EPA-600/2-77-003
January 1977
Environmental Protection Technology Series
PULSED FLUORESCENCE MONITOR FOR MEASURING
AMBIENT NITROGEN DIOXIDE
Development of a Laboratory Prototype
Environmental Sciences Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park, North Carolina 27711
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into five series. These five broad
categories were established to facilitate further development and application of
environmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
Th3 five series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
Th s report has been assigned to the ENVIRONMENTAL PROTECTION
TECHNOLOGY series. This series describes research performed to develop and
demonstrate instrumentation, equipment, and methodology to repair or prevent
environmental degradation from point and non-point sources of pollution. This
work provides the new or improved technology required for the control and
treatment of pollution sources to meet environmental quality standards.
Thi;s document is available to the public through the National Technical Informa-
tior Service, Springfield, Virginia 22161.
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PULSED FLUORESCENCE MONITOR
FOR MEASURING AMBIENT NITROGEN DIOXIDE
Development of a Laboratory Prototype
by
C.L. Fincher, A.W. Tucker, and M. Birnbaum
The Aerospace Corporation
Electronics Research Laboratory
Post Office Box 92957
Los Angeles, California 90009
EPA-600/2-77-003
January 1977
Contract Number 68-02-2246
Project Officer
Richard J. Paur
Atmospheric Chemistry and Physics Division
Environmental Sciences Research Laboratory
Research Triangle Park, North Carolina 27711
U.S. ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF RESEARCH AND DEVELOPMENT
ENVIRONMENTAL SCIENCES RESEARCH LABORATORY
RESEARCH TRIANGLE PARK, NORTH CAROLINA 27711
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PISCLAIMER
This report has been reviewed by the Environmental Sciences Research
Laboratory, U.S. Environmental Protection Agency, and approved for publication!
Approval does not signify that the contents necessarily reflect the views and
policies of the U.S. Environmental Protection Agency, nor does mention of
trade names or commercial products constitute endorsement or recommendation
for use.
ii
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ABSTRACT
A prototype pulsed flashlamp ambient NO monitor has been constructed and
tested. The basic principles are similar to a laser fluorescence NO monitor
developed some years earlier at the Electronics Research Laboratory of the
Aerospace Corporation. The prototype flashlamp unit has fully met the
contract design goal of a sensitivity of 5 ppb for an integration time of
approximately 1 min. Its operation has been successfully demonstrated in
monitoring the outside air. The unit provides a direct digital read-out in
real time of the NO^ concentration in ambient air. The prototype unit shows
great promise for further improvement. Its continued development will lead to
an instrument comparable in compactness to present chemiluminscence instru-
ments but probably more economical in cost and certainly more reliable in
operation. The freedom from interferences makes the present instrument
unique. Many new applications will undoubtedly be pursued which utilize the
unique characteristics of the pulsed lamp fluorescence monitor.
iii
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CONTENTS
Abstract • .,......,...,,...,,.,,... .,.,..,.,.....«...,...,., ill
Figures [[[ v±
Tables vi
Abbreviations and Symbols ............................................ vll
Acknowledgement vll
1. Introduction 1
2. Conclusions « 3
3. Description 4
4. Calibration and Linearity Tests 11
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FIGURES
Number Page
1 Pulsed NO Fluorescence Monitor ................................... 19
2 Block Diagram of Optical Subsystem ..................'; ............ 20
3 Block Diagram of Electronic Subsystem ...................... 21
4 Spectral Transmission of the Flashlamp Filter (5 cm Path
of 300 grams per liter CuSO, Solution plus Corning Glass
CS 0-51 Filter). Spectral Output of Xe Flashlamp
(Dashed Line) 22
5 Spectral Transmission of the PMT Filter (2 cm Path of
304 grams per liter Na-Cr-O-. 2H20 Solution Plus
Corning Glass CS 2-62 Filter). Spectral Response
of EMI 9659 QAM PMT (Dashed Line) • 23
6 Linearity Test for N02 Monitor in the 30 to 200 ppb
Range of NO. in Nitrogen 24
L
7 Atmospheric N02 Concentrations, El Segundo, California,
March 17, 1976 25
8 Atmospheric NO,, Concentrations, El Segundo, California,
March 18,1976 26
TABLE
Number Page
1 Instrument Sensitivity; Consecutive
Daily Calibrations .......................< ...;.,. ^....,;.. 13
vi
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LIST OF ABBREVIATIONS AND SYMBOLS
EPA Environmental Protection Agency
LINFM Laser Induced NO. Fluorescence Monitor
NERC/RTP National Environmental Research Center,
Research Triangle Park, North Carolina
ppb parts per billion
PMT photomultiplier tube
PNFM Pulsed NO- Fluorescence Monitor
ACKNOWLEDGEMENT
The cooperation of Dr. Richard Paur, EPA contract monitor, is gratefully
acknowledged. His suggestions throughout this contract period have been
helpful and pertinent.
vii
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SECTION 1
INTRODUCTION
The objective of this program Is to demonstrate the feasibility of a
pulsed N0? fluorescence monitor (PNFM) for ambient air which is compact and
of low cost. Excitation of the NO is provided by a small pulsed xenon filled
flashlamp and the concentration of NO is determined by measurement of the
characteristic N0« fluorescence. The physical principles involved are iden-
tical to those considered in the development of a laser-induced NO. fluores-
1 L
cence monitor.
