United States
Environmental Protection
Agency
Atmospheric Research and
Exposure Assessment Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S3-90/034
Sept. 1990
Project Summary
Laboratory and Field Evaluations
of Extrasensiive Sulfur Dioxide
and Nitrogen Dioxide Analyzers
for Acid Deposition Monitoring
EsStsvard E. Bietemani, Jr., Annette H. Green, Robert S. Wright, and
Josepito £. SieKles, III
Hie objective oi this project was to
evaluate the performance characteristics
of selected extrasensltive NO2 and SO2
analyzers in both laboratory and field
environments. The analyzers that were
evaluated are two Columbia Scientific
Industries (CSI) model 1600 NO2 ana-
lyzers, two Scintrex model LIMA 3 NO2
analyzers, and two Thermo Environmen-
tal (TECO) model 43S SO2 analyzers. The
laboratory test procedures for measuring
their performance characteristics were
adapted from the equivalency test proce-
dures given in Chapter 40, Part 53 of the
Code of Federal Regulations. The field
evaluation was conducted over a one-
month period at a site in Research
Triangle Park, North Carolina. The results
of the laboratory and field evaluations are
given in this project summary. Although
EPA has not established performance
specifications for extrasensitive ana-
lyzers, the results of the laboratory and
field evaluations suggest that the analyz-
ers, generally, will perform in an accept-
able fashion for the intended application.
Nevertheless, several specific areas
needing improvement were uncovered in
the evaluations.
This Project Summary was developed
by EPA's Atmospheric Research and
Exposure Assessment Laboratory,
Research Triangle Park, NC, to announce
key findings of the research program that
is fully documented in a separate, report'
of the same title (see Project Report
ordering information at back).
Introduction
The analyzers that were evaluated are
two Columbia Scientific Industries (CSI),
model 1600 NO2 analyzers, two Scintrex
model LMA 3 NO2 analyzers, and two
Thermo Environmental (TECO) model 43S
SO2 analyzers. The CSI model 1600 NO2
analyzers detect NO2 by catalytic conver-
sion of NO2 to nitric oxide (NO) followed
by the chemiluminescent reaction of NO
with ozone (O3). The Scintrex model LMA
3 NO2 analyzers detect NO2 by the
chemiluminescent reaction with luminol
in aqueous solution. The TECO model 43S
SO2 analyzers detect SO2 by ultraviolet
fluorescence using a pulsed light source.
The ranges that the instruments were
tested on are as follows:
CSI model
1600NO2
Scintrex model
LMA 3 NO2
TECO model
43S SO2
0-0.05 part per million
(ppm) (i.e., 0-50 ppb),
0-50 ppb at recorder
output, (0-20 ppb on
front panel meter), and
0-10 parts per billion
(ppb).
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Each instrument also has higher ranges
which were not used in this evaluation.
The two instruments within each model
are designated as A and B in this Project
Summary.
Before evaluating the analyzers' perfor-
mance characteristics, it was necessary
to consider what characteristics are
relevant to performance and how those
characteristics can be experimentally
measured. Performance specifications
and test procedures for ambient air quality
monitoring instruments have been estab-
lished in Chapter 40, Part 53 of the Code
of Federal Regulations. These specifica-
tions are referred to as "equivalency
specifications." In general, the proce-
dures used in the laboratory and field
evaluations are modified versions of the
procedures given in the equivalency
specifications.
A fundamental difference exists
between the purpose of the equivalency
specifications and the purpose of the
present study. The equivalency specifica-
tions were designed to certify that a given
instrument was acceptable for determin-
ing the compliance of a locality with the
National Ambient Air Quality Standards
for the given pollutant. Performance
specifications were established for these
methods and test procedures were given
to determine whether an instrument
demonstrated acceptable performance.
The purpose of the current study was to
evaluate instruments with substantially
better sensitivity than those used for
compliance monitoring. No similar perfor-
mance specifications exist for these
extrasensitive instruments. Thus, this
study estimated the performance parame-
ters of the instruments quantitatively
rather than determining whether these
instruments met predefined specifica-
tions. For this reason, many of the tests
described in the Code of Federal Reg-
ulations were modified for this study.
