UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, D C 20460
MAY 11 2010
OFFICE OF THE ADMINISTRATOR
SCIENCE ADVISORY BOARD
SUBJECT: Transmittal of Science Advisory Board Report
FROM:
TO:
Vanessa T. Vu
Director, Science Advisory Board Staff Office (HOOF)
Karen Sheffer
EPA Headquarters Library Repository (3404T)
This is to advise you that the Science Advisory Board, Clean Air Scientific Advisory
Committee (CASAC) Ambient Air Monitoring and Methods Subcommittee (AAMMS), issued
a report numbered EPA-CASAC-10-010, Review of the White Paper on Particulate Matter
(PM) Light Extinction Measurements dated, April 29,2010.
Two copies of the report are attached and a third copy has been sent electronically to
the attention of Ms. Jeannie Turner at turner.jeannie@epa.gov. The report is available in
electronic format on the Science Advisory Board's Web site at http://www.epa.gov/sab.
If you have any questions regarding this report, please contact the Designated Federal
Officer, Ms. Kyndall Berry directly at (202) 343-9868.
Attachments (2)
Internet Address (URL) • httpyAnvwepa.gov
Recycled/Recyclable • Pnnted with Vegetable Oil Based Inks on 100% Postconsumer, Process Chlorine Free Recycled Paper
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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON D.C. 20460
OFFICE OF THE ADMINISTRATOR
SCIENCE ADVISORY BOARD
April 29, 2010
EPA-CASAC-10-010
The Honorable Lisa P. Jackson
Administrator
U.S. Environmental Protection Agency
1200 Pennsylvania Avenue, N.W.
Washington, D.C. 20460
Subject: Review of the White Paper on Paniculate Matter (PM) Light Extinction
Measurements
Dear Administrator Jackson:
The Clear Air Scientific Advisory Committee (CASAC) Ambient Air Monitoring and
Methods Subcommittee (AAMMS, or the Subcommittee) met February 24-25, 2010, in
Washington, D.C. to review EPA's white paper on Paniculate Matter (PM) Light Extinction
Measurements. The Chartered CASAC held a public teleconference on March 26, 2010, to
review and approve the report. This letter provides CASAC's overall comments and evaluation.
The white paper was developed to assess possible technologies that could be used to
measure light extinction. The assessment supports the on-going PM NAAQS review where EPA
is considering a secondary standard that is protective of visibility. The white paper presented
options for direct measurement of light extinction as an indicator for a secondary standard and
included a brief discussion of potential instruments. The white paper focused primarily on the
use of a nephelometer to measure the scattering portion of the total light extinction due to PM
and a filter transmission-based instrument for the absorption portion. The Subcommittee was
asked to comment on the white paper and respond to the Agency's charge.
The Subcommittee thanks EPA for requesting input at this relatively early stage in the
process. The Subcommittee viewed the white paper as a good assessment of the potential
approaches to measuring light extinction due to atmospheric PM. The white paper identified
instruments in contention for use in a network were and tabulated their respective strengths and
weaknesses, though the Subcommittee recommended a few additional approaches for
consideration. If the Agency chooses to restrict the discussion of instruments to those whose
performance in routine monitoring applications has already been demonstrated, then the choice
of a nephelometer and filter transmission-based instrument is logical. The Subcommittee notes
that two approaches to measure light extinction directly may soon become available and might
be preferable. Both of the promising alternatives are "Cavity" technologies, with direct "closed
path" extinction measurement. As these devices are not currently commercially available, costs
are less certain and they would have to be thoroughly tested for use in a network. Data from
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existing, continuous PM2 5 Federal Equivalent Method (FEM) monitors could be used as a
surrogate for fine-mode visibility assessment until data from extinction monitoring methods are
more widely available. Subcommittee members view such mass-based methods as a practical
interim approach, or bridge, to extinction monitoring. These options are further discussed in our
responses to the charge questions.
The CASAC and AAMMS memberships are listed in Enclosure A. The Subcommittee's
consensus responses to the Agency's charge questions are presented in Enclosure B. Individual
review comments from the Panel are compiled in Enclosure C.
Sincerely,
/Signed/
Dr. Armistead (Ted) Russell, Chair
CASAC Ambient Air Monitoring &
Methods Committee
/Signed/
Dr. Jonathan M. Samet, Chair
Clean Air Scientific Advisory Committee
Enclosures
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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 of commercial products does not constitute a recommendation for use.
CASAC reports are posted on the EPA website at http://www.epa.gov/CASAC.
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Enclosure A
Rosters of the Ambient Air Monitoring & Methods Subcommittee and CASAC
U.S. Environmental Protection Agency
Clean Air Scientific Advisory Committee (CASAC)
Ambient Air Monitoring and Methods Subcommittee (AAMMS)
CHAIR
Dr. Armistead (Ted) Russell, Professor, Department of Civil and Environmental Engineering,
Georgia Institute of Technology, Atlanta, GA
MEMBERS
Mr. George A. Allen, Senior Scientist, Northeast States for Coordinated Air Use Management
(NESCAUM), Boston, MA
Dr. Judith Chow, Research Professor, Desert Research Institute, Air Resources Laboratory,
University of Nevada, Reno, NV
Mr. Bart Croes, Chief, Research Division, California Air Resources Board, Sacramento, CA
Dr. Kenneth Demerjian, Professor and Director, Atmospheric Sciences Research Center, State
University of New York, Albany, NY
Dr. Delbert Eatough, Professor of Chemistry, Department of Chemistry and Biochemistry ,
Brigham Young University, Provo, UT
Dr. Eric Edgerton, President, Atmospheric Research & Analysis, Inc., Gary, NC
Mr. Henry (Dirk) Felton, Research Scientist, Division of Air Resources, Bureau of Air Quality
Surveillance, New York State Department of Environmental Conservation, Albany, NY
Dr. Philip Hopke, Bayard D. Clarkson Distinguished Professor, Department of Chemical and
Biomolecular Engineering, Clarkson University, Potsdam, NY
Dr. Rudolf Husar, Professor, Mechanical Engineering, Engineering and Applied Science,
Washington University, St. Louis, MO
Dr. Kazuhiko Ito, Assistant Professor, Department of Environmental Medicine, School of
Medicine, New York University, Tuxedo, NY
Dr. Donna Kenski, Data Analysis Director, Lake Michigan Air Directors Consortium,
Rosemont, 1L
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Dr. Thomas Lumley, Associate Professor, Biostatistics, School of Public Health and
Community Medicine, University of Washington, Seattle, WA
Dr. Peter H. McMurry, Professor and Head, Department of Mechanical Engineering,
University of Minnesota, Minneapolis, MN
Mr. Richard L. Poirot, Environmental Analyst, Air Pollution Control Division, Department of
Environmental Conservation, Vermont Agency of Natural Resources, Waterbury, VT
Dr. Kimberly A. Prather, Professor, Department of Chemistry and Biochemistry, University of
California, San Diego, La Jolla, CA
Dr. Jay Turner, Associate Professor, Environmental & Chemical Engineering, Campus Box
1180 , Washington University , St Louis, MO
Dr. Warren H. White, Research Professor, Crocker Nuclear Laboratory, University of
California - Davis, Davis, CA
Dr. Yousheng Zeng, Air Quality Services Director, Providence Engineering & Environmental
Group LLC, Baton Rouge, LA
Dr. Barbara Zielinska, Research Professor, Division of Atmospheric Sciences, Desert Research
Institute, Reno, NV
SCIENCE ADVISORY BOARD STAFF
Ms. Kyndall Barry, Designated Federal Officer
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U.S. Environmental Protection Agency
Clean Air Scientific Advisory Committee
(CASAC)
CHAIR
Dr. Jonathan M. Samet, Professor and Flora L. Thornton Chair, Department of Preventive
Medicine, University of Southern California, Los Angeles, CA
MEMBERS
Dr. Joseph D. Brain, Cecil K. and Philip Drinker Professor of Environmental Physiology,
Department of Environmental Health, Harvard School of Public Health, Harvard University,
Boston, MA
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
Dr. Helen Sun, Associate Professor, Department of Environmental Health, School of Public
Health, Harvard University, Boston, MA
Dr. Kathleen Weathers, Senior Scientist, Gary Institute of Ecosystem Studies, Millbrook, NY
SCIENCE ADVISORY BOARD STAFF
Dr. Holly Stallworth, Designated Federal Officer, 1200 Pennsylvania Avenue, NW,
Washington, DC, Phone: 202-343-9867, Fax: 202-233-0643, (stallworth.holly@epa.gov)
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Enclosure B
CASAC Responses to the Agency 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
of550nm with an aerosol size fractionation ofPMio 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. Does the Subcommittee agree with the goal identified?
Overall the Subcommittee agrees that using the measurement of light extinction as the indicator
for specifying a secondary PM NAAQS is technically feasible. Although the white paper is a
good starting point, further documentation of studies and evaluations performed with current and
emerging technologies need to be considered. The measurement methods should not be dictated
by the status quo, but should promote innovation and continued improvement in promising
emerging technologies that meet desired operational measurement attributes. Comments on the
specific goals stated above follow.
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 opt ions for the specifications and a rationale for why a different
specification should be considered
a. Wavelength of 550 nm
The specification of 550 nm wavelength in measuring PM light extinction is too restrictive and
not justified given wavelengths associated with visual perception. The selection of wavelengths,
chosen appropriately within the visible range, should be driven by the overall precision,
accuracy, performance and costs of the instruments to make the desired measurement. Multiple
wavelength approaches which enhance the information provided by scattering and absorption
measurements should be encouraged.
b. Aerosol size fractionation at PM/o
The consensus is that PM25 is a better choice as it responsible for the great majority of the
scattering in typical urban conditions, and the measurement of coarse particle extinction presents
significant challenges requiring considerable resources to resolve, while providing minimal
contribution to the total extinction. Finally, the spatial distribution of PMI0-2 5 in urban areas is
likely locally-generated and not uniformly distributed along the sight path, minimizing the
visibility-relevance of PM 10-25 measurements made at any specific location.
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c. Operation at ambient relative humidity (RH)
The consensus is that relative humidity critically effects scattering measurements and therefore
tracking the effect of ambient relative humidity on PM light extinction will be essential.
Unfortunately PM light extinction and RH measurements are prone to significant errors at
humidity >90%. It remains an open issue as to how we might accommodate PM light extinction
measurements at high humidity levels (>90%) and if the application of a humidity cutoff is an
appropriate resolution. It may be advisable to use a smart heater or inlet dryer to bring RH down
to 90% or suitable drier to reduce the potential for fogging of optical surfaces of the instruments
during cold and damp conditions, and to reduce the frequency of required cleaning and
maintenance.
d. Overall accuracy and precision < 10%
Overall accuracy and precision is very much dependent on the PM size fraction, humidity cutoff
and the base PM light extinction to be considered. The 10% accuracy and precision in laboratory
settings appear to be reasonable goals, as well as 10% precision in the field. However, the
accuracy of ambient extinction measurements will be difficult to ascertain, and a 10% goal is
overly ambitions.
e. Range ofconditions from 10 Mm' to] 000 Mm
This seems like a reasonable range, but should be reviewed once the specification of the
secondary PM light extinction standard is set.
/ Valid measurements (with all other appropriate checks) when sampled at < 90%
relative humidity
The goal of 95% valid measurement data (excluding span and zero checks) when the RH is less
than 90% is a good target.
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
The measurement should be capable of producing 1-minute or 5-minute averages to address RH
relationships and data validation. Most technologies already report data at 1-minute intervals, or
better. For reporting purposes, agencies would include averages, plus min, max, s.d. and count
for each hour. The same statistics should be reported for relative humidity. Consideration
needs to given to the RH requirement and aggregation rules for data management. For example:
Do you exclude an hour of bext if RH exceeds 90% for 1 minute? Alternatively, do you make the
determination on a minute by minute basis? RH and temperature should be reported.
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b. Instrument specific parameters such as angular integration for nephelomelers?
If we accept the idea of correctable bias (Question 9c), then the only a priori specifications
would be for precision and accuracy for fine particle extinction measurements. Once accepted as
an FRM or FEM, manufacturers would provide their design specifications, including, for
example: light spectra, truncation error, flow rate, inlet configuration. Instrument specs for RH
could/should be posed up front.
c. Calibration with a gas that has known Rayleigh scattering properties.
In general, the light scattering properties of gases and particles and filters are different.
Therefore, gases and neutral density filters should be used only for "ranging" instruments and for
continuing verification, but not for calibration per se. Fundamental calibration of measurement
techniques should be done with particles of known size (or distribution) and well-defined
composition (sulfate, nitrate or carbon dominated). This is an essential component of laboratory
testing and demonstration. Ideally, an "aerosol in a can" approach would be developed for use as
a transfer standard in the field.
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?
Light Scattering by "wet" nephelometer (b«.at)
There is at least one commercially available nephelometer that is practical for operation in a
routine monitoring network. It does not utilize best available technologies and designs, but with
modest improvements and additional characterization tests it would be suitable. EPA may need
to work with vendors to clearly define performance goals such as minimal change in sample
conditions within the instrument and optimized optics to minimize sensitivity to different particle
sizes and scattering properties. Appropriate caveats with regard to limitations of the data,
especially for coarse mode particles, are needed.
Filter transmission light absorption (b^ surrogate")
There are at least two instruments potentially suitable for this measurement. Acknowledging the
limitations of this filter-based approach to babs> either instrument could be used with the caveat
that the measurement is only appropriate for sub-micron aerosols. Both instruments are expected
to be upgraded to new hardware platforms within the next year or two, improving performance
and reliability. The Aethalometer is widely used in existing monitoring networks, but (at least
for the current model) there may be need for a correction of bias (both at seasonal and sub-daily
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time-scales) resulting from sample aerosol matrix effects. Next-generation aethalometers may
address these artifacts, and should be evaluated when they are they become available. The
MAAP design should not need this correction, but there are insufficient published data to
properly evaluate its performance in routine networks at this time. Neither instrument measures
at or near 550 nm, but this could be implemented. The aethalometer can measure at multiple
wavelengths, a potential advantage for non-visibility related use of the data such as identifying
contributions from biomass burning.
b What are the Subcommittee's thoughts on alternative instrumental approaches
that should be considered to meet the light extinction goals7
Two promising alternatives not yet commercially available are "Cavity" technologies that are
direct "closed path" extinction measurement (b^ plus babs). These methods should be evaluated
when stable commercial versions are available. The Droplet Technologies Photo-Acoustic Soot
Spectrometer is a commercial instrument that measures babs at 780 nm. Another approach to babs
measurement is by difference. If robust bext and bscat measurements are available, babs can be
calculated without the data quality issues inherent in filter-based babs methods.
An indirect alternative visibility metric that could be considered is to use PM2 5 Class III FEM
hourly data from the existing national network, but the Subcommittee views this would be much
less accurate as a measure of light extinction. This approach does not reflect the enhanced
scattering at high humidities but is a practical alternative that could be rapidly implemented for
this revision of the NAAQS with relatively few additional resources. It is possible to apply a
generic RH correction to these data (between -60 and 90%) to better approximate visibility
under humid conditions. If this route were chosen, use of an instrument that may lose substantial
mass of semivolatile species (e.g., due to heating) should be avoided, and averaging times of
greater than 1 hour should be considered.
