National Ambient Air Quality Standards for Particulate Matter

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: [email protected] 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: [email protected].

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