United States.
Environmental Protection
Agency
Environmental Monitoring Systems
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S4-85/057 Jan. 1986
Project Summary
Preliminary Evaluation  of  a
Modified  NBS  PMio Sampler for
Indoor  Paniculate
Measurements

J. E. Howes, Jr., R. Vijayakumar, F. S. Howard, F. Doerfler, and F. J. Burmann
  The full report describes modifica-
tions to the prototype NBS portable
ambient air sampler and results of a
preliminary performance evaluation of
the modified unit. The study also  in-
cluded a limited performance evaluation
of the indoor particulate sampler devel-
oped by Harvard University.
  A prototype  portable sampler was
designed and constructed for the U.S.
Environmental Protection Agency (EPA)
by the National Bureau of Standards
(NBS) to measure the mass concentra-
tion of inhalable particulates in indoor
environments. The prototype was eval-
uated in field applications and labora-
tory tests and found to have several
design features that needed  improve-
ment. In this study, modifications were
made to overcome problems identified
in the NBS design.
  A series of collocated sampling exper-
iments were performed in an indoor
environment to compare  results ob-
tained with the modified NBS and the
Harvard samplers with data obtained
with a conventional ambient-type  di-
chotomous sampler. These experiments
indicated that over the mass concentra-
tion range studied (~8 -14 /jg/m3). the
modified NBS and the Harvard samplers
yielded PMio mass concentration data
comparable to the dichotomous sam-
pler. However, the NBS sampler did not
accurately classify particles in the fine
(< 3 £
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sored development of a portable PMi0
sampler by the National Bureau of Stand-
ards (NBS). The design criteria adopted by
NBS were as follows:

• The sampler should have well defined,
  stable cut points with respect to par-
  ticle size.
• The samples (as with the standard EPA
  area samplers) should be collected on
  filters for subsequent  weighing and
  chemical analysis.
• The sampler should have as high  a
  sampling flow rate as possible since
  ambient particle concentrations  are
  typically low and a  >100  //g particle
  sample per filter is  necessary for  a
  gravimetric measurement uncertainty
  of <10%.
• X-ray fluorescence, a  widely used
  method for elemental analysis of filter
  samples, requires the sample be in an
  even, homogeneous layer on the filter.
• To be acceptable to volunteers partic-
  ipating in  exposure studies, the sam-
  pler must be  light, quiet,  and incon-
  spicuous.
• To be able to sample at high flow rates
  for long time periods and still be small
  and  lightweight,  the sampler must
  have  energy-efficient components.
  This means that the sampler air path
  must have a  low pressure drop and
  that the motor-pump unit have  low
  internal losses.

  Several prototype NBS samplers were
constructed and evaluated by Research
Triangle  Institute (RTI) in previous field
studies. The work described in the full
report was undertaken to improve several
weaknesses in the design of the prototype
units identified by RTI in the earlier work.
In addition, performance data were also
obtained on the indoor sampler designed
for the Harvard Multicities Study.

