United States
Environmental Protection
Agency
Environmental Sciences
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S3-84-073 Aug. 1984
&EPA Project Summary
Investigation of the Performance
of Sulfation and Nitration Plates
J. E. Sickles II and R. M. Michie
An inexpensive method for measur-
ing atmospheric pollutants, which has
been in use for several decades, is the
passive reactive monitor. Such a device
contains a reactive substance that is
exposed to ambient air for a period of
time. The pollutants in the surrounding
air react with the reactive surface of the
monitor. A quantitative chemical
determination is made of the reaction
product. A calibration factor is then
applied to estimate the pollutant
concentration in the ambient air
averaged over the exposure period.
Passive monitors, known as sulfation
and nitration plates, are available from
commercial suppliers.
The objective of this study was to
evaluate the performance of
commercial sulfation and nitration
plates. The sensitivity of plate
calibration factors to pollutant concen-
tration (SO2 and NO2), simulated wind,
speed, humidity, and temperature was
determined. Sulfur dioxide was
collected as SOj" on both sulfation and
nitration plates. Nitrogen dioxide was
collected effectively only on the
nitration plates and is retained as NO ~2.
Nitration plates were subject to SOf~
and NO2 loss upon storage and should
be analyzed promptly after use. No
SO|' loss was observed for sulfation
plates.
For sulfation and nitration plates, test
results indicate that sulfation
calibration factors were extremely
sensitive to wind speed. A slight effect
of relative humidity was observed for
sulfation calibration factors on both
types of plates and for nitration
calibration factors on nitration plates.
Nitration calibration factors were
extremely sensitive not only to wind
speed, but to NO2 concentration and
temperature as well.
This Project Summary was developed
by EPA's Environmental Sciences
Research Laboratory. Research
Triangle Park, NC. to announce key
findings of the research project that are
fully documented in a separate report of
the same title (see Project Report order-
ing information at back).
Introduction
Sulfur dioxide (S02) and nitrogen
dioxide (NO2) are released m large
quantities by anthropogenic sources
These gases are the major precursors of
the acids that contribute to acidic
precipitation. Inexpensive, reliable,
accurate means are needed to monitor
SO2 and NO2 in the atmosphere.
One type of method, which has been
used with varying degrees of acceptance
for two decades, is the passive reactive
monitor. Such a device is exposed for a
period of time, so that the pollutant in the
surrounding ambient air reacts with the
reactive surface of the monitor The
monitor is then returned to the
laboratory, where a chemical
determination of the reaction product is
made. By considering the duration of
exposure and the reactive surface area, a
product flux (F) is determined. This
product flux is used along with a calibra-
tion factor (CF) to estimate the gas phase
concentration of the pollutant (C)
Historically this calibration factor, which
relates ambient air pollutant concentra-
tion to product flux (CF = C/F), has been
assumed to be constant. The magnitude
of this calibration factor may depend on
several variables, such as pollutant
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concentration, wind speed, humidity, and
temperature.
Passive monitors for S02 and N02 are
available from commercial suppliers.
These devices are known as sulfation and
nitration plates. The objective of this
study was to evaluate the performance of
commercial sulfation and nitration
plates. The study was designed to
determine the sensitivity of plate calibra-
tion factors to pollutant concentration,
simulated wind speed, humidity, and
temperature.
Experimental
An exposure apparatus was designed
and built; it permits the simultaneous
exposure of up to 40 plates, with half
exposed at a high gas velocity and half at
a low gas velocity. In the high velocity
(HV) channel, the gas velocity at the
centroid averaged 471 cm/s, and in the
low velocity (LV) channel, the centroidal
velocity averaged 66.6 cm/s. The test
section was approximately 152 cm long;
it contained two parallel channels, and
each channel had dimensions of 6.35 by
10.2 cm. The exposure apparatus was
contained in a temperature-controlled
environmental chamber that can
maintain temperature to within ± 0 5°C.
Air sampling occurred both up- and
downstream of the test section on a 30-
min cycle controlled by a Chrontrol timer.
A Monitor Labs Model 8440 Nitrogen
Oxides Analyzer and Model 8450 Sulfur
Monitor were used to determine the
pollutant concentrations in the test
apparatus. An EG&G Model 880 was
used to monitor dew point, and a shielded
thermistor was used to measure
temperature. Sulfation and nitration
plates were acquired from SERCO (P.O.
Box 625, Cedar Falls, Iowa 50613) for
testing The sulfation plate is a 5 0-cm
diameter petri dish with a lead dioxide
(PbO2) coating that is reactive with SO2.
The nitration plate is identical to the
sulfation plate, except it has a coating of
triethanolamme (TEA) that is reactive with
both S02 and N02.
