EPA-650/4-74-036
AUGUST 1974
Environmental Monitoring Series
SSi'S
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EPA-650/4-74-036
N02 ACTINOMETER FOR FIELD USE
Prepared by
D.E. Burch, R.C. Bean, andF.J. Gates
Philco-Ford Co. , Aeronutronic Division,
Ford Road
Newport Beach, California 92663
Contract No. 68-02-0798
ROAP No. 56AAI
Program Element No. 1A1003
EPA Project Officer: P.L. Hanst
Chemistry and Physics Laboratory
National Environmental Research Center
Research Triangle Park, North Carolina 27711
Prepared for
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, D.C. 20460
August 1974
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This report 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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ABSTRACT
Solar radiant energy in the ultraviolet and
short-wave length visible dissociate N02 in the
atmosphere to produce NO and 02• This photolytic
reaction plays an important role in the formation
of photochemical smog, and information about the
amount of actinic energy available in the lower
atmosphere is required for the development of
mathematical models of the atmospheric processes.
This report describes the development and testing
of an actinometer designed to measure the actinic
energy available for the photolytic dissociation
of N02- A spherical test bulb contains a mixture
of NO? and 0~ when it has been in the dark for
several minutes. When the bulb is exposed to
solar energy NO is formed; its concentration is
monitored by gas-cell correlation methods involving
the infrared absorption by NO. A shutter periodically
shades the test bulb from the sun for a one-minute
period each two minutes. During the shaded period,
part of the NO recombines with 02 to form N02- The
cyclic change in the NO concentration is related
to the actinic irradiance.
iii
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CONTENTS
Page
Abstract iii
List of Figures v
Sections
1 Introduction 1-1
2 Preliminary Tests 2-1
3 Description of Actinometer 3-1
A Tests Results and Instrument Performance 4-1
5 Electrical Diagrams and Details of Components 5-1
6 References 6-1
IV
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FIGURES
Wo. Page
1-1 Spectral plots of the absorption coefficiento 1-5
for N02 and N20^ between 3000 cm"1 and 4500 A
2-1 Optical diagram of actinometer used for pre- 2-2
liminary tests
2-2 Response of breadboard model NC>2 actinometer to 2-5
sunlight
2-3 Response of N02 actinometer to artificial uv and 2-6
visible light at different intensities and
different position in the test bulb
2-4 Plots of V" and AV" vs relative actinic irradiance 2-8
2-5 Plots of V" and AV" vs 02 partial pressure in the
test bulb for various partial pressures of N02
o
2-6 Transmittance of NO, from 4300 to 4500 A 2-12
3-1 Optical diagrams of the optics assembly of the 3-2
actinometer
3-2 Two views of the optics assembly of the actinometer 3-3
3-3 Photograph of the control assembly, amplifier and 3-4
recorder
4-1 Response of actinometer to solar energy with the 4-2
shutter in the automatic mode
4-2 Recorder tracings of actinometer output when the 4-3
sky contains broken clouds
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FIGURES (contd.)
Mo. Page
4-3 Plots of AV" vs relative actinic irradiance 4-5
4-4 Response of actinometer to different absorber 4-8
thicknesses of NO in a mixture of NO + ^ at
a total pressure of 120 torr
5-1 Simplified schematic diagram for ac power 5-4
5-2 Simplified diagram of interconnecting cables 5-5
5-3 Wiring diagram of electrical components on 5-5
the optics assembly
5-4 Wiring diagram of timer and control assembly 5-6
5-5 Schematic diagram of preamplifier used with 5-7
the InSb detector
5-6 Schematic diagram of the 350 Hz reference- 5-7
pickup
5-7 Power supply for thermo-electric cooler for 5-8
the detector
5-8 Power supply for preamp and reference circuits 5-8
vi
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SECTION 1
INTRODUCTION
BACKGROUND
Nitrogen dioxide is readily photolyzed to NO and 0- by short-wave
visible and near-ultraviolet light. Measurement of the rate of photolysis
of N©2 in polluted air is necessary in order to understand the complex
photochemical processes in the atmosphere. Photolysis of N0~ starts a
series of chemical reactions that yield ozone, peroxyacyl nitrates, and
other objectionable species that make up photochemical smog. There is
presently a need for a field instrument that can measure the amount of
light available to photolyze N02 for purposes of developing mathematical
models of the atmospheric processes.
Laboratory methods for measuring the intensity of light producing
photolysis usually involve a photochemical reaction chamber containing
N02 and an inert gas. The photolysis is measured by monitoring the dis-
appearance of N02« Such a method is impractical for a field instrument
because the gas must be replaced after each measurement. Photoelectric
instruments have generally been used in the field because they are simple
and can be used continuously. However, this type of instrument suffers
two serious drawbacks that make it difficult to relate a reading to the
rate of N02 photolysis.
Geometric corrections and spectral response corrections must be
applied to the reading of the photoelectric sensor. The response of a
photodetector is highly directional, with maximum response to light rays
perpendicular to the surface. It is clear that correcting for the direc-
tional response would be quite complicated and would require considerable
knowledge of the angular variation of the solar irradiance, which depends
on the solar zenith angle and on the conditions of clouds and haze.
1-1
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Spectral corrections are required because it is not possible to pro-
duce a photodetector surface with a spectral response that matches the
absorption spectrum of N02 • The rate of photolysis due to light in a
narrow spectral interval is proportional to the product of the solar spec-
tral irradiance and the absorptivity of the NO,* Similarly, the response
of a photodetector due to the same narrow spectral interval is proportional
to the product of the solar irradiance and the spectral responsivity of
the detector. Because the relative spectral responsivity of a photodetector
may change and the spectrum of sunlight reaching the lower atmosphere is
variable, the spectral correction is also very difficult.
SUGGESTED INSTRUMENT FEATURES
An ideal N0« actinometer would have exactly the same spectral re-
sponse as the N02 absorption in the- atmosphere and would be equally sensi-
tive to light arriving from any direction. Hanst* has suggested that this
ideal actinometer can be approximated by an instrument built along the
following lines. A spherical receiver, which we shall call a test bulb,
is made of quartz, which is transparent to all wavelengths of actinic
radiant energy reaching the lower atmosphere. A photosensitive mixture
of N02 + Oo» and possibly an inert gas such as ^ > is contained in the
test bulb. The system operation depends on the chemistry of N02 photolysis
and the reverse reaction given by
N02 + hv * NO + (1/2) 02. (1-1)
The bulb is mounted in the open, well above any parts of the instrument,
so it can receive radiant energy from all directions except for a few
steradians below. Most types of earth surface reflect weakly in the uv
and visible; therefore, it is unlikely that a significant fraction of the
actinic energy reaching a point in the lower atmosphere comes from the
lower hemisphere. Thus, the gas in the bulb will effectively integrate
the effects of the radiant energy arriving from all directions.
The problem of spectral response errors inherent in photoelectric
detectors is greatly reduced by having the N02 itself be the absorber of
actinic light in the actinometer. Since the absorptivity of NC>2 is not
the sar.ie for all wavelengths of actinic light, the correct spectral response
can be maintained only if the NOo concentration in the test bulb is low
enough that the gas is optically thin at all wavelengths.
