EPA-650/2-74-046-B
July 1974
Environmental Protection Technology Series
DE
i:iii^^^^^
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EPA-650/2-74-046-b
DEVELOPMENT OF A GAS LASER SYSTEM
TO MEASURE TRACE GASES
BY LONG PATH
ABSORPTION TECHNIQUES:
VOLUME II - FIELD EVALUATION OF GAS LASER
SYSTEM FOR OZONE MONITORING
FINAL REPORT
by
W. A. McClenny, F. W. Baity. Jr.,
R. E. Baumgardner, Jr. , and R. A. Gray
Chemistry and Physics Laboratory
National Environmental Research Center
Research Triangle Park , North Carolina 277]!
and
R. J . Gillmeisfer and L. R. .Snowman
General Electric Company
Electronic System Division
100 Plastics Avenue
Pillsfield. Massachusetts 01201
Contract No. 68-02-0757
ROAP No. 26ACX
Program Element No. 1AA010
EPA Project Officer: W. A. McClenny
Prepared for
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, D.C. 20460
July 1974
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This ruport has been reviewed by the Environmental Protection Agency
and approved for publication. Approval does not signify that the
contents necessarily reflect the views and policies of the Agency,
nor does mention of trade names or commercial products constitute
endorsement or recommendation for use.
11
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TABLE OF CONTENTS
Paze No.
A. INTRODUCTION 1
B. DESCRIPTION OF THE SYSTEM , 3
C. SYSTEM CALIBRATION 10
D. PATH-POINT MONITOR COMPARISON METHODOLOGY 14
E. FIELD MEASUREMENT RESULTS 31
F. CONCLUSIONS , 43
G. REFERENCES 4G
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LIST OF ILLUSTRATIONS
Page No.
Figure 1 - ILAMS Block Diagram 4
Figure 2 - "V" Laser Optical Layout G
Figure 3 - Data Collection and Reduction System 9
Figure 4 - Response to Ozone in Calibration Cell 12
Figure 5 - Space-Time Plane for Measurement Path 17
Figure 6 - Time-Delayed Correlation Between Ozone Point Monitors 21
Figure 7 - Ambient Ozone Variations About Mean Ozone Concentrations 23
Figure 8 - Open-Path Measurement Site 24
Figure 9 - Super-Imposed Recorder-Traces 27
Figure 10 - Super-Imposed Recorder-Traces (Space) 29
Figure 11 - Super-Imposed Recorder-Traces (Time) 30
Figure 12 - ILAMS and Auxiliary Equipment in Trailer 32
Figure 13 - A View of the Optical Path Over Which Ozone was Measured 33
Figure 14 - Comparison by Use of Procedure 3 36
Figure 15 - System Performance Effect from Beam Movement on the 38
Retroreflector
Figure 16 - Comparative Path/Moving Point Monitor Ozone Data 41
Figure 17 - Point and Path Monitor Comparison 44
LIST OF TABLES
Table I Syracuse N. Y. Ozone Concentrations 2
Table II Characterization of Ozone Variability Along Measurement Path 26
Table Til Point Monitor-Path Monitor Comparison Data by Procedure 1 34
Table IV Shift of Beam Position on Relroreflector with Time of Day as 42
Indicated by Focusing Lens Adjustment
IV
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A. INTRODUCTION
The Final Report of EPA Contract 68-02-0757, Development of a Gas Laser
System to measure Trace Gases by Long Path Absorption Techniques, consists of
two (2) volumes:
I. Gas Laser System Modifications for Ozone Monitoring
II. Field Evaluation of Gas Laser System for Ozone Monitoring
The work reported here stems from development activity begun in 1966 at
General Electric's Electronics Laboratory. Under this contract, a breadboard
laser long path monitor called ILAMS (Infrared Laser Atmospheric Monitoring
System) was modified to improve its sensitivity as indicated by previous field
experience. System parameters were selected to optimize system performance
for ozone monitoring. A field evaluation of the modified system was conducted.
Following completion of the system design investigations and hardware
modifications described in Volume I of the Final Report, the laser path monitor
called ILAMS was set up and operated over a range (one way distance) of . 67 kilo-
meter. Field tests were performed in cooperation with EPA personnel. The
object was to evaluate the monitoring capability of the breadboard ILAMS by
comparing its ambient air measurement of ozone concentrations with data from
chemiluminescence point monitors operated near the system's optical path. Table
I shows representative chemiluminescence ozone monitor data from Syracuse, New
York, where the measurements were made.
This portion (Volume II) of the Final Report, is a joint EPA-General Electric
Company effort. In the sections which follow, the system is described. Calibration
of the system with a multipass cell and point/path monitor comparison methodology
are discussed, field test results are presented and conclusions are given.
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Month + Year
Jan 1973
Feb 1973
Mar 1970
Apr 1973
May 1973
June 1973
July 1973
/ ug 1 973
Sept 1973
Oct 1973
Nov 1973
Dec 1 973
Jan 1974
Monitoring Station
No. 1
Average of
Daily Averages
10
15
14
20
21
27
35
32
17
11
9
10
9
Range of Maximum
Hourly Averages
5-35
0-40
1 -48
14 -59
17-75
16 -87
20 - 125
30 - 100
9-93
15 - 57
9-49
2-32
4 - 32
Downtown
Average of
Daily Averages
8
11
14
19
18
*
39
34
no dot
no dot
Range of Maximum
Hourly Averages
5-29
5 -57
6-96
12 - 108
10 - 76
24 - 124
30 - 132
22 - 132
a
a
no data
no dot
a
no data
Data from New York State Continuous Air Monitoring System
Number of daily averages (18) insufficient, per New York State data processing
criteria, for taking monthly average. Average for 18 days is 36 ppb.
Table I - Syracuse, N. Y. Ozone Concentrations During Recent Months
(in Parts per Billion )
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B. DESCRIPTION OF THE SYSTEM
The gas laser system used in this evaluation is a breadboard instrument.
It was constructed by General Electric Company under an internally funded program
and its performance evaluated in rural and urban atmospheres under EPA Contract
EHSD 71-8. Under this contract the system was modified as described in Volume I of
this document. The system operates in the middle region of the infrared spectrum
and identifies atmospheric constituents by absorption spectroscopy. It measures
average pollutant concentrations (total burden) over its optical path. Laser operation
is at relatively low, safe power densities of .001 to .01 watts/cm2 in the spectral
region where the eye does not transmit.
