[Federal Register Volume 63, Number 24 (Thursday, February 5, 1998)]
[Rules and Regulations]
[Pages 6032-6037]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 98-2878]
[[Page 6031]]
_______________________________________________________________________
Part VI
Environmental Protection Agency
_______________________________________________________________________
40 CFR Part 50
National Ambient Air Quality Standards for Particulate Matter; Final
Rule
Federal Register / Vol. 63, No. 24 / Thursday, February 5, 1998 /
Rules and Regulations
[[Page 6032]]
ENVIRONMENTAL PROTECTION AGENCY
40 CFR Part 50
[AD-FRL-5961-6]
National Ambient Air Quality Standards for Particulate Matter
AGENCY: Environmental Protection Agency (EPA).
ACTION: Final rule.
-----------------------------------------------------------------------
SUMMARY: On July 18, 1997, EPA announced a supplemental comment period
for the limited purpose of taking comments on certain field and
laboratory test results associated with the development of the
reference method (Appendix L of 40 CFR Part 50) for measuring particles
with an aerodynamic diameter less than or equal to a nominal 2.5
micrometers (PM2.5) in the ambient air. In the announcement,
EPA indicated that upon the close of the comment period it would decide
whether any further action would be appropriate. Having carefully
assessed the comments received, EPA has determined that no further
action is necessary.
ADDRESSES: The comments received during the supplemental comment period
and EPA's responses to those comments have been entered into Docket No.
A-95-54. The docket is available for public inspection in the Central
Docket Section of the U.S. Environmental Protection Agency, South
Conference Center, Rm. 4, 401 M St., SW., Washington, DC 20460. The
docket may be inspected between 8 a.m. and 3 p.m., Monday through
Friday, except legal holidays, and a reasonable fee may be charged for
copying.
FOR FURTHER INFORMATION CONTACT: John H. Haines, MD-15, Air Quality
Strategies and Standards Division, Office of Air Quality Planning and
Standards, Environmental Protection Agency, Research Triangle Park, NC
27711, telephone: (919) 541-5533, email: haines.john@epamail.epa.gov or
Neil H. Frank, MD-14, Emissions, Monitoring and Analysis Division,
Office of Air Quality Planning and Standards, Environmental Protection
Agency, Research Triangle Park, NC 27711, telephone: (919) 541-5560,
email: frank.neil@epamail.epa.gov.
SUPPLEMENTARY INFORMATION: On July 18, 1997, EPA published (62 FR
38652) a final rule revising the national ambient air quality standards
for particulate matter. In Unit VI.B. (Appendix L--New Reference Method
for PM2.5) of the preamble to the final rule, EPA concluded
that the proposed design and performance specifications for the
reference sampler, with modifications described in the final rule,
would achieve the design objectives set forth in the proposal.
Accordingly, EPA adopted the sampler and other method requirements
specified in the revised Appendix L as the reference method for
measuring PM2.5 in the ambient air. As discussed in the
preamble to the final rule, a series of field tests were performed
using prototype samplers manufactured in accordance with the proposed
design and performance specifications. The results of these field tests
confirmed that the prototype samplers performed in accordance with
design expectations. Operational experience gained through these field
tests did, however, identify the need for minor modifications as
discussed in the preamble to the final rule. As explained in that
preamble, EPA made other modifications to the proposed design and
performance specifications in response to public comment. As part of
this process, EPA performed laboratory tests to ensure that the
modifications achieved their intended objectives. While the results of
the field and laboratory tests were largely confirmatory in nature and
did not indicate a need to alter the basic design and performance
specifications, they did identify areas that needed further refinement.
Given that these tests were performed, by necessity, during and after
the close of the public comment period and because the results were not
available for placement in the docket until late in the rulemaking
process, the preamble to the final rule announced that a supplemental
comment period would be afforded for the limited purpose of taking
comments on these field and laboratory test results. The following
documents present the results of the field and laboratory tests and
associated analyses that EPA considered, as discussed in Unit VI.B. of
the preamble to the final rule, in making minor modifications or other
refinements to the proposed reference method for measuring
PM2.5 in the ambient air. The documents are:
1. Adaptation of the Low-Flowrate, PM10, Dichotomous
Sampler Inlet to Fine Particle Collection.