A new feature of the instrument suggested by Hanst involves the addi-
tion of 02 to the test bulb along with the NO,• In the presence of the
excess 02 the reverse reaction takes place to form N02 when the test bulb
1-2
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is shaded from actinic light. The second new feature of the suggested
instrument is a shutter that periodically opens to expose the test bulb
to actinic light then closes to allow the reverse reaction to occur. The
increase in the concentration of the NO, or the decrease in N02, during
the open portion of each cycle of the shutter is monitored spectroscopi-
cally. The monitor employs a gas correlation cell that is used as a
selective filter. The correlation cell contains either NO or N0~, which-
ever is being monitored in the test bulb.
We have previously employed gas correlation methods to monitor NO
at concentrations of interest for the present purposes. The fundamental
vibration-rotation absorption band near 5.4 |j,m is employed. The fractional
change in the NO concentration in the test bulb is expected to be quite
large during each period of the cyclic shutter. On the other hand, only
a small fraction of the N02 is expected to dissociate during each cycle.
Therefore, it seemed likely that the NO concentration could be measured
more accurately than the N02 concentration. Monitoring N02 has one ad-
vantage in that visible absorption bands could be employed to take
advantage of simple and efficient sources and detectors.
SUMMARY OF WORK PERFORMED
The objectives of the work reported here have been to demonstrate
the feasibility of an N02 actinometer based on Hanst's recommendations,
to perform experiments to determine the required design parameters, and
to build and demonstrate an instrument for field use and evaluation. The
preliminary tests performed before the final instrument was designed are
discussed in Section 2. Most of the tests were performed indoors under
controlled conditions with a uv lamp serving as the source of actinic
energy. Gas cell correlation methods were used to monitor the NO con-
centration in a cylindrical test bulb. A cylindrical bulb, rather than
a spherical one, was used because it facilitated the investigation of
possible wall effects and geometrical effects in the test bulb. Besides
demonstrating the feasibility of the instrument, the preliminary tests
provided information that was quite valuable in the design of the acti-
nometer to be built and demonstrated as part of the present contract.
Limited experimental data obtained on the measurement of N02 by gas
correlation methods tended to confirm our original belief that NO could
be better monitored in an actinometer test bulb than could NO,. A few
data were obtained by using hot NO as a source of radiant energy when
monitoring NO in a separate sample. As expected, a given small sample of
NO produces a larger fractional change in the detector signal when hot
NO is used as an energy source in the place of a continuum source, Better
discrimination against interfering HgO absorption can also be achieved.
However, simple NO cells cannot be operated at temperatures as high as
conventional continuum sources; therefore, their spectral radiance is less,
1-3
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even at wavelengths of high NO emissivity. We have chosen a hot ceramic
source for the actinometer because of its simplicity and the improved
signal-to-noise ratio obtainable with it.
Section 3 describes the actinometer built for laboratory and field
use. It has been designed so that components can be interchanged and gas
mixtures in the correlation cell and test bulb can be modified. Only a
limited amount of test data represented by those summarized in Section 4
have been obtained. Although the practicality of the instrument for field
use has been demonstrated and the tests indicate that it performs ade-
quately, subsequent tests to be performed by EPA scientists will probably
lead to suggestions for minor design changes.
SPECTRAL CHARACTERISTICS OF N02 ABSORPTION
When actinic radiant energy strikes a layer of NOo, the absorption is
a function of the wavelength with strong absorption throughout much of
the visible and near uv. ^0^, a compound that is always associated with
N02, is transparent in the visible but absorbs in the uv. Fig. 1-1 shows
plots of the absorption coefficients as given by Leighton2 for both species
over this portion of the spectrum. The spectral curves shown in Fig. 1-1
were not obtained with sufficiently good resolution to show all of the
vibrational structure that exists at the longer wavelengths. Below about
3700A the bands are diffuse with essentially no unresolved structure.
If I ^ photons of actinic energy of • wavelength X in a parallel beam
are incident on each cm per second, the rate of absorption of the photons
is given by
-3-1 P
N (photons cm sec ) = or c I , dX, (1-2)
a J o , A.
where a, the absorption coefficient, is expressed in cm2 /molecule and is a
function of wavelength, and C is the concentration of the absorbing gas
species in molecules /cm3. Equation (1-2) is restricted to optically thin
samples so that the incident intensity is essentially the same at all
points in the absorbing layer. If the sample is too thick, the intensity
is less near the surface where the beam leaves the absorbing layer than it
is on the front surface where the beam is incident. Additional data on
N02 absorption in the visible appear in Section 2.
The absorption coefficient shown as the right-hand ordinate scale
of Fig. 1-1 is expressed in atm'^m"1 and is convenient when relating the
transm'.ttance to the amount of N02 in the optical path. If p is the par-
tial pressure of the absorbing gas in atmospheres, 8 is the temperature in
degrees Kelvin, and L is the geometrical path length through the gas,
1-4
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= exp
-[•
X (atm^cm-^p
p(atm) L(cm) 273/9~|. (1-3)
The ratio of the concentration of N?0. to that of NCL decreases with
increasing temperature and increases in proportion to the NO? pressure.
Because of the very low partial pressure of NO? in the atmosphere, the
fraction that appears as ^A is extr~ertely low artd» therefore, negligible,
However, the N^O, absorption may be significant in laboratory samples or
in test cells containing higher concentrations of the gases. The rela-
tive equilibrium concentrations of NOj and N_0, can be determined from
data given by Giauque and Kemp^.
Most of the absorbed radiant energy of wavelengths greater than
approximately 4100A produces changes in the electronic and vibrational
energies of the absorbing molecules, but it does not produce dissociation.
However, a large fraction of the energy of shorter wavelengths produces
photolysis of the NO, molecules in accordance with Eq. (1-1). The quantum
yields of photolysis, according to Leighton at 4 representative wave-
lengths are as follows: 3130A, 0.97; 3660A, 0.92; 4047A, 0.36; and 4358A,
0.00. Because of the low quantum yield of photolysis at the longer wave-
lengths and the low spectral radiance of the sun at short wavelengths,
most of the photolysis of atmospheric N0« is due to radiant energy between
approximately 3200A and 4200A. By utilizing the quantum efficiency for
dissociation, the spectral absorption characteristics of N02, and the
actinic irradiance, the rate constant for dissociation reaction of N02 in
the atmosphere can be calculated. A value of 0.37 min"1 (0.0062 sec'l) is
accepted for typical clear atmospheric conditions^.
o
0)
i— i
o
e
*~s
o
4 x 10
-19
2 x 10
-19
3000
3500
4000
4500
and N_0
FIG. 1-1 Spectral plots of the absorption coefficient for NO, .
between 3000 cm'1 and 4500 X. The ordinate scale shown on Ehe rigfit is
convenient when the partial pressures of the gases are measured in atmospheres
and the optical path lengths are in cm.
1-5
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SECTION 2
PRELIMINARY TESTS
INSTRUMENTATION AND PROCEDURES
An optical diagram of the instrument used for the preliminary tests
is shown in Fig. 2-1. The test bulb is filled with a mixture of NO- and
Op- Actinic radiant energy in the near uv and visible from lamp A causes
a photochemical reaction that produces NO in the test bulb. The actinic
energy can be blocked from the test bulb by shutter Sh. The remainder
of the instrument is a gas-correlation cell analyzer that employs infra-
red energy to monitor the concentration of the NO in the test bulb.