Figure 1 is a block diagram of ILAMS. The output power from the laser is
directed to a 50 percent beamsplitter via a 1 mm spatial filter (cleanup aperture).
The energy reflected from the beamsplitter is focused down to a 0.1 mm aperture
that serves as an attenuator. Behind this aperture is the reference energy detector.
The transmitted power through the beamsplitter goes to a germanium lens which
focuses the energy near the focal point of an off-axis parabolic mirror, and the
expanded, nearly collimated beam is transmitted to the retroreflector. The return
energy from the retroreflector retraces the path through the beam-expanding parabolic
mirror and the germanium lens to the beamsplitter. The return energy reflected
from the beamsplitter Is collected by a germanium lens doublet and focused on the
signal detector. Preamplifiers are mounted directly behind the signal and reference
detectors. The preamp outputs go to the signal processor. The detectors used in
the system are thermistor bolometers, operating at ambient temperature (uncooled),
having a characteristic flat response across the middle infrared spectral region.
The attenuation of laser energy at many wavelengths produces absorption
patterns which are used to separate pollutant absorption from spectral interferences
1 *? 1 C
in the signal processor. C O laser lines in the P branch of the 00°1 - 02°0
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POLLUTED ATMOSPHERE
"7
RETROREFLECTOR
V /
LENS
REFERENCE
DETECTOR
ATTENUATOR
LINEAR
WEIGHTS
DOUBLET
LENS
SIGNAL
DETECTOR
SPATIAL
FILTER
SPECTRALLY
SCANNING-
LASER
DIGITAL
SIGNAL PROCESSOR
Figure 1 . ILAMS BLOCK DIAGRAM
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transition lie in the same spectral region as ozone absorption so that coincidences
between the two exist. Several of the absorption coefficients recently measured
by EPA personnel are listed in Volume I. Prior to the actual monitoring of ambient
ozone, a set of four wavelengths, optimized for ozone detection in the presence of
other atmospheric attenuators, was chosen by an optimization procedure. This
procedure and the calculation of linear weights is described in Volume I. During the
course of the evaluation some changes in wavelengths and weights occurred. The
final wavelengths were 9.2938, 9.5039, 9.5862 and 10.5321 microns. For these
wavelengths the linear weights were respectively -0.4215, 1.0000, -0.5820 and 0.0034.
The spectrally scanning laser itself includes a high gain CO flowing gas
laser as the radiation source and a wavelength selection mechanism, which
periodically (50 Hz) scans through a series of four laser wavelengths. A typical output
scan consists of laser pulses of 2. 5 msec, duration separated by time intervals of the
same duration during which laser action is interrupted and detector null signals are
recorded. The laser optical configuration is shown in Figure 2.
The laser cavity consists of a "V" shaped plasma tube and an external spectral
tuner, A relatively long laser cavity is used for sufficient gain to overcome the
losses inherent in the spectral tuner and to obtain lasing action on a large number of
spectral lines. A beam travels through the plasma tube with aid of a mirror at the
point of the "V". Leaving the tube through a germanium Brewster window, the beam is
directed by mirrors through an iris (for mode control) and onto a 105 lines/mm diffraction
grating, which disperses the beam spectrally and spatially. The four wavelengths of
interest are then relayed through holes in the chopper wheel to the four end mirrors of
the laser cavity. These holes in the chopper are so located that, as the wheel turns,
only one wavelength at a time is permitted to pass through to the end mirrors. The four
end or wavelength selection mirrors are adjusted so that the beams are directed back on
themselves through the laser cavity. In this way, selected laser wavelengths are
transmitted sequentially.
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Sr-..tia!
Hirer
Coupling Mi noi
Beam to/from
Col limator
Concave
Mirror
Signal Bolometer
Reference Concave Mirror
Bolometer
IRIS Stop / S
4 End Mirrors Chopper Wheel
Brewster
Window
Figure 2. "V" LASER OPTICAL LAYOUT
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The mini-computer signal processor includes a general purpose (stored
program) mini-computer and appropriate interface electronics. The collection
and reduction of data is entirely under computer, i.e. , program control; results
are displayed on simple displays incorporated in the equipment, and on an optional
teletype, which need not be used (or even be connected) during field or test range
exercise of the system.
The use of the stored program control and data reduction means:
changes in system design, or variations in data reduction algorithms,
may be accommodated without alteration of the data collection or re-
duction hardware; only changes in the control program will be required.
modification of signal processor parameters such as number of wave-
lengths (up to 8), gate locations, system response time, weighting
factors, etc. , do not even require software changes, these parameters
are expediently entered by the teletype input.
the precision of data processing may be made as accurate as desired;
similarly the impact of imprecise calculations may be assessed by direct
simulation for purposes of evaluating future low cost special purpose
instruments.
additional data, e.g., environmental conditions, time, date, signal
variability, laser parameters, etc. , may be measured and recorded
without modification of or addition to the existing system hardware.
the performance of one or more data processing and display systems
can be directly analyzed, e.g., data from several ozone monitors
could be crosscorrelated and recorded.
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The data collection and reduction system is sketched in Figure 3. A Digital
Equipment Corporation PDF 11/05 is used for the central processor. The data
collection and reduction equipment in Figure 3 consists of three major subsystems:
Interface Subsystem
This subsystem includes an 8 input analog signal multiplexer, which is
followed by a sample-ancl-hold amplifier and an analog-to-digital converter at
10-bit precision. (The analysis path detector preamplifier output is connected to
one multiplexer input, the reference path to a second multiplexer input, the re-
maining 6 are available for sensing other voltage levels of interest). Additional
L-.I=> system elements include an AGC attenuator, a wheel position counter and
demultiplexer/storage capability for analog data displays like the meters shown
in Figure 3.
Central Processor Subsystem
The central processor and its own control panel form this subsystem. Power
supplies for this equipment are contained within the CPU cabinet proper. The
central processor control panel ordinarily is disabled during operation.
Program Input and Data Logging Subsystem
A Teletype Corporation ASR-33 teletype with appropriate interface circuits
constitutes this subsystem. As indicated, it plays two roles. First, it permits
entry (ordinarily via paper tape) of the control program. Second, it permits detailed
reporting of directly measured quantities, or derived (computed) quantities.
The central processor is designed so that programs stored in its core memory
may be caused to remain intact during periods of no primary power. This option
is exercised, so that once a control program has been entered in the CPU, it
need not be reentered until there is a need to change it, regardless of whether the
CPU remains energized or not. The control program is designed so that it will
run properly regardless of whether the teletype is connected or not. Thus the
teletype unit is an optional data display device, not an essential component of the
system once the control program has been entered.