A program initiated at the Aerospace Corporation in 1970 has led to the
development of unique instrumentation for the monitoring of ambient NO con-
centrations. This instrumentation is based on NO- fluorescence induced by
specially selected sources. Several versions of laser-induced fluorescence
monitor (LINFM) have been developed. A prototype instrument which utilizes
a He-Cd laser at 442 nm was developed for the U.S. Environmental Protection
Agency (EPA) and has been used in both ambient air monitoring and smog chamber
experiments. Accumulated experience with this prototype has established the
sensitivity (<1 ppbv) of the technique as well as a lack of interference from
compounds such as water vapor, and various nitrogen containing compounds.
However, alternate solutions have been sought to reduce the size and cost of
the instrumentation. One such alternative using flashlamp induced fluores-
cence will be described in this paper. Preliminary design and feasibility
studies for the flashlamp system were conducted at EPA/RTP; subsequent devel-
opment was carried out at the Aerospace Corporation.
The current most widely used technique for measuring ambient levels of
N0« is chemiluminescence. Commercial chemiluminescence systems convert NO
to NO before introducing the sample stream into a reaction chamber in which
NO and excess ozone are reacted to give gas phase chemiluminescence. However,
the conversion process is not specific to N0» and other nitrogenous compounds
7
such as peracetyl nitrate (PAN) also yield NO. Winer, et al , has documented
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the extent to which a number of nitrogen-containing compounds cause a response
in a typical commercial monitor. Since the ratio of atmospheric NO to other
nitrogen containing compounds, such as PAN, is normally high, the chemilumi-
nescence monitors provide reliable data to determine compliance with national
air quality standards. Under conditions of low concentrations of N0« to other
nitrogenous compounds (which is a common situation in smog chamber studies but
is only rarely encountered in the atmosphere) the chemiluminescence analyzers
may suffer significant interference.
The development of a flash lamp NO fluorescence monitor (FLNM) is a
significant step towards new instrumentation which offers the advantages of
the LINFM with respect to freedom of interference, but, in addition, should
be economical, reliable and of simple construction. It is anticipated that
further development of the FLNM could result in instrumentation more advan-
tageous than present chemiluminescence instruments and could lead to a wide-
spread utilization of FLNM type of N0? monitors.
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SECTION 2
CONCLUSIONS
The goals of this contract have been successfully demonstrated, A proto-
type pulsed flashlamp NO- fluorescence monitor has been developed and tested.
The instrument displayed a linear response and a detectability of 5 ppb of
N09. The operating principles are similar to those of the laser fluorescence
N02 monitor developed earlier at Aerospace Corporation, which was shown to be
free of interferences, and consequently, the present instrument should not
exhibit any undesirable interferences. Admittedly, it is desirable to test
the prototype instrument directly to assess its freedom from interferences.
The instrument was demonstrated to perform satisfactorily in monitoring
ambient N02 in the vicinity of our laboratory. The prototype shows every
evidence of stable, reliable and long life operation.
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SECTION 3
DESCRIPTION.AND CHARACTERISTICS OF THE PULSED
FLUORESCENCE N02 MONITOR
DESCRIPTION
A photo of the prototype is shown in Figure 1. The NO chamber is
located along the back side of the optical table with the xenon flashlamp
housing mounted on the right arm of the chamber. The cooled photomultiplier
can be seen in the center foreground mounted on the side of the chamber. The
electronic packages are situated on top of the table on both sides of the
photomultiplier housing.
OPTICAL SUBSYSTEM
The layout of the optical system, depicted in the diagram of Figure 2,
shows the excitation xenon flashlamp and the N0« chamber with the side mounted
photomultiplier tube.
A USSI (United States Scientific Instruments, Inc.) type 3CP-1 xenon
flashlamp is used for excitation. This lamp has axial geometry with an arc
length of 1 mm and is mounted with the long axis vertical. The maximum
average power input to the lamp is rated at 100 watts (100 joules at 1 pulse
per second or .01 joules at 10,000 pulses per second). The life expectancy
of the lamp under these circumstances is 3 x 10 flashes, where life expec-
tancy is defined as the number of flashes allowed before the light output will
drop to 50% of its original value.
The lamp is driven by a discharge of a 10.6 microfarad capacitor charged
to 730 volts (2.8J stored energy). With an energy input of about 2.8 joules,
a pulse duration of 6.5 us and a pulse repetition rate of 13 pulses per
second, the average input power is 37 watts or about 37% of its maximum
rating. Under these circumstances the life expectancy of the lamp should be
about 10 flashes or close to one year in continuous operation. Typical
spectral output of the lamp within the region of interest is shown in Figure 4.