The experimental program consisted of
eight exposures. The exposures were
designed to study the effects of SO2 and
N02 concentrations, temperature, rela-
tive humidity, and wind speed on the
sulfation and nitration plates.
After exposure, each plate was
desorbed in 50 mL of sodium
bicarbonate-sodium carbonate eluent
solution; the solutions were subjected to
nitrite (NO2)1, nitrate (NOa), and sulfate
(SOv ) analysis by ion chromatography
(1C). 1C analysis provided a direct measure
of pollutant reaction product loading in
units of micrograms per plate. By using
the area by plate (20.27 cm2) and the
duration of exposure (day), the product
flux in units of micrograms per square
centimeter per day was determined. The
pollutant gas phase concentration was
determined by the gas monitor and con-
verted from parts per billion into micro-
grams per cubic meter. The calibration
factor is simply the ratio of the gas con-
centration to the product flux. It should be
noted that the reciprocal of the calibration
factor (adjusted by the appropriate stoi-
ichiometry and molecular weight) may be
expressed as the deposition velocity in
centimeters per second.
Results
The performance of commercially
available passive monitors for S02 and
N02 was evaluated. Sulfur dioxide can be
collected as S0l~ on both sulfation and
nitration plates Nitrogen dioxide can be
collected effectively only on the nitration
plates and is retained as NO2 Nitration
plates were subject to S04 and NOf
loss upon storage and should be analyzed
promptly after use. No S04 loss was
observed for sulfation plates.
The percentage changes in CF
corresponding to changes in the tested
environmental variables are given in
Table 1, along with precision estimates.
Sulfation CF, for both types of plates,
were relatively independent of SO2
concentration and temperature. They
were dependent on wind speed and
relative humidity They were also subject
to appreciable lot-to-lot differences, and
for sulfation plates, to smaller within-lot,
plate-to-plate differences. As the
simulated wind speed was reduced from
4.7 to 0.7 m/s, the CF increased by
factors of 3.1 and 4.5 on sulfation and
nitration plates. As the relative humidity
was reduced from 79 to 16% the sulfation
CF increased on the average by 45% for
sulfation and nitration plates. The lot-to-
lot precision of S02 deposition to nitration
plates was better than 1%; but for
sulfation plates, this difference averaged
30%.
Nitration CF were sensitive to most of
the tested variables. As the N02 concen-
tration was increased from 20 to 200
parts per billion, the CF increased by a
factor of 2.5. As the simulated wind speed
was reduced from 4.7 to 0.7 m/s, the CF
increased by a factor of 2.4. Reducing the
relative humidity from 79 to 16%
increased the nitration CF by 67%. A
temperature change from 27°C to 5.6°C
increased the CF by a factor of 4.6 at the
higher wind speed and 2.9 at the lower
wind speed.
The lot-to-lot variation in nitration CF
was small and was found to be less than
3%. At room temperature, the within-lot,
plate-to-plate variability of NO2 deposi-
tion to nitration plates averaged 2.7% at
the high wind speed and 7.5% at the
lower wind speed.
Test results indicated that for either
sulfation or nitration plates, sulfation CF
were extremely sensitive to wind speed.
Nitration CF were extremely sensitive not
only to wind speed, but to N02 concentra-
tion and temperature as well.
These findings identify and quantify
many of the uncertainties associated
with the use of sulfation or nitration
plates in ambient air quality monitoring.
They should provide insight toward the
interpretation of data collected with such
devices Finally, these results should
provide guidance on the quality of
information that can be expected from
these devices if their deployment is
considered in future air quality studies.
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rabla 1 • Summary of the Sensitivity and Precision of Calibration Factors for Sulfation and Nitration Plates
Plate Calibration Percentage Change in CF Corresponding to Indicated
Type Factor Change of Environmental Variable
Sulfation
Nitration
Nitration
Concentration
20 to 200 ppb
CFso2 ±25
CF$ol ±75
CFNQ- +250
Wind Speed
47 to 07 m/sec
+310
+450
+240
Humidity
79 to 16% RH
+39
+49
+67
Temperature
27 to 5 6°C
negligible a
negligible"
+460b
+290 c
Precision
Lot-to-Lot
±30
negligible3
negligible3
Plate-to-Plate
Withm-Lot
±5.3b
±16C
±35b
±14C
±27"
±75C
"On the average the difference is less than 5%.
b Result is for the high velocity wind speed.
c Result is for the low velocity wind speed.
J. E. Sickles, II and R. M. Michie are with Research Triangle Institute, Research
Triangle Park, NC 27709.
J. W. Spence is the EPA Project Officer (see below).
The complete report, entitled "Investigation of the Performance of Sulfation and
Nitration Plates," (Order No. PB 84-211 184; Cost: $8.50, subject to change)
will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
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, 1984 — 759-015/7781
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Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
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