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Analysis Path
Amplifier Output
Reference Path
Amplifier Output
Chopper Wheel
Encoder Timing H
Signals
Interface
Multiplexer,
Sample and
Hold, A/D
AGC
Attenuators,
D/A Converter
Storage Registers
(S)
Meters
Central Processor
PDP 1 1/05
CPU
Teletype and Paper Tape Reader
(Teletype Not Required for Equipment
Operation; May Be Removed After
Control Program Has Been Loaded
Program Input and
Data Logging
Data Display
Figure 3. DATA COLLECTION AND REDUCTION SYSTEM
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C. SYSTEM CALIBRATION
In day-to-day operation, system zero was set by comparing concentration data
on the teletype printout with the readings of a portable chemiluminescent ozone point
monitor operated along the system's optical path. An appropriate offset was entered
in the signal processor Parameter Table so that the value on the printout corresponded
to the average concentration indicated on the point monitor strip chart record.
For ozone, and pollutants with similar diurnal variations, a simple alternative
procedure can be followed. Typically, ozone concentrations were zero from sun-
set until shortly after sunrise. During this period, the system can be zei-oed by
offsetting to zero the log of transmission at each wavelength. Adding this fixed
offset in the signal processor is equivalent to introducing an optical filter that com-
pensates for the fixed absorption pattern introduced by the system optics.
Prior to field operation, the system was calibrated using a multipass (White)
cell* into which known ozone concentrations were introduced. In the experimental
setup, the uncoilimated laser beam was mirror-directed into the entrance port of
the multipass cell. Cell mirrors were so aligned that the beam was redirected
back on itself, rather than leaving the cell through the exit port. Two small ozone
generators employing UV illumination were arranged in series to produce a 1.6 ppm
ozone concentration in air flowing into the cell at four liters per minute. This
corresponds to S.O percent absorption at the strongly absorbing wavelength, A 9.505^,
and to an equivalent ozone burden along the measurement path of 50 ppb (parts per
billion). Ozone concentrations at the entrance and exit of the calibration cell were
monitored with a portable chemiluminescence ozone monitor** (Analytical Instrument
Development, Inc.) which had been previously calibrated by comparison with an
ozone generator, which was in turn calibrated by the neutral buffered potassium iodide
technique. No appreciable ozone loss was observed.
* Fabricated by EPA
contribution to field tests
10
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For another run under the same conditions, the ILAMS was used as the sensor
of ozone concentration buildup in the cell. In the system, the teletype printout is the
data display device. On the printout each group of digits (five at the time of the
experiment but now four) represents the sum of the weighted natural logarithms (in)
of the return signal to reference signal voltage ratios for each of the transmitted
wavelengths. This sum is the system response. It is proportional to a. CL, the
exponent of the Beer's Law equation, where a is the absorption coefficient, C the
concentration and L the system's optical path length. Since ap CL is dimensionless,
the units of a are the reciprocal product of those used for C and L as will be seen
below.
For this experiment, the system's four wavelengths (9.305, 9.504, 9.586 and
10. 532 microns) were weighted for ozone as the target gas and the sum appeared as
the left hand group of digits on the printout. The weights were, respectively,
-.4131, 1.0000, -.5869 and zero. As the ozone concentration in the multipass cell
increased to an equilibrium value, system response was recorded on the teletype
printout. At equilibrium the reading on the teletype printout was 00064, corresponding
to an a CL of . 128. This compares with an a CL of . 119 derived from multipass cell
parameters and the point monitor measurement of ozone concentration as shown in the
following calculations. Figure 4 shows how ILAMS response and ozone concentration
measurements relate, in terms of a CL, as cell ozone concentration increases with
time to an equilibrium value. Intermediate points on the plot are determined in the
same manner as shown below for equilibrium values.
a CL Calculation from Multipass Cell Parameters
Measured 0 absorption coefficients, a, at transmitted wavelengths are as follows:
O
01 (atm cm
9.305
9.504
9.586
10.532
0
12.659
.69183
0
11
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.130
.120
.110
_, .100-1
u
6
, .090
!£ .080-1
O
| .0701
2 .060-
z
O .050-
O
Q_
Z .040-
.030-
.020-
.010-
/
IDEAL CURVE
0 ,010 .020 .030 .040 .050 .060 .070 .080 .090 .100 .110 .120 J30
ILAMS RESPONSE - c* CL
Figure 4. RESPONSE TO OZONE IN CALIBRATION CELL
(ILAMS VS. POINT MONITOR)
12
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The resultant a , the sum of weighted absorption coefficients at the four
wavelengths is:
a- = 4131 x 0 + 1. 0000 x '12. 659 - . 5869 x . 69183 + 0 x 0
r
= 12.253 atm" cm"
The multipass cell was aligned to produce 42 passes through a 5 foot (1.524 meter)
cell length. For an assumed 1.52 ppm equilibrium On concentration in the cell
(obtained from previous point monitor measurements employing the same procedure)
we have:
ft
a CL = 12.253 x 1.52 x 10~ x 42 x 1.524 x 100 = .1192
r
13
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D. PATH-POINT MONITOR COMPARISON METHODOLOGY
There are several methods by which comparisons between point
monitors and path monitors can be accomplished. For each of these methods
it is important to assess the comparison'accuracy and to insure that
tolerances inherent to the method are not attributed to disagreement
between the monitors. The differences between methods correspond to the
ways in which point monitors are used to obtain values of the ambient trace
gas concentration for use in the comparison and to the statistical treat-
ment of this data. The sampling procedures for use with point monitors
are the following:
1. Recording the real time signal from a point monitor as it
is moved along the measurement path.
2. Simultaneously taking several bag samples along the path
and measuring each with a point monitor.
3. Monitoring the signals from stationary point monitors placed
along the measurement path.
4. Filling a sample container while moving along the measurement
path and measuring the path integrated sample with a suitable
point monitor.
In what follows, these various sampling procedures and the corres-
ponding point monitor, path monitor comparison methods are discussed
with special emphasis on their applicability for ozone.