Images of the radiant energy source are formed near the center of
the test bulb and on the slit. A narrow beam of nearly-collimated energy
is formed by the combination of the slit and the mask on the flat mirror
near the source. After the beam passes through the slit, a rotating mirror
chopper alternately directs it through one of two paths to a bandpass
filter and detector. The 0.15u-m wide bandpass of the filter centered near
5.3u-m includes a portion of the strong, fundamental band of NO. Approxi-
mately 640 torr of NO in the 1 cm correlation cell is essentially opaque
near the centers of the NO absorption lines and absorbs approximately 72%
of ihe radiant energy in the bandpass of the filter. With no NO in the
test bulb and with the blocking shutter B removed from the beam through
the attenuator, the attenuator is adjusted so that the average transmittances
of the two alternate beams are the same. Thus, the component of the de-
tector signal at 170 Hz, the frequency of the rotating mirror chopper, is
made zero.
When NO is formed in the test bulb, it absorbs a portion of the radi-
ant energy leaving the detector. Because the NO in the test bulb absorbs
2-1
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Det-
M
(r
10
i
IS)
Test Bulb
Reference
Synchronous
Amplifier
and
Gain Control
Strip Chart
Recorder
FIG. 2-1. Optical diagram of actinometer used for preliminary tests. S, infrared source.
M, mask to limit IR beam size. Ch, chopper. CC, correlation cell containing NO.
F; bandpass filter centered near 5.3 urn. Det, liquid-nitrogen cooled InSb
detector. Att, adjustable attenuator. B, blocking shutter that can be moved
into or out of the beam. A, source of uv and visible actinic energy. Sh, shutter
to shade the test bulb from the actinic energy. When the instrument was used to
investigate solar energy, the sky replaced A.
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most at wavelengths where the NO in the correlation cell is opaque, the NO
in the test bulb has less effect on the beam passing through the correlation
cell than on the beam passing through the attenuator. The two alternate
beams are no longer balanced and a detector signal at 170 Hz results. The
percentage of misbalance between the two alternate beams is a measure of the
concentration of NO in the test bulb. The spectroscopic principles of
operation of gas cell correlation instruments have been described in detail
in References 4 and 5.
In order to normalize the detector voltage, all of the NO is removed
from the test bulb and the blocking shutter is moved so that it blocks
the beam through the attenuator. The amplifier output for this condition
is defined as VD. The blocking shutter is removed from the beam and the
attenuator is adjusted to produce zero amplifier output when no NO is
present in the test bulb. As NO is formed in the test bulb, or introduced
into it, a detector signal Va results. The relationship between the ratio
Va/V, and the amount of NO in the test bulb depends on the spectral band-
pass as well as on the length of the correlation cell and the amount of
NO in it. This ratio, V /V, is defined as the normalized voltage V" and
is nearly independent of source brightness or detector sensitivity pro-
vided neither of these parameters change after Vb has been measured.
A photoelectric pick-up produces an electric signal at the frequency
of the mirror chopper and synchronized with it. This signal serves as a
reference for the phase-lock amplifier (Princeton Applied Research Model
JB-5). The amplifier output is recorded on a strip chart recorder. Elec-
trical noise due to the detector and amplifiers is less than 10"4Vb and
does not contribute significantly to the uncertainty in the measurements.
The test bulb is cylindrical, 5 cm diameter by 10 cm long, with a
quartz body and Ca?2 windows. Both the actinic source and the test bulb
can be moved in a horizontal plane perpendicular to the narrow monitoring
beam of infrared energy passing through the test bulb. Thus, the infrared
beam can sample different portions of the test bulb to check for non-
uniformity in the NO concentration. If mixing is not rapid, non-uniformi-
ties might be expected to result from gradients in the intensity of the
actinic energy. For test bulb mixtures containing enough N02 that the
absorption depth of the actinic energy is less than the diameter of the
test bulb, the actinic energy density is less on the side of the test bulb
away from the actinic source than it is on the opposite side. Reflection
from the quartz wall can also produce non-uniform actinic energy density
within the test bulb. For example, a line of high density may be expected
along the cylindrical test bulb midway between the center and the wall
farthest from the actinic source. Reflection of the nearly-parallel beam
of actinic energy is focused along this line. By using a cylindrical
test bulb, rather than a spherical one, it is possible to direct the infra-
red monitoring beam through a path parallel to the wall of the bulb and at
different distances from it. Therefore, the actinic energy density is
2-3
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nearly constant along the beam, and the effect of some of the geometrical
factors contributing to non-uniform actinic energy density can be investi-
gated more easily than with a spherical test bulb. Results obtained with
the cylindrical test bulb have been used in determining the optimum dia-
meter of a spherical test bulb and the NC^ + CL mixture to be contained in
it. The non-uniform directional sensitivity of the cylindrical test bulb
does not create a serious problem for preliminary tests.
The irradiance by the artificial actinic source was approximately
the same as that to be expected by solar energy near sea level on a typi-
cal clear day. Out-of-door measurements were made with the sun shining
directly on the cylindrical test bulb and the artificial actinic source
removed.
RESULTS OF PRELIMINARY TESTS WITH ACTINOMETER
An example of the recorder traces of the actinometer output during
exposure of the test bulb to sunlight is given in Fig. 2-2. Before the
trace was started, the test bulb had been shielded from actinic energy
sufficiently long for all of the NO to disappear. At t = 0, the shutter
was opened and the test bulb was exposed to sunlight; the increase in V"
is due to the formation of NO in the test bulb. After approximately
2 minutes of exposure to the sun, the NO concentration was approaching
equilibrium. A slight influence of a very thin cloud between the sun and
the actinometer can be seen from about t = 2 to t = 5 min. At approxi-
mately t = 5 min., the intermittent shutter was turned on so that the
test bulb was alternately dark for 30 seconds, then exposed to the sun for
30 seconds. We note that the amplitude, A", of the variation in V" due
to the changes in NO associated with the intermittent shutter stabilized
after only 1 or 2 minutes of intermittent exposure. Other data not shown
indicate that the drift in the amplitude when the sky was clear was less
than the drift in the steady-state value of V" when the shutter was left
open for continuous exposure to the sun.
With artificial actinic energy from a "black light" source
(Westinghouse H44-GS mercury vapor spot lamp) substituted for sunlight,
responses such as those of Fig. 2-3 were obtained. At t = 0, there was
essentially no NO in the test bulb because the shutter had been closed
for several minutes to block the actinic energy. At about t = 2 min., the
snutter was opened, exposing the test bulb to the artificial actinic source
placed a distance D = 20 cm from the center of the test bulb. The shutter
was then closed at about t = 6 min., and D was varied by moving the arti-
ficial source as indicated. The increased response after the shutter was
re-opened resulted from the increase in irradiance associated with the
decrease in D.
2-4
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0.02 .
e
ro
i
0.01 .
Intermittent Shutter
TIME (min)
FIG. 2-2. Response of breadboard model N09 actinometer to sunlight. The test bulb was filled with
55 torr N02 plus 155 torr 02> Exposure started at 12:29 P.M. (PDT) on June 29, 1973 near
Newport Beach, Ca. at an elevation of approximately 50 m above sea level. The sky was mostly
clear, but hazy with occasional thin clouds. The quantities AV" and V" for intermittent
shutter operation are illustrated.
-------
0.06
0.05
0.04
V"
0.03
0.02
0.01
-l.fc
30 60
Time (min)
90
FIG. 2-3. Response of N0» actinometer to artificial uv and visible
light at different intensities and different positions
in the test bulb. D is the distance from the center of
the test bulb to the face of the black light lamp used as
the artificial actinic light source. The distance from
the axis of the test bulb to the monitoring IR beam in cm
is denoted by R; positive R indicates that the monitoring
beam is between the axis of the test bulb and the actinic
energy source, and negative R indicates that the moni-
toring beam is displaced in the opposite direction. The
intermittent shutter is open 30 sec and closed 30 sec of
each cycle. The test bulb was filled with 55 torr NO.
plus 105 torr 09.