2. Filter Temperature Specification Report.
3. Flow Rate Specification Report.
4. Laboratory and Field Evaluation of FRM Sampler Report.
5. Prototype PM2.5 Federal Reference Method Field
Studies Report.
In a separate document published on July 18, 1997 (62 FR 38762),
EPA announced a supplemental comment period for the limited purpose of
taking public comment on the five documents specified above. The
document emphasized that comments received on the reference method for
PM2.5 that went beyond the scope of the five documents would
not be considered. The EPA also indicated in the document that upon the
close of the supplemental comment period, it would consider the
comments received and then decide whether any further action was
appropriate. In response to the July 18, 1997 document, EPA received
comments from three organizations. The EPA has conducted a careful
assessment of the comments and has concluded that they raise no issues
not considered prior to promulgation of Appendix L or addressed in the
quality assurance guidelines to be presented in Section 2.12 of the
Quality Assurance Manual for Air Pollution Measurement Systems.
Accordingly, EPA has concluded that no additional rulemaking action is
necessary as a result of the comments received during the supplemental
comment period. A summary of the significant issues raised by the
commenters and EPA's responses has been entered in Docket No. A-95-54
and is reproduced as Appendix A to this document.
Appendix A--Responses to Significant Comments on Field and Laboratory
Test Results Regarding Federal Reference Method for Measuring
PM2.5 in the Ambient Air, Docket No. A-95-54, October 1997
Summary
On July 18, 1997 (62 FR 38762), EPA announced a supplemental
comment period for the limited purpose of taking public comment on the
results of various laboratory and field tests and associated analyses
involving the new Federal Reference Method for measuring
PM2.5 in the ambient air (Appendix L of 40 CFR part 50). The
new Federal Reference Method (FRM) was adopted on July 18, 1997 (62 FR
38652) in conjunction with new national ambient air quality standards
for PM2.5 (40 CFR 50.7). During the supplemental comment
period announced on July 18, three organizations submitted comments.
The EPA has reviewed the comments received and has concluded that
none of them presents issues that were not previously considered in the
development of the FRM for PM2.5, or that have not been
addressed in the specific quality assurance guidelines to be presented
in Section 2.12 of the Quality Assurance Manual for Air Pollution
Measurement Systems. Accordingly, it is unnecessary to take further
rulemaking action or to postpone
[[Page 6033]]
implementation of the Federal Reference Method for PM2.5 as
a result of any of the comments.
Significant comments raised in each commenter's letter are
summarized below, together with EPA's responses.
Item VI-D-04 Author: EPRI.
Comment: FRM sampler provides biased results due to known losses of
volatile and semi-volatile aerosol components.
Response: The FRM sampler was never intended to collect and measure
all semi-volatile aerosol components. The sampler was designed to
closely approximate the measurements obtained by the type of samplers
used in the health studies that served as the basis for the
PM2.5 standards. Moreover, the new monitoring regulations
require supplemental monitoring at a 50-site national speciation
network in which volatile and semi-volatile aerosol components will be
measured, thus providing a more complete characterization of the
ambient aerosol.
Item VI-D-05 Author: American Petroleum Institute.
Comment: Efficacy of the rain shroud has not been demonstrated
regarding minimizing rain or snow intrusion.
Response: The EPA has been evaluating three identical prototype
inlets which meet the dimensional specifications of the new
PM2.5 FRM inlet. In these field tests conducted at Research
Triangle Park, NC, three prototype FRM samplers containing the
prototype inlets were collocated with six prototype FRM samplers
containing the older style PM10 inlet (as proposed for the
PM2.5 reference method sampler on December 13, 1996).
Although relatively few significant rain events occurred in the area
during this time period, inspection of the samplers appeared to
indicate that the new inlet design was more effective at minimizing
rain intrusion than the older design.
The performance of the prototype inlets was also evaluated under
artificial conditions designed to simulate periods of heavy rainfall.
For these tests, two identical prototype reference method samplers were
collocated outdoors such that their inlets were at the same elevation
but positioned approximately 0.7 m apart horizontally. One of the two
samplers used the prototype new PM2.5 inlet design while the
other sampler used the older PM10 inlet design. An
oscillating type sprinkler was then used to expose the two samplers to
conditions of accelerated rainfall. The sprinkler nozzle was oriented
to provide equal coverage to the two inlets and adjusted so the angle
of incidence continuously varied between 0 deg. and 90 deg. relative to
the inlet. A rain gauge was positioned between the two samplers and
used to measure the quantity of simulated rainfall to which the
samplers were exposed. Over a 2-day time period, eight discrete tests
were conducted, each having a duration of 3 hours. At the completion of
each test, the sprinkler was turned off, the rain gauge measurement was
noted, and the water volume was measured in each of the sampler's
collection jars. Prior to the next test, the rain gauge and collection
jars were emptied, and the inlet locations were alternated between
samplers in order to minimize any positional effects or flow system
effects on the test results.