In Procedure 1 a point monitor is moved with a constant velocity, V,
along the measurement path, thereby describing a trajectory on the space-
time graph of Figure 5, i.e., a straight line given by S = VT. This
procedure like others using
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point monitors does not give complete space-time coverage. It can be thought of as
providing a number, N, of concentration measurements, A., along the measurement
path. A specific A. corresponds to a spatial and temporal increment and a
coordinate (S., T.). The average value, n , obtained for the trace gas density during
a transit time, T , is £ A./N. Often the point monitor data can be obtained as an
i
analog signal recorded on a chart recorder. In this case
"!-'; -
.X = 0 (1)
where A is now a continuously recorded value for the trace gas concentration and
corresponds to position along a chart trace. If conditions remain the same along
the path until several traverses are made, the values of n associated with a set of
traverses (point monitor measurement sequence) form the statistical basis for a
determination of the precision with which the true mean value of path averaged con-
centrations can be stated. Specifically, if a sample mean n is obtained for the
individual values of n , then the 95% confidence interval n ± A includes the true
mean value for 95% of all possible measurement sequences. The value of A
is given by the expression (References 1 and 2).
l .05 ll (2
where s denotes the standard deviation for a set of traverses, k denotes the
number of traverses in a set, and t nr. is a tabulated statistical function which
. 05
approaches 1. 96 for large k .
15
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Once the confidence interval has been defined, a formal comparison can be
made between the value n and the average value, n*, of path monitor readings during
the point monitor measurement seouence. Experimentally, the number of path
monitor readings is usually much greater than k while the standard deviation for
path monitor readings is approximately the same. Thus, the value of A associated
with the confidence interval for the path monitor is much less than in the point
monitor case and the value of n* closely approximates the true mean. For a given
comparison seouence, if n* lies within the interval n + A 95% of the time, then
a statement that the two monitors agree can be made. Procedure 1 was used in a
comparison sequence discussed in a later section of this report.
Procedure 2 involves establishing a number of stationary positions along the
path at which bag samples are taken. For a bag filling time, T , the space-time
coverage consists of vertical channels in Figure 5 corresponding to complete time
coverage during T at the fixed sampling positions. The spatial interval A£ over
which the point monitor readings are representative, i.e. , the width of the vertical
channels in Figure 5, is determined by the main features in the spatial gradients
which exist along the path. In the case of ozone, these main features are caused by
topological proximity conditions, e.g. , trees or buildings which provide surfaces on
which ozone is destroyed, thereby constituting an ozone "sink" and by NO source
proximity such as automobile traffic near or across the measurement path. Ozone
disappearance occurs when NO concentrations comparable to ambient ozone con-
centrations are present. In this case the rapid reaction between these two gases
(Reference 3) can cause ozone changes to occur within a few seconds. Both of these
causes of spatial gradients are dependent on the residence time of ozone/NO near
sink/ source regions and therefore change as a result of wind speed and direction.
16
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SPACE
3. ^FAOK-Tr-I". PL-VS";-. FOR
l-"'AS'''HP!F.::T PAT!'
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For Procedure 2, placement of the sampling sites must be such that spatial
gradients are revealed.
Ozone concentrations must not be changed in the sampling and measurement
procedure. If NO levels are high the gas phase reaction of NO and O inside sample
O
bags can reduce the O level drastically. Ozone can also be destroyed on the walls
of the sampling bag. Typical decay curves for 100 liter Tcdlar and Mylar bags
indicate that ozone decay rates can be limited to less than S9c per hour if the sample
bags are conditioned with ozone prior to use and if the bags are filled to insure
high volume to surface ratios.
If the sample site selection is appropriate and the integrity of bag samples is
assured, an accurate point-path comparison can be made. Each bag sample
measurement represents a mean ozone concentration during T . If T. is short com-
pared with temporal variations in ozone concentrations, several samples should be
taken at each point. The average of a set of simultaneous samples taken along the path
can be treated analogously to the path-averaged reading obtained during a traverse
in Procedure 1. Each average defines a mean value n so that a confidence interval
n9 _L A ^ can be defined based on several values of n. n replaces n and A
replaces A] in the statement on point-path comparison. If T is long compared with
temporal ozone variations, the readings obtained from the sample bags closely
approximate the true mean values at the sample sites and the sum of readings from
one set of sample bags can be compared directly to an n* established during T .
Using Procedure 3, a close approximation to the true average trace gas con-
centration at some point along the measurement path can be obtained by monitoring
for a time period long compared to that typical of trace gas variations. This
experimental technique is illustrated for O later in this section. Once these values
are obtained, they arc simply averaged and compared with ;m n* established during
18
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the same time period. Just as in Procedure 2, stationary site locations must be
adequate to accurately represent any spatial gradients. This is obviously not the
case when only one or two point monitors are used along a long path near which
sources and sinks of the trace gas exist. Thus, the usefulness of Procedure 3 for
point-path comparisons depends on the choice of the path and prior knowledge of the
temporal and spatial variability of the target gas.
Procedure 4 is useful in those cases when a portable point monitor for the
target pollutant does not exist and when the path cannot be adequately characterized
by sampling at a number of fixed sites. A sample container is filled as the path is
traversed so that different portions of the path are equally weighted. The container
is carried to a suitable point monitor and a single reading representing the target gas
average along the path is obtained. Target pollutant loss while in the sample container
must again be considered. However, in the case of relatively inert tracer gases or a
primary pollutant such as CO, this loss can be negligible. The value of the reading
obtained for each traverse, n , is treated like n and the comparison technique is
the same as that using Procedure 1.
Measurements were made to determine the short term temporal/spatial
variability of ozone. Other than diurnal variations in ozone concentration and long
term trends established by NO or O sources, e.g., the effect of traffic or general
cloud cover patterns, variations within periods of several minutes occur which reflect
localized variations in target gas concentration in the air mass being carried across
measurement locations. These localized variations arise due both to disruption of the
NO, NO2, O , and ultraviolet radiation steady state (Reference 4) by changes in the
ultraviolet throughput to ground level due to segregated clouds and to pockets of gases,
e.g., NO or olefinic hydrocarbons, with which ozone reacts. These effects give rise
to short term structure in the ozone concentration in both space and time.
19
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Two ozone point monitors (Bendix Model 8002; were used in field tests to establish
the variability in ambient ozone concentrations. These monitors detect ozone by its
gas phase chemiluminescence with ethylene and have minimum detectable limits of
less than 5.0 ppb. One monitor was placed on a stationary platform and a second one
was mounted on an electric car and moved to different points along the measurement
path. Figure 6 shows chart recorder traces of the two monitors as a function of time.