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The lamp Is placed at the focal point of a 5 cm diameter, 2,5 cm focal
length lens. Spatial filtering and collimation of the beam was accomplished
by the use of two 5 cm focal length lenses placed 10 cm apart with a 4 mm
diameter aperture at the midpoint between them, In order to reject off axis
light, the first lens (5 cm diameter) has a 3.8 cm diameter mask directly in
front of it and the second lens (2.5 cm diameter) had a 2.5 cm diameter mask.
This whole assembly was placed 2 cm from the collecting lens. The beam is
introduced into the chamber through a Corning glass CS 0-51 filter and a 5 cm
path of saturated CuSO, solution, respectively. The pass band of this filter
combination is shown in Fig. 4. The energy delivered to the NO *jphple»uiaingsx
this filter combination was found to be 80uj per pulse for 2.8J input to lamp.
The Corning CS 0-51 filter is used to reject UV light below 400nm. Wave-
lengths shorter than 400 nm produce dissociation of the N0« molecules and are
very inefficient in exciting fluorescence. The CuSO, solution filter absorbs
light longer than 580 nm. The filter combination is placed 5 cm from the lamp
collimator and 33 cm from the center of the chamber. This distance could be
shortened to as little as 12 cm before the fluorescence of the quartz window
on the solution filter cell will cause an appreciable increase in background
signal.
The entrance to the central part of the chamber is sealed with an air-
tight quartz window. A 15 cm f .1. lens is used to focus the beam to a
diameter of approximately 1 cm at the center of the field of view of the
photomultiplier. Immediately following the lens there is a 1.8 cm mask and
6 cm beyond the lens another 1.5 cm aperture which is 6 cm from the center of
the field of view of the PMT. These apertures reduce scattered off-axis
light and thereby reduce the background fluorescence. The inside of the front
arm is blackened to reduce scattered light. The center portion of the chamber
is essentially a 5 cm diameter tube, 12.5 cm long with a 2.5 cm long side arm
on which the PMT Is mounted for viewing the fluorescence. The inside of this
tube was blackened to reduce scattered light. A mask with a 1 cm wide by
2.5 cm long rectangular slot is provided through which the fluorescence is
viewed by the PMT. A cylindrical front surface aluminized mirror positioned
around th.e back side of this center section to reflect the fluorescence light
into the PMT resulted in doubling the signal.
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After exiting the center section, the light beam passes into a 25 cm
long rear arm which is blackened on the inside and contains three baffles
placed 10 cm apart starting from the end of the center section. The first
baffle is 3.2 cm in diameter, while the last two are;3.8 cm in diameter, The
beam is absorbed on a blackened "wall" at the end of the rear arm. However,
in the center of this "wall", there is a small hole through which the detector
for the light integrator controller samples the beam energy, The chamber is
sealed with a fused silica window oriented 55.6° with respect to the direction
of the incident beam (Fig. 2).
All optical elements, apertures and masks, including their sizes and
locations, between the flash lamp and the end of the rear arm were chosen
with a view toward minimizing the background signal. Individual elements
themselves may only have contributed to a small reduction but in combination
with the other elements resulted in a substantial reduction in the background.
The side arm of the chamber is sealed with a quartz window, A 5 cm focal
length lens (5 cm in diameter) is used to collect and focus the fluorescence
onto the surface of the PMT. Situated between the viewing window and the lens
is a 2 cm path length of 304 grams per liter solution of Na Cr 0 ,2H 0. This
solution filter strongly absorbs the scattered light from the flashlamp with-
out producing detectable fluorescence. However, there is a small overlap in
the bandpass of the CuSO/ filter in front of the flashlamp and the Na Cr 0 ,
2H-0 solution which transmits about .05% at 560 nm. To eliminate the band
overlap a Corning CS 2-62 filter with a cutoff at 590 nm was placed in back
of the Na Cr-0 solution filter. The location of this Corning filter is
important. If placed in front of the solution filter, the Corning filter
would be exposed to the scattered flashlamp light and the resulting fluores-
cence would again increase the background.
The resultant fluorescence bandpass of the combination Na^Cr^O solution
filter and CS 2-62 Corning filter is shown in Fig. 5.
The detector used for observing the fluorescence is a EMI 9659 QAM
photomultiplier with an extended S—20 response, installed in a Products for
Research type TE 12 OTS-RF thermoelectric cooled housing with a PFR type
271TSA-9558 dynode chain assembly. The PMT was cooled to approximately -12°C.
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Tests on uncooled 9659 QAM PMT Indicated a detectabillty of 11 +_ 3 ppb
of NO.. At this point, it was noticed that the fluctuations of the back-
ground signal (which is the limiting factor for our detectability) were of
the same order of magnitude as the fluctuations of the PMT dark current
signal. Consequently, improved sensitivity could be obtained by cooling the
PMT.
On cooling the PMT, the dark current fluctuations were reduced by a
factor of 8. The fluctuations in the background signal were only reduced by
a factor of 2 and this is in part due to the reduction in the PMT dark current.
This reduction in the magnitude of the background fluctuation combined with
a 10% Increase in sensitivity of the PMT due to cooling, resulted in the
present level of detectability, 5.6 ppb +_ 1 ppb.