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 genericallyfor all types of
instruments or for specific makes and models A good starling 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
When FRJvls and FEMs are defined based only on technology available at the time of
designation, practical experience over a wider range of environmental conditions than were
evaluated before designation, development of new technology and methods, and more efficient
manufacturing methods reveal deficiencies in the FRM or FEM. Where design criteria are
necessary, they should consider the extent to which components are commonly available or must
be custom produced. Non-standard components can increase production costs with no
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improvement in quantification. The ideal performance criteria should be stated, with uncertainty
allowances to accommodate current technologies, but with periodic tightening of the
specifications. Co-funded development opportunities like the Small Business Innovative
Research (SBIR) and Science to Achieve Results (STAR) programs are good models, which
should be considered as a means to speed up the development process. The vendor specification
lists for different instruments are not always in agreement with published independent tests.
Changes to some instruments might include:
• Measuring at additional wavelengths with specified bandwidth.
• Using more energy-efficient and cooler solid-state illumination sources to minimize
heating of the nephelometer scattering chamber and to more precisely define the
wavelength and bandwidth.
• Adding temperature and RH sensing in the optical sensing zone.
• Combining scattering and absorption measurements into a single instrument (e.g., Photo-
Acoustic, Soot, and aerosol Sensor - three wavelength [PASS-3]).
• Acquiring less than five minute averages for the measurements, with stable hourly
measurements (e.g., including on-line data processing to minimize post processing for
hourly data).
• Upgrading data acquisition and analysis software to better meet the needs of a visibility
standard, and to allow real-time quality assurance and data reporting.
b. If applicable, what are the Subcommittee's suggestions for improvement of
alternative instrumental approaches for use in future routine monitoring
applications?
EPA should not dictate the measurement principles, designs, or manufacturers. It should set out
the extinction measurement goals as specifically as possible based on performance standards as
opposed to design standards and allow ingenuity to rise to the challenge.
Tests to be considered include:
• Effects of water and light-induced absorption on measurements.
• Equivalency and comparability between cavity-based spectrometers and other particle
light scattering and absorption methods.
• Characterization of nephelometer truncation angle over the relevant range of fine mode
aerosol (~ 0.15 to 1 u.m).
Questions Regarding the Establishment of Specifications and Procedures for Approval of
Federal Reference Methods (FRM's) and Federal Equivalent Methods (FEM's).
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.
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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?
This option is preferred over the others because it will permit the greatest latitude for innovation,
however if not properly implemented, it also has a greater potential to lead to the approval of
methods that do not work well in environments other than where the method was evaluated.
Specifying a performance standard alone allows instrument manufacturers to propose methods
that are unrelated to the conventional methods in use today. This can lead to truly innovative
technologies and may also lead to the approval of methods that have complimentary co-benefits
for the monitoring communities and other stakeholder groups such as health effects researchers.
The disadvantage of this option is that the use of methods that are not uniformly used throughout
a monitoring network can lead to bias between different approved methods. Due to the nature of
how different monitoring methods work, this bias is most likely going to be manifest unevenly
across the country. Methods that are more sensitive to humidity or the concentration of one
atmospheric component over another will tend to have a regional bias. This type of non-
uniformity is not ideal for a FRM or FEM and must be reduced to the extent possible through the
development of very precise performance standards.
The performance standards must include acceptable ranges for instrument response in any
environment where the regulatory standard is expected to apply. These environments include
widely varying atmospheric component concentrations and environmental factors such as
temperature, humidity, direct sunlight and elevation. The starting point for the performance
standards must be based on the response to laboratory generated aerosols that are generated with
component concentrations of specific interest that exceed what is likely to be found in the
environment. These laboratory evaluations should include as many known interferences as can
be accommodated with synthetic aerosol. Manufacturers can also take advantage of models that
can estimate or interpolate instrument responses across other particle size fractions not evaluated
in the laboratory.
The performance standards must include specifications for instrument maintenance, data
availability and calibration. The equipment manufacturers are of course free to choose how this
is done as long as the recommended procedures ensure that once in an operational environment,
the instrument users will be able to demonstrate that the instrument can reliably meet precision
and accuracy goals over a multi-year period. To demonstrate that candidate instruments work
reliably, instrument manufacturers are encouraged to operate their instruments in several areas of
the country that represent different mixes of aerosols over at least a one year period. This is
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necessary in order to determine if the instrument has a significant seasonal bias in relation to
other methods. Costs could be reduced for long term field evaluations by partnering with state
and local monitoring agencies and research organizations. The EPA should be notified of
proposed field work before it is initiated so they can comment on the suitability of the location
and also visit the site while the candidate method is being operated.
The use of performance standards for the selection of FRMs and FEMs will lead to a situation
where new methods could be developed that are vastly superior to methods currently designated.
In expectation of this occurrence, the EPA must have a procedures and appropriate resources to
review the acceptable methods quickly. Periodically, performance standards should be reviewed
and adjusted as appropriate. These decisions could be handled internally by the EPA, however,
it would be advantageous to have input on these decisions from representatives of state and local
monitoring agencies and the data user community.
The disadvantage of the performance approach is that EPA would need to develop three or four
separate sets of performance standards: one each for bscat, babs> and bext, and possibly one for babs
by difference. This is awkward, and would require more resources and provides more
possibilities for important specifications to be omitted.
b Specify the measurement principle(s), calibration procedure(s), and operational
performance requirements and demonstration procedures7 What aspects of
performance should the standards cover7
This option is easier than option (a) because limiting the method to one measurement principle
reduces the number of variables that have to be considered in the method. The simplicity stems
from the assumption that a single measurement principle built into analyzers from any acceptable
instrument vendor will respond similarly in a variety of monitoring locations and atmospheric
conditions. The end result of this approach is a fairly uniform and consistent database, however,
it is still possible that the database will suffer from inaccuracies in certain regions or
environmental conditions due to biases in the specified measurement principle.
This option encourages innovation but only as related to the specific measurement principle.
This kind of "linear" innovation is helpful and will likely result in future instrumental
improvements but only as related to the specific measurement principle. This approach will still
eventually result in an obsolete method.
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?
This approach will stifle innovation because the manufacturer of the specified instrument model
has no incentive to improve upon an accepted method. Manufacturers of potential FEMs will
also be stifled because even if a newly proposed method is superior to the designated instrument,
the FEM equivalency evaluation process forces the new technology to emulate the old method.
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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 Standard™, Methods for
sampling and analysis of ambient air, Method 12.1. Determination of light
scattering — Integrating 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 nephelomelers.
This option is not recommended because it suffers from the disadvantages of both option b and c.
The specification of measurement principle limits the number of candidate methods and the
designation of specific makes and models will force proponents of new instruments to compare
their data to the designated models.
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
The proposed phased approach outlined in the white paper presents a logical sequence in which a
careful assessment of currently available information would help identify and prioritize those
aspects of "aerosol light extinction" measurements that can and should be most effectively
assessed in laboratory vs. field evaluations.
Certain aspects of instrumental response, such as effects of varying temperature, RH, aerosol size
distribution and chemical composition and consistent responses to calibration gases or aerosol
mixtures can and should be evaluated in rigorous laboratory testing. For babs by filter methods,
"spot loading" effects need to be evaluated using a sooty (black) "worst case" aerosol. A serious
application of existing modeling technologies should be applied to define and determine
theoretical compliance with performance specifications. Several laboratory-generated aerosol
mixtures could be presented to each candidate to determine how the instrument will respond
when compared to a primary standard. Nephelometer truncation errors should be characterized
as a function of aerosol size.
Following laboratory testing, a limited number of field or laboratory intercomparison studies
could help assess the performance and operational characteristics of various candidate methods
under a range of challenging conditions. Then finally, a small pilot network deploying the most
promising methods, directly operated by a limited number of state/local agencies, in areas with
varied aerosol composition, size distribution, temperature, and/or humidity conditions would
provide the most realistic operational and performance feedback.
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7 Would some aspects of performance be better addressed through a design standard, e g,
for the flow rate and the geometry of the PMio inlet, rather than a performance
specification and demonstration requirement?
In the past, design specification for FRM has resulted in considerable implementation problems
and impeded the development and use of more suitable measurement methods. Performance
specification is a more flexible approach if the performance for scattering and absorption can be
specified and adequately evaluated. Therefore, the consensus of the Subcommittee is that
performance specification is recommended over design specification.
8. What data and analysis does the Subcommittee believe EPA staff should have studied or
performed in establishing some kind of FRM (5 a-d)for use in regulatory decisions and
to help inform the public?
The data collection and analysis for establishing an FRM should include systematic observations
that determine the detection limit, interferences and precision in different environments. A
critical set of closure measurements should be focused on establishing consistency between
direct extinction measurement and scattering plus absorption. The relative humidity cutoff
should be a special focus of the measurement design and analyses. A subset of the pre-FRM
studies should include the role of the chosen wavelength, both for the scattering and absorption
measurements.
The pre-FRM data analysis should include the examination of the available data from both
routine and special study monitoring (e.g. EPA Supersite program, MANE-VU RAIN,
IMPROVE and SEARCH). Field studies should include other candidate instruments and
continuous measurements and their respective performance for providing a light-extinction
indicator should be evaluated and compared. For informing the public, the use of visibility-
cams, WinHaze images, and airport visibility (e.g., ASOS) should be included as an
augmentation of the chosen instrumental visibility indicator.
As recommended in the white paper, a more detailed assessment of available information should
be conducted. The preparation of the FRM should be supported by the development of a
guidance document on visibility measurements.
9. 9 As detailed in the while 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
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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?
We believe that performance based standards are preferable given that there are multiple
approaches that could provide adequate measurements of extinction or scattering and extinction.
As noted in the white paper, each method has strengths and weaknesses, but it is likely that in
many cases, the identified weaknesses can be addressed by the vendors sufficient to meet
appropriate performance standards. The ability to meet performance standards will be easier for
some techniques if the range of particle sizes is restricted to PM2 5.
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?
Again, there are methods that are currently better developed and implemented than others, but
that should not preclude the vendors from working to meet the appropriate standards to become
an FRM or FEM. There is certainly no instrument that has been demonstrated as the "gold
standard", but there are several very promising in situ methods on the horizon for extinction and
absorption measurements.
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?
At this time, there is some literature on particular instruments, but without a "gold standard"
biases are undefined. Some methods can be tested for "internal consistency" to evaluate
reproducibility of short-term data (spot loading artifacts for filter-based babs for example). The
critical issue is the development of some basic calibration methods (gases, known composition
and concentration aerosol with known particle size distribution) that can be used to test the
instruments with respect to their basic operating principles. There is a particular problem with
respect to coarse particles though the Subcommittee has recommended that the measurement of
the indicator be limited to PM25.
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;
li. current availability,
Hi record of successful field experience; and
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iv. ability to generate supplemental information (e.g. multiwavelength
scattering/absorption, albedo, forward/backscaltering, scattering
polarization, etc.)?
Unique advantages of a PM light extinction indicator include the facts that light extinction is an
actual characteristic of the ambient atmosphere, and that its relationships to human visual
perception, as well as to the physical and chemical characteristics of ambient PM and associated
meteorological variables are reasonably well known. For these reasons, a secondary standard
based on a PM light extinction indicator can confidently be established to protect against adverse
visibility effects, without advance knowledge of the exact method or methods by which PM light
extinction would be measured. Currently available methods could provide reasonable
measurements or estimates in the near future, with performance improvements from promising
new methods forthcoming within a few years.
The Subcommittee commends EPA for looking at ways to enhance the scientific value of the
instruments that would be deployed and the measurements thereby obtained. In general, the
approach should be to obtain sound basic instruments that meet the performance standards.
Current availability of the instrument is not critical. Clearly all instruments should be robust in
the field, easy to calibrate, and easy to monitor and troubleshoot through the internet. Having the
instruments available as needed in the development and implementation of the new PM
secondary NAAQS process is the critical criterion. Instrument need to be field tested to ascertain
their robustness and ease of operation under realistic conditions. All else equal, instruments that
provide additional information are preferred.
Questions Regarding Network Design and Probe and Siting Criteria
EPA anticipates that a network design strategy -wouldfocus 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 ofPM 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 ofPM2 5
mass and speciation measurements In the Visibility Assessment Document,' 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 ofPM in the respective city EPA expects that a review of available data
1 Available at http //www eDa.gov/ttn/naaas/standards/pm/data/20100121 UFVAforCASAC pdf
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within each city combined with information from networks assessments2 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 ofPM 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 infra-urban variability ofPM 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?
Some members made the point that multiple sites in an urban area would allow better
characterization of area-wide visibility. The consensus of the Subcommittee was that the spatial
averaging inherent in vision through the atmosphere typically would allow a single extinction
measurement site to adequately characterize visibility in urban centers. The Subcommittee
strongly favored collocation of extinction measurements with PM mass, PM speciation, and
precursor gas measurements, identifying continuous PM mass and speciation measurements as
being of particular value. National Core (NCore) multi-pollutant monitoring sites were
identified as worth considering even though these would not necessarily capture maximum
concentrations and visibility impairment in an urban area. There was general support for making
public communication an important consideration in network design, for example by selecting a
monitoring site that can be associated with a vista that is recognized by a significant fraction of
the local population.
12 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?
The Subcommittee members thought that the relative humidity issue with local surface
conditions might not be limited to thermal effects. Any deviation from the prevailing surface
characteristics of the site in question (e.g., grass surface, proximity to a large body of water, near
a vent outlet of a large building HVAC system) may create local relative humidity conditions
that produce light extinction data not representative of the city. Clearly, these aspects need to be
taken into consideration when developing siting criteria and choosing a site.
2 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
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13 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?
The Subcommittee members identified several factors that may need to be considered in
selecting probe height including: (1) avoiding the influence of unrepresentative emissions of
particles in the immediate vicinity; (2) heights that represent aerosol mixing representative of the
city (e.g., sulfate vs. carbon); (3) heights relevant to viewing the scenery of the city (e.g., on a
higher floor of a building); and (4) using NCore sites where possible. The Subcommittee
thought that the probe height should be at least four meters above the ground. For bscat
measurements in the IMPROVE network using a nephelometer, the entire instrument along with
ambient temperature and RH sensors are typically mounted on a meteorological tower
approximately four meters above any surface. The four-meter height is to insure that surface
solar heat does not unduly influence the bscat measurement by drying it out, which may be a
factor in this application. This issue should be further examined for the suite of instrument(s)
under consideration for assessing attainment of a secondary visibility standard.
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Enclosure C
Compendium of Individual Review Comments from the AAMMS on EPA's Light
Extinction Measurement white paper
Comments received:
Mr. George Allen 21
Dr. Judith C. Chow 26
Mr. BartCroes 45
Dr. Kenneth L. Demerjian 49
Dr. Delbert Eatough 52
Mr. Dirk Felton 54
Dr. Kazuhiko Ito 56
Dr. Donna Kenski 58
Dr. Peter McMurry 60
Mr. Rich Poirot 63
Dr. Warren H. White 72
Dr. Yousheng Zeng 75
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Mr. George Allen
General background
Ideally an open path method (transmissometer) would be used for b-ext measurements (requiring
subtracting out b-ext from gases), since it is an in-situ direct measurement. But this method is not
practical for use in routine networks. It may have a role as an FRM, but even that limited use
presents many challenges, both operational and budget related. Thus I conditionally support the
use of some combination of closed path b-ext, b-scat, and b-abs measurements for any urban
visibility characterization network. Currently available b-scat and b-abs instruments that are
practical for routine network deployment are not ideal but could be useable with modest
improvements and appropriate caveats with regard to limitations of the data. Single instruments
that measure b-ext are not yet commercially available but may be candidate methods in the
future.