Both monitors were stationary and they were separated by 42 meters. Variation in
ozone concentrations are the result of wind conditions, of NO source and O. sink
O
proximity, and of cloud cover patterns, i.e., reduction of the short wavelength
O
radiation ( ^ < 4000A) necessary to photolyze NO . Air mass transport from one
monitor site to the other is obvious from the time delay correlation of signal features
on the two chart recordings. Correlations of this type vary depending on the
projection of the wind vector along the path joining the two point monitors. Intake
ports for the monitors were established at a height of six feet above the ground.
These ports were open Teflon tubes of 8 mm inside diameter and approximately 100
cm long. Flow rate into the instrument was approximately 1.0 liters per minute
and the time constant for the instrument was set at 1.0 seconds. With these experimental
conditions the ozone concentrations monitored by the instruments were essentially
real time.
In order to characterize temporal variations for one point monitor given
meterological and source/sink proximity conditions, a method of data handling was
developed. A single chart recording is characterized by two quantities, an average
concentration c and a measure of the temporal variability of ozone. Data from the
chart recording was digitized at six second intervals and c was determined by averaging
the set of digitized data values. The digitized readings were fitted by a linear re-
gression analysis over a short time period, typically ten minutes. The differences
20
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50
40
o 30
i-
cc
I
LU
O
o 20
cr>
O
10
"i i 1 1i 1 1 1 1 1 1 1 1ii 1 1i r
CORRELATION (DELAYED)
CORRELATION
(DELAYED)
I I I I I I I I I I I I I I I
I I
10
TIME, min
15
20
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between the linear approximation and actual recorded values were obtained and the
number N of these differences within given intervals, AC, of the straightline
value were tabulated. The distribution of N with Ac was determined to be approx-
imately a Gaussian distribution and hence was determined uniquely by a mean value
and standard deviation. A typical distribution with a superimposed Gaussian is shown
in Figure 7. As expected the standard deviation changes in response to variations
in source/sink proximity conditions and in meteorological parameters such as wind
speed and direction. Values of the standard deviation could be used to obtain a value
of the true mean ozone concentration during the period of measurement. Typically,
100 values were determined so that a 95% confidence interval of "£"±0.196s was
defined. In most cases c is determined within one or two ppb and can be used for
point-path comparison as suggested in the discussion of Procedure 3.
Of the procedures discussed for point/path comparisons, the first and a
variation of the third were used in field tests described in the next section. The
major advantage in using these two procedures is that minimal ozone loss occurs
due to ozone confinement prior to measurement. A secondary advantage is the
requirement of only one person to perform that part of the comparison involving the
point monitor. Field measurements along a measurement path of 0.67 Ion have
indicated some of the characteristic variations in ozone concentrations to be
expected. Both a hand-carried Model 560 portable ozone monitor made by Analytical
Instrument Development Inc., and a Bendix Model 8002 ozone monitor were used to
obtain real time ozone concentrations by moving along the measurement path. The
Bendix instrument was placed on an electric car and powered by batteries with DC
inverters to render it portable. A stationary monitoring station was established near
one end of the measurement path as shown in Figure 8. One ozone monitor and a
Climet wind speed and direction indicator were placed at the stationary site. A path
along which the point monitors could be moved was established from the laser source
22
-------
Viy:.;? ,; . ''1S!.Yi 0?.0.\r VAT AT TO/'S
o
Q_
U_
O
O
MEAN = 0.001
STANDARD DEVIATION = 4.05
DIFFERENCE AC (ppb)
-------
pE\:-p..\TH 'T>\STJRP 'FNT S
(G.F-.. Sy?ACIiSF,, N'.Y.)
O O
FUEL OIL
-------
to its retro-reflector. Runs with the moving point monitors prior to
actual comparison trials indicated that the concentration variability
of ozone was strongly dependent on the wind direction and the constancy
of cloud cover conditions. Arteries of traffic existed to the west and
south of the stationary monitoring station while a forested residential
area was to the north and east. During periods of two to three hours
after 2:00 p.m. (DST) on days with consistent cloud cover, almost constant
ozone concentrations existed. Consistent light wind conditions from the
northeast were optimum for low ozone concentration variability.
In preparation for comparison tests, a number of simulated point
monitor comparison sequences were run. The objectives of these tests
were to establish the spatial gradients and the variability of ozone
concentrations along the selected measurement path. The distribution of
the average concentration values (n-^ values) during the individual
traverses was assumed to be normal and the comparison was treated as
specified in Procedure 1. Table II summarizes the point monitor data.
Test 1 of Table II corresponds to the information obtained from six path
traverses. A reproduction of the recorder chart outputs from these tra-
verses are shown superimposed in Figure 9. Each trace was obtained by
hand-carrying the AID ozone monitor along the measurement path of 0.67
km in a time of approximately eight minutes. A mean ozone concentration
of 68.2 ppb is shown in Figure 9 along with dotted lines indicating the
95% confidence interval. For each traverse the recorder chart data
was digitized to give approximately 100 values for the ozone concentration.
25
-------
Test
1
2*
3
nl
69.9
73.6
64.2
66.9
68.5
66.2
62.5
63.5
64.3
65.5
64.0
63.2
68.9
70.4
70.8
69.9
70.7
nl
68.2
63.8
70.1
i
Al
3.4
1.1
1.0
stl
4.9
5.4
6.4
7.1
7.0
10.0
1.0
2.2
2.4
1.8
2.4
2.2
2.2
2.7
2.1
2.4
2.2
SH
6.8
2.0
2.3
Mode
Moving
Moving
Stationary
Figure
9
10
11
* Three of the traverses were made with a 1.0 second time constant
and the remaining three with a 10.0 second time constant.
Table II Characterization of Ozone Variability along Measurement Path
26
-------
100
80
60
-------
The standard deviation, S , of these values varied appreciably from
traverse to traverse as seen in Table II. This variation is caused by
the contribution to the concentration average of the large decreases
evident in Figure 9. Decreases of this type were caused by the inter-
action of ozone with pockets of air having high concentrations of NO.
This is an example of the practical limitation to accuracy of a compar-
ison test in which the ambient atmospheric conditions are not sufficiently
equivalent. However, in other simulated comparisons, during another day,
the values of the standard deviations were approximately equal, indi-
cating constant atmospheric conditions. Figure 10 and the corresponding
treatment of the data, i.e., Test 2 of Table II illustrate this point.
