality monitoring; however, the mixing of 

aerosols impacting light extinction occurs throughout the boundary layer. 

Considering site path, aerosol mixing, the goal of PM light extinction 

measurements, site logistics, and the location of other air monitoring equipment 

inlets5

, what should be the acceptable range for probe height? 

4

 Network Assessments are required of each State or delegated monitoring agency every five years with 

the next assessment due to EPA Regional Offices by July 1, 2010. 

5

 The appropriate range for PM inlets at neighborhood and larger scales is 2 – 15 meters. See: Table E4 to Part 58. 

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