Results of these simulated rainfall tests are summarized in Table
1. The simulated rainfall during each 3-hour time period ranged between
3.5 inches and 7 inches with a mean value of 4.75 inches. Inspection of
Table 1 reveals that the older style PM10 inlet collected a
range of 80 ml to 450 ml of water during each rain event. As expected,
observations during the simulated tests indicated that rain intrusion
into the inlet was maximum when rain impinged at an angle normal to the
face of the sampler's insect screen. This phenomenon is typically
observed in the field during periods of rain accompanied by elevated
horizontal wind speeds. In contrast to the older PM10 inlet,
no water droplets were observed to collect inside the prototype
PM2.5 inlet during any of the eight replicate tests. During
the entire testing totaling 38 inches of simulated rainfall, the new
PM2.5 inlet collected no water while the older
PM10 inlet collected over 1600 ml of water. Although these
simulated rainfall tests cannot exactly simulate all the conditions
that the samplers might encounter in the field, these results indicate
that the new PM2.5 inlet design was much more effective at
minimizing rain intrusion than the older, original PM10
design.
Table 1.--Results of Simulated Rainfall Tests for PM2.5 Inlet Evaluation
------------------------------------------------------------------------
Volume of water in
Simulated collection jar (ml)
Test No. rainfall ---------------------------
(inches) PM10 inlet PM2.5 inlet
------------------------------------------------------------------------
1............................. 4.5......... 100......... 0
2............................. 4.5......... 220......... 0
3............................. 4.0......... 80.......... 0
4............................. 4.5......... 200......... 0
5............................. 5.0......... 450......... 0
6............................. 5.0......... 80.......... 0
7............................. 3.5......... 80.......... 0
8............................. 7.0......... 420......... 0
Mean =...... Mean =...... Mean =
4.75 in..... 204 ml...... 0 ml
------------------------------------------------------------------------
Comment: Filter temperature overheats measured in February do not
adequately represent those which might be measured in summer.
Response: Evaluation of prototype FRM at RTP, NC after February
indicated that overheats of 3 deg. C were occasionally observed but
5 deg. C overheats were not observed even on days when radiant fluxes
at the sampling site exceeded 1200 W/m\2\.
Comment: The 6/30/97 McElroy/Frank memorandum provides a tabular
summary of FRM PM2.5 precision measurements used to revise
upward the method detection limit (MDL) specification from 1
g/m3 to 2 g/m3. Detailed
analysis is difficult since individual data are not provided or cited.
However, inserting the reported mean daily precisions into the
definition of MDL (and assuming that blank means=0) yields minimum MDLs
of 2.3 g/m3 for Denver and RTP locations and 3.7
g/m3 for Azusa, values that differ from those
reported in the table where Denver = 2 g/m3, RTP =
3 g/m3, Azusa = 2 g/m3.
Response: The change in estimated method detection limit from 1
g/m3 to 2 g/m3 was due to
information gained through field use of prototype samplers since the
regulation was initially proposed. As specified originally in the
December 13, 1996 proposal, the detection limit of the PM2.5
mass concentration measurement ``* * * is determined primarily by the
repeatability (precision) of filter blanks * * *.'' At the time the
regulation was proposed, field data had not yet been collected to
determine the variability of field blanks. For this reason, laboratory
blanks were used to provide a preliminary estimate of the method's
precision. Once prototype samplers became available, specialized field
studies conducted in Denver, Azusa, and RTP provided a data base upon
which to provide actual estimates of the method's detection limit. The
final regulation as promulgated on July 18, 1997 updated the
preliminary estimate and modified the text to indicate that field
blanks were used for estimating the method detection limit. In
particular, Section 3.1 was modified to read, ``The
[[Page 6034]]
lower detection limit of the mass concentration measurement range is
estimated to be approximately 2 g/m3, based on
noted mass changes in field blanks * * *.'' Thus, the use of actual
field data in conjunction with a minor modification in the MDL's
definition accounted for the revision in the method detection limit.