For Test 2, a Bendix ozone monitor with response characteristics equi-
valent to the AID instrument was used. The confidence interval was
approximately three times smaller than that of Test 1. Thus, ambient
conditions could be chosen during which the accuracy of point-path
comparison sequences would be high. Specifically, the atmospheric condi-
tions prevalent during Test 1, related mainly to wind direction, result
in a wider confidence interval, + A , about a sample mean than do those
prevalent during Test 2.
Figure 11 shows superimposed recorder chart traces of five
consecutive time intervals taken under the same atmospheric conditions
as the traces of Figure 10. The data was recorded on a second Bendix
ozone monitor at a stationary position along the path. Near equality
of the standard deviations associated with the stationary (Test 3) and
and moving (Test 2)point monitors demonstrate the equivalence of temporal
variability associated with a stationary monitor and the temporal-spatial
variability associated with a moving point monitor.
28
-------
100
BENDIX MOVING MONITOR
SIX TRAVERSES
80
60
o
o
o
co
O
40
20
0
LASER
140
280 420
DISTANCE, meters
560
RETRO
ripfi:"re
r'PPFFT) PF.r^P^1"" - TRACK? (SF.^CF!^ 11 ."rjOPFP "1°7T"
-------
100
80
60
-------
E. FIELD MEASUREMENT RESULTS
Field tests employing ILAMS over a 0. 67 km path were conducted during the
period October through December of 1973. The laser beam was positioned between
1 and 2 meters above ground level and pointed along a horizontal path inside the plant
boundaries of General Electric Company in Syracuse, New York. The
ILAMS was housed in a small, air-conditioned trailer at one end of the path. In-
dependent suspension of the laser and associated optics was provided in order to
maintain the pointing accuracy of the laser since small changes of the order of a few
thousandths of an inch at the source correspond to displacements on the order of
inches at a remotely positioned retroreflector. The beam was transmitted to a two-
mirror type retroreflector, consisting of an aluminized parabolic reflector of 122 cm
focal length and a small secondary mirror (1.0 cm diameter) at the focal point, was
2
positioned at the other end of the path. An area of 600 cm (30 cm x 20 cm) was inter-
cepted by the retroreflector.
Figure 12 is a picture of the system and auxiliary equipment in the trailer.
Figure 13 is a view from the laser of the optical path over which ozone was measured.
Actual path/point monitor comparison tests were made in two ways. In the first,
Procedure 1, described in the previous section, was used. The individual averages,
n were obtained by measuring the area under chart recorder traces. Two sets of six
such values are shown in Tests 1 and 2 of Table III. Ozone variability was similar to
that shown in Figure 9. Some variation in individual values of n occurred due to changing
conditions along the entire path. As specified in Procedure 1,5, and A were
determined. The average path monitor reading, n*, during each set of six traverses
was obtained by averaging a digital printout from the signal processing unit of the
monitor (4 second update) which consisted of approximately 600 determinations of the
path averaged ozone concentration. Under ideal test conditions n* should fall within
31
-------
Lrl
tsj
Figure 12. \LAMS AND AUXILLIARY EQUIPMENT IN TRAILER - SYRACUSE, N.Y
-------
Figure 13. A VIEW OF THE OPTICAL PATH OVER WHICH OZONE WAS MEASURED.
33
-------
Test
1
2
nl
31.6
32.6
33.8
34.1
37.7
35.1
49.8
56.5
43.7
43.2
44.5
47.0
nl
34.2
47.5
Al
2.2
5.3
n*
31.3
49.3
Table III Point Monitor-path Monitor Comparison Data by Procedure 1
34
-------
the confidence interval of n 95% of the time. For the data presented in Table III,
Test 1, n* = 31.3 ppb falls just outside the confidence interval 34.2 + 2.2 ppb. For
Table 2, Test 2, n* = 49.3 ppb falls well within the confidence interval 47.5 + 5.3 ppb.
To obtain a better measure of the ability of one monitor to track the other,
comparison sequences during which ozone concentration gradients occur were deemed
appropriate. In this case individual values of n were compared directly with n*,
i. e. , the average value of the ozone concentration along the path as determined by one
traverse was compared with the average value'of the path monitor readings during
the time of traverse. If concentration gradients do not exist, such as is the case
shown in Figure 9, each point along the measurement path is equivalent. By dividing
the chart recording of a single traverse into a number of segments and assigning a
concentration value to each, a data base for a statistical treatment to determine the
confidence interval for n is established. Equation 2 applies where now A defines the
confidence interval about n, s denotes the standard deviation in a sample of y values
for the concentration. For y^ 100, t * 1. 96 and using values of s from Table II
UD *
Test 3, A is less than 1.0 ppb. Even with s as high as 10.0 ppb, the confidence
interval is only 4 ppb.
In actual practice, the recorder chart trace for each traverse was measured
with a planimeter and the value obtained was compared directly with the average of the
path monitor readings during 1^ Figure 14 shows a comparison of this type extending
from noon until sunset and includes the points presented in Table III as Test 2. In
this comparison sequence good agreement is obtained although the disparity between
path monitor and point monitor readings for several pairs of points is greater than
would be expected for a common measurement path, under equivalent atmospheric
conditions. Review of the chart recording show that departures from
equivalence occur for comparisons which show the largest disparity . In
this case path monitor and point monitor may see quite different concentration
profiles. This is illustrated in Figure 1 by the placement of areas labeled
A. and B. Significant decreases in ozone concentration in these areas of the
space-time plane will cause measurement-disparities.
35
-------
-------
In the period following the October comparative path/point monitor tests, the
ILAMS system was operated extensively. In the course of this work a number of areas
were investigated. Studies of system alignment, beam position on retroieflector and
distance between retro primary and secondary, were made and optimal conditions
established. These studies utilized alternative entries in the Parameter Table that
were preserved on paper tape for reference and future use. Laser performance was
improved by adjustments and modifications. The teletype printout format was
changed from 4 groups of five digits to eight groups of four digits. Other program
changes to improve system performance were implemented. Minor linear weighting
changes were made to correct previous errors. Signal processor sensitivity to
elevated ambient temperature in the trailer was found to be caused by a defective
component. System operation ended with a series of path/point monitor comparative
tests 26 November through 4 December.
Again, as in the October tests, system performance effects from movement of
the laser beam on the retroreflector were noted. Figure 15 shows the rapid change
on the teletype printout as the beam moves off the retro. Data is from 3 December tests.
