August 7, 1996
EPA-SAB-CASAC-LTR-96-009
Honorable Carol M. Browner
Administrator
U.S. Environmental Protection Agency
401 M Street SW
Washington, DC 20460
Subject: Report of the Clean Air Scientific Advisory Committee (CASAC) Technical
Subcommittee for Fine Particle Monitoring
Dear Ms. Browner:
Enclosed is a report of the (CASAC) Technical Subcommittee for Fine Particle
Monitoring. The Subcommittee was formed following the December 14-15, 1995, CASAC public
meeting on the particulate matter criteria document and staff paper. In discussions during that
meeting it became clear that there were substantial concerns in the scientific community about
the EPA's planned path for development of reference and equivalent methods for monitoring
PM25. In response to those discussions, EPA staff proposed formation of a CASAC
subcommittee to provide technical input about fine particulate monitoring.
The Subcommittee and EPA staff held a public meeting with significant public input on
March 1, 1996. At the meeting there was significant progress toward a consensus concerning a
reasonable approach to fine particulate monitoring, but important questions and technical
concerns remained. After clarifications from EPA staff and development of a new approach to
setting priorities for monitoring to optimize protection of public health from excessive
concentrations of fine particulate matter, the Subcommittee has reached a consensus on its
recommendations contained in the enclosed report.
The membership of CASAC has reviewed the Subcommittee's report and endorses it. I
want to join the Subcommittee in thanking the EPA staff for the openness with which they have
reviewed PM2 5 monitoring issues with the Subcommittee. I believe that EPA's responsiveness to
the Subcommittee's review and continuing responsiveness to scientific review of proposals for
fine particulate monitoring are important in development of possible new fine particulate
standards.
Sincerely,
Dr. GeorgT. Wolff, Chair
Clean Air Scientific Advisory Committee
Attachment
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August 1, 1996
Dr. George T. Wolff, Chair
Clean Air Scientific Advisory Committee
Science Advisory Board
U.S. Environmental Protection Agency
401 M Street, SW
Washington. DC 20460
Subject: Report of the Clean Air Scientific Advisory Committee (CASAC)
Technical Subcommittee for Fine Particle Monitoring
Dear Dr. Wolff:
A CASAC Technical Subcommittee for Fine Particulate Monitoring (the
Subcommittee) was formed in response to discussion at the December 14-15, 1995
CASAC meeting to review the Particulate Matter Criteria Document. The
Subcommittee was established to provide advice and comment to EPA on appropriate
methods and network strategies for monitoring fine particles in the context of
implementing a possible revised national ambient air quality standard (NAAQS) for
particulate matter (PM). The Subcommittee held a public meeting March 1, 1996 in
Chapel Hill, North Carolina. In response to discussions and input from the
Subcommittee and the public at this meeting, EPA staff prepared and on March 19,
1996 provided to the Subcommittee a table titled Summary of Key Fine Particle Federal
Reference Method Issues/Recommendations. In response to questions from a number
of Subcommittee members the EPA staff revised and expanded the table and
redistributed it to the members on April 14, 1996.
In a letter dated February 12, 1996, Mr. John Bachmann, Associate Director for
Science/Policy in the Office of Air Quality Planning and Standards (OAQPS), asked for
advice and comment on appropriate methods and network strategies for monitoring
PM25 and specifically listed a number of key issues:
a) the appropriateness of the overall approach taken for the federal
reference method (FRM) [for fine particulate matter (PM2 5)],
b) the efficacy of a hybrid-design approach for achieving precision and cost
savings,
c) the relation of the measurement method to those that have been used in
the key epidemiological studies,
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d) the appropriateness of the recommended sampling frequency (i.e., daily
as opposed to one-in-six day sampling for core fine particle monitoring
sites and less frequently at supplementary monitors),
e) that appropriateness of using actual temperature conditions for reporting
instead of standard temperature and pressure,
f) the appropriateness of the recommendations for monitor placement with
an emphasis on community-wide exposure,
g) the extent to which the approach, as implemented by State and local
agencies, would provide reliable information on the status of ambient fine
particle concentrations with respect to a potential national ambient air
quality standard (NAAQS) for fine particles, and
h) other technical issues such as the need to use a particular dichotomous
sampler head for maintaining wind speed independence (aspiration
efficiency), the desirability of sample heating, and the desirability of
having a method that is within the public domain.