The commenter apparently misinterpreted the precision table
included in the docket (reproduced in Table 2 below). The values
reported in the last column of the table refer to the precision of
measured PM2.5 concentrations and have no relationship with
measured precision of field blanks. This apparent misinterpretation led
to the commenter's conclusion that the original method detection limit
calculations were in error. The enclosed Table 3 below presents actual
data from the three field sites relating to the observed mass changes
in the field blanks. As indicated in the final column of Table 3, the
method detection limits determined at Denver, Azusa, and RTP were 2
g/m3, 2 g/m3, and 3 g/
m3, respectively. This actual field information was the
basis for the July 18, 1997 text which stated that the method detection
limit ``* * * is estimated to be approximately 2 g/
m3.''
Table 2.--Summary of Precision Tests at 3 Separate Sites
[Method Detection Limit (Field Blanks) = |Mean| + 10 * (Std. Dev.)]
--------------------------------------------------------------------------------------------------------------------------------------------------------
Method Mean daily
Mean PM2.5 detection precision
Site Dates No. days Prototype samplers PM2.5 range (g/ conc. limit (std. dev.)
evaluated m\3\) (g/ (g/ (g/
m\3\) m\3\) m\3\)
--------------------------------------------------------------------------------------------------------------------------------------------------------
DENVER, CO...................... Dec. 10-22......... 10 6 Graseby-Andersen. 1.4 to 20.6............ 10.9 2 0.23
AZUSA, CA....................... March 25-April 10, 9 6 Graseby-Andersen. 6.0 to 32.1............ 18.6 2 0.37
1997.
RTP, NC......................... April 4-30, 1997... 13 3 R&P.............. 7.2 to 18.5............ 11.7 3 0.23
--------------------------------------------------------------------------------------------------------------------------------------------------------
Table 3.--Calculated Method Detection Limit at 3 Separate Sites
[Method Detection Limit (Field Blanks) = Mean + 10 * (Std. Dev.)]
--------------------------------------------------------------------------------------------------------------------------------------------------------
Standard
Total Mean of deviation of Method
Number of number of daily field daily field detection
Site Dates sampling field blanks blanks limit
days blanks (g/ (g/ (g/
m\3\) m\3\) m\3\)
--------------------------------------------------------------------------------------------------------------------------------------------------------
Denver, Co...................................... Dec. 10-22, 1996.................. 10 30 -.010 0.19 2
Azusa, CA....................................... March 25--April 10, 1997.......... 8 24 0.18 0.22 2
RTP, NC......................................... April 4-30, 1997.................. 8 24 0.52 0.27 3
--------------------------------------------------------------------------------------------------------------------------------------------------------
Comment: The 25 C limit should be termed ``post-acquisition''
rather than ``post-sampling.''
Response: This is a good suggestion, and this terminology will be
employed in Section 2.12 of the Quality Assurance Handbook for Air
Pollution Measurement Systems.
Comment: The 9/96 G. H. Achtelik report offers at best a lower
bound estimate of filter volatiles loss.
Response: Studies are currently being performed in Riverside, CA to
further characterize the effects of volatile losses. In addition, EPA
requires a 50-site chemical speciation network in which volatile and
semi-volatile aerosol components will be measured.
Comment: Midnight to midnight sampling may provide different
measured concentrations than noon to noon sampling due to water of
crystallization effects.
Response: It was necessary to maintain the midnight to midnight
sampling for PM2.5 to be consistent with the sampling
schedules for other particulate measurements and to not unduly
constrain the work schedules of site operators. However, if such
effects are suspected, operators are encouraged to re-weigh filters
after additional conditioning (beyond the minimum 24 hours).
Comment: A number of lingering problems were identified in the
field tests.
Response: One of the purposes of these field tests was to develop
preventative maintenance guidelines for routine operation of these
samplers. None of these problems was unexpected, and each will be
addressed in Section 2.12 of the Quality Assurance Handbook for Air
Pollution Measurement Systems. Note also that these tests were
performed using prototype and not production model PM2.5
samplers.
Comment: A field calibration protocol should be developed to test
the performance of the inlets.
Response: While the intent of the comment is understood, the
recommended calibration protocol would be cumbersome, time consuming,
and not precise enough to measure any realistic changes in fractionator
performance.