Printout speed is twelve lines per minute. Each count on the left hand, ozone column,
represents 2.44 ppb of O^ That is, 0010 indicates 24. 4 parts per billion of ozone.
As can be seen from the figure, in about two minutes, counts went from 0013 to 0001,
a change of 29 ppb.
It is possible to compensate for beam motion through movement of the focus-
ing lens in the system. This lens focuses the laser beam on the entrance aperture
of the collimator, an off-axis parabola optical system which serves as the breadboard
ILAMS transmit-receive optic. It is mounted in a fixture on the laser channel. The
fixture is oriented so that the lens can be moved along the laser beam axially,
vertically (V) or horizontally (H) in relation to it. Consequently, V and H axis
adjustment of the focusing lens can be used to move and position the system's transmit
beam on the retro.
37
-------
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Figure 15. SYSTEM PERFORMANCE EFFECT FROM BEAM MOVEMENT
ON THE RETROREFLECTOR
38
-------
Like the teletype printout, the return (analytic) signal amplitude reflects
beam position on the retro. Maximum amplitude indicates the beam is centrally
located on the retroreflector. With return signal amplitude oscilloscope-displayed,
centering the beam on the retro can be done by V and H axis adjustment of the
focussing lens. This approach was used to check beam position and center the beam
on the retro. Signal amplitude variation with V and H movement was an indication
of beam location relative to the retro center. Thus, maximizing or "peaking" the
signal on the oscilloscope with V and H adjust would center the beam. When centering
the beam, it was decided that final V and H settings would be the mid-point between
the micrometer screw settings where the return signal rose and fell significantly as
the beam was moved across the retro by the motion of axes adjustment screws.
A series of comparative path-point monitor field tests in the period 26 November
through 4 December duplicated some of the results obtained in October and provided
additional information on the system's performance. Comparative data was collected
in the same manner as that employed in the earlier tests. Retroreflector alignment
checks, secondary mirror position relative to primary, took place 28 November. The
secondary was moved .025 Inches further away from the primary. Also a weight change was
made on the ozone channel to reflect an earlier shift in A , from the R14 line to the
R16 line of the (00 1 - 02 0) transition. In microns, the wavelength shift was from
9.3054 to 9.2938. It was made to avoid water vapor Interference noted after the original
wavelength selection process was completed.
The weather during this period in November and early December was quite variable
with snow and rain occurring. The system was operative in all but extreme weather
conditions, even when the retroreflector site was not visible from the trailer. December
3rd was an unusually warm winter day. The high temperature was 58° F, a near-
record for Syracuse. It was bright and sunny, reminiscent of important data-taking
days in late October. The laser was turned on at 0650. The system was balanced at
0830. Around 0900 a steady negative drift in concentration-related counts on the
teletype printout prompted freo^ient checks of beam position on the retroreflector
throughout the day.
39
-------
The comparative data for this clay is shown in Figure 1G. Noteworthy is the
genera] absence of large excursions in path monitor relative to point monitor data
noted in October data during mornings and late afternoons, fit was felt that adjust-
ment of beam position in the period 1345 to 1430 could have been more frequent to
reduce the excursion at 142.S on the figure). Table IV shows beam position shift with
time of day in terms of focussing lens adjustments made to maximize the return
signal. Xote that H axis adjustment is constant (within operator reading error
tolerance for an awkward location) while V axis setting rises and falls with time of
clay and coincidentallv, the effect of localized atmospheric heating. This represents
about a seven centimeter beam excursion on the retro. Similar data for 23 October
shows the laser beam raised 15 centimeters on the retro reflector. As the retro
diameter was 20 centimeters, this motion could be expected to have considerable effect
on the system. The time of day variation in beam position gives rise to speculation
that beam motion at the retro is largely attributable to atmospheric looming.
40
-------
0
* PATH MONITOR
MOVING POINT MONITOI
ESTIMATED FROM
LATER READING
900
1000
1100
1200
TIME OF DAY
1600
Figure 16. COMPARATIVE PATH/MOVING POINT MONITOR OZONE DATA - 3 DEC 1973
-------
3 December 1973
Time
Start*
0815
0928
1006
1022
1135
1259
1410
1440
1513
1540
1615
V
379.6
376.5
378.0
378.9
H
308.5
308.4
308.5
308.4
no change
379.6
308.4
no change
379.3
379.0
378.5
378.0
377.4
308.5
308.5
308.5
308.5
308.5
Laser was turned on at 0650. Initial V and H settings
were those made at 1520 30 November.
Table IV Shift of Beam Position on Retroreflector with Time of Day
as Indicated by Focusing Lens Adjustment
42
-------
F. CONCLUSIONS
An example of point monitor and path monitor comparisons is shown in
Figure 17. It includes the afternoon data of Figure 14 with the morning data
taken on 23 October. The erratic behavior of the T.LAMS during the morning
hours has not been definitely identified, but is believed to be related to the
alignment of the laser beam at the retroreflector. Spurious absorptions result
whenever the relationship between return and reference signals arc changed by
other than known atmospheric attenuators. In this case the spurious absorption
coincided with drift of the transmitted laser beam away from the retroreflector
caused by atmospheric looming as the ground warmed up and cooled down during
the day to produce vertical temperature gradients in the system's optical path.
The accuracy with which measurement of ozone concentrations could be
made was limited by optical noise related to atmospheric turbulence and thermal
gradients. The transmit and receive optics produced spectra.! attenuation because
the diameter of the far field diffraction pattern of the laser beam is proportional
to wavelength. The retroreflector is an aperture stop that acts upon these spatial
variations to produce spectral attenuation. Hence, when the transmitted beam was
focused on the retroreflector, one would expect a spectral return in the presence
of atmospheric looming, which is precisely what we observed. It was shown possible
to produce a similar error by deliberately aiming the beam slightly off the edge of
the retroreflector. The experiments with defocusing the beam to alleviate this
problem, however, insulted in an increase in the observed optical noise and an
increase in measurement error. The aggravation of the noise condition with de-
focusing indicated that spectral non-uniformities were still present in the near field
in spite of the spatial filter (clean up aperture) in the laser transmitter. Direct
43
-------
o POINT MONITOR
o PATH MONITOR
SYSTEM
REZEROED
0800
1000
1200 1400 1600
TIME, hours (23 Oct.)
Figure J.7. POriT ANT) PAT1! MONITOR COMPARISON
1800
-------
measurements of the intensity pattern of the near field showed that there is indeed
some "clutter" in the near field beam and this has been tentatively attributed to the
optical surfaces of the laser transmitter both preceding and following the clean up
aperture.