Sampler Modification and
Evaluation
  Modifications were made in the proto-
type NBS portable sampler to overcome
several  design flaws. The modifications
included: (1) replacement of the sampler
case  which  was highly susceptible to
leakage, (2) redesign of the filter  holder
which was inaccessible and susceptible
to leakage, and  (3) addition  of a  motor
control circuit to stabilize the flow rate
and extend the motor lifetime.
  A  brief series of experiments were
performed in an indoor environment to
compare results  obtained with the mod-
ified NBS samplers with those obtained
with collocated dichotomous samplers.
The  comparison showed  that the NBS
sampler yielded  PMio mass concentra-
tion measurements that were in reason-
able agreement with the corresponding
dichotomous data. The mean difference
between the NBS and dichotomous PM10
measurements was approximately 9 per-
cent. The differences in NBS and dichot-
omous results for individual experiments
ranged  from  approximately 2  to  -36
percent.  The  greatest  difference was
observed during the period with  the
lowest mass concentration (~ 5  /ug/m3)
and, thus, the largest weighing error. The
precision of paired measurements with
the NBS sampler was excellent; differ-
ences between duplicate measurements
was <9 percent. The mean difference
between duplicate dichotomous sampler
determinations was approximately 5 per-
cent.
  The mass of the fine and coarse  particle
fractions determined with  the NBS sam-
plers did not agree well with measure-
ments made with collocated dichotomous
samplers. The NBS sampler which uses a
filter  pack  for  separation of the size
fractions,  yielded  significantly  higher
values than the dichotomous sampler for
the coarse size  range (~ 3.5 to 10 //m).
The difference is attributed to the inability
of the filter pack to accurately separate
particles on the basis  of aerodynamic
diameter.
  The experience with the NBS sampler
in the performance tests indicated the
most serious problem with the unit is the
short pump/motor lifetime (<100 hours
for some units) and the unfavorable flow
rate—pressure drop relationship at the
operating flow rate. The premature failure
and unfavorable operating characteristics
are attributed to operation  of the pump at
over twice its design flow rate in the NBS
design.
  The battery pack (six 1.25V Ni-Cd  D
cells) used with the modified NBS sampler
provided at least 24 hours of operation at
a flow rate of 6 liters per minute (Ipm). The
constant current motor control  circuit
added  to the sampler provided  a very
uniform flow rate  (within ±5 percent)
over a  24-hour sampling period. The
circuit also automatically  decouples the
motor  when  the battery output drops
below a voltage that delivers a flow rate of
6 Ipm.
  The  performance  of an indoor  air
sampler developed by Harvard University
was also evaluated  in this study. The
mean difference between results obtained
with collocated pairs of the Harvard and
conventional dichotomous samplers dur-
ing nine days of sampling was less than 2
percent. The range of differences in PM10
mass concentration for the nine compari-
sons was 13.7 to  -14.3 percent. The
mean of  differences between duplicate
measurements with the Harvard samplers
was 11  percent. The results obtained
with the Harvard sampler exhibit excellent
agreement with the dichotomous data
considering the weighing errors associ-
ated with the measurements and the
possible  inhomogeneity of  particles in
test atmosphere.

Conclusions
  The conclusions drawn from results of
this study may be summarized as follows:
• This work and a previous field evalua-
   tion have disclosed several flaws in
   the design of the prototype NBS port-
   able particulate sampler. The problems
   that have been  identified include (a)
   potentially inaccurate sampling rates
   due to susceptibility  of the sampler
   case to leakage, (b) susceptibility of the
   filter cassette to leakage and accidental
   disassembly  if jarred or  bumped, (c)
   premature failure  of the  sampling
   pump  unit, and (d) possible movement
   of the  inlet components (funnel insert
   and oil-soaked frit) thus changing the
   inlet cut point. Other features which
   may affect user acceptance of the NBS
   prototype sampler are the difficulty in
   changing filters due to inaccessibility
   of  the filter cassette,  poor weight
   distribution, and unsuitability of the
   sampler configuration for  personal
   monitoring applications.
• A modified version of the NBS sampler
   was assembled in this study with the
   objective of eliminating  the initial
   problems in the NBS design. A limited
   evaluation of the modified sampler has
   indicated that the weakest component
   in the system is  the sampling pump.
   The Sensidyne (formerly Bendix) pump
   assembly used in the NBS design and
   the modified sampler has an unac-
   ceptably short lifetime (at least in this
   application); one new assembly failed
   after less than 100 hours of operation.
   In this study all sampler failures were
   attributed to the motor rather than the
   pump. The pump also has marginally
   acceptable flow characteristics for this
   application.  The flow  rate  is very
   sensitive to pressure changes on the
   fore and backside of the pump. This
   poses a problem in performing flow
   calibrations on the sampler.