2-6
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At about t = 32 min., the actinic source and test bulb were moved
as a unit relative to the IR monitoring beam to check for non-uniformities
in the NO concentration in the test bulb. The apparent shift in response
when the test bulb was moved is due primarily to a "zero" shift arising
from non-uniformity in the CaF_ windows on the test bulb. The amount of
zero shift due to the windows was measured with the test bulb evacuated.
After the shift was accounted for, the results indicated there was no
significant difference in the NO concentration at the three different
positions of the test bulb for the N02 and 02 mixture given in the legend
of Fig. 2-3. When the intermittent shutter was used, the amplitude AV"
was also essentially the same at all three values of R when D remained
fixed. This provides additional evidence that the NO is mixed under these
conditions.
Similar tests for non-uniform NO concentrations were also made with
higher and lower pressures of N02 and 02 in the test bulb. With N02 pres-
sures greater than approximately 100 torr and 02 pressures greater than
approximately 150 torr, there was evidence that the NO concentration was
higher on the side of the test bulb toward the actinic light source. At
the higher N02 pressures, a large fraction of the actinic energy is ab-
sorbed and forms NO before it has penetrated through the cell. The higher
pressure also reduces the mixing rate in the test bulb. Thus, there are
upper limits on the pressures of N02 and 02 that can be used in a spherical
test bulb of a field instrument in order that the concentration along the
diametric path of the monitoring beam represent the average for the entire
bulb. This is required if the response is to be independent of the direc-
tion of the incident actinic energy.
From Fig. 2-3 we see that the steady-state values of V" obtained
with the shutter open and the amplitude AV" of this quantity when the
shutter is intermittent both increase as the actinic irradiance is increased
by decreasing D. The dependence of both V" and AV" on the actinic irradi-
ance are shown in Fig. 2-4 for the conditions represented by Fig. 2-3. The
relative irradiances were measured with the probe of a "lite-Mike" (EG&G,
Inc. Model 560B) at the normal position of the center of the test bulb.
The steady state value of V" tends to saturate at much lower actinic
irradiance than does Av" obtained with the intermittent shutter. A near-
linear relationship exists between V" and the actinic irradiance over a
wide range of irradiance values.
Figure 2-5 illustrates the dependence of the actinometer response on
the partial pressures of NO- and 02« Values of AV" increase nearly in
proportion to the %QO «ith little or no 02 in the test bulb. Adding 02
to a fixed amount of N02 increases the amplitude AV because it increases
the rate at which the NO returns to N02 during the dark half of each cycle.
In the upper panel, the plot of V" for PNQ = 72 torr illustrates the de-
crease in the equilibrium amount of NO witfi increasing 0 when the test
2-7
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0.01
0.008 _
0.006 _
0.004
0.002 _
0
20
60
80
100
Actinic Irradiance (Arbitrary Units)
FIG. 2-4. Plots of V" and AV" vs relative actinic irradiance.
The ordinate for each curve is labelled.
2-8
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0.04
0.03
0.02
V"
0.01
0.010
..i
0.008
72
AV"
0.006
0.004
0.002
/ // /
{/// *
J_
X
50 100 150 200
0, partial pressure (torr)
250
FIG. 2t5. Plots of V" and AV" vs 0 partial pressure in the test bulb
for various partial pressures of NO-,. The partial pressures of
NO. in torr are indicated for each curve. A uv black light
without a filter served as the source of actinic energy. The
monitoring beam passed along the center of the 10 cm long test
bulb. While measuring AV", each shutter cycle consisted of
periods of 30 sec open and 30 sec closed; V" was measured after
the shutter had been open several minutes.
2-9
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bulb is irradiated continuously. This result is to be expected because
of the increase in the rate of the reverse reaction producted by the addi-
tional $2- The measurements of V" for the lower values of I^Q were not
reproducible and were inconclusive. Variations in temperature and the
amounts of impurities in the test bulb probably account for the changes
in V" while the actinic irradiance is maintained constant. These vari-
ations apparently have less influence on AV" than on V".
Increasing the pressure of either N02 or 0, increases AV"; however,
as pointed out above, if the pressures are too high the actinic energy
density and the NO concentration are not uniform throughout the test bulb,
and the interference by ^0^ is significant. The optimum pressures for a
test bulb of a given diameter in a field instrument are determined by the
best compromise among the spectral response, uniform directional response,
and signal-to-noise ratio. Therefore, the optimum pressures depend upon the
detectivity of the detector and the radiance of the IR source and can best
be determined after an instrument is assembled. Decreasing the 0 pressure
Lncreases the required period for the shutter because of the decrease in the
rate at which the NO disappears during the dark portion of the cycle.
A few tests were made on the influence of temperature changes in the
test bulb while maintaining the amount of gas in it and the actinic irradi-
auce constant. An infrared heat lamp heated the test bulb, and a thermo-
couple, inserted into the interior of the bulb but shielded from the
direct radiation of the heat lamp, measured the temperature of the gas in
the bulb. For a typical gas mixture, AV" increased by approximately 97
as the temperature changed from 300 K to 320 K; an additional 20 K increase
produced an additional 5% increase in V". The value of AV" quickly
stabilized after the temperature had been changed. Steady-state values
of V" obtained with the shutter open were more dependent on temperature
and much less stable than those of AV".
We have performed other tests on NO detection in our laboratory with
a gas cell correlation instrument employing the same bandpass filter and
correlation cell as those illustrated in Fig. 2-1. Absorption by water
vapor in the atmospheric path in the instrument interferes with the NO
measurement and produces a shift in the zero reading that corresponds to
the test bulb being free of any NO. If the H»0 concentration is constant
it can be accounted for by adjusting the zero setting when the test bulb
has been shaded from actinic energy sufficiently long for all of the NO
to disappear. However, variations in the H?0 concentration can cause
zero shifts that produce significant errors in measurements of V". Errors
in AV" produced by H»0 interference are much smaller because this quantity
represents the decrease or increase in V" each time the shutter is closed
or opened. If several measurements are repeated, the errors cancel each
other out.
2-10
-------
Results obtained in the sunlight with a test bulb having a boro-
silicate glass body were similar to those obtained with a test bulb made
of quartz, which is known to be transparent in the spectral interval of
the actinic energy. Although the glass is known to absorb strongly at
the shorter wavelengths of the actinic energy, values of AV" were only
about 10% less for the glass test bulb than for the quartz one. A greater
difference might be observed where shorter wavelengths comprise a greater
proportion of the actinic energy than that transmitted through the hazy
atmosphere of these tests.
MONITORING OF N02 VS NO
Nitrogen dioxide has at least three absorption bands in the infra-
red and visible that are worthy of consideration for a gas cell correlation
instrument designed to monitor this gas. The strongest infrared band
near 6.2 \i,m is probably not practical for an instrument with a signifi-
cant optical path in the atmosphere because of excessive interference by
the strong, over lapping'lUO absorption. Furthermore, no convenient de-
tector with adequate detectivity without cryogenic cooling exists for this
band. Considerable structure exists in the spectrum of the 3.4 u-m band
so that it lends itself to the use of gas cell correlation techniques.