The signal output of this laser detection system is proportional to the log
of the ratio of the signal returned from the retroreflector to the signal on the
reference detector. Since it is the relative signal behavior of the two detectors
to which the output is sensitive, it is equally possible that drift in the optics pre-
ceding the reference detector is also partially responsible for the measured error
in the output. However, because the major drift error occurred in the mornings
on clear clays following clear nights, the strong change in humidity and ground
temperature during the morning hours are considered significant.
Water vapor is fairly well understood spectral interference, but ground fog
and liquid water adhering to participates (haze) is not. In addition, unknown spectral
interferences may have entered the beam to offset the system's zero baseline. (A
method called factor analysis, to analyze and compensate for unknown spectral
effects is discussed in Volume I).
The effect of looming on the laser beam was noted on many occasions. Direct
measurement of the effect has been reported in the previous section. Reaiming the
laser by moving the focussing lens preceding the beam expander to correct for the
looming only appeared to compensate for about 50% of the measurement error associated
with this time of day phenomenon. The considerable drift in the laser system output
signal which has been tentatively attributed to looming was greatest during the mornings
on clear days when the wind velocity was low. In these periods, the RMS error
between the ILAMS and the point monitor ozone measurements was approximately 28
parts per billion. At other times, the system was relatively stable and the RMS error
was about 6 parts per billion. It is expected that the effects of atmospheric looming on
the signal are still the dominant source of error even during stable operation and that
correcting non-uniformities in the transmitted laser beam will reduce the system
error and improve its accuracy.
45
-------
REFERENCES
1. G.W. Snedecor, Statistical Methods, The Iowa Suite College Press, Ames, Iowa.
2. B. E. Saltzman, Journal of the Air Pollution Control Association, _22_(No. 2),
99, February 1972.
3. J.A. Ghormley, R. L. Ellsworth, and C. J. Hochanaclel, J. Phys. Chem. , 77,
1341, 1973.
4. D.H. Stedman, E.E. Daby, F. Stuhl, and H. Niki, Journal of the Air Pollution
Control Association, _22_(No. 4), 260, April 1972.
5. Final Report: Field Study on Application of Laser Coincidence Absorption
Measurement Techniques, Contract EHSD 71-8, February 1972, prepared
for the Environmental Protection Agency by General Electric Electronics
Laboratory, Syracuse, N. Y. The system used in the comparison tests is a
modified form of the system described in this final report. Modifications
include those specified in Reference 5.
46
-------
4. TITLE AND SUBTITLE
Development of a Gas Laser System to Measure Trace
Gases by Long Path Absorption Techniques Volume II
Field Evaluation of Gas Laser System for Ozone Mom'tori
J. f'ErU'GRMING ORGANIZATION NAME AND ADDRESS
Joint Report: Environmental Protection Agency, Researc[i
Triangle Park, North Carolina 27711 and
General Electric Company, Electronic Systems Division,
Pittsfield, Massachusetts 01201
TECHNICAL REPORT DATA
(I'leasc rcail [nuruciiciHS on t/ic rcvcrsi1 before c
. REPORT NO.
EPA-650/2-74-046-b
5 REPORT DATE
July 1974
6. PERFORMING ORGANIZATION CODE
la.
8. PERFORMING ORGANIZATION REPORT NO.
W. A. McClenny, F. W Baity, Jr., R. E. Baumgardner, Jr.
R. A. Gray, EPA, R. J. Gillmeister and L. R. Snowman, G
12. SPONSORING AGENCY NAME AND ADDRESS
Environmental Protection Agency
National Environmental Research Center
Chemistry and Physics Laboratory
Research Triangle Park, North Carolina
27711
RECIPIENT'S ACCESSIOC*NO.
_ __ __
"lO. PROGRAM F LEMEM NO.
1AA010
11. CONTRACT'GRANr NO.
68-02-0757
13. TYPb Oi REPORT AND PERIOD COvi nbD
Final_ _ _
14. SPONSORING AGENCY CODE
15 SUPPLEMENTARY NOTES
Joint Field Study by G. E. and EPA of Open-Path Monitor
1G. ABSTRACT
Ambient ozone measurements in real time using an open-path monitor are described.
These studies establish the sensitivity of an open-path monitor, based on transmissiv
ity measurements of C02 laser lines, at <_ 5 ppb and validate the values obtained during
real-time monitoring of ambient ozone by establishing and using a methodology for
the comparison of point monitor readings and open-path monitor readings over a common
path.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
Lasers
Atomospheric Absorption
Ozone
Air Pollution Monitoring
b.IDENTIFIERS/OPEN ENDED TERMS
I LAMS
Methodology for Point
Monitor, Path Monitor
Comparisons
c. COSATI I'ickl/Oruup
1705
'>'-:. _M5TRIBUTIO'Ni STATEMENT
Release Unlimited
19. SECURITY CLASS (This Report)
Unclassified
21. NO. OF PAGES
51
20. SECURITY CLASS (This pugt!)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
47
-------
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ataloging
(b) IDENTIFIERS AND OPEN-ENDED TERMS - Use identifiers for project names, code names, equipment designators etc Use open-
ended terms written in descriptor form for those subjects for which no descriptor exists.
(i) COS ATI HELD GROUP - Field and group assignments are to be taken from the 1965 COSATI Subject Category List Since the ma-
jority ol documents are mullidisciplmary in nature, (he Primary Field/Group assignment!.-*) will be specific discipline area of human
endeavor, or type of physical object. The application(s) will be cross-referenced with secondary Field/Group assiunmcim thai will follow
the primary postmg(s). ' - "
18. DISTRIBUTION STATEMENT
Denote reusability to the public or limitation for reasons other than security for example "Release Unlimited " Cite anv ",viihhililv to
the public, with address and price. ' ' '
19. & 20. SECURITY CLASSIFICATION
DO NOT submit classified reports to the National Technical Information service.
21. NUMBER OF PAGES
Insert the tola! number of pages, including this one and unnumbered pages, but exclude distribution list, if any.
22. PRICE
; Insert the price set by the National Technical Information Service or (be Government Printing Office, if known.
(a) DESCRIPTORS - Select from the Thesaurus of Engineering and Scientific Terms the proper authorized terms that identify the nnior
concept ol the research and are sufficiently specific and precise to be used as index entries for cataloging.
:PA Forrr 2220-1 (9-73) (Reverse)
48
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