The Subcommittee started with the understanding that it was to assume that the
size cut to be used to differentiate coarse-mode and fine-mode particulate matter was
an aerodynamic diameter of 2.5 //m.
Concerning the appropriateness of the overall approach taken for the FRM,
while several of the Subcommittee members would prefer to see a reference method
that defined PM25 as the in situ PM mass below a 2.5 //m pre-collector size cut, there
are serious practical difficulties with attempting to measure that quantity. Under the
circumstances, EPA has made an appropriate choice to establish a good practice
standard for filter sampling and analysis technology. The Subcommittee expects that
such a specified mass concentration can be measured reliably and reproducibly. The
requirement for a monitoring method that produces an analyzable sample is
appropriate. Analyzable samples will often be needed to deal with exceptional events,
with identification of sources for designing control strategies, and for determining
coarse mode intrusion into fine mode samples. Since the recent epidemiological
studies have used a variety of methods with different performance characteristics, no
one FRM can match them all; however, it matches most in the choice not to use a more
complex design that includes denuding and backup filtration to improve the sampling of
the ambient particulate nitrate compounds.
The Subcommittee understands the U.S. Environmental Protection Agency's
(EPA's) frustration about the lack of agreement among the reference and equivalent
methods for measuring particulate matter regulated under the current NAAQS for
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participate matter with an aerodynamic mass median diameter smaller than 10 //m
(PM10). We agree with EPA's desire to avoid the problems resulting from the current
situation in which reference and equivalent methods do not produce comparable
results. The Subcommittee understands that this frustration has led to EPA's plan to
proceed with a hybrid design and performance specification for a reference method.
We conclude that EPA's plan to designate a reference monitoring method for field use
to evaluate the acceptability of other PM2 5 monitoring methods under actual field
conditions is appropriate.
The Subcommittee's recommendation for the most appropriate procedure for
obtaining comparability of results from reference and equivalent method sampling for
PM25 would be the following sequence: a) development of an appropriate performance
specification tight enough to limit comparability problems; b) an open competition to
develop a reference method; c) testing in the laboratory and the field under a variety of
geographic and meteorological conditions those prototypes that make a prima facie
case that they have met the performance criteria; d) choice of a "reference sampler"
from among the submitted prototypes that best meets the performance criteria,
including evaluation of field use criteria; and e) designation of equivalent methods, with
the possibility that some may be restricted geographically or otherwise to account for
differing conditions.
If EPA rejects this recommendation and continues with the development of a
hybrid design and performance reference method, we urge EPA first to demonstrate the
repeatability and precision of the reference method by testing colocated samplers.
Further, we urge EPA to build into the process enough time for independent agencies
and interested parties to test the proposed reference method monitor in a variety of
geographic and meteorological conditions before the final rule is issued. This
Subcommittee would be an appropriate group to review and critique EPA's testing and
evaluation program. In any case, the sharpening of the PM25 size cut for the FRM to
achieve 5g < 1.2 after accounting for losses is appropriate.
If, as the Subcommittee understands, EPA staff are working only on a single-day
FRM sampler and any development of multi-day samplers will be left to the private
sector, that arrangement is a reasonable choice. Only after there has been an
opportunity for testing to determine the precision and field performance of the proposed
FRM would there be a meaningful opportunity for public comment and for review by this
Subcommittee of the proposed performance requirements for equivalent instruments.
The proposed plan to certify geographically limited equivalency if a sampler or monitor
performs acceptably in some areas but not in others is reasonable and appropriate.
The proposed requirement that the sampler be maintained between ambient
temperature and 3' C above ambient temperature during sampling and until the sample
is removed is a major step forward. It will preserve more of the semivolatile compounds
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than most current gravimetric methods retain. We look forward to reviewing the
proposed sample handling and quality assurance requirements that will be necessary
to complement this requirement.
At the March 1, 1996 public meeting EPA's proposal was to require some daily
sampling in all areas except ones with PM concentrations far below the standards. The
argument has been made that daily FRM sampling provides additional protection of
public health. Since the data from FRM sampling will not be available immediately, it
cannot provide the basis for emergency emission controls or of timely public
notification. Since EPA's analysis projects approximately a 5-to-7 year lag time
between collection of FRM or equivalent method gravimetric data and implementation
of source controls to reduce PM25 concentrations, the public health advantage of daily
FRM sampling appears minimal at best. The needs for public notification about
excessive PM25 concentrations and for managing episodes require real-time
information about PM25 concentrations, not daily filter samples.