This band is also relatively free of interfering absorption by H»0 or other
atmospheric gases. However, its band strength is only moderate so that
the sensitivity of an instrument employing this band is limited. A
relatively convenient detector with a thermoelectrically cooled PbSe ele-
ment would probably be adequate for the 3.4 pm region.
As pointed out in Section 1, most of the NO? absorption in the uv is
continuous without enough structure in the spectrum for a gas cell corre-
lation instrument to operate efficiently. We have investigated the use
of the absorption features in the visible spectrum. These features are
quite distinct and consist of many very closely spaced lines that overlap
except for very low sample pressures. Fig. 2-6 shows spectral curves of
transmittance obtained with a spectral slitwidth of approximately 2 A.
Absorption is quite strong, even at wavelengths adjacent to the stronger
absorption features. Thus, the strong absorption features are not sepa-
rated by narrow regions of little or no absorption as are the lines in
the fundamental NO band near 5.4 M>m. Because of the absorption between
the wavelengths of strongest N0« absorption, a gas cell correlation in-
strument is less efficient than we had originally expected. Our tests
on these visible NO- bands were limited to the measurement of N02 in a
conventional absorption cell, rather than in a test bulb with NO2 being
dissociated photochemically.
We have used the actinometer illustrated in Fig. 2-1 to obtain data
such as those represented by Fig. 2-2 as well as to study stable samples
of NO + N2 in the test bulb. By comparing the results, we have been able
2-11
-------
100%
to
I
u
u
z
s
£
07,
4300
4400
WAVELENGTH
4500
(X)
FIG. 2-6. Transmittance of NC>2 from 4300 to 4500 A. The sample cell length
was 3.96 cm, the pressure of NO 9 was 25 torr, and the temperature
was 51"C.
-------
to estimate the NO concentration in the test bulb from the measurement
of V". During the period when the shutter was cycling to obtain the data
shown in Fig. 2-2, the concentration of NO was oscillating from about 4%
to 6% of that of the N02- It follows that the N02 concentration was
varying from approximately 94% to 96% of its original value. If the acti-
nometer were made to measure the N02 concentration, rather than NO, it
would be required to measure the small relative change quite accurately.
Because the relative change in the NO concentration is much greater, the
required accuracy for its measurement is much less. For example, the
temperature of the test bulb can be expected to oscillate slightly in
synchronism with the opening and closing of the shutter over the test
bulb. The small temperature change can influence slightly the absorption
characteristics of both the N02 and the NO and produce an error in the
measurement of AV". Such an error would be more serious when measuring
the small relative changes in N02 than when measuring the larger relative
changes in NO.
After considering the encouraging results obtained with the actinometer
measuring NO concentrations and the problems anticipated in measuring the
changes in NO. concentrations, we decided to build the final instrument
to measure the changes in NO.
HOT NO AS AN IR ENERGY SOURCE
The use of a hot gas cell containing NO as a source of infrared energy
in place of a conventional continuous source has been considered, and a
few related experiments have been performed. Such a source would provide
the energy for the monitoring beam in an actinometer that monitored the
formation of NO in the test bulb. The hot NO in the source cell emits
strongly near the centers of the absorption lines which, of course, are at
the wavelengths of strong absorption by the NO in the correlation cell and
in the test bulb. The length, L, of the hot source cell and the pressure
PNO, of NO in it should be adjusted so that the product L PNO is somewhat
greater than the corresponding quantity for the NO in the correlation cell.
On the other hand, it should be small enough that there is not appreciable
emission at wavelengths midway between adjacent lines.
Little information about NO in the test bulb can be obtained from the
wavelengths well separated from the line centers. Therefore, the extra
energy emitted by a continuous source at these wavelengths produces little
information, but it can contribute to misbalance, drift and interference
by other gases that may be present in the optical path. Because of the
coincidence of the emission lines of the source gas and the absorption
lines of the absorbing gas in the test bulb, a fixed amount of absorbing
gas absorbs a larger fraction of energy from the hot gas source than from
a continuous source. For a low concentration C of NO in the test bulb of
optical path length LXB, the normalized voltage V" is related to C by
2-13
-------
V" = Va/Vfa = k C 4B- (2-1)
It follows from the above discussion that the constant k can be larger
when using a hot NO source than when using a continuous source. Thus a
given percentage error in balance of the instrument yields a smaller error
in the measurement of C when using the hot NO source.
Hot gas cells generally cannot be operated at temperatures as high as
continuous sources, with the result that both Va and V, are lower for hot
gas sources. In the case that the instrument accuracy is limited pri-
marily by detector noise, the increased energy available from the con-
tinuous source makes it preferable to the hot gas source. The results of
the tests made with the actinometer shown in Fig. 2-1 indicated that the
larger values of k and the better discrimination available with the hot
NO source would not be as important for the actinometer to be built as
would the extra signal available with the continuous source. The extra
signal is required because it is desirable that the actinometer not re-
quire a cryogenically-cooled detector; therefore, the detector detectivity
is much lower than that of the liquid nitrogen-cooled detector used in the
preliminary tests.
CONCLUSIONS DRAWN FROM PRELIMINARY TESTS
The results of the tests reported here indicate the feasibility of an
N02 actinometer for field use in which the concentration of photochemically-
produced NO is monitored by non-dispersive infrared techniques. Accurate
steady-state values of NO concentration are difficult to determine because
of instabilities in the optical components and variations in the tempera-
ture of the test bulb. Other slow chemical reactions not considered here
may also produce slow changes in the NO concentration. In contrast, re-
sults that are more consistent and easier to interpret can be obtained by
using alternate light and dark periods of approximately 30 to 60 seconds
each to produce rapid changes of NO concentration around a mean level.
The alternating method has the problem that the total response is reduced
substantially and the cycle periods must be controlled carefully. How-
ever, these problems are more than offset by the improved stability. The
relatively slow response in both methods precludes the measurement of
small, rapid fluctuations of the actinic light level. Monitoring the con-
centration of NO can apparently provide a more accurate measure of the
actinic energy than is possible by monitoring the N02«
Although the optimum instrument parameters depend on a variety of
operating conditions, it is possible to prescribe approximate parameters
for a potentially useful tool for measuring the relative available actinic
light under most natural conditions of interest. A spherical, quartz
test bulb 10 cm in diameter filled with a mixture of N02 + 02 can be made
2-14
-------
with its sensitivity to light nearly the same from all directions in the
upper hemisphere. The amount of NO produced under normal atmospheric con-
ditions is sufficient to be measured by non-dispersive infrared techniques
employing a gas correlation cell. The addition of an inert gas such as N_
to the mixture in the test bulb is not required and would reduce the rate
of mixing of the gases and increase non-uniformities in the NO concentra-
tion.
A small, readily available InSb detector with thermoelectric cooling
to about 250 K can probably provide an adequate signal-to-noise ratio
when used with a conventional infrared energy source. Cooling the de-
tector to lower temperatures with liquid nitrogen is probably not required.
The advantage of improved discrimination that could be achieved by using
hot NO as the Infrared source is more than offset by the increased com-
plexity and decrease in signal.
Windows on the test bulb for the transmission of the IR monitoring
beam and associated mirrors necessarily decrease the sensitivity of the
system to actinic light arriving from certain directions. By properly
orienting the test bulb relative to the sun so that the windows and mirrors
do not interfere with the direct sunlight, it is possible to minimize the
error caused by these interfering optical components.