For the present review of the PM NAAQS, I support a PM2.5 FEM sub-daily approach, perhaps
with some generic RH adjustment factor when RH is in the range of-60 to 90%. I do not support
a full network of b-scat and/or b-abs measurements at this time; the length and complexity of
these charge questions reflect the wide range of complex and unresolved issues for a visibility
network using these measurements.
CQ1: "Does the Subcommittee agree with the goal identified?"
(where the goal is to use PM light extinction...to support light extinction measurements for a PM
visibility standard).
I do not support this goal for the present revision process of the secondary PM NAAQS. As
noted above, 1 support a simple PM2.5 FEM measurement, limited to daytime or mid-day hours.
1 do not support a full network of b-scat and/or b-abs measurements at this time. I do support and
encourage a limited pilot program, both laboratory and field based, to better understand and
assess possible technologies for use in a future "true" visibility network.
CQ3a: Available Technology
Scattering measurements (nephelometiV)
There are three commercial nephelometers that could be considered for this work: NGN, TSI,
and Ecotech (the Radiance Research M903 nephelometer is not a practical candidate). Of these,
the NGN and TSI are more research oriented, have better optics (e.g., smaller truncation errors),
and have been more intensively characterized. The Ecotech is a good candidate for routine use in
SLT monitoring networks because of its robust design. The Ecotech "Aurora" model
is the most recent version of the Ecotech model 9003 nephelometer (http://www.aurora-
nephelometer.com'). This instrument is well designed, but needs modest changes to be suitable
for use outdoors at ambient T and RH, such as ambient and in-chamber temperature and RH
measurements, and the ability to be "tower mounted" with appropriate solar and rain shields.
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Characterization of the optical performance (truncation error) over a range of relevant fine-
particle mode sizes is critical for any nephelometer. Truncation error may be improved
somewhat by using a broader-band light source ("white", centered at 550 nm) instead of the near-
monochromatic sources now used. For a detailed discussion of performance issues with the TSI
and the older 9003 Ecotech nephelometers, see: Miiller et al. (2009), "Angular Illumination and
Truncation of Three Different Integrating Nephelometers: Implications for Empirical, Size-
Based Corrections".
Aerosol Science and Technology, 43:581-586, DOI: 10.1080/02786820902798484
If a broad-band light source is shown to improve performance, "white" LEDs are now readily
available. The Ecotech instrument supports measurements at multiple wavelengths, so it would
be possible to get scattering at discrete wavelengths as well as "white" (broad -550 nm). The
hardware package is user friendly, and chamber cleaning is not difficult.
Nephelometer inlet considerations:
Measuring coarse mode particle scattering is problematic with nephelometery, as discussed in:
Massoli et al. (2009), "Uncertainty in Light Scattering Measurements by TSI Nephelometer:
Results from Laboratory Studies and Implications for Ambient Measurements", Aerosol Science
and Technology, 43:1064-1074, DOI: 10.1080/02786820903156542
One consideration is to constrain the inlet to fine-mode particles; this would also keep the
instrument chamber cleaner. In the eastern US, coarse mode aerosols usually make only a minor
contribution to b-scat, and the larger uncertainty of coarse mode aerosol b-scat measurements by
nephelometer decreases the value of measuring this size fraction. One option for areas with a
substantial coarse mode contribution to visibility impairment might be to use on-line PM-coarse
measurements to estimate the b-scat from this particle mode; composition and relative humidity
growth are not significant factors for coarse particles.
Absorption (b-abs) measurements:
The best commercial measurement method for b-abs is the photo-acoustic method (Moosmiiller
and Arnott) from Droplet Technologies; this method could be considered for use as an FRM but
may be too expensive and complex for wide deployment. For routine network use, a surrogate
measurement of light absorption of atmospheric particles can be done with optical transmission
(optical density) measurements. However, care must be taken in interpreting these data for
visibility use, since the optical extinction of the aerosol is modified by the filter and sample
matrix.
There are several commercial methods for b-abs by filter optical transmission. Two are practical
instruments for network use: the Thermo Scientific MAAP and the Magee Scientific
Aethalometer. In the U.S., the Aethalometer has been used widely as a surrogate for BC or soot
mass concentration, in the NATTS and other measurement programs. The Aethalometer uses
multiple wavelengths; the 2-channel (880 and 370 nm) configuration is the most common, but
other wavelengths such as 520 and/or 565 nm could be used. The current version of this method
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has a substantial "spot loading effect" that biases the data low as the filter loads with aerosol; the
error is a strong function of the aerosol matrix and is largest when the aerosol is dominated by
black aerosols. Compensation methods have been developed that can remove the worst of the
error, but only on a time scale of many days to weeks. See:
Virkkula et al. (2007), "A Simple Procedure for Correcting Loading Effects of
Aethalometer Data", J. Air & Waste Manage. Assoc. 57:1214-1222, DO!: 10.3155/1047-
3289.57.10.1214
Kirchstetter and Novakov (2007). "Controlled generation of black carbon particles from
a diffusion flame and applications in evaluating black carbon measurement methods."
Atmospheric Environment 41, 1874-1888, doi: 10.1016/j.atmosenv.2006.10.067
Turner, Hansen, Allen (2007). "Methodologies to Compensate for Optical Saturation and
Scattering in Aethalometer Black Carbon Measurements". Paper No. 37, Symposium on
Air Quality Measurement Methods and Technology, San Francisco, CA, April 30 - May
2, 2007.
Coen et al., 2009. "Minimizing light absorption measurement artifacts of the
Aethalometer: evaluation of five correction algorithms." Atmos. Meas. Tech. Discuss., 2,
1725-1770.
http://www.atmos-meas-tech-discuss.net /2/1725/2009/amtd-2-l 725-2009.html
It remains to be seen if the next version of the Aethalometer (the "next-gen" instrument) will
properly measure (e.g., without significant filter loading and aerosol composition matrix effects)
b-abs at a sub-daily time-scale without this bias; this needs to be re-evaluated when the
instrument is available, perhaps later in 2010.
http://mageesci.com/products/upcoming_products.htm
The Thermo MAAP is a more sophisticated measurement method, incorporating scatter from the
filter media into the measurement. This should (in theory) minimize the variability of b-abs
measurements from filter spot loading aerosol matrix effects. However, there is only limited
published ambient data that demonstrates that the MAAP achieves this goal: Petzold et al.
(2005), " Evaluation of Multiangle Absorption Photometry for Measuring Aerosol Light
Absorption". Aerosol Science and Technology, 39:40-51, DOI: 10.1080/027868290901945
The MAAP is a single wavelength instrument, using a 670 nm source.
http://www.thermo.eom/com/cda/product/detail/l „ 19884,00.html
To be used for b-abs, the wavelength would need to be changed to -550 nm. It is important to
note the strong b-abs spectral dependance of biomass combustion (wood smoke). A b-abs
measurement at 880 or 670 will underestimate the b-abs at 550nm, since wood smoke has
substantially enhanced b-abs at shorter wavelengths. A suitable light source near 550 is needed
for proper b-abs measurements, since wood smoke is a significant component of urban aerosols
in areas with cold winters, making up approximately 20% of cold-month PM2.5.
The current production versions of both the MAAP and the Aethalometer are based on old
hardware designs. Both instruments are expected to be updated in the near future, using current
technologies. This will improve reliability, but it is unclear at this time what changes in
performance may result from these updated methods.
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CQ3b: Alternative Approaches
For the current review of the PM NAAQS, PM2.5 from a FEM continuous monitor is an
adequate indicator for the secondary standard even though it does not reflect the effects of
humidity or aerosol composition. A daytime (mid-day) 4- to 8-hour mean could be used instead
of the 24-hour average used for the primary NAAQS. This approach was suggested during the
last review of the PM-NAAQS. It has several advantages over a wide-deployment of b-scat
and/or b-abs measurements in a new network for the present PM NAAQS review; the FEM
PM2.5 network is or soon will be widely deployed, the technology is reasonably mature, and air
agencies are familiar with the operation of these methods. I do not support an averaging time of
less than 4 hours both in terms of a stable and relevant design value and limitations of the
precision of 1-hour data from FEM PM2.5 instruments.
This FEM PM2.5 network could be supplemented with a pilot network of b-scat arid b-abs
measurements at a few sites. This would provide a comparison with the FEM estimates of visual
range. It would also allow a field evaluation of routine use for these methods in the context of
routine state/local monitoring networks, and allow refinements of these methods to make them
more appropriate for this use, potentially under future revisions of the secondary PM NAAQS. It
is not advisable to proceed with wide deployment of a b-scat and/or b-abs measurement network
at this time, both for technical and resource limitations.
Two promising alternatives not yet commercially available are the "Cavity" technologies that are
direct "closed path" extinction measurement (b-scat plus b-abs). These methods should be
evaluated when a stable commercial version is available. The Droplet Technologies Photo-
Acoustic Soot Spectrometer is a commercial instrument that measures b-abs at 780 nm. Another
approach to b-abs measurement is by difference. If robust b-ext and b-scat measurements are
available, b-abs can be calculated without the data quality issues inherent in filter-based b-abs
methods.
CQ5: Issues with development of FRMs for b-ext measurements
I agree with the panel's sentiment that a performance-based standard should be used. However,
EPA may find itself in the position of needing to define multiple standards given the options of
various combinations of methods and measurements. It should be noted that although a direct
PM2.5 b-ext measurement would satisfy the PM Light Extinction Measurement Goal, it may be
desirable to have some information on the relative contributions of b-scat and b-abs to support
control measures. Given that the NAAQS and thus the FRM metric is b-ext, if that is measured
by a combination of instruments for b-scat and b-abs, would an FR.M or FEM need to be defined
as a matched pair of these methods, or could b-scat and b-abs methods be defined separately
even though neither is a b-ext method?
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Questions Regarding Network Design and Probe and Siting Criteria
Siting For urban visibility monitoring should be neighborhood or larger scale, within the core
urban area. Visibility sites should be collocated with NCore monitoring if at all possible. Probe
height is problematic for proper b-scat or b-ext measurements because of the strong influence of
chamber temperature on RH and thus b-scat measurements. To avoid local surface heating
effects, b-ext or b-scat measurements must be made several meters (10?) above any surface.
While this is not easy to implement, it is a critical siting aspect.
A related topic is the proper assessment of the instrument chamber temperature and/or RH for b-
ext or b-scat instruments. The ambient temperature used to generate a chamber deltatemperature
(above ambient) measurement must be at 10 meters or higher to be reasonably free from local
surface heating effects. This is especially critical given that the metric will be a 4 to 8-hour
daytime measurement, emphasizing the effects of solar radiation on these measurements.
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Dr. Judith C. Chow
PM Light Extinction Measurement Goal and Method
CQ1: Does the Subcommittee agree with the goal identified?
The concepts presented in the white paper are a good starting point. The white paper
recognizes the need to take better advantage of previous studies, to more thoroughly evaluate
currently available instruments, and to identify emerging technologies that might better achieve
measurement objectives.
The measurement method goal should be more ambitious than dictated by current
technology. An ambitious goal would encourage more innovation and continued improvement
in monitoring technology. On the other hand, the goal should not sacrifice the good in pursuit of
the perfect. The sole focus on compliance hinders with the utility of data for a wider range of
applications, such as climate assessments, source zones of influence (including hot-spots), and
source and receptor modeling (Scheffe et al., 2009; U.S.EPA, 2008).
When Federal Reference Methods (FRMs) and Federal Equivalent Methods (FEMs) are
defined based on technology available at the time of designation, practical experience over a
wider range of environmental conditions than available before designation, development of new
technology and methods, and more efficient manufacturing methods reveal deficiencies in the
FRM or FEM. Examples are the freezing of oil and rapid overloading of the WINS impactor in
the PM25 FRM (Kenny et al., 2004; Pitchford et al., 1997), the heated PMio TEOM that
underestimates PMio mass for semi-volatile aerosols (Allen et al., 1997; Chow et al., 2006b), and
changes in PMio inlet cut-points with inlet loading (Rodes et al., 1985a; Rodes et al., 1985b;
Wedding et al., 1985a; Wedding et al., 1985b; Wedding et al., 1985c). Where design criteria are
necessary, they should consider the extent to which components are commonly available or must
be custom produced, thereby increasing production costs with no improvement in quantification.
An example of this is the PM25 inlet tube (U.S.EPA, 1997a) that specifies dimensions are not
available as common tubing stock, thereby increasing the complexity of manufacture and the
cost of the instrument.
A possible way to address this is to set performance standards that approach an ideal, but
that also allow for fairly large deviations around these standards with reductions in these
deviations at ~5 year intervals (a reasonable lifetime for most monitors). This type of
performance standard (Watson et al., 1995) would encourage innovation and improvement, as
opposed to the current motivation to degrade new instrument performance so that it mimics the
older FRMs. More specifics are given in the answers to the questions.
a. Wavelength of 550 nm
The 550 nm wavelength is specified because it is near the peak (555 nm) International
Commission on Illumination's (CE1) photopic response curve for a "standard observer (Fairman
et al., 1997; Smith and Guild, 1931)." Visual perception is more complex and depends on a
melding of the different wavelengths perceived, usually in the red, green, and blue regions of the
spectrum (Fairman, 1995; Fulton, 2009; Vienot, 1980). The goal should be to acquire extinction
at several wavelengths that might be better related to what people see than extinction at a single
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wavelength. Scattering and absorption at several wavelengths would provide information on
particle size (Thielke et al., 1972) and black vs. brown carbon (Andreae and Gelencser, 2006)
that would be useful to determine the causes of haze episodes.
A wavelength centerpoint of 520 to 530 nm would be more practical as a starting point
since there are several light emitting diodes (LEDs) and laser diodes available within this region.
A bandwidth needs to be specified, as it is for the TSI 3563 three-color nephelometer. The
narrower the bandwidth, the better for estimating scattering properties (Ruby and Waggoner,
1981). Wavelengths used for currently available light scattering and absorption instruments
include:
• 450±40, 550±40, and 700±40 nm for the TSI 3563 nephelometer (http://www.tsi.com/en-
1033/models/3158/3563.aspx). Wavelength specifications are close to those reported by
(Anderson et al., 1996)
• Broad band peaking at -620 nm for the OPTEC NGN-2 and NGN-3 open air
nephelometer (Molenar, 1997).
• 450, 525, and 635 nm for the Ecotech nephelometer
(http://www.ecotech.com.au/ecotech/nenav.nsf/LinkView/A2619E971A03E075CA25727
2001 OF 11FD82CO114BA147F41CA25715600207006.
• 530 nm for the Radiance M903 nephelometer (Richards et al., 2001).
• 655 nm for the TSI DustTrak II and DRX nephelometer/optical particle counter
combinations
(http://www.tsi .com/en-
1033/products/14000/dusttrak%C3%A2%E2%80%9E%C2%A2_aerosol_monitors.aspx)
• 405, 532 and 781 nm for the DMT PASS-3 Photoacoustic instrument
(www.dropletmeasurement.com/products/carbon-sensing-instruments/55). PASS-1 uses
the 781 nm wavelength.
• 370, 470, 520, 590, 660, 880 and 950 nm for the AE31 aethalometer
(mageesci.com/products/rack_mount_aethalometer.htm). The AE22 and OT21 use 370
and 880 nm, and the AE51 uses 880 nm.
• 670 nm for the Thermo Scientific Model 5012 Multi Angle Absorption Photometer
(MAAP) (http://www.thermo.eom/com/cda/product/detail/0.1055.19884.00.htmn.