In addition to the laboratory tests for
evaluating performance parameters, a
field evaluation of the instruments was
performed. The field evaluation was
conducted over a 31 -day period at RTI's
Materials Exposure Site in Research
Triangle Park, North Carolina. The CSI
1600, LM A 3 and TECO 43S extrasensitive
analyzers and other analyzers at the site
were operated under typical ambient air
quality monitoring conditions. This por-
tion of the study provided a real-world
evaluation of the extrasensitive analyzers'
performance. The primary goals of the
field evaluation were to measure the
analyzers' zero and span drifts, their
precision, and any operational failures.
Procedure
Description of the
Laboratory Evaluation
The laboratory evaluation of the NO2
analyzers was done separately from that
of the SO2 analyzers. The laboratory
evaluation consisted of a nine-day test
cycle and separate interference tests. The
test cycle was conducted inside an
environmental chamber where the
temperature and line voltage were con-
trolled according to the schedule given
in Table 1. Day 0 was for setup and initial
calibration.
Various performance parameters for the
extrasensitive analyzers were evaluated
on each test day. These performance
parameters are defined below:
Noise is the standard deviation of
twenty-five consecutive instantaneous
measurements of zero air taken at two-
minute intervals;
Precision is the standard deviation of
six instantaneous measurements of
pollutant concentrations correspond-
ing to 0, 20, 50 or 80% full scale (%
FS) readings. The pollutant concentra-
tion was alternately spiked positively or
negatively between each measurement
and then was allowed to restabilize at
the original concentration;
Limit of detection is three times the
precision at 0% FS;
Limit of quantitation is ten times the
precision at 0% FS;
Lower detection limit is two times the
noise at 0% FS;
Zero drift is the shift in an analyzer's
response to zero air over approximately
24 hours;
Span factor drift is the shift in an
analyzer's span factor over approxi-
mately 24 hours. The daily span factor
is the slope of the linear regression of
the mean of the precision measure-
ments at 0, 20, 50 and 80% FS;
Rise time is the period between the
initial analyzer response and the time
of 95% of the final stable reading after
a step concentration change from 0 to
80% FS; and
Fall time is the corresponding period
after a step concentration change from
80 to 0% FS.
The test manifold for the laboratory
evaluation was fabricated from 0.95-cm
ID PFA Teflon tubing. Test atmospheres
containing NO2 were prepared by gas-
phase titration and dilution of 5 parts per
Table 1. Laboratory Evaluation Test Schedule
Test
Day
0
1
2
3
4
5
6
7
8
Line Voltage
(Volts AC)
115
125
105
125
105
125
105
125
105
Temperature
(Degrees Celsius)
25
20
20
30
30
20
20
30
30
million (ppm) NO in nitrogen contained
in a compressed gas cylinder. Test
atmospheres containing SO2 were pre-
pared by dilution of 0.8 ppm SO2 in
nitrogen contained in a co'mpressed gas
cylinder. The diluent gas was ambient air
that had passed through a permeation
drier, a heated catalyst bed and chemical
scrubbers. The dilution air for the NO2
analyzers was' humidified to approxi-
mately 20% relative humidity. The total
sample flowrate was 10 liters per minute.
The diluted pollutants were mixed turbu-
lently by a 36-cm length of 0.17-cm ID
PFA Teflon tubing.
Interference tests were conducted after
the nine-day test cycle had been com-
pleted. In many cases, test atmospheres
containing interferent gases were pro-
duced by dilution of interferent gases
contained in compressed gas cylinders.
O3 was produced by the ultraviolet
irradiation of air, nitric acid (HNO3) by a
permeation tube, nitrous acid (HNO2) by
a sublimation generator, peroxyacetyl
nitrate (PAN) by gaseous dilution of a
PAN/iso-octane solution in Tedlar bags,
and water vapor (H2O) by passing the
dilution air through a glass bulb contain-
ing water.