2-15
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SECTION 3
DESCRIPTION OF ACTINOMETER
OPTICAL LAYOUT
The requirements described above and the results of the preliminary
tests described in Section 2 led to the design and construction of the
instrument illustrated in Figs. 3-1, 3-2 and 3-3. The upper panel of
Fig. 3-1 shows an optical schematic diagram as viewed from above. Infra-
red radiant energy is directed by mirror Ml through a correlation cell
with an image of the source formed at the reflecting plane of the mirr&r
chopper. When the notched, mirror chopper is in the open position, the
beam continues on and another image of the source is formed on the small,
flat mirror, M8, which is tilted approximately 22.5° from the vertical so
that the beam is reflected at 45° upward to mirror M9, which returns the
beam displaced slightly sidewise to a CaF2 lens. A reduced image of the
source is formed on the detector.
During the other half of each cycle the mirror chopper, the energy
falling on the detector is directed by mirror M3 through an attenuator to
the reflecting surface of the mirror chopper. The fixed portion of the
attenuator is a sapphire window, and the adjustable portion blocks a
portion of the beam. Beyond the mirror chopper, the beam travels the same
path during both halves of the cycle. Mirror M8 is sufficiently short that
it limits the useful length of the source from which radiant energy is
collected. Spherical mirror M7 images mirror M6 near mirror M9, which
is bonded to the quartz test bulb and forms the aperture stop. The mask
on mirror M6 reduces scattered light that would not be collected by M9.
A narrow bandpass filter similar to the one shown in Fig. 2-1 is placed
in front of the detector and limits the spectral band to a region approxi-
mately 0.15 microns wide centered near 5.3 microns.
3-1
-------
MIRROR
CHOPPCR
Fig. 3-1. Optical diagrams of the optics assembly of the actinometer. The upper
panel is a top view with the instrument in its normal operating position.
A side view of a portion of the instrument is shown in the lower panel.
3-2
-------
FIG. 3-2. Two views of the optics assembly of the actinometer.
The upper panel shows the instrument completely assembled
with the shutter open. The lower panel shows a top view
with the shutter open and the lid removed. S, source;
CC, correlation cell; Ch, mirror chopper; TB, test bulb;
Pre, preamp.; P, power supply.
3-3
-------
FIG. 3-3. Photograph of the control assembly, amplifier and recorder.
The control assembly contains switches and a timer assembly
for remote control of the shutter and the recorder.
3-4
-------
Approximately 10 torr of NOo plus 110 torr of 0_ has been added to
the quartz test bulb. As the test bulb is irradiated with actinic energy,
from the sun or from an artificial source, part of the N02 is photolyzed to
produce NO and 0~ according to Eq. (1-1). The shutter periodically covers
the quartz test bulb to shade it from actinic energy. Out of each 2 min
period of the shutter, it is open approximately 55 sec and closed for 55
sec. Approximately 5 sec are required for the shutter to open or close.
The primary purpose of the instrument is to measure the change in the
concentration of NO in the test bulb during the two halves of the shutter
cycle.
The spectroscopic principles of operation of this instrument are the
same as those of the breadboard instrument shown in Fig. 2-1. After the
shutter has been closed sufficiently long that essentially all of the NO
has disappeared from the test bulb, the attenuator is adjusted so that
the same amount of infrared energy is incident on the detector during
both halves of cycle of the mirror chopper. Thus, there is no detector
signal at 350 Hz, the chopper frequency. The adjustable attenuator is
mounted directly to a micrometer screw and is moved into or out of the
beam. After the instrument has been balanced, the quantity Vj, is measured
by moving the blocking attenuator to the position indicated by the broken
line in the upper panel of Fig. 3-1. This blocks all of the energy that
normally passes through the attenuator with the result that the infrared
energy reaching the detector is completely modulated by the mirror chopper.
After the quantity VD has been measured and recorded,the blocking attenu-
ator is moved to its normal position before measurements are made.
DESCRIPTION OF OPTICAL AND MECHANICAL COMPONENTS
The source is a ceramic-coated resistance wire that is heated elec-
trically by approximately 10 amperes of alternating current at 2.5 V.
This source is commercially available from Perkin-Elmer Corporation and
receives power from a small transformer. Its spectral radiance in the
region of interest, near 5.3 microns, is only approximately half that of
a Nernst glower under normal operating conditions. However, this source
is easier to operate, and.its durability makes it more practical for a
field instrument.
The mirror chopper is machined from stainless steel and has 6 reflec-
ting blades and 6 openings of equal width. The reflecting surface has
been coated with a thin layer of aluminum and polished to produce good
reflectivity. The chopper is mounted directly to the shaft of a small
induction motor that rotates at approximately 3500 rpm to produce a
chopping frequency near 350 Hz. This frequency is sufficiently high
that the detectivity of the detector is not seriously limited by the 1/f
noise. In order to obtain higher chopper frequencies, either more blades
would be required on the chopper or it would need to be rotated at a
higher frequency. Either of these methods has obvious disadvantages.
3-5
-------
The detector contains a 2 mm x 2 mm sensitive element of InSb that is
cooled to approximately -25°C by a small thermo-electric cooler that re-
ceives its power from a dc power supply mounted in the position shown in
Fig. 3-2. A separate supply powers the preamplifier and the reference
pick-up and produces bias voltage for the detector.
Calcium fluoride windows are sealed by epoxy cement to the 1 cm long
by 2.5 cm diameter correlation cell. Approximately 680 torr of NO in
the correlation has an average transmittance of 72% in the spectral band
of the bandpass filter.
The test bulb is approximately 10 cm in diameter and is made from the
spherical bulb of a quartz beaker. Holes approximately 2.2 cm in dia-
meter have been cut in the walls; CaF« windows are bonded with epoxy
cement over the holes. The quartz wall had to be removed from the windows
because quartz is opaque to the 5.3 micron infrared energy in the moni-
toring beam.. A piece of Monel tubing has been bonded in a smaller hole
cut in the bottom of the test bulb. A small Monel valve welded to the
tubing makes it possible to evacuate the test bulb and refill it with
any desired mixture at a pressure less than 1 atm. It is anticipated
that the mixture of NO- + 02 in the test bulb can be used for several
months without refilling.
The test bulb is mounted above the remainder of the instrument in
order that it can receive actinic energy from as much of the upper hemis-
phere as possible. It seems likely that a negligible portion of actinic
energy would be reflected from the earth's surface. Thus, most of it
would be incident on the test bulb from a direction above the horizon.
Ideally, the bulb should have no obstruction that would shade any part of
it from any part of the upper hemisphere. However, it is necessary to
obstruct small portions of the test bulb in order to pass the infrared
monitoring beam across a diameter. The CaF. window on the lower side of
the test bulb is not expected to obstruct a significant portion of the
actinic energy from the sky. The window and mirror M9 on the upper
portion of the test bulb probably cause the most obstruction and produce
the biggest deviation in the sensitivity from all directions. On most
days, it is expected that a large fraction of the actinic energy will
come from a direction within several degrees of the sun. Therefore, the
influence of the windows and mirror M9 can be minimized by orienting the
instrument so that these parts produce a minimum of obstruction of light
coming directly from the sun. This is best accomplished by locating the
instrument with the end containing mirrors Ml and M3 pointed in the
azimuthal direction of the sun. Unless the sun is very high in the sky,
shading by mirror M9 should not be serious. It seems likely that the
shading of the test bulb by the windows and the mirror is less under most
circumstances when oriented in this manner than would be possible if
mirror M9 were located on the top of the bulb with the monitoring beam
3-6
-------
traveling along a vertical diameter. Double passing the test bulb with
the infrared monitoring beam increases the sensitivity of the system
without enlarging the diameter of the test bulb. Although mirror M9
shades part of the test bulb, removing it so the beam single-passes the
test bulb would not significantly decrease the shading because another
mirror would need to be placed near the bulb to direct the beam to the
detector.