ELECTRONICS SUBSYSTEM
A block diagram of the electronics system of the laboratory model pulsed
NO. fluorescence monitor (PNFM) is shown in Fig. 3. The overall operational
scheme of the PNFM is that fluorescence photons are generated when light from
a pulsed xenon flashlamp excites NO. molecules in air drawn through the mon-
itor's examination chamber. A photomultiplier tube senses the fluorescence.
Electronic circuitry processes the electrical signal generated by the PMT to
provide the NO concentration measurement data. A more detailed operational
discussion follows.
During a measurement cycle the flashlamp is controlled either by a flash
counter or by an optical energy meter. In the flash counter control mode a
specific, selectable number of lamp flashes will occur during a measurement
cycle, In the energy meter control mode a fixed amount of optical energy
passing through the air examination chamber sets the duration of a measure-
ment cycle. Both lamp control modes were used during the testing phase of
the PNFM. For test purposes the number of lamp flashes during one measure-
ment cycle was arbitrarily set to exactly 1000 flashes for the flash counter
mode and the optical energy meter controller was set to stop a measurement
cycle when an accumulated energy measurement corresponding to about 1000 of
these flashes or 80 mJ had passed through the air examination chamber.
Although our test- results indicate that both lamp control modes provide
almost identical NO detecting capability, the optical energy controller
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could be used to compensate for lamp variations, i.e., if a flashlamp's energy
per flash, were to degrade with, time, the optical energy controller would
assure that a sufficient number of lamp flashes were added per measurement
cycle to produce the pre-selected optical energy. Conversely, if the lamp or
its operating conditions were to change such that more energy per flash was
emitted, the controller would reduce the number of flashes per measurement
cycle to attain the pre-selected energy.
The electronic charge signal (output of the PMT) is processed by the
PNFM's electronic circuitry to provide the NCL measurement data. The opera-
tion of the circuitry is as follows. A capacitor connected to the output of
the photomultiplier tube receives the electronic charge when enabled to do so
by an electronic switch connected to the terminals of the capacitor, The
switch maintains a short circuit across the capacitor terminals at times other
than a fixed period that includes the flashlamp operating time. The switch
opens for a period of 100 microseconds during which time the lamp flash starts
and ends. Since the fluorescence lifetime of the NO molecule is only about
-9
10 seconds, the photomultiplier responds to the total fluorescence signal
by producing an electronic charge on the capacitor that is proportional to
the fluorescence signal. The resulting capacitor voltage is buffered by a
unity gain operational amplifier stage and presented to another unity gain
operational amplifier operating in a sample and hold circuit configuration.
The sample and hold stage acquires the capacitor voltage at its final ampli-
tude and stores the amplitude of this signal in a "holding" memory capacitor.
The sample and hold circuit commences its holding operation before the 100
microsecond switch opening period is completed and continues to hold the
amplitude for further processing after the capacitor shorting switch has
closed. The output signal of the sample and hold stage that results from a
sample taken when the lamp was flashed is fed to a high gain (x500) amplifier
and then presented to the add/subtract integrator where a fixed time duration
sample of the signal voltage is taken and stored as a charge in the "long
duration" memory capacitor of the add/subtract integrator. Before the next
lamp flash occurs, a control circuit initiates signals that erases the signal
stored in the sample and hold memory and starts a new sampling period by
sampling the output from the photomultiplier for a 100 microsecond period.
But, this time the flashlamp does not flash, so, no fluorescence signal
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component Is present in the processed signal at the output of the high gain
amplifier. The control circuitry routes this signal through a unity gain
polarity inverter and then presents it to the input of the add/subtract inte-
grator. Again a fixed time duration sample of the amplitude is taken and
added to the memory. The result is that the sample taken when the lamp did
not flash is subtracted from the sample taken when the lamp did flash by the
add/subtract integrator. This add/subtract routine is repeated for each
flash of the measurement cycle, i.e., about 1000 times, The partial sums
from the add/subtract routines accumulate to a final value at the end of the
measurement cycle. What is accomplished by this signal processing operation
is explained below.
The sample that appears at the output of the high gain amplifier stage,
corresponding to when the lamp did flash, includes the NO fluorescence
signal, the photomultiplier tube dark noise, the dc offset noise introduced
by the electronic circuitry and a noise component that arises when the light
from the flashlamp shines through the air examination chamber while a NO -
free gas is present. The sample that appears at the output of the high gain
amplifier when the lamp does not flash includes only the photomultiplier dark
noise and the electronic circuitry dc offset noise. So, by inverting the
polarity of the sample taken with,the lamp off and adding it to the sample
taken with the lamp on we expected to cancel the electronic circuitry offset
noise and the photomultiplier dark noise. The electronic circuitry noise
cancelled as expected. However, the dark noise of the uncooled 9659 QAM
photomultiplier is of a statistical character and complete cancellation is
not expected. Statistical fluctuations in the output of the PMT resulted in
an NO. detectivity of~11 ppb. With the photomultiplier tube cooled to
12°C the photomultiplier dark noise was reduced and detectivity was
improved to ~ 5 ppb.