In general, the interference tests were
performed in the presence of a pollutant
concentration equivalent to a 50% FS
reading. The exceptions were O3, HN03
and HNO2 for the NO2 analyzers. These
interference tests were performed in the
absence of NO2 to eliminate gas-phase
reactions or because of experimental
constraints. The CSI NO2 analyzers were
used to measure the PAN and HNO2
concentrations for the interference tests
of the Scintrex NO2 analyzers. Based on
the reported performance of other gas-
phase chemiluminescenceNO2 analyzers
to atmospheres containing PAN or HNO2,
it was assumed that the CSI NO2 analyzers
respond quantitatively to these com-
pounds.
A summary of the results of the labor-
atory evaluation (excluding the interfer-
-------
ence tests) is given in Table 2. The results
of the interference tests are given in
Table 3.
Description of the
Field Evaluation
Following the laboratory evaluation, the
extrasensitive analyzers underwent a 31 -
day field evaluation at a monitoring site
on the RTI campus. The purpose of the
field evaluation was to test the analyzers'
performance under realistic field condi-
tions. The field evaluation tested for
operational failures, zero and span factor
drifts and precision.
The monitoring site is a grassy, open,
2.4-hectare area located near a four-lane
highway. The extrasensitive analyzers
were housed in a temperature-controlled
trailer. Continuous monitors for several air
pollutants and meteorological parameters
are also operated in this trailer in con-
junction with other RTI projects.
The extrasensitive analyzers were
calibrated on a daily basis for the first 17
days of the field evaluation and on a bi-
daily basis for the remainder of the field
evaluation. The calibration apparatus was
the same as was used during the labor-
atory evaluation with the exceptions that
the NO2 and SO2 analyzers were cali-
brated simultaneously and that the diluent
air was not humidified. The SO2 analyzers'
data were corrected for the NO remaining
from the gas phase titration. In general,
the extrasensitive analyzers' zero and
span pots were not adjusted during the
field evaluation.
The extrasensitive analyzers sampled
ambient air during those portions of the
field evaluation when they were not being
calibrated. The purpose of these ambient
air quality measurements was to produce
matched sets of NO2 and SO2 data to
compare the analyzers and from which
analyzer precision estimates could be
calculated. The NO2 analyzers produced
483 sets of hourly mean concentrations,
and the SO2 analyzers produced 463 sets
of hourly mean concentrations. The SO2
analyzers' data were corrected for the NO
interference using the relative interfer-
ence responses to NO and the NO
measurements from the CSI NO2 ana-
lyzers.
Maintenance and Failures
During the laboratory evaluation, the
CSI 1600 and TECO 43S analyzers
required only scheduled maintenance
and experienced no operational failures.
One LMA 3 analyzer (i.e., LMA B) expe-
rienced problems with fluid backup in the
air intake trap. The problem was traced
to worn pump tubing and a misaligned
tensioning bracket on the pump. Realign-
ing the bracket and replacing the tubing
at two-week intervals solved that prob-
lem. Erratic responses were noted after
several days operation of the LMA 3
analyzers. This problem was eliminated
by backflushing the LMA 3 analyzers on
a daily basis.
During the field evaluation, the TECO
43S and CS11600 analyzers required only
scheduled maintenance. One CSI 1600
analyzer (i.e., CSI A) experienced a
temporary failure in its NOX channel due
to an air conditioner failure at the field
site. All three analyzers experienced
noticeable but temporary changes in
calibration during the air conditioner
failure. One LMA 3 analyzer (i.e., LMA A)
had a solution backup into its ozone trap.
It is possible that an earlier replacement
in pump tubing could have prevented this
problem. The other LMA 3 analyzer (i.e.,
LMA B) exhibited a significant span factor
drift that appeared to be associated with
excessive wear of the pump tubing. This
problem is discussed further below.