An improved proposal that has been put forward since the March 1st meeting is
that, where an appropriate continuous monitoring method or a method that reports data
every hour or so is available, EPA allow the use of such a method supplemented by
every nth. day gravimetric sampling. This proposal is a substantial improvement over
requiring daily reference method sampling, which would consume large amounts of
resources for samplers and sample handling. That drain on already limited state and
local resources would strongly inhibit the use and further development of real-time
monitoring technologies. The alternative proposal would save resources that would
have had to go into daily gravimetric monitoring and would encourage development
and application of improved monitoring technologies. It would also provide gravimetric
data to which to compare the time-resolved data. If the continuous monitoring and the
reference method agree within certain limits, the continuous monitoring method could
be designated an equivalent method for the area. A less restrictive requirement would
be appropriate for pollutant standard index (PSI) reporting and public notification about
elevated PM25 concentrations.
There have been suggestions that more frequent or daily sampling should be
required when PM2 5 concentrations are near either the annual average or the 24-hour
standard. The rationale is that the increased sampling frequency will help resolve the
question of whether the area is attainment or nonattainment by increasing the accuracy
of the estimates of the annual average PM2 5 concentration and the distribution of high
24-hour PM2 5 concentrations. For areas that are near either an annual or a 24-hour
standard, year to year variations in concentrations can easily cause the areas to flip
into and out of attainment. The problems associated with such an area's flipping
between attainment and nonattainment are primarily administrative problems that
cannot be solved by improving the accuracy of estimating mean values and
distributions.
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At the May 17, 1996 CASAC public review of the OAQPS Staff Paper, the
members of CASAC who spoke to the subject strongly recommended that the statistical
form of a short-term fine particulate matter standard be a percentile standard or another
similarly robust form, not an annual second-high standard. If EPA follows this
recommendation, there will be less of a need for daily FRM sampling to determine
whether an area is in attainment or nonattainment for the 24-hour PM2 5 standard.
The question of reporting PM25 concentrations using actual temperature and
pressure rather than standard temperature and pressure involves a moderate change
in the stringency of a standard at high altitude. The question of the theoretical
appropriateness of the choice is related both to atmospheric physics and to
physiological and toxicological considerations. Since the latter are outside this
Subcommittee's areas of expertise, a larger panel would be needed to provide
integrated advice on this matter. On the other hand, the wide range of opinion about
the appropriate levels of the standards makes this consideration a comparatively small
one that can be handled appropriately in EPA's proposing and setting a PM25 standard.
The proposed requirement for monitoring to identify the contribution of transport
and regional PM25 levels to urban PM25 is appropriate. The Subcommittee agrees that
there can be a major fraction of PM2 5 that is relatively uniform over regions, but a
number of us are uncomfortable with the suggestion that PM25 varies fairly smoothly
and is relatively uniform over substantial parts of urban areas. The question of the
appropriateness of monitor placement with an emphasis on community-wide exposure
is difficult. It would be appropriate to base decisions that have community-wide impact
on monitors that represent community-wide PM25 and to base decisions that have
region-wide impact on regionally representative monitors. That approach does not,
however, deal with the problem of high public exposures that can be caused by some
point sources and by some localized area sources. Data from monitors that measure
localized public exposure to high PM25 concentrations are inappropriate for making
decisions about area or regional controls.
The Subcommittee encourages EPA to give careful consideration to harmonizing
the definition of PM10 with internationally accepted definitions of thoracic particulate
matter. Since some agencies and persons may choose to monitor for PM2 5 and PM10_2 5
using a single sampler, there is an advantage to accomplishing this harmonization
before the samplers are designed and built. The decision whether to grandfather
existing PM10 samplers is an appropriate one for EPA to make.
On behalf of the Subcommittee, I want to thank EPA staff for the openness with
which they have reviewed PM2 5 monitoring issues with us. We look forward to
reviewing the detailed monitor siting guidance, the equivalency determination
procedures and specifications, and the sample handling requirements when they are
available.
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Sincerely,
James H. Price, Jr., Ph.D., Chair
Technical Subcommittee for Fine
Particle Monitoring,
Clean Air Scientific Advisory Committee
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