In order to further minimize any obstruction of the test bulb from
light in the upper hemisphere, the legs on the instrument are made so
that it slopes downward at about 5° from the test bulb toward the end
containing mirrors Ml and M3. The shutter, when open, also is below
the horizon from any point on the test bulb.
The cycling shutter is chain driven by a small gear motor that is
turned on and off and reversed in direction by a timing mechanism mounted
in the control panel described below. Microswitches actuated by the
shutter turn the motor off as it approaches either the closed or open
position. A small slip-clutch mounted to the shaft of the motor allows
the shaft of the gear motor to continue turning for a short time after
the power has been turned off and the shutter has reached the end of its
travel. It is necessary to allow for this small amount of slippage because
of the angular momentum of the armature of the motor.
Fig. 3-3 shows the control unit, the synchronous amplifier, and the
strip chart recorder. On-off switches on the front panel of the control
assembly operate the chopper, the source, and the detector. When the
shutter-control switch is in the manual position, the shutter can be
either opened or closed by holding a spring-loaded switch in the appropri-
ate position. The automatic timer controls the action of the shutter
when the shutter-control switch is in the automatic position. If no
electrical power is available, the shutter can be opened or closed by
forcing it slowly and allowing the clutch to slip.
When the chaxt recorder-control switch is in the manual position, the
pen remains in the writing position and the chart is controlled by the
switch on the front of the recorder. With the chart recorder-control
switch in the automatic position, both the chart drive and the pen may
be deactivated by the timer. In the automatic mode, the chart drives
and the pen writes only for two intervals of about 10 seconds each during
each shutter cycle when V" is near its minimum and maximum values. By
recording only during these intervals, the amplitude that corresponds to
iiv" can be measured easily and less chart paper is required. Recorder
tracings that illustrate the use of the chart recorder in the automatic
mode appear in Section 4. The period of time that the pen writes on
the strip chart can be changed by adjusting the appropriate cams in the
timer assembly.
3-7
-------
A push-button switch on the front panel of the control unit actuates
an "event-marker" pen that records on the right-hand side of the strip
chart. The electrical cables that connect the optics assembly to the
control unit, recorder, and amplifier are approximately 8 meters long and
can be removed for easy handling. Detailed wiring diagrams are shown in
Section 5.
SET-UP AND OPERATION
The instrument is designed for use in the position shown in the upper
portion of Fig. 3-2. The bench or table on which it is placed should be
small so that it subtends a small solid angle from the test bulb and thus
blocks a minimum of actinic energy. Other structures that block actinic
energy from the bulb should also be avoided. As pointed out previously,
the amount of energy blocked by mirror M9 on the test bulb is minimized
by orienting the instrument so that the right-hand end in Fig. 3-2 is in
the azimuthal direction of the sun. The outside surface is blackened to
minimize reflection of actinic energy to the test bulb.
The power supplies and the source produce enough heat inside the
instrument cover to increase the temperature by several degrees. Over-
heating of the power supply for the detector coolers is avoided by heat-
sinking it to one side of the box. It is recommended that a fan be used
to circulate air over the optics assembly to reduce its temperature, parti-
cularly if it is being used in hot, sunny weather or in a hot test chamber.
Increased temperature degrades the signal-to-noise ratio of the cooled
detector. Excessive temperature changes in the test bulb also change the
response of the instrument.
All of the electrical plugs and connectors are labeled so that the
instrument can easily be connected in accordance with Fig. 5-2. A few
minutes should be allowed for warm-up before the following adjustments
are made. The beam in the attenuator side of the alternator is blocked
by moving the knob for the blocking attenuator to the BLOCK position. The
knob is located near the chain-drive for the shutter. Adjustments are
then made to the amplifier frequency and phase to produce a maximum posi-
tive output of the amplifier. The amplifier frequency reading should be
approximately 350 Hz, the chopper frequency. Positive output corresponds
to a reading to the right of center on the meter on the amplifier panel
with the meter-monitor switch in the OUT x 1 or OUT x 10 position.
Zero and span adjustments on the amplifier and recorder are made in
the following manner. Set the meter-monitor switch to the OUT x 10
position and disconnect the signal input cable to the amplifier. Use the
zero adjustment on the amplifier to set the meter reading to zero (center).
Next, adjust the recorder zero potentiometer on the front panel of the
recorder to produce a zero reading of the recorder. Set the meter-monitor
3-8
-------
switch on the amplifier to the OUT x 1 position and rotate the zero-
adjustment potentiometer on the amplifier clockwise until the panel meter
reads full-scale (10) to the right. Now, use the recorder-adjust poten-
tiometer on the rear of the amplifier to produce a full-scale reading
of the recorder. Return the zero-adjustment potentiometer of the ampli-
fier to the position that produces a zero, or mid-scale, reading on the
meter. Reconnect the signal input cable to the amplifier. The recorder
is now spanned so that a full-scale reading corresponds to full-scale of
the amplifier panel meter and is within the useful dynamic range of the
amplifier. A 1 K ohm resistor across the input terminals of the recorder
matches the recorder to the amplifier output. A zero off-set of the
recorder can be obtained by adjusting the zero-potentiometer on the
recorder.
In normal operation, the chart speed-control switches on the recorder
and on the control assembly are in the LO position. When the control
switch of the control assembly is in the FAST position, the chart speed
is opposite to that indicated by the switch on the recorder. The chart
speed-control switch on the control assembly is not needed but was in-
stalled before the recorder was delivered.
From Section 2, we recall that the instrument output of Interest is
AV", which is proportional to AV/V, > where VL is the output with the
attenuator leg of the alternator blocked, and AV is the change in the
output as the shutter periodically opens and closes. (See Fig. 2.2.)
Before starting a series of measurements, the blocking attenuator is
moved to the BLOCK position to block the beam in the attenuator leg and
V, is measured. At the time the instrument was delivered to EPA, Vj,
corresponded to a recorder-pen of approximately 17 cm with the amplifier
on 20 mv scale. In order to simplify the conversion of AV to AV", the
pen deflection can be adjusted to produce an even-numbered deflection,
such as 20 cm. This is done by adjusting the calibration-potentiometer
on the input of the amplifier. It is best if this is done after the
shutter has been closed sufficiently long that little NO remains in the
test bulb.
After V, has been measured and recorded, the blocking attenuator is
removed from the beam and the alternator is balanced with the adjustable
attenuator while the shutter is kept closed to avoid the formation of NO
in the test bulb. The instrument is then ready for operationin the manner
described previously. Ordinarily, the system is balanced with the ampli-
fier on a more sensitive scale, either 1 mv or 2 mv full-scale. Measure-
ments of AV are also made on the most sensitive scale. In relating the
pen deflections to V" or AV", the different sensitivities of the ampli-
fier must be accounted for. For example, assume that a recorder-pen
deflection of 20 cm was observed when measuring Vy, with the amplifier
on the 20 mv scale. When the actinic irradiance is being measured, the
amplifier is on the 1 mv scale. As the shutter opens and closes during
3-9
-------
its cycle, the pen deflection varies from a maximum of 13 cm to a mini-
mum of 9 cm, giving a difference of 4 cm. If V, had been measured on this
scale, it would have corresponded to a pen deflection of 20 x 20/1 = 400
cm. Therefore, AV" = 4/400 = 0.010.