Intra-Model Agreement
From the field evaluation data, the intra-
model agreement is estimated by the
concentration difference between the
hourly average ambient air concentra-
tions indicated by the two analyzers within
a given model. The 30-day, overall mean
concentration difference for the LMA 3
analyzers is 0.733 ppb. On a daily basis,
the mean concentration difference varies
from 1.577 to 1.793 ppb. The pattern of
the day-to-day variation in the mean
concentration differences parallels the
Table 2. Summary of Results for the Laboratory Evaluation of Extrasensitive SOZ and NO2 Analyzers (Excluding Interference Tests)
Performance
Parameter
Mean Noise (am)
Mean Noise (pm)
Mean Precision (0% FS)
Mean Precision (20% FS)
Mean Precision (50% FS)
Mean Precision (80% FS)
Limit of Detection"
Limit of Quantitatione
Lower Detection Limit
Mean Daily Zero Drift (am)
Mean Daily Zero Drift (pm)
Mean Daily Span Drift (20% FS)
Mean Daily Span Drift (50% FS)
Mean Daily Span Drift (80% FS)
Mean Daily Span Factor Drift
Mean Rise Time
Mean Fall Time
Nitrogen Dioxide Analyzers
Units
ppb
PPb
ppb
PPb
ppb
ppb
ppb
ppb
PPb
PPb
ppb
%
%
%
%
min
mm
LMA A
0.003
0.002
0.003
0.040
0.065
0.105
0.009
0.030
0.005
-0.00
-0.00
-1.3
0.4
0.7
-1.0
1.8
0.2
LMAB
0.004
0.003
0.003
0.050
0.078
0.165
0.009
0.030
0.006
0.00
-0.00
-3.3
-1.2
-0.5
-0.0
1.9
0.3
CSI A
0.280
0.303
0.221
0.300
0.300
0.297
0.663
2.210
0.560
0.01
-0.00
0.3
0.6
0.5
-0.6
4.9
4.2
CSIB
0.335
0.358
0.363
0.408
0.364
0.426
1.089
3.630
0.670
0.04
0.02
0.1
0.6
0.3
-0.4
4.3
3.8
Sulfur Dioxide Analyzers
TECO A TECO B
0.032
0.033
0.029
0.038
0.046
0.052
0.088
0.292
0.065
0.010"
0.011"
0.1"
0.4"
0.5"
-0.4
3.5
3.4
0.016
0.015
0.016
0.017
0.027
0.035
0.049
0.163
0.031
0.003"
0.003"
0.7"
0.7"
0.8"
-0.9
3.4
3.5
a These values are calculated from the mean precision at 0% FS. Slightly higher values for the LMA 3 and TECO analyzers are obtained when
the concentration-dependent precision estimates are used.
"Higher values for the zero and span drifts are obtained when Days 3 and 7 are excluded from the calculations. These revised zero and span
drifts are equivalent to approximately 1.1%FS per day.
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Tab/e 3. Interference Test Results for Extrasensitive Analyzers
Relative Interferent Response
(A [NOJ or JSP J Response/A. [Interferent])
Interferent
Species
02
C02
H2O
SO2
HCI
NH3
03«
PAN
HN03"
HNO-f
02
CO2
NO
H2S
H20
Xy/ene
/.M/M
3X10(-8)
-4X10(-6)
-1X10(-7)
1 X 10(-3)
2X 10(-3)
ND
ND
7X 10(-1)
ND
, 4X10(-1)
TECOA
ND
ND
3X1'0(~2)
1X10(-3)
ND
1 X 10(-2)
LMAB
3X10 (-8)
-5X 10 (-6)
-2X 10 (-7)
ND
6X10 (-4)
ND
ND
6X 10(-1)
ND
6X10(-1)
TECOA
ND
ND
2X10 (-2)
1 X 10(-3)
-7X 10(~9)
8X10(-3)
CSIA
ND
ND
-6X 10(-8)
1 X 10 (-3)
ND
6X10 (-2)
ND
NA
1 X 10(0)
NA
CSIB
ND
ND
-7X 10(-8)
; ND
ND
2X 10(-1)
ND
NA
1 X 10(0)
NA
I
l'
'
I
I
ND = Not detected. The change in the analyzer response is less than the 95% confidence interval
for the precision at 50% full scale.
NA = Not applicable. The CS11600 analyzers were used to measure the interferent concentration.
The WO2 concentration 0 ppb for this interference test. For the remaining interference tests,
the pollutant (i.e., NO2 or SOJ concentrations corresponded to analyzer readings of 50% full
scale.
pattern of the day-to-day variation in LMA
B's span factor (see below). The two
phenomena appear to be related in some
manner.