At the output of the add/subtract integrator there is, in addition to
•the fluorescence signal component, the noise component that arises when the
flash lamp light illuminates a NO -free gas, i.e., the air with "zero" N02
present. It was expected and experimentally verified that this noise would
be essentially a constant value when the lamp energy was held constant during
a measurement cycle. A "background suppression" bias control was incorpor-
ated to supply a selectable, fixed voltage of opposite polarity to cancel
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this noise. The processed signal with the background correction is essentially
all due to fluorescence from the NO. so the instrument can be calibrated for
N02 response by measuring the voltage produced at the end of a measurement
cycle when a known concentration of NO is present in the air or carrier gas
drawn through the chamber during a measurement cycle. It was convenient for
us to scale the NCL calibration voltage with a ten-turn linear potentiometer
by setting the potentiometer dial to a fractional part of the full scale value
that equated 1 ppb NO concentration with--1 millivolt of signal, The scaled
voltage was presented to the digital voltmeter. The voltmeter reading printed
at the end of a reading cycle was in units of millivolts, hence directly
equivalent to ppb of N0« concentration in the air being examined. Each time
a measurement cycle was started the electronic control circuitry erased the
accumulated sum in the add/subtract integrator before proceeding with the new
measurement. Typically, a calibration measurement yielded 3 millivolts per
part per billion NO,, concentration before scaling by the potentiometer. ConP
sidering that this resulted from about 1000 flashes of the lamp it is seen
that a single flash of the lamp produced only about 3 microvolts per part per
billion N0« concentration,. Actually, the signal at the output of the photo-
multiplier tube was a nominal 30 microvolts per flash per ppb N02 concentration,
but the combined gain of the high gain amplifier and the add/subtract inte-
grator in the PNFM was chosen to be 0.1 from other considerations.
Integration of the fluorescence signal resulting from more than one
flash of the lamp is desirable mainly because the gain of the photomultiplier
tube is statistical in nature. The averaging technique described above is-t! "'
useful in reducing the statistical fluctuation effect in the measurement
signals. Also, the higher voltage amplitudes resulting from the summation of
many fluorescence signal samples is more compatible with the electronics of
the display system.
The photodiode and optical energy meter controller were identical to
those employed in the prototype laser NO monitor developed for EPA under
contract No. 68-02-1255. The add/subtract integrator used another of those
type units that was modified to perform the integration of electronic signals.
The operation controller was constructed using standard TTL logic modules,
This unit was designed to facilitate laboratory investigation of several
operating regimes of the PNFM.
10
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SECTION 4
CALIBRATION AND LINEARITY TESTS
Calibration and operation of the system was performed in the excitation
light control mode in which a readout is obtained after a preset energy has
been delivered to the sample from the flashlamp. The controller was adjusted
to sense 80 mJ of energy which corresponded to approximately 1000 pulses of
the flashlamp. This means that a single measurement of the NO. concentration
requires about 77 seconds.
As stated earlier, a background offset was incorporated into the system
to zero out the background. In addition, the output may be scaled so that
the system will readout directly the NO. concentration in ppb. In light of
this, the system was calibrated and adjusted in the following manner.
First, with pure nitrogen flowing through the chamber, 54 measurements
of the background signal were obtained. The average value of the background
signal was 508 +_ 11 mV which corresponds to 156 + 3 ppb (based on calibration
constant determination to be discussed next).
The background offset control was then adjusted to zero out the back-
ground. After the adjustment was made, 25 readings were taken of the zero
level. The average zero level was -1 +_ 9 mV.
Next a known sample of N07 was delivered to the chamber using a standard
1 cm N0_ permeation tube. Based on the manufacturer's permeation rate data,
flow rate, pressure and temperature, the sample NO concentration was deter-
mined to be 133 +_ 10 ppb. To provide some information on the measurements
involved in the determination of the calibration constant we list the fol-
lowing values: temperature, 24,61 + .02°C: pressure, 758+3 Torr; flow rate,
4.34 + 0.17 ^/min; permeation rate, 1083 +72 ng/min; NO concentration, 133
+_ 10 ppb, Twenty-five readings were taken of the NO. sample. The average
value obtained was 433 + 4 mV. With a zero level of -1 +9 mV, the NO
measurement would be 434 HH 10 mV, giving a calibration constant of 3.26 +
11
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.26 mV/ppb or an accuracy of 8%. The system is then scaled for a 1.00 +_
0,08 mV/ppb readout, which provides a direct readout of the NO concentration.
Scaling the system, by a factor of 3,26 also scales down the background
fluctuations by a factor of 3,26. This means with a 1,00 mV/ppb scale factor
and a zero level of 0 + 4 mV, the minimum detectable level of NO is ( ft)
(A) /I or 5,7 ppb .
Table 1 lists the calibrations performed on 5 consecutive working days.