• 567 nm for the Radiance Research Particle Soot Absorption Photometer (PSAP) (Bond et
al., 1999).
There should be a near correspondence between the scattering and absorption
measurements, which seems to be possible at 450-470 nm, 520-550 nm, and 630-700 nm. The
value of the 370 nm absorption wavelength should not be discounted, as this has been found
useful for separating biomass burning smoldering soot from higher temperature fuel combustion
soot (Kirchstetter et al., 2004; Sandradewi et al., 2008a; Sandradewi et al., 2008b).
b. Aerosol Size Fractionation at PM|0.
A PM25 size cut is a better choice than PMio- The rationale for a PMio size fraction to
measure urban haze is not given in the white paper. Under most urban circumstances (i.e., PM25
mass as half of PMio), PM2s will cause >90% of the scattering at 550 nm. If the particles are on
the large side of the PMio-zs fraction in urban areas, they are probably locally-generated and are
unlikely to be as uniformly distributed along the sight path as PM^s (Burton et al., 1996; Chow
et al., 1992; Chow et al., 1999; Chow et al., 2000; Chow et al., 2002a; Magliano et al., 1999;
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Monn et al., 1997; Wilson et al.5 2005). This will be aggravated by slanted sight paths that
probably experience a stronger decrease of PMio-2 5 relative to PM2 5 (Chan and Kwok, 2000).
c. Operation at ambient relative humidity (RH) for RH<90%.
This is a good idea for the scattering measurement, but it is not necessary for the
absorption measurement when these are measured separately. In photoacoustic measurements
the particle heating evaporates water, thereby decreasing the acoustic intensity that corresponds
with light absorption (Arnott et al., 2003; Murphy, 2009). An acceptable interval needs to be
specified (e.g. ambient ± 5% RH, similar to the specification for PM2s FRM filter equilibration).
It may be advisable to use a smart heater to bring higher humidity down to 90% to reduce the
potential for fogging of optical surfaces during cold and damp conditions.
d. Overall accuracy and precision < 10%
Accuracy and precision should be defined separately. There should be separate
requirements for the scattering and absorption measurements. A ±10% interval seems reasonable
for precision. A ±10% accuracy could be attained for consistent primary standards (e.g., light
scattering or absorbing gases, neutral density filters, and particles generated with a known
composition and size distribution), but would probably experience higher deviations among
instruments for more complex urban aerosols.
Methods for precision estimation should be specified, possibly following the collocated
sampling in different environments currently in use for PlVhs FRMs or with respect to a variety
of laboratory-generated aerosols (Sheridan et al., 2005) that cover a broad range of conditions.
e. Range of conditions from 10 Mm'1 to 1000 Mm"1
This range is reasonable and has been attained by current technology. There may be
some non-linearity in the concentration response at high concentrations that needs to be
evaluated for a specific configuration. This has been observed for filter light transmission
measurements (Lin et al., 1973; Watson and Chow, 2002).
f. Valid measurements (with all other appropriate checks) when sampled at < 90%
relative humidity
A more complete validation procedure is needed to elaborate on this. U.S. EPA (2008)
is a good starting point. It requires at least 45 minutes of data to represent an hourly average, as
well as specifying frequencies for performance tests (e.g., zero and span), re-calibrations, and
audits. Other checks could include extreme values and sudden increases in measurements that
might be from electronic noise rather than a change in particle extinction, runs tests to determine
that there is some change over a period of time, sensing chamber temperature and RH variability,
and correlations or lack thereof with collocated readings.
CQ2: Please comment on inclusion of the following additional performance specifications:
a. Measurement averaging times
The one hour averaging time is reasonable, but data should be acquired for <5 min
averages. Sharp spikes of short duration probably represent localized emitters (Watson and
Chow, 2001) that should not be included in a longer average intended to represent the uniform
distribution along a sight path.
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b. Instrument specific parameters such as angular integration for nephelometers?
Nephelometer truncation errors have been evaluated for various configurations (Abu-
Rahmah et al., 2006; Bond et al., 2009; Ensor and Pilat, 1971; Ensor and Waggoner, 1970;
Guyon et al., 2003; Heintzenberg et al., 2006; Jonasz, 1990; Moosmuller and Arnott, 2003;
Muller et al., 2009; Penaloza, 1999; Quenzel et al., 1975; Quirantes et al., 2008; Rabinoff and
Herman, 1973; Reed and Howser, 1995; Rosen et al., 1997; Shkuratov et al., 2007; Varma et al.,
2003), and their results indicate that unmeasured forward and backward scattering is not
important for the 2-4 urn end of the PMio-25 distribution expected along a sight path.
Nevertheless, an evaluation of truncation biases should be part of the FRM certification process.
This might use the same urban size distribution required for evaluation of PMio inlet sampling
efficiencies (U.S.EPA, 1987) and one or more of the methods described in the previously cited
articles.
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.
Except for ultrafine particles, scattering by gases differs from scattering by particles,
especially when the scattered wavelength is about the size of the particle circumference
(Moosmuller and Arnott, 2009). The goal for transfer standards should be a consistently
generated ambient aerosol that mimics one or more urban aerosols and size distribution. A high
sulfate content might be specified for the eastern U.S. with a high organic carbon content
specified for the western U.S. to reflect these obvious differences in PM composition (DeBell et
al., 2006). Several aerosol generation systems have demonstrated the ability to do this (Evans et
al., 2003a; Evans et al., 2003b; Gerde et al., 2004; Gill et al., 2006; Guo and Kennedy, 2007;
Horvath and Gangl, 2003; Kim et al., 2006; Kirchstetter and Novakov, 2007; Mikhailov et al.,
2006; Sheridan et al., 2005; Teague et al., 2005; Veranth et al., 2000; Vlasenko et al., 2005;
Widmann et al., 2005). This standard would be applicable to light scattering, light absorption,
and light extinction instruments.
Refrigerant gases and COi are often used for nephelometer calibration, but these gases do
not mimic PM characteristics (Horvath and Kaller, 1994). Neutral density filters (Macleod,
2001) are a long-accepted standard for densitometry (i.e., filter light transmission), but they do
not separate the optical properties of particles on a filter surface from scattering and absorption
of the filter media. The wavelength dependence of gases (XT4) and neutral density filters does not
follow the relationship for particles of different particle sizes, shapes, and compositions.
Primary standard instruments could be established using more advanced, but not
necessarily the commercially-available or cost-effective technologies needed for a widespread
network, to characterize these transfer standards that could be used for field calibration of more
practical, cost-effective, and commonly available instruments.
CQ3: Suitability of currently available nephelometers and filter transmission systems.
a. To what extent does the Subcommittee support the staffs position that currently
available nephelometer light scattering and filter transmission light absorption
measurement instruments are suitable to meet the light extinction goals?
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Currently available nephelometer and filter transmission light absorption instruments are
sub-optimal for a robust urban visibility measurement goal. Current commercial technology
should be considered as the starting point, not the end-point, for a more ambitious goal.
Currently available commercial instruments have been designed and marketed by small
businesses with limited markets (e.g., researchers), limited research and development budgets,
and limited testing in a variety of environments. With the prospect for orders of magnitude
increases in sales resulting from a FRM designation, these and other manufacturers would attract
the investments needed to develop better, more versatile, and more cost-effective instruments.
An opportunity is lost if the bar isn't raised to encourage the next generation, rather than locking
users into old technology for decades to come. It would be worth a few years of Small Business
Innovative Research (SBIR) initiatives or research solicitation to encourage the development and
commercialization of the next generation of instrumentation.
b. What are the Subcommittees thoughts on alternative instrumental approaches
that should be considered to meet the light extinction goals?
Wang et al. (2009) describe a nephelometer for PIv^s combined with an optical particle
counter (OPC) for larger particles that allows a better measure of scattering, the potential effect
of PMio-25 on scattering, and the distribution of large (i.e., locally-emitted, >10 um) vs. smaller
(more widely dispersed, 2.5-4 jim) coarse particles.
Adaptations of nephelometers in commercial smoke detectors (Edwards et al., 2006;
Litton et al., 2004) and portable AE 51 aethalometers could be located along a sight path at low
cost offer the opportunity to obtain a more representative measure than quantification at a single
location.
The National Weather Service's Automated Surface Observing System (ASOS) (ASOS,
2002; Powell, 1993; SAO, 2002) that replaced human observer airport measurements should be
considered. These are currently truncated at 10 miles visual range, but they are valid over longer
distances that are not well defined.
Currently-expensive multiwavelength photoacoustic instruments for light absorption, as
noted in the white paper, might be used initially as a primary standard at a few regionally
distributed locations to certify aerosol generation transfer standards, then used to replace filter
transmission methods in the future as their size and costs are reduced with the advent of newer
technologies.
CQ4: Instrument Improvements
a. Suggestions for improvement to the commercial versions of these technologies
for optimization in future routine monitoring applications for light extinction.
When FRMs and FEMs are defined based on technology available at the time of
designation, practical experience over a wider range of environmental conditions than available
before designation, development of new technology and methods, and more efficient
manufacturing methods reveal deficiencies in the FRM or FEM. Where design criteria are
necessary, they should consider the extent to which components are commonly available or must
be custom produced. Non-standard components increase production costs with no improvement
in quantification. The ideal performance criteria should be stated, with uncertainty allowances to
accommodate current technologies, but with periodic tightening of the specifications. Co-funded
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development opportunities, with funds allocated by a competitive proposal process (the SBIR
and STAR programs are good models) should be planned to speed up the development process.
A small initial investment will result in large long-term savings in terms of field operation, data
processing, data validation, data dissemination, and legal expenses.
The vendor specification lists for different instruments are not always in agreement with
published independent tests. Changes to some instruments might include:
• Measuring at additional wavelengths with specified bandwidth.
• Using more energy-efficient and cooler solid-state illumination sources to minimize heating
of the nephelometer scattering chamber and to more precisely define the wavelength and
bandwidth.
• Adding temperature and RH sensing in the sensing zone.
• Combining scattering and absorption measurements into a single instrument (e.g., Photo-
Acoustic, Soot, and aerosol Sensor - three wavelength [PASS-3]).
• Acquire less than five minute averages for the measurements, with stable hourly
measurements (e.g., including on-line data processing to minimize post processing for
hourly data).
• Upgrading data acquisition and analysis software to better meet the needs of an urban
visibility standard.
b. If applicable, what are the Subcommittees suggestions for improvement of
alternative instrumental approaches for use in future routine monitoring
applications?
EPA should not dictate the measurement principles, designs, or manufacturers. It should
set out the visibility characterization goals as specifically as possible based on performance
standards as opposed to design standards and allow American ingenuity to rise to the challenge.
Tests to be considered include:
• Effects of water on absorption measurements.
• Equivalency and comparability between cavity ring-down spectrometers and other particle
light scattering methods.
Pros and Cons of Different Procedures for Approval of Federal Reference Methods
(FRM's) and Federal Equivalent Methods (FEM's).
CQ5: Performance vs. Design Standards
a. Translate measurement goal to performance standards and methods to demonstrate
performance.
This should be the preferred alternative. The white paper and the time available for these
comments are insufficient to define these standards and the ways to attain them. Demonstration
methods might include:
• Theoretical analyses of size cuts, particle transmission, and changes in particle size and
composition: Computerized fluid dynamics (CFD) (Chen et al., 2005; Gimbun et al..
2005; Hari et al., 2007; Hu et al., 2007; Wang and McMurry, 2006), non-spherical optics
(Fuller et al., 1999; Kalashnikova and Sokolik, 2004; Mishra and Tripathi, 2008; Wind et
al., 2004), particle/filter interactions (Chen et al., 2004), and equilibrium (Nenes et al.,
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1998) models are now accurate enough to estimate the performance of instrumentation
under a variety of different conditions.
• Replication of values from primary and transfer standards: Several suggestions are given
above.
• Temperature and RH deviations between the sample measurement zone and ambient air.
Tolerances could be set based on findings from the previous analyses.
• Fast averaging times. < 5 min averages should be attained to separate local from urban-
scale contributions that would better represent extinction along the sight path.
b. 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.
This approach should be avoided. It will lead to difficulties and controversies, as it has
done in the past.
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.
This approach should be avoided. If adopted, it should be only on an interim basis, to be
terminated within 5 years in favor of methods that achieve a more ambitious, but attainable, goal.
CQ6: Which aspects of a light extinction measurement could be adequately assessed in
laboratory and which require field studies (perhaps across multiple air sheds).
As noted above, a serious application of existing modeling technologies should be
applied to determine theoretical compliance with performance specifications. Several
laboratory-generated aerosol mixtures could be presented to each candidate to determine how the
instrument will respond when compared to a primary standard.
CQ7: Would some aspects of performance be better addressed through a design standard,
e.g., for the flow rate and the geometry of the PM|0 inlet, rather than a performance
specification and demonstration requirement?
No. Several examples of the failure of this approach have been cited above, and many
more examples could be assembled with some effort.
CQ8: What data and analysis does the Subcommittee believe EPA staff should have
studies or performed in establishing some kind of FRM for use in regulatory
decisions and to help inform the public?
There is a wide literature on this subject that has not yet been completely reviewed and
evaluated (Andreae and Gelencser, 2006; Bond and Bergstrom, 2006; Chow, 1995; Chow et al.,
2002c; Chow et al., 2008; Hand and Malm, 2007; Heintzenberg and Charlson, 1996; Horvath,
1993a; Hyslop, 2009; Kerker, 1997; Kokhanovsky and Zege, 1997; Liou and Takano, 1994;
Moosmuller et al., 2009; Moosmuller and Arnott, 2009; Sorensen, 2001; Watson, 2002; Watson
et al., 2005; Watson and Chow, 1994; Wilson et al., 2002). A substantial expansion of the white
paper or a guidance document on urban visibility measurement should be commissioned from
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researchers with broad experience in these measurements to document and evaluate what is
already known. This would evaluate detection limits, interferences, collocated precision in
different environments, calibration methods, data validation techniques, and data analysis
approaches applied in prior studies.
There is substantial potential for additional analyses from the supersites data bases
(CARJB, 2009; NARSTO, 2009) and the IMPROVE filter and continuous measurement sites
(VIEWS, 2009) that can look at relationships between aerosol size and composition as well as
scattering and absorption under different emissions and environmental conditions.
CQ9: Evaluation of current measurement technology
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?
Each of the currently available instruments has advantages and limitations that need to be
more explicitly stated and referenced in the white paper. Much of this information can be
extracted from published reports and articles and from interviews with users. It is beyond the
scope of these comments or the time available to prepare them to do this here.
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?
There is no "gold standard" as yet. Certainly newer technologies are superior to older
technologies. The aethalometer is much more stable and better referenced than its Coefficient of
Haze (COH) predecessor. The earlier MRI/Belfort nephelometers were often better indicators of
temperature fluctuations than light scattering.