The overall mean concentration differ-
ence for the CS11600 analyzers is -0.259
ppb. On a daily basis, the mean concen-
tration difference varies from 1.134 to
0.212 ppb. The daily mean values are
generally consistent for the first 15 days
of the field evaluation until the air
conditioner failure and show greater
variability afterwards.
The overall mean concentration differ-
ence for the TECO 43S analyzers is 0.036
ppb. On a daily basis, the mean concen-
tration difference varies from 0.008 to
0.112 ppb. Relatively little day-to-day
variation is seen in these daily mean
values.
Noise
The LMA 3 NO2 analyzers had consid-
erably less noise than the CSI 1600 NO2
analyzers. The noise level for the TECO
43S SO2 analyzers was between that of
the two NO2 analyzer models. During the
field evaluation, ambient air measure-
ments using the LMA 3 analyzers exhi-
bited more short-term variability than the
corresponding measurements for the CSI
1600 analyzers. However, the LMA 3
analyzers' responses were less noisy
during field calibration than those of the
CSI 1600 analyzers. It appears that the
LMA 3 analyzers were responding to
short-term fluctuations in the ambient air
NO2 concentration which were being
integrated over longer time periods by the
CS11600 analyzers.
Precision I
In the laboratory evaluation, the LMA
3 NO2 analyzers had lower precision
values (i.e., better precision), expressed
as standard deviation than the CSI1600
NO2 analyzers. The precision values for
the LMA 3 NO2 analyzers increased with
increasing concentration. The precision
values for the CS11600 NO2 analyzers did
not exhibit a concentration dependence.
The precision values for the TECO 43S
SO2 analyzers were lower tha.n those of
the NO2 analyzers and showed a con-
centration dependence.
From the field evaluation data, esti-
mates of the intra-model precision and
the effective operating precision are
calculated from ambient air measure-
ments. The first estimate is obtained by
dividing the standard deviation of the
hourly mean concentration differences
between the two analyzers within a given
model by the square root of 2. The
precision estimate is somewhat in error
for the LMA 3 and TECO 43S analyzers
because of its assumption that the
precision is independent of concentra-
tion. The laboratory evaluation demon-
1 strated that the precision of these ana-
lyzers is concentration dependent. The
second estimate is obtained from the
hourly mean concentrations using the
principal component analysis program
that was developed by Holland and
McElroy. The second estimate includes
both the intra-model precision and the
uncertainty associated with assumed
random interference effects.
The overall standard deviation of the
concentration differences jbetween the
two LMA 3 analyzers is 0.930 ppb, which
results in an intra-model precision esti-
mate of 0.658 ppb. On a daily basis, the
standard deviation varies from 0.091 to
1.901 ppb. This precision estimate may
be influenced by the day-to-day variation
in LMA B's span factor (see below).
The overall standard deviation of the
concentration differences between the
two CSI 1600 analyzers is 0.554 ppb,
which results in an intra-model precision
estimate of 0.392 ppb. On a daily basis,
the standard deviation varies from 0.149
to 1.039 ppb.
The overall standard deviation of the
concentration differences ibetween the
two TECO 43S analyzers is 0.042 ppb
which results in an intra-model precision
estimate of 0.030 ppb. On a daily basis,
the standard deviation varies from 0.012
to 0.061 ppb.
It should be noted that; these intra-
model precision estimates cannot be
compared validly to those obtained during
the laboratory evaluation. The field
precision estimate is for hourly average
measurements of varying concentrations
and the laboratory precision estimate is
for instantaneous measurements of a
constant concentration. Nevertheless, the
precision estimate for the LMA 3 ana-
lyzers obtained from the field evaluation
data is approximately an order of mag-
nitude greater than those obtained from
the laboratory evaluation data. The
difference between the laboratory and the
field precision estimates that is seen for
the LMA 3 analyzers is not seen for the
CSI 1600 and TECO 43S analyzers. The
laboratory and field precision estimates
-------
are in good agreement for the latter ana-
lyzers. ,
The effective operating precisions of
the extrasensitive analyzers during the
field evaluation were calculated using the
principal component analysis program
that Holland and McElroy developed. The
hourly mean concentrations were used in
the calculations. This statistical method
has a number of assumptions which must
be satisfied if its results are to be
statistically valid. Among these assump-
tions are:
At least three analyzers' data must be
compared;
The range of measured concentrations
is large relative to the precision;
The precision must be concentration-
independent;
The interference effect must be random
and independent of the pollutant con-
centration; and
The analyzers to be compared must be
sufficiently different in their sensitivity
to interfering substances.