The purpose of these tests was to demonstrate the repeatability and realia-
bility of the PNFM. The last two entries were made on the two days of ambient
air monitoring, Several important facts should be noted from the table:
(1) The 3% variation in the scale factor is well within the original 8% un-
certainty in determination of the 1.00 mV/ppb scale factor and is mostly due
to the 7,5% uncertainty in the N0? sample and 6% statistical fluctuation in
the NO. reading, (2) The apparent change in the zero level is within the
statistical fluctuation about the zero level of jf 4 ppb and well within the
original uncertainty of the zero setting, (3) The accuracy at the 100 ppb
level is +_ 8 ppb, (4) Finally, the average minimum level of detectability is
5,6+1 ppb based on the average fluctuations of the zero level of + 4 ppb and
the average scale factor of 0.991 detectability Zerc
One of the most important parameters of the PNFM is that of linearity
of response. This has been checked over the range of 30 to 200 ppb and the
results are shown in Figure 6. The curve is linear within experimental error.
It should be noted that the vertical error bars reflecting the statistical
uncertainty of the N0« monitor reading are on the order of 10 ppb near zero.
This is due to the fact that the data was taken prior to cooling the photo-
multiplier at which time the detectability had been only 11 ppb. This means
knowledge of the zero was certain only to within 9 ppb and could account for
the fact that the line does not pass through zero and that all readings are
slightly high,
12
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TABLE 1
INSTRUMENT SENSITIVITY
CONSECUTIVE DAILY CALIBRATIONS
Instrument NO,
Day in
March '76
8
9
9
9
10
10
10
10
17
18
Zero
Calibration Level
' Number (i»pb)
1
2
3
4
5
6
7
8
9
10
-1+4
-1 + 4
-1 + 4
-4 + 4
0 + 5
-4 + 3
0 + 4
0 + 4
+2 + 4
0 + 3
Instrument
Sample NO NO
Concentration Reading
(apb) (ppb)
136 + 10
136 + 10
139 + 10
138 + 10
136 + 10
138 + 10
136 + 10
139 + 10
123 + 10
129 + 10
133 + 5
140 + 6
140 + 6
134 + 7
133 + 6
138 + 6
131 + 8
137 + 7
123 + 8
121 + 8
Concentrations
Including Scale
Zero Shift Factor
(Wb) (mV/t>t»b)
134 + 6
141 + 7
141 + 7
138 + 8
133 + 8
142 + 6
131 + 9
137 + 8
121 + 9
121 + 8
0.985
1.037
1.01*
1.000
0.978
1.029
0.963
0.986
0.984
0.938
Detecta'
bility
(ppb)
5.7
5.5
5.6
5.6
7.2
4.1
5.9
5.7
5.7
4.5
INSTRUMENT CAPABILITY =5.6+1 ppb
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SECTION 5
AMBIENT AIR NO LEVELS AT LOCATION OF THE AEROSPACE CORPORATION
After completion of the calibration and linearity testa, the PNFM was
used to monitor the ambient air in the vicinity of our building • The general
geographical location is just southeast of Los Angeles International Airport
near the intersection of Aviation and El Segundo Boulevards. The air sample
was drawn into the lab through PVC tubing with an orifice 15 ft above the
roof of our building; the overall length of the piping to our chamber was
approximately 40 ft, A flow rate of 4.34 liters/min. was used and the air
sample was filtered to remove aerosols. Continuous monitoring was performed
for 10 hours a day on both the 17th and 18th of March, 1976, with readouts
obtained every 2 minutes. The recorded data is shown by the graphs in Figures
7 and 8. The calibration data is also shown for each day. The background
fluctuations are only 4 ppb and the undertainty at the sample NO. level is
about 8 ppb. This is indicated by the error bars on the baseline and 100 ppb
level respectively.
On the 17th of March, monitoring began at 7.00 AM with overcast skies
and light fog. The relative humidity and temperature was 92% and 60°F
respectively. The average N0? concentration at this time was only 20 ppb.
By 10:30 AM, the temperature had risen to 70° F, the relative humidity had
dropped to 38% and the N0« concentration had peaked at an average value of
70 ppb. Throughout the day, there was considerable haze with an average
temperature and relative humidity of 70 F and 40% respectively. Wind was
blowing from the southwest at about 10 MPH. A noteworthy feature is the low
level of N02 in the afternoon except for the singular strong peak (150 ppb)
at 2:15 PM occurring over a half hour interval. The presence of this peak is
not fully understood. On the 18th of March, monitoring began at 7:30 AM with
o
heavy fog. The relative humidity was again 92% with a temperature of 56 F.
But in contrast to yesterday at this time the average NO concentration was
14
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55 ppb. By 10:00 AM, the temperature had risen to 60 F, the relative humid-
ity had dropped to 50%* and the NO- concentration had again peaked at about
75 ppb, Light haze existed throughout the morning but by 2:00 PM the haze had
disappeared with clear skies and extremely good visibility. Wind was still
from the south-west with gusts up to 18 MPH, During the afternoon the lowest
levels of N0? concentration, from 5 to 10 ppb, were observed. At 5:00 PM a
second peak of about 35 ppb was observed which may be correlated with the on-
set of automobile traffic,
A feature of our measurement technique was the ability to monitor rapid
variations in concentration. Much detail of this type is clearly exhibited
by the data in Figures 7 and 8.