There are several published comparison studies for light scattering, absorption, and
extinction (Adams et al., 1989; Allen et al., 1999; Arnott et al., 2003; Arnott et al., 2005a; Arnott
et al., 2005b; Arnott et al., 2006; Bennett, Jr. and Patty, 1982; Bond et al., 1999; Bundke et al.,
2002; Cappa et al., 2008; CARB, 2003; Chakrabarti et al., 2004; Chow et al., 2006a; Chow et al.,
2006b; Clarke et al., 1987; Edwards et al., 1983; Fischer and Koshland, 2007; Foot and Kilsby,
1989; Heintzenberg et al., 2006; Hitzenberger et al., 1984; Horvath, 1993b; Japar et al., 1990;
Kashuba and Scheff, 2008; Lack et al., 2008; Liu et al., 2002; Malm et al., 2000a; Malm et al.,
2000b; Merles et al., 2003; Moosmuller et al., 1998; Park et al., 2006; Petzold et al., 2005; Reid
et al., 1998; Ruoss et al., 1991; Ruoss et al., 1992; Ruoss et al., 1993; Saathoff et al., 2003;
Sioutas et al., 2000; Slowik et al., 2007; Snyder and Schauer, 2007; Turpin et al., 1990; Virkkula
et al., 2005; Wallace, 2005; Watson et al., 1989; Watson et al., 2005; Watson and Chow, 2002;
Weingartner et al., 2003; Weiss and Waggoner, 1984; Wu et al., 2005) that have been
insufficiently evaluated. Many of these have insights and suggestions for improvement that have
not yet been catalogued and pursued.
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?
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Many of these have been identified above or in the literature cited. A comprehensive list
of potential issues needs to be assembled from a careful review of published articles. This
information should be used to adapt some of the CFD, optical, and equilibrium technologies
cited above into a practical model that can be used to evaluate different instrument designs. As
noted, primary standards and transfer aerosol generation systems are needed to truly evaluate the
measurement accuracy.
d. What weight should EPA give to other factors in establishing a reference method
for routine PM light extinction monitoring?
i. Current availability
Zero weight. Current technology is more than a decade old and was not designed for this
purpose. Commercially available instruments should only be used as a stop-gap measure and
should not dictate the desired performance standard goals.
ii. Record of successful field experience
Zero weight. There is no quantitative record. The only data base is anecdotal, so there is
no basis on which to quantify such a record. One cannot compare an older technology, which is
sub-optimal but has gone through several iterations, with a newer and better technology that is in
the improvement process.
iv. Ability to generate supplemental information (e.g.
multiwavelength scattering/absorption, albedo,
forward/backscattering, scattering polarization, etc.)?
High weight. As noted above, the measurements should go "beyond compliance" (Chow
and Watson, 2008) in addressing issues beyond a secondary urban visibility standard.
Network Design and Probe and Siting Criteria
CQ10: To what extent should network design characterize maximum visibility impairment
across an urban area? What other considerations should EPA include in setting a network
design strategy?
The goal should be to determine average extinction along a sight path. Valued views
(and their accompanying sight paths) will vary with location, so any value will be imperfect.
PM2s network design guidance (Chow et al., 2002b; U.S.EPA, 1997b) should be adaptable to
this application, as it discusses spatial averaging, special purpose monitors at hot-spots, and
setback distances from nearby sources. It would be advisable to locate monitors along a valued
sight path, possibly with one in a maximum Plv^s concentration area (neighborhood-scale), one
in a suburban area (urban-scale), and one in a rural area (regional-scale). Subtraction of high-
frequency signals and of neighborhood-scale contributions as described by Watson and Chow
(2001) might be considered.
CQ11
a. To what extent does the Subcommittee support collocation of PM mass and light
extinction measurements to complement each of the measurements systems
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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.
Continuous visibility measurements should be collocated with PIVUs sites, especially
those with speciation measurements, wherever the siting criteria are attained. Even at hotspot
sites, the high time resolution will allow nearby source contributions to be subtracted and will
allow for better understanding of local contributions to the 24-hour PIVhs sample. Site- and
season-specific relationships can be established between PIVh 5 mass and light scattering (Chow
et al., 2006a) and between elemental carbon and light absorption (Park et al., 2006) that can
determine what is happening within a 24-hour period and between the 3- to 6-day filter samples
(U.S.EPA, 2010) acquired at many locations
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?
Multiple sites along a sight path are essential, as described above. Some precision and
accuracy for a single point measurement might be sacrificed in favor of lower cost and greater
portability for several instruments that can be located along a sight path.
CQ12. 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?
This is a minor consideration compared to other uncertainties. Other considerations are
more important, such as surface moisture, snow cover, low inversion pockets that might trap
pollutants in a small region around the monitor.
CQ13. Considering site path, aerosol mixing, the goal of PM light extinction measurements,
site logistics, and the location of other air monitoring equipment inlets, what should be the
acceptable range for probe height?
Inlets should be 3 to 10m above ground level, on the rooftops of 1 to 3 story buildings
and at least 1 m above the rooftop. PM2 5 network design and continuous monitoring guidance
documents (U.S.EPA, 1997b; U.S.EPA, 1998) provide a good starting point for sampler siting
criteria.
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Wind, L ; Hofer, L ; Nagy, A ; Winkler, P , Vrtala, A ; Szymanski, W.W (2004) Light scattering from droplets with
inclusions and the impact on optical measurement of aerosols J Aerosol Sci, 35(9). 1173-1188
Wu, C F ; Delfino, R J , Floro, J.N ; Samimi, B S., Qumtana, P.J E ; Kleinman, M T., Liu, L J S. (2005) Evaluation
and quality control of personal nephelometers in indoor, outdoor and personal environments J Expo Anal
Environ Epidemiol, \ 5( 1) 99-110
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Mr. Bart Croes
These comments also reflect input from California Air Resources Board (ARB) staff responsible
for implementing U.S. EPA monitoring requirements and using the data in source apportionment
and health studies.
Questions regarding a PM Light Extinction Measurement Goal and Method
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 PMJO
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
a. The 550 nm wavelength is the peak of the solar visible spectrum (seen as green light), and is
often used as a monochromatic surrogate for all visible light. This specification is reasonable,
but it needs to be refined by adding a defined spectral range and sensitivity, so that photometric
instruments used to make this measurement are comparable.
b. Fractionation to PM10 is appropriate. Although most combustion-derived light attenuation is
due to particles in the range of 0.5 to 2.5 u.m, a significant fraction of PM optical effects is due to
larger particles, particularly in the case of soil dust or mechanically produced anthropogenic
particles. A smaller cutpoint would be inappropriate as it ignores a major contributor to reduced
visibility in many industrial and rural settings.
d. Accuracy and precision of 10% is reasonable in light of the necessity that a point measurement
will be used to represent a phenomenon (atmospheric turbidity) that is only meaningful (in a
public perception sense) over distances of multiple kilometers and which is also variable across
viewing environments. A 10% uncertainty is acceptable, so long as the difference among
observing systems (multiples of the same instrument, or between competing instruments) is
unbiased. Any adopted method must be defined so as to prevent "cherry picking" between
instruments to bias monitoring statistics. Striving for higher performance, per se, would be a
waste of resources.
e(l). The upper range of conditions for accurate measurement is appropriate, as it is higher than
the expected range of optical conditions possible due to variation of aerosol composition when
concentrations approach the current health-based PM NAAQS. If the measurement is to be
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applied in situations outside the constraints of PM NAAQS, then the upper limit of the range
should be considered in light of other legal or physical constraints.
e(2). The lower limit, which approximates Rayleigh scattering for green light in particle free air
at about 2 km above sea level (about 20% clearer than "clean" dry air at sea level) is appropriate
for most urban areas in the U.S. If the measurement is to be deployed at high altitude, especially
in remote areas, such as Western National Parks or Wilderness Areas, a more sensitive lower
limit should be required to insure good measurement performance in clean conditions. This,
however, would require a fairly sophisticated approach to field calibration, as merely filtering air
would not allow a lower limit test.
f. Establishing a humidity cutoff is appropriate to prevent condensation on particles from turning
otherwise acceptable air quality into an exceedance of PM optical criteria, when they are in
effect. The relative humidity limit of 90% is acceptable, so long as EPA approaches this with
appropriate understanding of the consequences. At very high humidity, particle growth is
dominated by water, and it may be inappropriate to "penalize" wet conditions. California uses a
70% cutoff, which may be too low for the more humid conditions found in the rest of the U.S.,
but a compromise (say, 85%?) may be more appropriate. EPA should examine the number of
hours that would be exempted in some very humid locations, and make a determination based on
practicality and measurement reliability.
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 parameters) and an appropriate goal to support a PM
light extinction measurement
a. Averaging times are very important in using a point measurement to represent an areally
dispersed phenomenon. Short averaging times would make the measurement unduly sensitive to
local "puff' emissions or short term variations in PM composition or concentration. In the
context of using this measurement to supplement health protective PM mass concentration
regulations, the averaging time should be set to approximate the relevant exposure time (e.g., 24-
hr light attenuation to supplement 24-hr PM mass regulations). If the goal is to provide a welfare
benefit of good regional visibility, then the averaging time should reflect human perception of
"good visibility" based on survey or laboratory studies of human responses to short term
visibility degradation. The latter would probably use shorter averaging time, on the order of one
or a few hours, rather than the 24-hr criterion derived from current health-based PM NAAQS.
b. The angular integration of a nephelometer is strongly sensitive to particle size. Since real
aerosols may exhibit anything from strong backscatter to strong forward scatter, or may
approximate isotropic scattering, a wide field of view of the nephelometer is the best way to
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measure light scattering without introducing unintentional particle size or humidity weighting
into the measurement.
c. Calibration with a highly scattering gas is desirable as it is both simple and repeatable with
limited technical sophistication. The historic practice of calibrating with Freon (CFC-12), a
strong greenhouse gas and stratospheric ozone depleter, should be explicitly banned in any new
measurement specification; any gases proposed for this use should be carefully reviewed for
their suitability for use over multiple decades. "Milk glass" standards have been used in the past
as an alternative to gas calibration. This approach, using a solid scattering medium, should be
considered, but caution is needed to prevent creating an opportunity to down-bias instruments if
used improperly.
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 goals7
b What are the Subcommittees thoughts on alternative instrumental approaches that
should be considered to meet the light extinction goals7
a(l). Existing nephelometers, such as those used in the IMPROVE network are quite suitable to
the task, and offer the benefits of an existing installed basis for a network for those agencies
which currently use them.
a(2). Filter transmission measurements of light absorption need tightly defined protocols and
specification of the filter medium to be reliable. The principle, as applied by the IMPROVE
network, is workable, but EPA should be cognizant of the critical role of the filter medium in this
measurement. In order to measure only absorption, light scattering by material on the filter
needs to be overwhelmed by scattering by the filter itself, and filter loadings need to be modest
(little more than a mono-layer). This last constraint is a weakness of the commonly used
aethalometer. EPA should be wary of accepting existing aethalometers for this purpose; at
minimum, instrument operations protocols should be reviewed, and careful laboratory and field
studies done to quantify the uncertainty, bias, and environmental (temperature and humidity)
responses of current production models. Thin, non-filamentous filter substrates, such as Teflon,
should not be used for transmission absorption measurements as they violate the physical
assumptions of the measurement.
b. Alternative measurements of light absorption are available. Although subject to some siting
constraints, subtraction of nephelometer scattering from long path light extinction can yield light
absorption. Within the context of keeping the measurement compatible with traditional
monitoring site operations, two alternatives are available. Switching to a reflection measurement
with the filter set against a white background (and measuring the base transmission of an
unexposed portion of the filter as IQ) is a viable method, compatible with existing FR.M samplers
(and assuming that the protocol takes account of the problem of heavily loaded filters - a
problem also present in the transmission measurement). Alternatively, the nephelometer can be
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replaced with a cavity ring-down optical measurement3 which can give both total extinction (on-
axis decay) and scattering (off-axis intensity), and thus absorption by subtraction, all in a single
instrument. Such a machine has been built by Dr. Anthony Strawa for use by NASA in airborne
atmospheric research, and could be easily commercialized if EPA elected to pursue this method.
The cavity ring-down instrument would eliminate the dual instrument problem and facilitate
unified calibration.
4. Considering the potential need to deploy nephelometer light scattering and filter ransmission
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
PMmass 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 applications7
a. Existing nephelometers are adequate, but calibration methods should be reviewed. Existing
aethalometers are inadequate and unreliable, especially as they respond to temperature and
humidity variation; these should be viewed with suspicion for regulatory applications.
Absorption measured by integrating sphere, as developed for the IMPROVE program is suitable,
so long as relatively open weave filamentous filter substrates are used.
b. EPA should explore alternatives, especially the unified measurement of both scattering and
total extinction possible with cavity ringdown technology.
3 Strawa, A W, R Castaneda, T Owano, D. Baer, B Paldus, The Measurement of Aerosol Optical
Properties Using Continuous Wave Cavity Ring-Down Techniques, Journal of Atmospheric and Oceanic
Technology 20, 454-465, 2002
DOI 101175/1520-0426(2003)20<454 TMOAOP>2 0 CO,2
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Dr. Kenneth L. Demerjian
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 PMio under ambient relative humidity conditions with
overall accuracy and precision < 10% in a range of condition from 10 Mm-i to 1000 Mm-i for
relative humidity conditions <90%. EPA staff believes that such a goal would be reasonable
starting point for establishing performance specifications to support light extinction
measurements for a PM visibility standard.
Adequacy of the goal (1)
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 - There is nothing magic about the SSOnm wavelength in measuring
PM light extinction. The choice of wavelength (within the visible range) should be driven by
the overall precision, accuracy, performance and costs of the instruments to make the desired
measurement. Jf multiple wavelengths are to be considered, justification should be provided
with respect to the value added information and it's utility in supporting mitigation strategies.
b. Aerosol size fractionation at PMio- Choice of PMIO needs to be further assessed in terms of
its robustness in attributing PM light extinction. Further documentation ofPM2.5 and PMIO
contributions to PM extinction by region and season is needed to determine the optimal PM
size cut. Measuring PM course particle extinction contributions will be challenging.
c. Operation at ambient relative humidity - Tracking the effect of ambient relative humidity on
PM light extinction is essential in development of management strategies
d. Overall accuracy and precision < 10% - Overall accuracy and precision will be very much
dependent on the PM size fraction, humidity cutoff and base PM light extinction to be
considered. The 10% accuracy and precision seems a very ambitious goal
e. Range of conditions from 10 Mm-1 to 1000 Mm-1 - This seems like a reasonable range, but
should be reviewed once the specification of the secondary PM light extinction standard is set.
f. Valid measurements (with all other appropriate checks) when sampled at < 90% relative
humidity - / would think 95% valid measurement data (excluding span and zero checks) is a
good target for routine monitoring systems and is obtainable for many of the systems
identified.
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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 — High time resolution (e.g. minute average) data should be
collected and stored if available. High time resolution data will support in reconciling PM
extinction through consideration of with PM component species, relative humidity and
absorptive gases.
b. Instrument specific parameters such as angular integration for nephelometers?
c. Calibration with a gas that has known Rayleigh scattering properties. Instrument
manufacturers should provide standard calibration procedures for which Rayleigh scattering
of gases is likely necessary, but not sufficient calibration.
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 believes 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 staffs position that currently available
nephelometer light scattering and filter transmission light absorption measurement instruments
are suitable to meet the light extinction goals? - This may be true, but seems to be reverting to
the lowest common denominator from a technology point of view. New technologies as
mentioned in section b) look promising, have gotten significant SBIR support and will likely
be commercially available within the next year.
b. What are the Subcommittees thoughts on alternative instrumental approaches that should be
considered to meet the light extinction goals? - New technologies such as Cavity Ring Down
CRD and Cavity Attenuation Phase Shift (CAPS) look extremely promising and should be
evaluated and considered as a possible alternative to nephelometer
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. - Laboratory aerosol chamber systems exist which
can generate and characterize primary and secondary aerosols to evaluate light extinction
measurement technologies. These aerosol environments can also he perturbed by changes in
temperature and/or relative humidity to test the measurement systems sensitivity to these
factors.