Comment: Poor correlation achieved by the Tucson site technician
might indicate the samplers are not user-friendly and/or require
special field personnel.
Response: It should be noted that all of these studies were
performed using prototype samplers that were operated using procedures
that were at that time still under development. Taking this under
consideration, the intramethod and intermethod results from all the
other studies could have been interpreted as being closer than
expected. The lower intramethod precision observed at the Tucson site
can no doubt be attributed to a combination of contributing factors. As
noted in the EPA staff report, ``* * * the Tucson study was operated by
a site technician as additional and unassisted duties to his normal
work load * * *.'' Of equal importance is the fact that the mean
concentration at the Tucson site was appreciably lower than at any of
the other five sampling sites. At low ambient concentrations, the
effect of
[[Page 6035]]
sample handling, conditioning, and weighing uncertainties becomes much
more important than at higher concentrations. It is reasonable to
expect, therefore, that higher intrasampler variability would be
observed at the Tucson site than at the other sampling sites. An
assertive quality assurance program will be included within the
implementation of the national monitoring network.
Specialized tests were conducted in Azusa, CA to determine if local
site personnel would experience significantly more variability with the
prototype FRM samplers than would be experienced by specially trained
researchers. First, aerosol researchers conducted 6 days of 22-hour
sampling using six identical PM2.5 samplers. Mean precision
in PM2.5 concentrations was measured to be 0.4 g/
m3. Using the same procedures, site operators from the South
Coast Air Quality Management District then conducted their own
precision tests with the same samplers. Mean precision in
PM2.5 concentrations was also measured to be 0.4 g/
m3. Incidentally, this measured intrasampler variability was
appreciably less than the 2 g/m3 maximum value
allowed by the regulations.
Item VI-D-06 Author: National Cotton Council of America.
Comment: Based on impactor theory developed by Ranz and Wong,
Parnell et al contend that the impactor cutpoint is actually 2.74
m rather than the 2.5 m design value.
Response: There are basically two problems associated with the
Parnell et al approach. First, although the 1952 Ranz and Wong research
led to important insights regarding impactor theory, it was an early
work which could not properly account for the effects of complex
impactor design parameters such as jet-to-plate distance, throat
length, and fluid Reynolds number. Only the development of
sophisticated numerical analysis techniques in conjunction with the
advent of high speed computers allowed detailed analysis of fluid flow
fields and of particle trajectories within the flow fields. In
particular, important advances in our understanding of inertial
impactors were made by Marple (1970) and Marple and Liu (1975). It was
upon these improved design guidelines that the EPA prototype WINS was
developed. Based on this well-accepted inertial impactor theory, one
would predict a cutpoint of 2.44 m aerodynamic diameter for
the WINS impactor rather than the 2.74 m value predicted by
the simplistic approach of Ranz and Wong.
The second problem associated with the Parnell et al. approach is
that impactor theory can never be used to reliably predict an actual
impactor's performance. Despite advances since the Ranz and Wong work,
conventional impactor theory only provides starting guidelines upon
which to base impactor design. In reality, a number of factors can
affect a given impactor's performance including actual component
dimensions, flow rate, particle bounce, particle re-entrainment, wall
losses, and electrostatic effects. If one is interested in determining
an impactor's actual performance, therefore, the impactor must be
calibrated in the laboratory under carefully controlled conditions
using primary calibration aerosols. The novel geometry of the WINS
impactor reinforced the need for laboratory calibration to determine
its actual performance. As described in ``Modification and Evaluations
of the WINS Impactor,'' the experimentally determined cutpoint of the
WINS impactor was measured to be approximately 2.48 m
aerodynamic diameter at standard temperature and pressure conditions.
References: Marple V.A. and Willeke K. (1976) Impactor design.
Atmos. Envir. 10:891-896.
Marple V. A. and Liu B.Y.H. (1975) On fluid flow and aerosol
impaction in inertial impactors. J. Coll. & Interface Sci. 53:31-34.
Comment: PM from agricultural operations has different
characteristics than that used in the laboratory calibration. Actual
performance of the WINS may be different in the field.
Response: Laboratory tests showed that there was no difference in
collection between liquid and solid aerosols. Fractionation of the
aerosol using its aerodynamic properties automatically accounts for the
particle's physical size, shape, and density.