15
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SECTION 6
SUGGESTIONS FOR IMPROVEMENTS IN PERFORMANCE OF THE PNFM
It io almost axiomatic that improvements in the PNFM will be required as
the range of application expands. These design modifications will encompass
virtually all aspects of the instrument, In particular, designs leading to
greater compactness and mechanical conveniences are desirable. It is almost
superfluous to state that increased sensitivity will be required.
Tests have shown that the length of the input arm which contains the
flashlamp (Fig, 2) can be substantially reduced, It is obvious that other
dimensions can also be reduced, We estimate that the longest dimension of a
fully packaged unit should be less than 75 cm,
The electronic circuitry developed for the PNFM can certainly be mina-
turized and redesigned for even higher performance. Nevertheless, the elec-
tronics package is probably furthest along in refinement as compared to the
mechanical and optical design of the unit,
Increased sensitivity can be obtained by incorporation of the following
suggested design modifications (1) lamp output - the flash lamp is currently
used at 1/3 its output rating. By increasing the light output per flash,
greater sensitivity can be obtained. A well designed reflector, placed in
back of the flashlamp to redirect this light along the input arm of the tube
might almost double the useful light output of the lamp. (2) reduced back-
ground - it appears that the potential for radical improvement in sensitivity
is in the direction of reduction of background. Considerable effort was
expended along these lines but much remains to be done, such as improved
collimation of the pump light and improved coatings for the fluorescence
chamber (coatings that absorb the scattered blue light with an absolute
minimum of red fluorescence).
The statistical fluctuations of the PMT dark current from an uncooled
16
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PMT presently limits the detectability to about 11 ppb. Cooling the PMT
eliminates the dark current and increases the detectability; an alternative to
cooling is to redesign the electronics to accept dark current pulses for a
shorter length of time, At present, the electronic control packages record,
PMT currents for a duration of 100 us for each flashlamp light pulse which is
approximately 7 us in duration, A reduction in this gate width could lead to
a reduction in PMT dark current and other noise signals without adversely re-
ducing the fluorescence signals. Optimizing the duration of this gate and its
position with respect to the flash lamp pulse should improve the sensitivity
of the unit,
It appears to us, that a four-fold improvement in sensitivity should be
readily achievable. Thus we can look forward to an improved instrument, more
compact than the present phototype with a sensitivity of about 1 ppb.
17
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REFERENCES
1, M, Birnbaum and A, W, Tucker, "NO- Measuring System", Final Report
on Contract No, 68-02-1255, EPA-650/2-74-059, 31 pp., May 1974.
2, J, A, Gelbwachs, M. Birnbaum, A, W. Tucker and C, L. Fincher,
"Fluorescence Determination of Atmospheric NO ", Opto Electronics,
4: 155-160, 1972. *
3, A, W, Tucker, A, Petersen and M, Birnbaum, "Fluorescence Determination
of Atmospheric NO and N02", Appl. Optics, 12(9): 2036-2038, 1973.
4. A. W. Tucker, M, Birnbaum and C. L. Fincher, "Atmospheric N0_ Determi-
nation by 442 nm Laser Induced Fluorescence", Applied Optics, 14(6):
1418-1422, 1975,
5. M, Birnbaum, "Laser-Excited Fluorescence Techniques in Air Pollution
Monitoring", Modern Fluorescence Spectroscopy; Vol. 1, edited by
E, L, Wehry, Plenum Publishing Corp., New York, 1976, pp. 121-157.
6, A, M, Winer, J, W, Peters, J.^P. Smith and J. N. Pitts, Jr., "Response
of Commercial Chemiluminescent NO - N0? Analyzers to Other Nitrogen-
Containing Compounds", Environ, Science and Technology, Vol. 8,
pp, 1118-1121, 13 December 1974,
18
-------
D
Figure 1. Pulsed NO Fluorescence Monitor,
-------
PHOTOMULTIPLIER
TUBE
COLLfCTING
LENS-
CS 2-62
COLLIMATING
LENSES
WINDOWS
TO ELECTRONICS
SHUTTER
LENS
SOLUTION
FILTER
I
MIRROR
FOCUSING"
LENS c±±>- AEROSOL
|| FILTER |
I AIR OUTLET
SOLUTION FILTER I
AND 0-51 AIR INLET
APERTURES
PHOTODIODE
TO ENERGY METER
WINDOW
Figure 2. Block Diagram of Optical Subsystem.
-------
FLASHLAMP-i
AIR IN —+-
FLUORESCENCE
AIR OUT<«-
STORAGE
CAPACITOR
H.V.