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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? - Site selection should characterize representative (area-wide) visual impairment of a
vista recognized by a significant fraction of the local population.
11. EPA and the State monitoring programs have an extensive historical dataset of Plvh 5 mass
and speciation measurements. In the Visibility Assessment Document, 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 information from
networks assessments 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 co-locating both types of measurements systems in an
area-wide location of expected maximum concentration. Co-location light extinction
measurements with PM mass, PM species composition and precursor gases is essential in
attributing primary PM sources and understanding processes and the attribution of secondary
PM production sources. As mentioned above site selection should characterize representative
(area-wide) visual impairment of a vista recognized by a significant fraction of the local
population.
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?
// is unlikely that intra-urban variability ofPM is sufficiently large to warrant multiple PM
extinction measurements with any urban center
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Dr. Delbert Eatough
I am agreement with AAMMS response to the White Paper. This documents provides
additional comments on issues related to the developing EPA document on the consideration of
measurement of light extinction by both an extinction measurement and a PM mass measurement
as they relate to the AAMMS subcommittee charge questions from EPA.
I. Questions Regarding a PM Light Extinction Measurement Goal and Method.
I have a few comments in connection with this charge question in connection with the
EPA Viability Assessment referenced in the White Paper which I think are relevant to the current
charge to AAMMS. The Assessment includes extensive evaluation of both the magnitude of a
possible secondary PM standard based on human response studies and an evaluation of current
visibility conditions in several urban areas.
With respect to the observer studies, a couple of comments. In general the studies are
well suited to contribute to the Urban Visibility Goal decisions. However the observer is
sensitive to contrast and not to absolute extinction. In this regard, I think the Washington D.C.
studies are outliers (as they appear in Figure 2-16, page 2-26 of the Assessment) because there
are no appropriate landmarks in the Washington study from which the observer can judge
contrast in the same context as the other cities. This results in the larger suggested threshold
observed in these studies. Thus, I think they should be discounted when the standard is set. I
also wonder to what extent time of day versus location has been evaluated in connection with the
difference in perceived visibility quality as influenced by observation of a scene in the forward
scattering and the back-scattering mode. The response of a human subject will be dependent on
the location of the observer to the observed distant scene in the morning and the afternoon.
Finally, I found the evaluation of current extinction in the urban areas studied in the
Assessment less than persuasive, largely because of the crudeness of many of the assumptions. I
understand these assumptions were necessary because of the data sets that EPA chose to evaluate
and that EPA is fully aware of the limitations of the approach used in the Assessment. I think
EPA should consider additional efforts to shore this area up with studies which focus on
measured extinction, measured hourly mass and, where available, measured hourly average
composition. When measured mass is included in the evaluation, I strongly urge that
conventional TEOM data not be used in this evaluation because of the serious problems with the
measurement of semi-volatile ammonium nitrate and organic material.
We have worked ourselves into a difficult hole with respect to the primary standard by
the use of inferior mass measurements for the establishment of the standard. Now that much
better methods are available for semi-continuous measurement, we cannot use them easily for the
primary standard because they do not reproduce the mass measurements as measured by the
FRM method for the standard. The secondary standard evaluation will be immutable connected
to the quality of the mass measurements to which it is compared. We do currently have several
instruments which will measure fine participate mass with minimization of the loss of semi-
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volatile material such as ammonium nitrate and sem-volatile organic material. These include the
FDMS TEOM, the BAM and the GRIMM PM monitors. Generally these instruments do not
measure total aerosol water content. However, we have shown that the GRIMM can either
measure aerosol water (Grimm, H. and Eatough D.J. "Aerosol Measurement: The Use of Optical
Light Scattering for the Determination of Particulate Size Distribution, and Paniculate Mass,
Including the Semi-volatile Fraction," J. Air & Waste Manage Assoc 2009,59; 101-107) or
exclude aerosol water (Hansen J., et al., "Semi-continuous Particulate Matter (PM2 5 and PMio)
Mass and Composition Measurements in Lindon Utah During Winter 2007." J. Air & Waste
Manage. Assoc. 2010,60: 346-355), depending on the instrument configuration chosen. 1 urge
evaluation of areas (such as SLC) where good FDMS TEOM (or comparable techniques)
measurements are available be focused on. Reasonable extinction measurements on an hourly
basis are available at essentially all urban areas as airport ASOS data for this evaluation. We
have demonstrated the use of the ASOS data in evaluation of sources of visibility degradation in
urban areas. (Eatough, D.J. and Farber R. "Apportioning Visibility Degradation to Sources of
PM2 5 Using Positive Matrix Factorization," J Air & Waste Manage. Assoc 2009, 59; 1092-
1110). At a minimum, the use of ASOS extinction, FDMS TEOM fine paniculate mass and any
measurement of coarse mass would give a better evaluation of the current status of visibility
impairment in an Urban area in my opinion. Adding fine paniculate composition which
measures semi-volatile ammonium nitrate and organic material would further add. This would
allow the better assumption of an f(RH) factor to the analysis
The evaluation of such data would be a great assist when a choice is made of a potential
mass measurement which could also be considered for an FRM for a future PM secondary
standard. It is important as EPA moves forward in this evaluation that a performance based
approach be taken in the choice of the mass monitor, as the committee has urged for the
identification of a method for the measurement of extinction.
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Mr. Dirk Felton
General Comments:
Visibility Based Secondary PM NAAQS:
It is premature to institute a secondary PM NAAQS for visibility at this time. The EPA should
conduct further investigation of other welfare based effects that ambient PM has on all areas, not
just urban areas. Two of the more significant aspects of ambient PM are the contribution to
climate change and the impact from wet and dry deposition. The impacts of climate change and
deposition could easily be more significant than visibility for both human well being and for the
health of the ecosystem. The timing is also not right for a secondary standard based on visibility.
Many other PM and visibility related NAAQS are currently in the midst of their biggest change
in the past 30 years. NO2 is directly related to visibility and the new NAAQS will likely lead to
future controls on NO2 which will lead to improvements in visibility. Other recently proposed
NAAQS including SC>2 and the upcoming proposals for CO and PM will also have both direct
and indirect effects on visibility. It is more appropriate to determine if a secondary visibility
related standard is necessary after these NAAQS have been fully implemented.
If it is determined that the measurement of visibility is necessary at this time, other simpler
options should be considered. Visibility can be calculated from PM component concentration
data and these calculations can be compared to existing airport visual range instruments and air
pollution cameras. The accuracy of a light extinction measurement is not necessary for the rather
subjective goal of determining when 50% of the population determines that a specific view is
impacted. Light extinction measurements are also of limited value to health effects researchers
and to agencies that must design PM control strategies. The visibility data will not be well
correlated with data collected for the primary PM NAAQS because the determination of light
extinction, as laid out in the assessment paper includes the effects of relative humidity.
Urban Focused Visibility Standard:
The EPA has proposed PM standards in the past that attempted to apply different air quality
standards to areas of the country that were classified as either urban or rural. This approach did
not sit well with the public at the time and the proposal was not implemented. An urban focused
visibility standard is likely to suffer a similar fate. State and local air monitoring agencies do not
want to be put in the awkward position of explaining to the public that a national secondary air
quality standard was written specifically for people living in urban areas but does not equally
cover people living in rural areas. If the EPA wants to implement a secondary visibility standard
that covers all areas of the country, the proposal will have to include a monitoring network
design that includes non-urban areas or include a plausible explanation of why rural areas do not
need to be covered by this standard.
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Specific Comment for Charge Question 3a:
Instruments Potentially suitable for filter transmission light absorption (b-abs surrogate)
This list should include the Sunset EC/OC analyzer and the Magee OT2I Transmissometer. The
Sunset instrument provides a thermal as well as a laser transmission based measurement of EC
on an interval from 30 minutes to 3 hours. This instrument is specifically designed to
differentiate OC fractions from EC fractions which is not necessary for the determination of b-
abs. The Sunset is usually operated by state and local agencies interested in source attribution
research, however the data should be used for as many purposes as possible.
The Magee Transmissometer is a simple instrument that determines the amount of light
absorption through manually loaded sample filters. This type of instrument is not suitable for
continuous or short interval data but its advantage is that it can use archived filters from multiple
sampling locations to estimate spatial gradients of b-abs.
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Dr. Kazuhiko Ito
Questions regarding a PM Light Extinction Measurement Goal and Method
1. Does the Subcommittee agree with the goal identified?
General Comment: It is good to have a concrete set of goals identified, but, given the potential
uncertainties regarding the definition of the visual impairment of sight path to "valued urban
scenes" across cities, it may be premature to set specific numerical values with these goals.
c. Operation at ambient relative humidity
Comment: Frequency and its diurnal profile of ambient relative humidity likely vary across
regions, cities, and even within a city. Therefore, such information needs to be characterized to
determine what the feasible range of the ambient relative humidity is.
d. Overall accuracy and precision < 10%
Comment: Does this goal incorporate the range of "acceptable" visibility found in the urban
visibility preference studies across cities? I am not sure if it is essential to have a numerical
value set now.
Questions Regarding Network Design and Probe and Siting Criteria
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 area7 What other considerations should EPA include in setting a network design
strategy?
Comment: EPA talks about the visual impairment of sight path to "valued urban scenes." 1
wonder if EPA can come up with a concrete list of such valued urban scenes.
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?
Comment: 1 am not sure how big a problem this is, but satellite surface temperature data and land
use data (e.g., imperviousness) may help to identify such "dry spots". However, the extent of "a
local heat island" effects may vary across cities, depending on what fraction of the city is
considered a "heat island" (e.g., Manhattan).
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
ofPM light extinction measurements, site logistics, and the location of other air monitoring
equipment inlets, what should be the acceptable range for probe height7
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Comment: Ideally, monitoring probes should be placed so that the sampled air represents the air
along the sight path to the "valued urban scene". However, logistically, setting the probe height
at such sight path may be difficult. The answer also depends on whether EPA will use a "closed
path" method or an "open path" method.
For a "closed path" method, as long as the light extinction measured at a location is highly
correlated with the actual visibility impairment occurring at the sight path, I think it is
acceptable. Therefore, it may not necessarily be a range of probe height that is important. What
is important may be the lack of too strong or too local source impacts, or "dry spots" mentioned
above, around the monitor so that the light extinction at the site is still highly correlated with the
visibility impairment relevant along the sight path to the valued urban scene. Thus, it is possible
that we may still be able to use existing air quality sites for light extinction measurements. We
just need assurance (data) that they correlate well with the relevant visual impairment. The issue
then becomes how high correlation is acceptable.
For an "open path" method, there is less excuse for the location to be away from the actual sight
path to the "valued urban scene". In this case, the probe height can be in any height where
people actually observe the urban scenes.
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Dr. Donna Kenski
These comments on the White Paper are more general than EPA requested in its charge
questions. Regretfully, I lack expertise in the specifics of visibility monitoring and equipment.
However, there were aspects of the White Paper that raised some concerns that 1 want to express.
The White Paper did a fine job laying out some of the issues that need to be discussed at the
upcoming meeting. Nevertheless, the discussion on FRMs/FEMs and measurement goals seems
quite premature and 1 fear that EPA is rushing to establish a secondary standard and
accompanying monitoring method without laying the appropriate groundwork or considering
reasonable alternatives. While the Visibility Assessment made a solid case for determining what
visibility conditions are acceptable to the public, it did not provide any discussion on the level,
form, or averaging time of a potential standard. The White Paper seems to presume that a
standard would be set in terms of light extinction and measured at hourly intervals, and that those
measurements should be made with a combination of nephelometers and aethalometers, which
are to be operated in ways not currently accepted as standard practice (i.e., without drying the
sample stream before measurement). While that MAY be the optimum way of determining light
extinction, I believe it is possible to propose a standard that is protective of visibility and yet
does not require rolling out hundreds of untested monitors at a time when states are struggling to
maintain existing criteria networks and meet new monitoring regulations that are inadequately
funded. EPA needs to more carefully examine the alternatives, and take a more inclusive view
of methods. Specifically, methods based on mass measurements should be included for
consideration - both total PM2.5 and speciated PM2.5. Long path instruments and photographic
methods also seem to have been dismissed without adequate discussion of their potential
advantages and disadvantages. The measurement goals, as laid out in the White Paper, are much
too restrictive at this point in the process. Our need to measure visibility with accuracy and
precision should not supersede a common sense approach that tolerates greater measurement
variability but yields significant benefits through ease of use, dependability, and economy. A
standard that protects visibility could be posed in a number of ways that take advantage of
existing networks and data—just one of which might be light extinction as calculated from the
IMPROVE equation from speciated PM2.5 data (as done in the Visibility Assessment) or from
hourly PM2.5 mass measurements and RH. The hourly PM mass measurements could be
incorporated into a sub-24-hour but more than 1-hour standard to help smooth out the greater
variability in these measurements. Along those lines, the White Paper and Visibility
Assessment seem to put undue weight on the coarse mass contribution to visibility impairment.
From the data presented there (with the possible exception of Phoenix) coarse mass contributes
little to visibility impairment and its presence could generally be ignored, or perhaps
incorporated only when it is a significant fraction of total PM as established by historic data.
1 urge EPA and CASAC to carefully consider the pragmatic aspects of a visibility standard.
Visibility has been determined for years from speciated and mass measurements of PM. Neither
the Visibility Assessment document nor the White Paper make a strong case for discarding this
time-tested and practical method. It simply is not feasible, in our current economic climate, to
consider requiring states to implement a new network of monitors without first showing
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decisively that such equipment is vastly superior to alternatives. By all means, new technology
should be encouraged, but it needs to be thoroughly vetted in the field. It has been my
experience that technologies which appear promising in the lab almost inevitably exhibit
significant flaws when deployed under real-world monitoring conditions. Thus any proposed
FR.M/FEM technology must be first be demonstrated in a pilot study that compares its
performance with these older, time- and field-tested technologies. It seems unlikely that such a
study could be completed in this review cycle. Consequently any secondary standard proposed
as part of this review should not require measurements that meet the very tightly prescribed goals
in the White Paper, but rather allow for extinction to be measured or estimated from the data
being collected now as part of the PM2.5 network.
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Dr. Peter McMurry
EPA staff are recommending that extinction (scattering + absorption) be measured by using
integrating nephelometry to measure scattering and filter light transmission to measure
absorption. I am writing to provide my opinions on those recommendations.
Suitability of Nephelometer and alternative approaches
1.1 support the idea of independently measuring scattering and absorption, since this provides
information on types of species that contribute to extinction.
2.1 support the use of point measurements rather than the use of long path transmissometers,
especially if they are co-located at speciation measurement sites. This will allow more detailed
analyses on the contributions of different species to extinction. Also, point measurement methods
allow for the use of PM10 inlets, so that the optical properties of particles smaller than 10 urn
aerodynamic diameter can be measured. Long path measurements do not allow for that
possibility.
3.1 support the use of integrating nephelometers. They have been used for more than 30 years,
and have provided valuable information on aerosol scattering coefficients. Their limitations are
quite well understood, although not always easy to overcome.
4.1 have reservations about the use of filter transmission measurements to measure absorption
coefficients. On the positive side, commercial instruments are available and have been used
extensively. Furthermore, they provide data with high time resolution, enabling analyses in
concert with scattering data from integrating nephelometers. Concerns include:
(i) The optical properties of particles deposited on filter substrates are different from the
optical properties of airborne particles because morphology and mixing characteristics
are altered by deposition onto filter surfaces. Also, filter transmission is affected by
multiple scattering within the filters. For example, Cappa et al (2008) found "the
presence of this OA [organic aerosol] in an external mixture of absorbing aerosol and
OA can cause an increase in the light absorption measured by the PSAP, relative to that
measured by the PAS [photoacoustic aerosol spectrometer], by more than a factor of
two."