These assumptions are violated to some
degree by the extrasensitive analyzer data
from the field evaluation. Accordingly,
precision estimates from principal com-
ponent analysis of these data are some-
what questionable.
The effective operating precisions for
the two LMA 3 analyzers are 0.504 and
0.628 ppb, which compare well with the
corresponding intra-model precision of
0.658 ppb. These precision estimates may
be partially compromised by the span
factor drift that was observed in LMA B.
The frequent calibration of these analyz-
ers tended to reduce the effect of this drift
on the NO2 concentration measurements
and the precision estimates obtained from
these analyzers.
The effective operating precisions for
the two CS11600 analyzers are 0.284 and
0.185 ppb, which are appreciably lower
than the corresponding intra-model
precision of 0.392 ppb. These values are
considerably lower than the value of 2.7
ppb obtained by Holland and McElroy for
the same analyzer model. Their data set
was collected on the analyzer's 0 to 500
ppb range, while this data set was
collected on its 0 to 50 ppb range. The
range that is used may have an effect on
the analyzer's precision.
The principal component analysis of the
SO2 data included measurements from
the RTI trailer's TECO model 43A SO2
analyzer. This analyzer was calibrated
using a different calibration apparatus
from that used for the extrasensitive
analyzers. It was operated on its 0 to 200
ppb FS range. This inclusion was neces-
sary to satisfy the requirement of principal
component analysis that at least three
analyzers' data must be compared.
The effective operating precisions for
the TECO 43S analyzers are 0.0003 and
0.0003 ppb, which are approximately two
orders of magnitude less than the cor-
responding intra-model precision of 0.030
ppb. It is hypothesized that the small
precision estimate from principal compo-
nent analysis is a result of violating the
assumption that the analyzers to be
compared must be different. The effective
operating precision for RTI's TECO 43A
analyzer is 0.111 ppb.
Detection Limits
The detection limits for the LMA 3 NO2
analyzers were lower than those of the
CSI 1600 NO2 analyzers. There was only
a very small difference between the
detection limit estimates for the LMA 3
NO2 and TECO 43S analyzers with and
without compensation for their increase
in precision with concentration. The
detection limits for the TECO 43S SO2
analyzers were between those for the two
NO2 analyzer models.
Zero and Span Drifts
Analysis of the zero and span factors
that were obtained during the laboratory
evaluation for the CS11600 NO2 analyzers
did not show any obvious dependence
on room temperature or line voltage. In
addition, no time-dependent change (i.e.,
secular trend) of either zero values or
span factors was seen for the CSI 1600
analyzer.
The LMA 3 NO2 analyzer showed a
slight dependence of zero values on
temperature (approximately 0.003 to
-0.006 ppb/degree C). This is in agree-
ment with the value of 0.009 ppb/degree
C obtained by Bubacz and Pleil. The LMA
3 analyzers did not show the temperature-
dependent span drift seen in earlier
models by Bubacz and Pleil. No secular
trend or voltage dependence in zero or
span drift was seen for the LMA 3
analyzer.
The TECO 43S analyzers showed a
secular trend in their zero values of
approximately -0.1 ppb/day during the
laboratory evaluation. This made the span
values appear to also drift because the
span drift calculations do not provide for
a zero-drift correction. In addition, the
TECO 43S analyzers' zero values showed
a slight dependence on temperature
(approximately 0.024 ppb/degree C). No
change in span factors with time, temper-
ature, or line voltage was seen for the
TECO 43S analyzer.
During the field evaluation, no secular
trends were observed in the zero values
for the CSI 1600 and LMA 3 analyzers.