Comment: The data presented in ``Flow Rate Specification Report''
seems to indicate that flow rate errors in FRM prototype samplers are
not random but systematically understate the actual flow rates. As a
consequence, the sampled particles actually have a higher momentum than
the FRM measurements imply, adversely affecting the interpretation of
the penetration curves.
Response: It is important to understand that no flow control system
is inherently accurate and that all systems require periodic
calibration. There are several factors which affect the flow rate
accuracy of any individual FRM sampler. Because automatic volumetric
flow control involves separate measurements of several key parameters
(e.g., ambient temperature, ambient pressure, etc.), any inaccuracies
in their actual measurements will naturally result in inaccuracies in
flow control. Although these parameters are typically calibrated at the
same time as the initial flow calibration, any drift in their response
since the time of calibration will naturally result in variations in
flow control. For example, if pressure transducer circuitry is not
properly compensated for temperature, significant reductions in ambient
temperature can result in directional biases in ambient pressure
measurements. These pressure measurement biases can, in turn, naturally
result in directional biases in flow control.
Because collocated, identical instruments are typically calibrated
in the field using the same flow transfer standard, it is reasonable to
expect that any directional bias in the transfer standard's calibration
will also result in biases among the group of collocated samplers in
the same direction as that of the transfer standard. Thus, if the flow
transfer standard and NIST traceable audit device do not agree exactly,
we tend to observe directional differences in flow response among a set
of samplers. In the case of the sample flow data provided in the
docket, the actual flow rates measured by the NIST traceable flow
standard were always higher (mean value = 0.9 percent higher) than the
flow value indicated by the instruments. Actual flow rates are
positively biased, therefore, which accounts for the percent error
direction used in reporting the flow audit results.
Regardless of one's individual choice of bias direction, the effect
of the flow bias can be predicted with respect to magnitude and
direction. These effects can be conveniently grouped into aspiration
and particle transport effects, effects of flow bias on fractionator
performance, and effects of flow bias on calculated PM2.5
concentrations. These factors are considered separately below.
Aspiration and Particle Transport Effects: Although major biases in
sampler flow rate can adversely effect the sampler's inlet aspiration,
minor flow rate biases should have negligible effects on the inlet's
ability to withdraw representative aerosol samples from the ambient air
and transport the aspirated aerosol efficiently throughout the sampling
system. The FRM specifications for flow rate control were designed to
ensure that large errors in flow control would be identified during
sampling and that appropriate action (i.e., sampler shutdown and/or
warning flags) would be automatically taken.
[[Page 6036]]
Effects on Fractionator Performance: Similar to the effect of flow
rate bias on the sampler's aspiration performance, minor flow rate
biases should have negligible effects on the sampler's ability to
accurately fractionate an aspirated aerosol. For small variations in
flow rate (such that the jet Reynolds number is not significantly
altered), the fractionator's cutpoint is inversely proportional to the
square root of the volumetric flow rate. For the EPA WINS impactor
which possesses a cutpoint of 2.48 m at 16.67 L/min., for
example, a 2 percent increase in flow rate would result in only a 1
percent decrease in cutpoint to 2.46 m. Similarly, a 2 percent
decrease in flow rate would result in only a 1 percent increase in
cutpoint to 2.50 m. Moreover, these 1 percent predicted
changes in fractionator cutpoint would result in an even smaller bias
in collected PM2.5 mass concentration. Since the expected
mass collected is a function of both the fractionation curve and the
mass size distribution of the aerosol to which it is exposed, numerical
sensitivity analysis has been performed on three idealized ambient
distributions. Assumed parameters for the distribution are identical to
those used in 40 CFR part 53 Table F-3 for coarse, ``typical,'' and
fine ambient aerosol distributions. Since only the cutpoint of the
fractionator curve can be expected to change at low flow rate biases,
the predicted fractionation curve can numerically integrate with each
of the ambient distributions to calculate the expected measured mass
concentration as a function of flow rate bias.
Results presented in the table below indicate that a maximum bias
in expected mass concentration of approximately 0.6 percent would be
associated with flow biases of 2 percent. Note that higher flow rates
result in lower fractionator cutpoints, which results in lower mass
gains than would normally occur.