(ii) Laboratory studies (Zhang et al. 2008; Khalizov et al. 2009; Lack et al. 2009; Murphy
2009), field studies (Lack et al. 2009) and modeling work (Nessler et al. 2005) have
shown that the absorption coefficients of soot are altered by transparent coatings of
materials such as sulfuric acid and oily organic compounds. This sensitivity arises in
three ways: (i) transparent liquid coatings lead to enhanced absorption, (ii) hygroscopic
coatings absorb water, thereby affecting the amount of transparent material condensed
on the absorbing particles, and (iii) changes in morphology due to the
evaporation/condensation cycles of water onto hygroscopic particles lead to more
compact structures that are more absorptive. These observations are based primarily on
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measurements that have been carried out on gasbome particles using cavity ring down
or photoacoustic spectrometry. It is highly unlikely that accurate information on
relative humidity-dependent absorption could be measured with by filter light
absorption, yet the proposed standard requires accurate measurements in the 10%-
90% RH range.
The types of measurement errors mentioned above are the source of my reservations regarding
the use of filter transmission methods for a secondary standard. I think it is likely that filter
transmission instruments would eventually be replaced with instruments designed for in-situ
measurements. It would be unfortunate to make a major investment in instrumentation that will
be replaced. An alternative might be to use measurements of BC or EC to estimate absorption
coefficients. This would not be a good long-term solution since those are not optical
measurements and they do not provide adequate time resolution. However, EC/BC data are now
available at speciation sites.
Potential improvements ot commercial nephelometers and future alternatives.
1. The White Paper discusses uncertainties regarding sampling efficiencies of instruments
such as nephelometers for particles up to 10 um. This is an important question that needs
to be resolved.
2. In-situ measurement methods that include cavity ring down spectrometers and
photoacoustic spectrometers (PAS) have been studied extensively in recent years. Cavity
ring down spectrometers measure extinction, while PAS measure absorption. In-situ
measurements of aerosol optical properties are likely to be more accurate than filter
transmission methods. However, in-situ techniques are also prone to measurement
artifacts. For example, Moosmuller et al (2009) state that "these [ in situ] methods may
suffer from some interference due to light-induced particle evaporation." Murphy (2009)
found that evaporation of water from coated soot particles reduces the photoacoustic
effect, leading to measured values of absorption coefficients that are below true values.
To avoid such errors, Murphy recommends that water be removed prior to measuring
absorption coefficients, which is inconsistent with EPAs recommendation that optical
properties be measured at ambient RH. In ambient aircraft measurements, Strawa and
coworkers (2006) reported good agreement (2%) for extinction measured by cavity ring
down and by nephelometry and filter transmission. Use of their "reciprical nephelometer"
also enabled them to obtain scattering coefficients that agreed with values measured with
the TSI nephelometer to within 2%.
Given the fundamental limitations of filter transmission methods for determining absorption
coefficients, the long-term goal should be to adopt in-situ measurement methods. 1 would prefer
to see the agency focus on supporting the development of such methods rather than investing in
methods that we know to be fundamentally flawed. Because single scattering albedos are
typically in the 0.8-0.9 range, scattering dominates extinction. Delaying the measurement of
absorption until a better instrument is available might not be a bad tactic. A great deal of
research is underway to develop better in-situ measurements of absorption. This work is driven
primarily by the need of understand absorption so as to better quantify the effects of aerosols on
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the earth's radiation balance and on local and regional precipitation patterns. Hopefully, those
efforts should lead to better commercial instrumentation in the near future.
The White Paper discusses the possibility of sponsoring an "invitational measurement
intercomparison study." The cost would be limited if "instrument manufacturers covered their
own cost to participate." // is important that EPA support the participation of researchers with
prototype instruments in such a workshop, especially for in situ measurements of absorption
coefficients
References Cited:
Cappa, C. D., D. A. Lack, J. B. Burkholder and A. R. Ravishankara (2008). "Bias in filter-based
aerosol light absorption measurements due to organic aerosol loading: Evidence from
laboratory measurements." Aerosol Science and Technology 42(12): 1022-1032.
Khalizov, A. F., H. X. Xue, L. Wang, J. Zheng and R. Y. Zhang (2009). "Enhanced Light
Absorption and Scattering by Carbon Soot Aerosol Internally Mixed with Sulfuric Acid."
Journal of Physical Chemistry A 113(6): 1066-1074.
Lack, D. A., C. D. Cappa, E. S. Cross, P. Massoli, A. T. Ahern, P. Davidovits and T. B. Onasch
(2009). "Absorption Enhancement of Coated Absorbing Aerosols: Validation of the
Photo-Acoustic Technique for Measuring the Enhancement." Aerosol Science and
Technology 43(10): 1006-1012.
Lack, D. A., P. K. Quinn, P. Massoli, T. S. Bates, D. Coffman, D. S. Covert, B. Sierau, S.
Tucker, T. Baynard, E. Lovejoy, D. M. Murphy and A. R. Ravishankara (2009).
"Relative humidity dependence of light absorption by mineral dust after long-range
atmospheric transport from the Sahara." Geophysical Research Letters 36.
Moosmuller, H., R. K. Chakrabarty and W. P. Arnott (2009). "Aerosol light absorption and its
measurement: A review." Journal of Quantitative Spectroscopy & Radiative Transfer
110(11): 844-878.
Murphy, D. M. (2009). "The Effect of Water Evaporation on Photoacoustic Signals in Transition
and Molecular Flow." Aerosol Science and Technology 43(4): 356-363.
Nessler, R., E. Weingartner and U. Baltensperger (2005). "Effect of humidity on aerosol light
absorption and its implications for extinction and the single scattering albedo illustrated
for a site in the lower free troposphere." Journal of Aerosol Science 36(8): 958-972.
Strawa, A. W., R. Elleman, A. G. Hallar, D. Covert, K. Ricci, R. Provencal, T. W. Owano, H. H.
Jonsson, B. Schmid, A. P. Luu, K. Bokarius and E. Andrews (2006). "Comparison of in
situ aerosol extinction and scattering coefficient measurements made during the Aerosol
Intensive Operating Period." Journal of Geophysical Research-Atmospheres 111(D5).
Zhang, R. Y., A. F. Khalizov, J. Pagels, D. Zhang, H. X. Xue and P. H. McMurry (2008).
"Variability in morphology, hygroscopicity, and optical properties of soot aerosols during
atmospheric processing." Proceedings of the National Academy of Sciences of the United
States of America 105(30): 10291-10296.
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Mr. Rich Poirot
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. \Vavelengthof550nm
This (wavelength of maximum human visual sensitivity) is a reasonable choice, but it may
not be critically important to focus exclusively on one wavelength, if instrumental responses
to other wavelengths or ranges of wavelengths can be obtained and reasonably scaled to
represent human perceptual responses. Scattering and absorption Information for other
wavelengths could also provide useful information on particle size distributions and/or
composition.
b. Aerosol size fractionation at PM10
This is not an unreasonable choice, although off-hand I can't think of any logical reason to
specify 10 microns as an upper size limit. Also, I think the arguments to include (or attempt
to approximate) coarse particle light scattering (and/or coarse particle adsorption) in this
indicator may be overstated. Coarse particles will make relatively minor contributions to
light extinction at most locations - and especially at locations likely to exceed a secondary
standard (which are not likely to be Phoenix). For example, a PM extinction level of 100
Mm"1, about mid-range of what's been proposed for a new secondary standard, would be
exceeded by 25 ug/m of non-hygroscopic PM2 5 organic matter (30% below the level of the
current PM25 standard). But it would require a PMio-25 concentration of 170 ug/m3 - well
above the level of the current PMio standard - to contribute a similar level of extinction. If
the 25 ug/m3 of PIVhs was hygroscopic ammonium sulfate at 80% RH, the PM extinction
would exceed the upper end of the range being considered, contributing over 200 Mm"1,
which would require over 325 ug/m3 of PM 10-25, more than double the current PMio standard,
to produce a similar optical effect.
Attempts to include coarse particle scattering &/or adsorption in the indicator also add
substantial measurement challenges and will cause instrument maintenance problems.
Neither nephelometers nor aethalometers respond with the same efficiency to coarse particles
than to fine ones, so simply employing a PMio inlet is not an ideal approach, and doubling
the number of samplers or employing periodically switching, size-fractionating heads adds
substantial costs and/or complexity. There will also be added maintenance costs if PMio
heads are used on nephelometers, especially in humid, urban environments. Conceivably,
methods measuring scattering or scattering plus adsorption from fine particles only could be
specified except for locations where the coarse/fine ratio exceeds X. Possibly also,
continuous PMio (&PM2s) mass measurements could be used if/where needed to estimate
coarse scattering, with an assumption of no hygroscopic growth, as most coarse particles are
hydrophobic, except (natural) sea salt.
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c. Operation at ambient relative humidity
This is a reasonable choice if measuring optical effects at under ambient air conditions is the
objective. However, a PM extinction indicator does not necessarily need to be constrained to
ambient RH conditions, and might well be defined in terms of "at 70% RH", at "70% RH or
less", or "at ambient conditions but only considering hours of less than 70% RH", etc. As
with coarse particles, some maintenance issues might be avoided if highly humidified
aerosols (droplets) were excluded from samplers. If it were feasible, valuable added
information content would be provided if both wet & dry scattering - at various RH levels
below ambient - could be obtained.
d. Overall accuracy and precision < 10%
This is not unreasonable, but seems difficult to justify given the very wide range of combined
levels + forms currently being considered as appropriate for a secondary NAAQS - from as
low as 64 Mm"1,98th percentile to as high as 191 Mm"1 90th percentile. That seems like a
very wide range for which to require both accuracy and precision of <10%. Accuracy will be
impossible to determine for "ambient aerosols" and will most likely be limited to laboratory
testing using surrogates.
e. Range of conditions from 10 Mm"1 to 1000 Mm"1
This is not unreasonable, but may be overly restrictive as it requires a low end well below the
64 Mm"1 to 191 Mm'1 range currently being considered. Also, if a very lenient form like 90th
%tile is employed, the standard is more based on a "counting" metric than on a precise
minimum threshold.
f. Valid measurements (with all other appropriate checks) when sampled at < 90%
relative humidity
This is reasonable, although, as indicated above, 90% RH may be an unnecessarily high
upper RH bound, and alternative and possibly more effective regulatory metrics might be
considered for visibility effects at (lower) RH levels "below XX% RH" or even
"standardized to YY% RH". Such lower RH limits might also reduce maintenance problems
and increase data capture efficiency (within more narrowly constrained RH limits). A lower
RH limit will also reduce effects of measurement errors or occurrences of spatially varying
RH, where higher RH or even fog may occur within the sight path but not at the monitor.
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
An hourly averaging time is not unreasonable, given the nearly instantaneous nature of
human perception of impaired visibility. However, longer averaging times - such as 4 to 8-
hour daylight averages, might make for a much more stable regulatory metric, place less
emphasis on early morning water, and allow use of a wider variety of instruments for which
1 -hour data can be noisy.
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b. Instrument specific parameters such as angular integration for nephelometers?
This may be necessary, but my preference would be to avoid instrument-specific
specifications, if possible, and as indicated above, 1 think coarse particles and their related
angular truncation issues might more efficiently be eliminated from the regulatory metric,
rather than compromising the method to accommodate a relatively unimportant influence.
c. Calibration with a gas that has known Rayleigh scattering properties.
1 don't know of viable alternatives, although it would be desirable (for this and other uses) to
have a standard "aerosol in a can" for calibration and audits. Also, a scattering gas tells
nothing about light absorption.
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 staffs position that currently
available nephelometer light scattering and filter transmission light absorption
measurement instruments are suitable to meet the light extinction goals?
A nephelometer or combination of nephelometer + aethalometer would seem to be the
methods that come closest to being considered both readily available and also suitable for use
in routine network operations. That being said, I don't think either method is fully ready for
deployment in routine network operations and that both methods would have problems
associated with the proposed PMio size limit. It may be a very costly undertaking to require
both methods and also to accommodate differences in fine and coarse particle responses.
While the potential use of these semi-standard methods should be evaluated, I would not like
to see these methods pushed to the exclusion of other optional approaches, especially as there
appear to be several promising techniques on the horizon (and lingering bugs in the "standard
methods").
b. What are the Subcommittees thoughts on alternative instrumental approaches that
should be considered to meet the light extinction goals?
The currently-stated "light extinction goals" should not be viewed as set in concrete, and it
might be productive to consider measurement goals and potential methods together. For
example, an indicator based on fine particle scattering and absorption or just fine particle
scattering could be very protective of visibility most times and places (especially where/when
impairment is greatest), would avoid coarse particle measurement and maintenance issues
and substantially reduce costs. Along similar lines, a sub-daily 4 to 8-hr PMj 5 mass
indicator such as was recommended by both EPA staff and CAS AC in the last review cycle
could certainly be protective of visibility, and could be measured by existing continuous
instrumentation, with little or no added cost. If need be, a generic "mixed aerosol f(RH)
function" could be developed (perhaps on a regional basis) and used to "humidify" the fine
mass to make it more extinction-like . There would be some uncertainty in such an
adjustment, but I think it would be rather small compared to the difference between 20 dv
98th percentile and 30 dv 90th percentile, which is currently being considered as the range
within which a standard might be selected. Conversely, a generic mixed aerosol f(RH)
adjustment might be applied (backward) to extinction levels within the 20 to 30 dv range of
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extinction considered adverse to select a reasonably protective level of fine particle mass -
perhaps averaged over 4 to 8 daylight hours - that would be beneficial for visibility,
measurable by current networks, and which would substantially reduce the large East/West
differences that would characterize a wet extinction indicator- if the Agency finds id
necessary to require a "threshold-based form", rather than a "progress-based form" of a
secondary standard as was recommended by the CASAC PM panel.
I would also like to see the existing, widely deployed Belfort 6230a (or other) foreword
scatter meters - widely employed at nearly 1000 FAA, NWS or DOD sites in the US (and
many more worldwide) - at least considered in this pilot evaluation process, and use of the
extensive existing measurement network (though there are many limitations) should be given
some consideration. Indeed there could be substantial benefits for both aviation safety and
aerosol visibility protection if EPA and NOAA could cooperate more closely in the
collection, processing, archival and redistribution of these valuable data. At a minimum, an
effort should be made to make a substantial subset of these data available in their raw
uncensored and un-binned form. In the recent past (2003-05?), Sonoma Technology, with
modest funding support from EPA, used to provide such raw ASOS data (complete with an
inverse f(RH) function to estimate PM2 5 mass) as part of the AIRNOW program. It would
be very useful to resurrect something like this as a component of or complement to an urban
visibility pilot network.
Lastly, some consideration should be given to automated digital camera techniques, from
which optical extinction estimates can be extracted, which may be crude compared to more
precise (point) measures, but which can provide integrated long path information on
combined effects from scattering & absorption, fine and coarse (over many different parts of
a scene) and would have obvious huge advantages as a public communication (and future
assessment) tool. Conceivably, photo-derived extinction estimates might initially be used to
determine compliance with a relatively lenient secondary standard, and then support
establishment of refined "local" target visibility goals or rates of progress in the
implementation phase.