The TECO 43S analyzers showed a
secular trend in their zero values of
approximately -0.01 ppb/day. This trend
is approximately an order of magnitude
less than that observed during the
laboratory evaluation in these same
analyzers. No secular trends were
observed in the span factors for the CSI
1600 analyzers and one of the two LMA
3 analyzers. The other LMA 3 analyzer
(i.e., LMA B) exhibited a span factor drift
of approximately 2.2%/day. It is hypothe-
sized that this drift was caused by
excessive wear in the analyzer's pump
tubing because large span factor shifts
were observed when the pump tubing
was replaced in this analyzer. The two
TECO 43S analyzers exhibited span
factor drifts of approximately -0.13 and
-0.28%/day.
Rise and Fall Times
All the extrasensitive analyzers' rise and
fall times were within 5 minutes. The LMA
3 NO2 analyzers' values were noticeably
faster than those of the other extrasen-
sitive analyzers. Additionally, the fall times
for the LMA 3 NO2 analyzers were much
faster than the corresponding rise times.
The LMA 3 NO2 and TECO 43S SO2
analyzers took long periods of time (i.e.,
several hours) to rise from approximately
95 to 100% of their final, stable readings
during each test day's initial change from
an overnight zero air exposure to an 80%
full scale concentration. This pheno-
menon was not observed for the CS11600
NO2 analyzers. In the case of the LMA
3 NO2 analyzers, it is hypothesized that
their ozone traps required conditioning.
In the case of the TECO 43S SO2 ana-
lyzers, it is hypothesized that this phe-
nomenon is associated with the daily
initial conditioning of the calibration
apparatus and the sample lines after the
overnight zero air exposure. This pheno-
menon was not observed after subse-
quent step concentration changes during
each test day.
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Interferences
Each of the extrasensitive analyzer
models responded to one or more inter-
ferents. The LMA 3 NO2 analyzers
responded to oxygen, carbon dioxide,
water, PAN, and nitrous acid. The relative
response to PAN was lower than the
quantitative response reported by Wen-
dell et at. The nitrous acid interference
has not been reported previously in the
literature. Interference from O3 on
luminol-based chemiluminescent analyz-
ers, which was reported in the literature,
was not seen in this study with the LMA
3 analyzers. It appears that the 03 traps
which were supplied by Scintrex are
effective in eliminating the O3 interference
in the LMA 3 NO2 analyzers. The CS11600
N02 analyzers showed responses to
water, ammonia, PAN, nitric acid, and
nitrous acid. The ammonia response was
not found in a previous post-designation
equivalency testing of the CSI 1600 NO2
analyzer. It is hypothesized that insuffi-
cient test manifold stabilization time was
allowed in the earlier testing. The TECO
43S SO2 analyzer responded to nitric
oxide and xylene. The nitric oxide
response was greater than what has been
reported by TECO for the model 43A SO2
instrument
Ambient Air Quality
Measurements
The field evaluation was conducted
from April 26 through May 27, 1988.
During this period, the CS11600 and LMA
3 analyzers produced 483 sets of matched
hourly mean NO2 concentrations. The
TECO 43S and TECO 43A analyzers
produced 463 sets of matched hourly
mean SO2 concentrations during the
same period. Overall mean concentra-
tions for the field evaluation are given in
Table 4.
Tables 4. Overall Mean Concentrations During
the Field Evaluation
Parameter
NOs,
SO2
Analyzer
CSI A
CSIB '
LMA A
LMAB
TECO A
TECOB
RTI-SO2
Overall Mean
Concentration
13.022 ppb
13.314 ppb
11. 309 ppb
10.512 ppb
1.691 ppb
1.653 ppb
1.398 ppb
The SO2 data are in good agreement with
SO2 data collected with a filter pack at
a nearby EPA National Dry Deposition
Network site in Research Triaingle Park.
The EPA site measured an overall mean
SO2 concentration of 1.61 ppb for the
period from April 26 through May 24,1988.