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• Performance evaluation of the original
   design by NBS and evaluation of the
   modified version developed during this
   study shows that the sampler yields
   PMio  measurements (i.e. total  mass
   concentration <10 fjm)  which is in
   reasonable agreement with collocated
   dichotomous sampler data. However,
   the results of this study suggest that
   there  is some doubt as to the ability
   and usefulness of the NBS sampler to
   resolve the distribution  of  particles
   within the 0 to 10 //m aerodynamic
   equivalent size range.
• The NBS and the modified systems
   have demonstrated that battery life-
   time will provide at least 24 hours of
   sampler operation. The motor control
   circuitry incorporated in the modified
   sampler  will provide  more uniform
   pump performance and longer sampler
   lifetime.
• Both the original and modified version
   of the NBS sampler use two filters; an
   Apiezon-coated  Nuclepore*  filter for
   collection of the ~3 to  10 //m size
   fraction and a Teflon filter for the 0 to
   ~3 fjm size  range. The errors associ-
   ated with these respective measure-
   ments must be assessed. The results
   of this study indicate that resolution of
   the fine  and coarse particle fraction
   may be impractical andthat the general
   application of the NBS sampler should
   be directed  toward determination of
   the mass of the particle size fraction
   < 10  /um in aerodynamic equivalent
   diameter.
• In this study, the average  of  mass
   concentration measurements  deter-
   mined by collocated dichotomous and
   Harvard  samplers  agreed  within 2
   percent based on tests conducted over
   nine sampling periods (of at least 24
   hours in duration).


Recommendations
  The preliminary evaluation of the NBS
sampler and the modified, portable ver-
sion developed in this study suggests the
following recommendations:

• The most crucial issue in developing a
   field-worthy sampler  is  the pump
   reliability. Either the Sensidyne unit
   must be upgraded to provide reliable
   performance or  an  alternate pump
   must be identified.
 • The resolution of the fine and coarse
   particle fractions using the Nuclepore
   filter does not appear practical. It is
   recommended for routine applications
   that a single filter (2 or 3 fjm pore size
   Teflon with polyolefin support ring) be
   used. This will also simplify the use of
   the sampler  and  permit  a further
   reduction in size.

 • When final design criteria are set and
   reliable components are found, a per-
   sonal version of the sampler can be
   constructed. It is recommended that
   configuration  of the personal sampler
   should consist of an inlet that can be
   clipped on an individual's lapel or shirt
   pocket connected by a flexible line to a
   pump unit either worn on a belt or in
   soft  case, e.g., camera bag with a
   shoulder strap. Consideration should
   be given to further size reduction of the
   inlet  assembly so that it is relatively
   unobtrusive when clipped to a lapel or
   pocket.

  It may also be useful to explore sampler
modifications  for alternate  flow rates
rather than  6 Ipm. Then, it would be
possible to develop a personal sampler
based on the availability of pumps and
motor rather than being forced to select
hardware from a limited choice that meets
the design requirements.
•Mention of trademarks or commercial products does
 not constitute endorsement or recommendation for
 use by the U.S. Environmental Protection Agency.

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J.  £". Howes, Jr., R.  Vijayakumar, F.  Doerfler,  and F. J. Burmann are with
  Environmental Monitoring and Services, Inc., Newbury Park, CA 91320; and F.
  S. Howard is with Esoteric Systems, Inc., Newbury Park, CA 91320.
Robert Jungers is the EPA Project Officer (see below).
The complete report,  entitled "Preliminary Evaluation of a Modified NBS PMio
  Sampler for Indoor Paniculate Measurements." (Order No. PB 85-24 7 005/A S;
  Cost: $9.95, subject to change) will be available only from:
        National Technical Information Service
        5285 Port Royal Road
        Springfield, VA 22161
        Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
        Environmental Monitoring Systems Laboratory
        U.S. Environmental Protection Agency
        Research Triangle Park, NC 27711
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