SUPPLY
TRIGGER
EXCITATION
r- PHOTOMULTIPLIER
\TUBE
INTEGRATOR
BUFFER
(single sample)
Lj
T
SAMPLE AND
HOLD
(single sample)
T
HIGH GAIN
AMPLIFIER
(X500)
T
ADD/SUBTRACT
INTEGRATOR
(X 2 x 10"4)
PHOTODIODE
LAMP ENERGY PER
MEASUREMENT
CYCLE SELECTOR
FLASHES PER
MEASUREMENT
CYCLE SELECTOR
No. CONSECUTIVE
MEASUREMENTS
AND PRINTOUTS
LAMP FLASH
RATE
SELECTOR
START
MEASUREMENT
MAN /AUTO
1
J
^^•H
L
I
— »
•
FLASHLAMP CONTROL
i
OPERA
CONTR
-
TION
OLLER
.
ELECTRONIC
LAMP FLASH
COUNTER
— + F
— *• S
— * D
BACKGROUND
SUPPRESSION
BIAS
i
I
SCALER
POTENTIO-
METER
LJ.
DIGITAL
PRINTER
DIGITAL
VOLTMETER
DIRECT READING AND PRINTOUT
[NOj ppb
FLASHLAMP CONTROL
SIGNAL PROCESSING CONTROL
DISPLAY AND PRINTOUT CONTROL
Figure 3. Block Diagram of Electronic Subsystem.
-------
100 r-
-.100
N3
to
FLASHLAMP OUTPUT
(ARB units)
FLASH LAMP OUTPUT
FILTER TRANSMISSION
- 20
380
420
460
500 540 580
WAVELENGTH (nm)
620
660
700
740
0
Figure 4. Spectral Transmission of the Flashlamp Filter (5 cm. 'Path
of 300 grams per liter CuSO, Solution plus Corning Glass
CS 0-51 Filter), Spectral Output p| Xe Flashlamp.
(Dashed Line)
-------
100
80
60
40
20
0
10
PMT QUANTUM EFFICIENCY (%)
I
FILTER TRANSMISSION
PMT QUANTUM EFFICIENCY
0
580
620
660 700
WAVELENGTH (nm)
740
780
820
Figure 5. Spectral Transmission of the PMT Filter (2 cm Path of
304 grams per liter Na2Cr2<57. 2H 0 Solution Plus
Corning Glass CS 2-62 Filter). Spectral Response
of EKE 9659 QAM PMT (Dashed Line).
-------
200
180
160
>
£ 140
120
<
UJ
100
o
~
o
~ 80
60
40
20
0
l
1
1
1 20 40 60 80 100 120 140 160 180 200
N02 CONCENTRATION (ppb)
Figure 6. Linearity Test for NO Monitor in the
30 to 200 ppb Range of NO in Nitrogen
24
-------
160 r-
to
MARCH 17, 1976
PACIFIC STANDARD TIME Ihr)
0
Figure 7. Atmospheric NO Concentrations, El Segundo, California,
March 17, 1976
-------
160 r-
0
MARCH 18, 1976
PACIFIC STANDARD TIME (hr)
11
12
13
14
15
16
17
18
Figure 8. Atmospheric NO- Concentrations, El Segundo, California,
March 18, 1976.
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/2-77-003
3. RECIPIENT'S ACCESSION NO.
4. TITLE AND SUBTITLE
PULSED FLUORESCENCE MONITOR FOR MEASURING AMBIENT
NITROGEN DIOXIDE
Development of a Laboratory Prototype
5. REPORT DATE i
January 1977
6. PERFORMING ORGANIZATION CODE
7. AUTHOH(S)
8. PERFORMING ORGANIZATION REPORT NO
C. L. Fincher, A. W. Tucker, and M. Birnbaum
9. PERFORMING ORGANIZATION NAME AND ADDRESS
The Aerospace Corp.
Electronics Research Lab., P.O. Box 92957
Los Angeles, California 90009
10. PROGRAM ELEMENT NO.
1AD605
11. CONTRACT/GRANT NO.
68-02-2246
12. SPONSORING AGENCY NAME AND ADDRESS
Environmental Sciences Research Laboratory
Office of Research and Development
U. S. Environmental Protection Agency
Research Triangle Park, N. C. 27711
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
EPA-ORD
15. SUPPLEMENTARY NOTES
16. ABSTRACT
A prototype pulsed flashlamp monitor for measuring ambient N0» has been
developed, constructed and tested. The basic principles are similar to a
laser fluorescence NO- monitor developed 3 years earlier by the Electronics
Research Lab of the Aerospace Corp. The pulsed system has met the contract
design goal of a sensitivity of 5 ppb for an integration time of 1 minute,
and shows great promise for further improvement. The systems operation has
been successfully demonstrated in monitoring outside air. Continued
development will lead to an instrument comparable in compactness to present
chemiluminescence instruments while being relatively interference free and
more reliable.
7.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS C. COS AT I Field/Group
AAir pollution
ANitrogen dioxide
^Measuring instruments
Prototypes
^Fluorescence
Pulse duration modulation
13B
07B
14B
2 OF
2 ON
8. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19. SECURITY CLASS (ThisReport)
UNCLASSIFIED
21. NO. OF PAGES
35
20. SECURITY CLASS (Thispage)
UNCLASSIFIED
22. PRICE
EPA Form 2220-1 (9-73)
27
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