Calibration Apparatus
It was necessary to design and fabri-
cate a calibration apparatus for the
laboratory and field evaluations that is
capable of generating ppb-level SO2 and
NO2 concentrations. In general, the effort
was successful. The laboratory evalua-
tion demonstrated that the apparatus
could generate N02 at low concentrations
via gas-phase titration. It alsio demon-
strated that the turbulent mixing loop
permits the rapid concentration changes
needed for response time measurements.
The daily span factors from the field
evaluation demonstrated that the calibra-
tion apparatus has good repeatability on
a day-to-day basis. However, it was not
possible to evaluate the accuiracy of the
concentrations that the calibration appa-
ratus generated.
The only negative feature of the cal-
ibration apparatus that was discovered is
the long stabilization time (approximately
2 hours) for the TECO 43S SO2 analyzers.
After an overnight zero air exposure and
the initial change to an 80% full scale
concentration, the TECO 43S SO2 ana-
lyzer took a long time to go from 95% to
100% of the final, stable residing. This
phenomenon was not observed for
subsequent step concentration changes
during each test day. It is hypothesized
that the long stabilization time is due to
the need to condition the PlrA Teflon
walls of the calibration apparatus after the
overnight zero air exposure.
A similar phenomenon was observed
for the LMA 3 NO2 analyzers, but it is not
attributed to the calibration apparatus
because it was not seen for the CS11600
NO2 analyzers. It is hypothesized that the
LMA 3 NO2 analyzers' ozone traps
required conditioning.
Conclusions
EPA has not established performance
specifications for extrasensitive analyzers
to be used in acid deposition monitoring.
As a result, conclusions concerning the
specific applicability and acceptability of
these analyzers cannot be drawn. Never-
theless, the results of the laboratory and
field evaluations suggest that Ithe analyz-
ers will perform in a generally acceptable
fashion for the intended application.
Several specific areas of the analyzers'
performance that need improvement were
uncovered in the evaluations.
Recommendations
Performance specifications should be
established for extrasensitive analyzers to
be used in acid deposition monitoring.
Users of monitoring data obtained with
these extrasensitive analyzers should be
made aware of the performance charac-
teristics of the analyzers (especially with
respect to interference effects).
To prevent changes in O2 concentration
from affecting analyzer response, calibra-
tion mixtures for all three types of
instruments should be made using dilu-
tion with air rather than nitrogen or ,
another dilutant. In addition, the LMA 3
NO2 analyzer is sufficiently sensitive to
O2 concentration that the percentage of,
O2 in synthetic zero air must be closely
specified. Similarly, the CO2 and water
concentrations of calibration mixtures for
the LMA 3 NO2 analyzer should be similar
to those in ambient air to prevent changes
in analyzer response due to differing
concentrations of CO2 and water. The
water concentration of calibration mix-
tures for the CSI 1600 NO2 analyzer
should be similar to that in ambient air
to prevent interference effects. Sites using
the TECO 43S SO2 analyzer should
monitor NO concentrations to be able to
detect and possibly correct for NO
interference.
Sites using the LMA 3 NO2 analyzer
should backflush the analyzer daily and
change the pump tubing every two weeks
to prevent operational failures. All three
types of instruments should be operated
in buildings with heating and air condi-
tioning, as evidenced by the temporary
change in zero and span values which
occurred when the air conditioner at the
Materials Exposure Site malfunctioned.
Further investigations of some of the
instrumental characteristics revealed in
this laboratory evaluation should be
pursued. In particular, more extensive
interference tests may be needed for
those compounds which demonstrated
an interference in this study. Additional
work is needed to identify the cause of
the span drift observed in one LMA 3 NO2
analyzer during the field evaluation.
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£ £ Rickman, Jr., A. H. Green, R. S. Wright, and J. E. Sickles, II are with Research
Triangle Institute, Research Triangle Park, NC 27711.
Darryl J. von Lehmden is the EPA Project Officer (see below).
The complete report, entitled "Laboratory and Field Evaluations of Extrasensitive
Sulfur Dioxide and Nitrogen Dioxide Analyzers for Acid Deposition Monitoring,"
(Order No. PB 90-201 062/AS; Cost: $23.00, 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
Atmospheric Research and Exposure Assessment Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S3-90/034
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