------------------------------------------------------------------------
Expected bias in measured mass
concentration solely as a function of
flow-induced cutpoint changes
--------------------------------------
Distribution -2% flow 0% flow +2% flow
bias bias bias
(Dp50=2.46 (Dp50=2.48 (Dp50=2.50
m) m) m)
(percent) (percent) (percent)
------------------------------------------------------------------------
Coarse........................... +0.5 0 -0.6
``Typical''...................... +0.2 0 -0.2
Fine............................. +0.2 0 -0.2
------------------------------------------------------------------------
Effects on Calculated PM2.5 Mass Concentration: As
discussed above, the effects of flow biases on inlet aspiration
performance and fractionator cutpoint are essentially negligible. The
primary effect of flow rate biases on PM2.5 measurements
concerns the calculation of PM2.5 concentration from the
measured mass gain of the filter divided by the volume of air sampled
as reported by the sampler. Because the FRM samplers are designed to
continuously adjust volumetric flow rate to the design setpoint flow
rate of 16.67 actual L/min., the sampled air volume reported by the
instrument is typically very close to the design flow rate times the
sampling duration. If, for example, the flow rate reported by the
sampler was in fact low by 2 percent, the sampler would have sampled,
fractionated, and collected a fine particulate mass which was
approximately 2 percent higher than it should have been. Since the
calculated PM2.5 concentration is simply the measured mass
divided by the indicated sampled air volume, the calculated
PM2.5 concentration would be positively biased by
approximately 2 percent. Note that the effects of flow biases on
fractionator performance and collected aerosol mass are in opposite
directions, thus partially offsetting each other.
Comment: The fractionator used in the FRM should be evaluated in
the laboratory after collecting appreciable quantities of polydisperse
particles on the impaction plate.
Response: These sensitivity tests were in fact conducted in the
laboratory and described in ``Modification and Evaluation of the WINS
Impactor.'' The WINS impactor was exposed to laboratory generated
polydisperse Arizona test dust for three 24-hour periods where the mean
dust concentration was measured to be 330 g/m3.
After each 24-hour collection period, the performance of the loaded
substrate was evaluated in the laboratory using primary calibration
aerosols. Results showed that the fractionator could be exposed to
ambient aerosol concentrations averaging 330 g/m\3\ for 6
consecutive days before a 5 percent bias in measured PM2.5
concentration would be expected.
Comment: Favorable results of collocated field tests should not
imply that the samplers are accurately measuring PM2.5
values, only that similar samplers produce similar results. To verify
accuracy, the six samplers should be simultaneously tested in the
laboratory using a known and typical aerosol as described in the
previous comment.
Response: Because the size and volatility of particles comprising
fine ambient particulates vary over a wide range of environmental and
sampling conditions, the accuracy of PM2.5 measurements
cannot be defined in an absolute sense. Instead, EPA defines
PM2.5 sampler accuracy based on how well the sampler meets
all design, construction, and operational specifications set forth for
samplers approved for determining compliance with the PM2.5
regulations. In particular, field accuracy can be defined by the level
of agreement between a given PM2.5 sampler and a collocated
PM2.5 reference audit sampler operating simultaneously. In
the case of collocated prototype FRM samplers, favorable agreement
among the samplers implies that adequate control is being exercised
over uncertainties associated with the sampler's construction,
calibration, setup, and operation.
Laboratory calibration of size selective components requires
accurate generation and measurement of primary aerosol standards under
very carefully controlled conditions. Simultaneous calibration of six
identical samplers under these conditions would be impractical. To
ensure that production samplers accurately meet the required
specifications, the samplers must be manufactured in an ISO-9001
registered facility, and the facility must be maintained in compliance
with all applicable ISO 9001 requirements. The manufacturer must also
conduct specific tests and submit supporting evidence to EPA
demonstrating conformance to critical component specifications such as
materials, dimensions, tolerances,
[[Page 6037]]
and surface finishes. In conjunction with final assembly and inspection
requirements, field tests are used to demonstrate that the samplers
meet required performance specifications.
List of Subjects in 40 CFR Part 50
Environmental protection, Air pollution control, Carbon monoxide,
Lead, Nitrogen dioxide, Ozone, Particulate matter, Sulfur oxides.
Authority: Secs. 109 and 301(a), Clean Air Act, as amended (42
U.S.C. 7409, 7601(a)).
Dated: January 29, 1998.
Carol M. Browner,
Administrator.
[FR Doc. 98-2878 Filed 2-4-98; 8:45 am]
BILLING CODE 6560-50-P
