United States
Environmental Protection
Agency
Environmental Sciences Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S2-81-146 Oct. 1981
Project Summary
The Tapered Element
Oscillating Microbalance:
A Monitor for Short-Term
Measurement of Fine Aerosol
Mass Concentration
Charles W. Lewis
A new detector, based on a device
called the Tapered Element Oscillating
Microbalance (TEOM), has recently
been developed for short-term moni-
toring of ambient aerosol fine mass
concentration. The main element of
the detector is a tapered hollow tube
(the TEOM) fixed at the wide end and
holding an exchangeable filter car-
tridge at the narrow end. Air is drawn
through the filter cartridge and down
the hollow tube. As air is pumped into
the tube, particulates deposit on the
filter cartridge. The tapered tube oscil-
lates continuously in a clamped-free
mode with the frequency accurately
monitored. As particulates load onto
the filter cartridge, the frequency
changes in relation to the mass added.
A microprocessor-based electronics
system connected to the detector then
collects and processes the data.
Output is printed in a format con-
sisting of time and aerosol concentra-
tion level in /
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neither can be strictly regarded as
having a re.al-time capability, each has
the potential for performing aerosol
mass measurements with a time
resolution ranging from several
minutes (QCM) to several hours (beta-
attenuation). The beta-attenuation
method is based on the decrease in the
number of electrons reaching an elec-
tron counter when a thin layer of
material (aerosol collected on a filter) is
interposed between the counter and a
radioactive source of electrons. The
QCM method utilizes the piezoelectric
effect, with which the mass of aerosol
deposited on an oscillating quartz
crystal is inferred from the measured
frequency change of the crystal. Both
the beta-attenuation and QCM
approaches have been employed in
commercially available instruments.
As a result of recent work supported
by the U.S. Environmental Protection
Agency and summarized here, a new
instrument is now commercially avail-
able (Rupprecht and Patashnick Co.,
Englewood, CO) for the short-term
monitoring of fine aerosol mass concen-
tration. The instrument is based on the
newly developed Tapered Element
Oscillating Microbalance (TEOM),
which has been assigned U.S. Patent
No. 3,926,271. As in the QCM method,
mass is determined through measure-
ment of frequency change. In nearly all
other respects, however, the TEOM is
distinctly different from the QCM and is
intended to avoid well-documented dif-
ficulties of the latter.
Description
Figure 1 shows the operation of the
TEOM in simplified terms. The TEOM is
a tube constructed of material with a
high mechanical quality factor, and it
has a special taper. The tube is firmly
mounted at the wide end, while the
other end supports a filter cartridge. The
tube is hollow, sothat a pump will cause
aerosol to be deposited on the filter
cartridge while the remaining clean air
passes through the tube. The free end of
the tube is set into transverse oscillation
and the natural frequency changes in
relation to the mass deposited on the
filter. The tapered element is kept in
oscillation by a feed-back system: the
oscillation of the element is converted
into an electrical signal by a light emit-
ting diode(LED)-phototransistor
combination, the output of the photo-
transistor being modulated by the light
Positive
Field
Plate
Phototransistor
Vacuum Pump
Data
Processor
Figure 1.
The TEOM in schematic
form.
blocking effect of the vibrating element.
The modulated voltage is amplified and
applied to a conductive coating on the
vibrating element. Interaction between
the latter signal and a steady external
electric field in which the vibrating
element moves is the driving force
which maintains the oscillation.
Figure 2 shows the TEOM in its
packaged field instrument form. The
system includes two modules — the
detector and supporting electronics. A
third component, not shown, is the
external vacuum pump which moves
the aerosol through the detector
module. The detector module consists
of the TEOM filter unit, cyclone pre-
separator, air heater, flow controller,
and air buffer. The electronics module
consists of a feed-back amplifier,
frequency counter, control circuits,
microprocessor, LED display, and
printer. The cyclone preseparator
removes aerosol particles larger than
2.5^minaerodynamicdiameter, sothat
the detected responds only to the
ambient fine particle mass concentra-
tion. The air heater maintains the
temperature of the incoming aerosol to
a narrow range near 60°C. This temper-
ature is required for frequency stability
and minimization of humidity effects on
the aerosol measurements. The flow
controller maintains the flow rate at 7.1
l/min. Once set into operation the
printer gives automatic outputs (on a Vz
to 1 h schedule) of the time of day and
fine particle mass concentration, in
jug/m3. The sensitivity is such that a fine
mass concentration of 10/ug/m3can be
measured with a precision of 10% in a
30 min sampling interval.
Conclusions
The TEOM is a new and unique
instrument capable of short-term
aerosol fine mass concentration meas-
Figure '2. The TEOM field instrument.
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urements. The TEOM measures the
mass of collected participates, indepen-
dent of their composition, atomic
number, optical properties, shapes, or
other properties. It is potentially a strong
competitor for existing beta-attenuation
and QCM methods for measuring short-
term ambient aerosol trends.
Recommendations
Although the detector appears very
promising, extensive field testing needs
to be conducted with the TEOM to
confirm its expected usefulness in
ambient air monitoring. This testing
should include intercomparisons with
existing short-term monitors such as
beta-attenuation and QCM devices,
integrating nephelometers, and con-
ventional long-term samplers such as
the dichotomous sampler.
The EPA author of this Project Summary is Charles W. Lewi's (also the EPA
Project Officer, see below), who is with the Environmental Sciences Research
Laboratory. Research Triangle Park, NC 27711.^
The complete report, entitled "The Tapered Element Oscillating Microbalance:
A Monitor for Short- Term Measurement of Fine Aerosol Mass Concentration,"
was authored by Harvey Patashnick and Georg Rupprecht of Rupprecht and
Patashnick Co.. Englewood. C080111 (Order No. PB81-245219; Cost: $6.50,
subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA2216J
Telephone: 703-487-4650
The,EPA Project Officer can be contacted at:
Environmental Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
U.S. GOVERNMENT PRINTING OFFICE; 1981 — 559-017/7414
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