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.
Others on the panel will have much better suggestions than I can offer here. Generally,
nephelometers currently used as PM mass monitors are heated and/or cut (at 2.5 um) and are not
necessarily likely to be the best starting points for measuring fine and coarse scattering under
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ambient conditions. Currently deployed aethalometers would benefit from improved guidance
for more standardized operating and data processing procedures.
b. If applicable, what are the Subcommittees suggestions for improvement of alternative
instrumental approaches for use in future routine monitoring applications?
Others on the panel will have much better suggestions than I can offer here. Although
transmissometers do not seem to be advocated in the white paper, they have for whatever reasons
been utilized for light extinction measurements to determine compliance with visibility standards
in the few locations (Denver and Phoenix) where standards have been developed. As such, a
more careful consideration of this approach, and possible ways to improve upon it (or to rule it
out) seems warranted. The various photoacoustic, CDR and CAPS methods all seem promising
but (absent EPA encouragement) there doesn't seem to be much incentive to develop them
further in the near-term. Perhaps one of the sites in a pilot exploratory urban visibility network
(or a few small laboratory research grants) could be used to enhance and further evaluate these
evolving methods. It would also be useful to evaluate the extent to which the presumably
superior approach of nephelometer + aethalometer with PMio heads approach can be
demonstrated to provide a superior, visibility-relevant regulatory metric to that which might be
provided by using existing hourly PM2 5 mass, airport ASOS or camera techniques.
Questions Regarding the Establishment of Specifications and Procedures for
Approval of Federal Reference Methods (FRM's) and Federal Equivalent Methods
(FEM's).
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?
See below.
b. Specify the measurement principle(s), calibration procedure(s), and operational
performance requirements and demonstration procedures? What aspects of
performance should the standards cover?
See below.
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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?
See below.
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 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.
Other panel members will have more informed opinions on these questions. As 1 see it, it
does not make sense to propose a specific secondary PM light extinction NAAQS without
knowing clearly in advance and specifying a method by which it could be widely measured
(at urban, suburban and rural - all non-class 1 areas) throughout the country. At best, a
small, pilot exploratory urban visibility monitoring network may be in place at the time the
NAAQS will need to be promulgated. So unless some form of existing measurements - such
as continuous PM2 5 mass or ASOS visibility is employed it seems like a requirement for a
strictly defined PM light extinction indicator essentially pushes any secondary NAAQS
decision into the next PM review cycle, and renders many of these detailed questions on
performance standards, calibration methods and equivalent methods to be premature.
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.
Certain aspects of instrumental response, such as effects of varying temperature, RH, aerosol size
distribution and chemical composition and consistent responses to calibration gases or aerosol
mixtures can and should be evaluated in laboratory testing. However, since there has been very
limited experience in the consistent, long-term application of the proposed methods in the field
(outside of relatively clean Class 1 areas), there is a need to rigorously confirm that any proposed
methods will perform as expected under challenging and varied field conditions.
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?
As indicated above, I don't necessarily agree that a PM10 inlet is desirable. Attempting to
include coarse particle effects has several disadvantages and offers little payback. I would prefer
limiting to PM 2.5 or developing methods that might periodically switch back and forth from PM
2.5 to 10 inlets.
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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?
As indicated above, I think a PlV^s mass indicator, existing ASOS measurement or camera
techniques are all worth considering. Impaired visibility is/should be the most readily and
publicly perceptible effect of air pollution, and care should be taken to make any regulatory
metric based on it to be very simply and clearly communicable to the public. Conveying
(processed, uncensored) ASOS data and camera views of impaired visibility to the public in
n3ear-real-time via AIRNOW would be very useful.
9. 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?
Other panel members will have more informed opinions on this question. I think it is way
too early to be thinking about any FEM procurement advice to SLTs - if there were money
or methods - of which there are neither.
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?
No.
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?
No opinion.
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;
This is obviously important, especially given the many new (unfunded) monitoring
requirements (for source-specific Pb, source-specific SO2, roadside NO2, rural ozone,
etc. that EPA has recently imposed, and current state budget crises and hiring freezes,
and total lack of current extinction measurements. Consequently any approach like
PM2.5 mass or ASOS that could utilize existing measurements should be carefully
considered.
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ii. current availability;
Since it doesn't seem likely a new PM light extinction standard can actually be
implemented in this review cycle, current availability may not be critical.
Conversely, if a protective secondary standard is mandated by the court decision, then
priority should be given to using currently available measurements.
iii. record of successful field experience; and
Obviously more important for near-term deployment and less important for more
future applications.
iv. ability to generate supplemental information (e.g. multiwavelength
scattering/absorption, albedo, fonvard/backscattering, scattering polarization, etc.)?
Assuming a more distant future application of new methods and increasing future
importance for climate forcing analyses, such supplemental information is always
desirable, pending costs.
Questions Regarding Network Design and Probe and Siting Criteria
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?
I'm not sure the "maximum" impairment is necessarily the best/only focus. Maybe consider the
"most typically perceived" impairment, or that which most impacts scenic vistas, etc.
11. EPA and the State monitoring programs have an extensive historical dataset of PM2.5
mass and speciation measurements. In the Visibility Assessment Document, EPA used
existing PM speciation and mass data to evaluate visibility impairment at a single site in
each of IS 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 information from networks assessments 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.
Collocated PM mass and visibility measurements are highly desirable for assessing causality, as
a form of quality assurance and to facilitate public communication.
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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?
It seems likely that intra-urban variability should be greater for light absorption than for fine
particle scattering. To the extent that such variability is caused by individual sources or coarse
particles, 1 think it should be avoided rather than sought after, its spatial variability is likely too
complex to capture in routine monitoring networks. Conversely, perhaps those are the kinds of
situations where long (integrating) path measurements would be most appropriate.
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?
The least of our problems...
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 inlets, what should be the acceptable range for probe
height?
Good question. I don't know, but another reason some sort of path measurement might be
desirable.
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Dr. Warren H. White
Measurement goal for light extinction
The details of the measurement goal - the 550 nm wavelength and the 10 um size cut - create
unnecessary difficulties for the measurement.
The wavelength is the easier of the two to dismiss as an arbitrary choice. The specified
550 nm is, indeed, the approximate wavelength to which "humans are most sensitive". But it is
not the wavelength to which the eye is most exposed, which is shorter (box A). Nor is it the
wavelength carrying the most information from distant objects, which is longer (box B).
olor speclrnl irrfidiance and oBQO K Planck rurrtion
Fig. 1. The solar spectrum plotted in wavelength units peaks
near 500 nm. The luminous efficiency of the eye peaks at 560
nm. All three curves appear to peak near 500-560 nm, a
wavelength region generally perceived as being green.
A. B.H. Soffer and O.K.
Lynch (1999) American
Journal of Physics 67, 946-
953.
B. H. Horvath(1981)
Atmospheric Environment 15,
1785-1796.
Any instrument that is used for indirect visibility
determinations has a spectral sensitivity different to
the human eye. In order to compare readings by one of
the instruments and visibility observations, the wave-
length of 550 nm is generally used, since this is the
wavelength of maximum sensitivity of the human eye.
In the atmosphere the brightness difference of a distant
target is highest in the red, and smaller in the green.
Therefore the most important wavelength for visibility
is not at 550nm, but approximately at 580 nm, because
hcrcthe higher contrast of the target is more important
than the slightly smaller sensitivity of the eye (Horvath,
1975). This wavelength should be used for converting
telephotometer and ncphelometer data into visibilities.
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It would be no more arbitrary, in other words, to pick any other wavelength between, say.
520 nm and 580 as our measurement goal. But such flexibility would then allow the use of
lasers operating at 531 nm, as noted in Table 1 of the white paper. This, in turn, would open the
door to cavity ring-down and photo-acoustic instruments, which are directly based on physical
principles and many view as the preferred techniques of the future. Sure, Agency guidance can
always specify empirical Angstrom exponents with which data can later be translated to different
wavelengths, but why create the need for such "corrections" if we don't have to?
Like the 550 nm wavelength, the PMio size fraction is another arbitrary choice. Particles
larger than 10 um contribute very little extinction, but particles larger than 2.5 um don't
contribute all that much either. Consider the size-resolved extinction data in Box C, from
measurements in the arid Southwest: even within the dust mode, composed overwhelmingly of
particles larger than 2.5 um ("coarse"), half or more of the PMis extinction was from the sub-2.5-
um ("fine") tail.
C. W.H. White, E.S. Macias, R.C. Nininger and D Schorran (1994) Atmospheric Environment
28,909-921.
Table 3 Mass and scattering budgets for fine and coarse haze and dust
Spint Mountain, NV Meadview. A
Mass |°/.) Mass (%)
Haze Oust Sum Haze Dust Sum
Fine K-' 9'-1 38 Fine 22'2 IOiJ 32
Coarse T-' S6-* 62 Coarse 5i2 63-*2 68
Sum 351* 6S-* 100 Sum 27iJ 73lJ 100
Scan f any (%) Scattering (%)
Haze Dust Sum Haze Dust Sum
Fine 51 = ' 23-3 74 Fine 461* 22** 68
Coarse II-1 \f>'-' 7* Coarse 8*' 2dl! 32
Sum 62 = 4 38s* 100 Sum 54=» 46" 100
Values represent daytime (1100-1900) averages from April to September. 1989 Superscripts indicate the variability
introduced by varying estimates of Ihe haze and dust size distributions.
Fine and coarse mass concentrations were determined by filter sampling behind fine (0^,0<2.5^m) and total
(D.,,. < 1S /im) inlets. Haze and dust mass concentrations are inferred from measured fine and coarse mass concentrations,
based on estimated size distributions of haze and dust
Fine and coarse scattering coefficients were determined by nephelometry behind fine (DMIO<2.Sjim) and total
(unrestricted) inlets. Measured coarse scattering is doubled to correct for angular truncation error. Haze and dust
contributions are derived by regression of size-resolved scattering nn ha?e and Hiitl mats
Much of the extinction by particles greater than 2.5 um in diameter takes the form of
extreme-forward scattering. As the white paper notes, this is under-measured by all
nephelometers. (Just as forward-scattered light is less evident to nephelometers, it has less
impact on visibility under most viewing conditions.) The angular truncation error of
nephelometers will be well over the 10% target for any aerosol dominated by coarse particles,
and will be sensitive not only to the particles' size distribution, but also to their almost certainly
non-spherical shape. It is hard to see how the effects of minor design differences between
instruments could be accounted for by any manageable inter-calibration procedure.
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The 10 um cut-point was originally designed to reflect aerodynamic characteristics of the
upper respiratory system, a rationalization relevant to a health standard but irrelevant to
visibility. The other PM NAAQS size cut, at 2.5 um, is also grounded in a health rationale, but is
much better suited to optical monitoring. It delivers a fraction accounting for most of the
extinction but much less affected by sampling losses and measurement artifacts.
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Dr. Yousheng Zeng
General Comments:
EPA is considering either light extinction or a mass concentration based metric as the indicator
for the PM Secondary Standard. Although I agree that light extinction is a more direct
measurement of visibility degradation than a mass concentration metric, using light extinction
may bring more challenges in implementing the standard, particularly as it relates to the New
Source Review (NSR) program under the Clean Air Act.
One of the reasons to set an ambient air quality standard is to manage air pollution sources,
especially anthropologenic sources that can be managed. If EPA plans to implement the PM
Secondary Standard in the same way as current SC>2 Secondary Standard, a proposed source
subject to the NSR in attainment area will undergo a Prevention of Significant Deterioration
(PSD) review. As part of PSD review, the source will perform an air quality modeling analysis.
The modeling analysis will limit the mass emission rate so that the new source will not exceed or
contribute to exceedance of the NAAQS and PSD Increment. This linkage between the emission
limit and the NAAQS and PSD Increment will have to be on mass basis. If the PM Secondary
Standard (and presumably corresponding PSD Increment) is based on light extinction, EPA
presumably will establish an algorithm to convert the initial model output, which is mass
concentration (e g., ug/m3), to light extinction.
The algorithm to convert modeled PM concentration to light extinction must be part of the
regulations and enforceable. If this conversion is required by regulations anyway, it will be easier
to just set the PM Secondary Standard in ug/m3 rather than light extinction. Although using light
extinction as the indicator makes scientific sense, using PM mass concentration as the indicator
leads to the same policy and regulatory endpoint and brings multiple practical benefits. These
benefits include: (1) the same PM monitoring instruments (maybe with relative humidity
instrument added) can be used for both PM Primary and Secondary Standards - a significant cost
saving, (2) data needed for attainment designation may be derived from existing PM monitoring
instruments - shorten the designation time, and (3) both regulating and regulated communities
are already familiar with the PM mass concentration in ug/m3 - minimizing confusion and
training effort.
Using light extinction as the indicator may be advantageous if EPA allows use of different
algorithms in different geographic areas or under different environmental conditions. In that
case, there will be one light extinction standard across the country, but different corresponding
mass concentration levels depending on local conditions. However, this will further increase
complexity of the implementation related regulations.
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CQ 8 - What data and analysis does the Subcommittee believe EPA staff should have studied
or performed in establishing some kind of FRM (S.a-d) for use in regulatory decisions
and to help inform the public?
As part of the effort to establish FRM for light extinction measurement, EPA should conduct a
study on candidate methods at a broad wavelength band centered around 550 nm vs. the
monolithic wavelength of 550 nm (or 531 nm), or multi-wavelength vs. 550 nm. The study
should include not only instrument measurement, but also corresponding images for a sample of
population to view, similar to the urban visibility preference studies.
The wavelength is one of the multiple aspects in evaluating candidate methods for FRM. Other
aspects should also be studied such as relative humidity, aerosol size fraction, precision,
accuracy, and operability. In field studies, PM mass concentrations should be measured side-by-
side with candidate light extinction measurement methods. The comparative study between light
extinction measurement and PM mass concentration measurement is very important in
addressing the implementation issues discussed in the General Comments section above.
CQ 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?
If there is an urban scale heat island, the scale is actually consistent with the objective of light
extinction measurement, i.e., how people living in the city feel about visibility. In this case, there
is no need to avoid urban heat island.
As long as the monitoring site is not in close proximity of a large heat source, this does not seem
to be a major concern. Typical monitoring sites have sufficient distance from major sources.
Localized hot air moves upward and have less impact to the measurement. The ground surface or
surface of the structure where light extinction instrument stands may have some influence on
localized relative humidity change, eg., a concrete pad below the instrument may cause
localized heating and reduction in relative humidity. This type of surface should be avoided,
especially if the sample inlet is low. Other significant heat sources, such as discharge vent of a
large building ventilation system.
A site near water body may not be representative. Some industrial stationary sources also emit a
lot of water vapor. EPA may consider some dispersion modeling study to establish distance
guideline, heat output/distance guideline, or even heat output/moisture output/distance guideline.
CQ 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 sight
path, aerosol mixing, the goal ofPM light extinction measurements, site logistics, and
the location of other air monitoring equipment inlets, what should be the acceptable
range for probe height?
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From the viewpoint of the purpose of light extinction measurement, sight path should be the
most important factor in probe height consideration. For this reason, the probe height should be
higher than typical PM mass concentration monitors. A higher probe will also minimize near
ground impact of large particles. A height comparable to a typical met tower at a monitoring
station (e..g., 10 m) seems a good starting point. To minimize aerosol composition change, the
distance from the probe intake to actual measurement chamber or filter should be as short as
possible and as straight as possible.
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