After several minutes of warm-up, Vu remains nearly constant for
several hours unless the instrument temperature changes significantly.
Thus, the frequency that V. needs to be checked depends on the weather
conditions i£ the instrument is outside, or upon the temperature in a
test chamber.
3-10
-------
SECTION 4
TEST RESULTS AND INSTRUMENT PERFORMANCE
RESPONSE TO SOLAR ENERGY
The recorder tracing shown in Fig. 4-1 illustrates the actinometer
output in the afternoon of a clear day in early June in Newport Beach,
California. The instrument was located on a roof top with essentially
clear vision above the horizon in all directions. Before time t = 0,
the actinometer had been balanced with the shutter closed so that no NO
was present in the test bulb. At t = 0, the shutter was opened; thereafter
the chart drive and the pen operated continuously while the shutter opened
and closed with a two-minute period. At t = 0, 2, 4, 6 minutes the
shutter opened; it closed at t = 1, 3, 5, 7 minutes. While the shutter
was open, V" did not increase linearly with time during the full one-minute
period. This non-linearity contributes significantly to the non-linear
relationship between AV" and actinic irradiance.
The first peak is not as high as the following ones because the NO
concentration had not reached its maximum value when the shutter closed
at t = 1 minute. After two cycles of opening and closing the NO concen-
tration has essentially reached the point so that the maximum and minimum
values of V" repeat themselves if the actinic irradiance is constant.
Typically, the maximum value of V" attained during cyclic operation of the
shutter is between 2\> and 3 times AV", the difference between the maximum
and minimum values of V". Detector noise is the major cause of small,
high-frequency variations in V". The peak-to-peak noise equivalent V"
is typically less than 0.001 with an amplifier time constant of 1 second.
Recorder tracings of the actinometer output are shown in the upper
panel of Fig. 4-2 for both the recorder and shutter operating in the
4-1
-------
0.03
0.02
V"
0.01
TIME (roin)
FIG. 4-1. Response of actinometer to solar energy with the shutter in the automatic mode.
-------
0.03
^y
0.02
0.01
D C
L
"I
V
o.oo
TIME
_ 0.03
— 0.02
Fig. 4-2. Recorder tracings of actinometer output when the sky contains broken clouds. L'pper panel: The shutter is cycling
and the recorder chart .drive and pen are in the automatic mode. Lower panel. The shutter is closed and the
recorder is running continuously.
-------
automatic mode. From point A to point B, the shutter has been closed
sufficiently long that NO exists in the test bulb. At point B, both the
recorder and shutter are started in the automatic mode. In this mode, the
chart drives and the pen is in the writing position only for short periods
when V" is near its minimum and maximum values. The shutter opens or
closes during these short periods. Values of AV" can be determined from
these condensed strips of chart by measuring between the minimum and maxi-
mum values of V" as illustrated in Fig. 2-2.
From point B to point C, the sky contained several broken clouds with
some thin clouds moving into and out of the direct path between the sun
and the test bulb. At point C, no apparent clouds were in the direct path.
From point D to point E, a cloud formed in the path so that at point E
there was enough scattering by the clouds that no apparent shadows were
cast by trees or buildings in the neighborhood of the instrument. This
type of cloud cover remained from point E to point F.
At point F, the shutter was closed and remained closed while the chart
drove continuously and recorded the curve shown in the lower panel of
Fig. 4-2. From point F to point G, V" decreased continuously, indicating
a decrease in the concentration of NO in the test bulb. It is apparent
that the rate of disappearance of NO decreases as its concentration decreases.
Between points G and H the infrared monitoring beam was blocked in order
to record the pen deflection corresponding to the absence of NO in the test
bulb.
RESPONSE VS ACTINIC IRRADIANCE
The curve in Fig. 4-3 relates AV" to the relative actinic irradiance.
A "black-light" placed approximately 20 cm from the test bulb served as the
source of actinic energy to obtain the data on which the curve is based.
A large, segmented chopper blade rotating at 1 revolution per second
blocked the beam of actinic energy from the test bulb for different fractions
•of the time. The data point corresponding to a relative irradiance of
100 was obtained with this chopper blade stopped in the open position so
that the test bulb was fully irradiated. By rotating the chopper with the
blades adjusted so that the chopper was half open and half closed, it was
possible to obtain the data point corresponding to a relative irradiance
of 50. Similarly, a blade that blocked the beam 25% of the time was used
for a relative irradiance of 75%, etc. Because of the 1 rps rate of the
chopper blade the length of time that the beam was transmitted, or blocked,
is short compared to the reaction time for the formation or decay or NO in
the test bulb. Therefore, it is expected that decreasing the duty cycle
'in this manner has the same effect as decreasing the actinic irradiance by
the same factor.
The non-linearity in the relationship between actinic irradiance and
AV" arises from a combination of two factors. When the NO concentration
4-4
-------
0.012
0.010
0.008
AV"
0.006
0.004
0.002
60
80
100
Relative Actinic Irradiance
FIG. 4-3. Plot of AV" vs relative actinic irradiance.
4-5
-------
in the test bulb is sufficiently high that the absorption is appreciable
at the centers of the absorption lines, the amount of energy absorbed no
longer increases linearly with the NO concentration. Secondly, the net
rate at which NO is formed during each 1-minute period that the shutter
is open decreases as the NO concentration increases. By reducing the
period of the cycling shutter, the non-linearity resulting from the change
in the rate of NO formation could be reduced. However, this would also
reduce AV", and thus the signal-to-noise ratio.
RESPONSE VS DIRECTION OF ACTINIC ENERGY
As discussed in the first part of this section, we expect the response
of the actinometer to be somewhat dependent upon its orientation relative
to the sun, particularly when there are but a few clouds in the sky and
most of the actinic energy arrives from a direction near the sun. In
order to approximate the effect of direction we obtained the data summarized
in Table 4-1 in the P.M. of May 1, 1974 when the sun was approximately
45° above the horizon. The first measurement was obtained with the acti-
nometer sitting on a horizontal table so that angle a = 0. The long side
of the actinometer was in the azimuthal direction of the sun with the
end containing the test bulb farthest from the sun. The other data were
obtained by rotating the actinometer clockwise around a vertical axis so
that ot was equal to the angles indicated. Approximately 10 minutes were
required for the measurement at each angle. Three of four cycles of the
shutter were used, and the data for all of the cycles were averaged.
As expected, the maximum response was observed when a = 0 , and the
minimum occurred at a = 180° when M9 provided the maximum shade on the
Lest bulb. A few wispy clouds in the sky probably caused a few percent
variation in the measurements. During the 50-60 minutes between the two
times that the measurements were made at a = 0, the change in AV" corres-
ponded to approximately art 8% decrease in irradiance. Part of this
decrease probably resulted from the increase in the zenith angle of the
sun and the corresponding increase in the atmospheric path. Some of the
decrease may have resulted from increased thickness of the thin clouds.
Although the actinometer response is somewhat directionalt a relatively
small fraction of the energy is usually incident from the portion of the
sky opposite the sun. Therefore, it is expected that errors due to the
direction-dependent response will not exceed a few percent if the instru-
ment is oriented as prescribed above with 01 = 0°.
4-6
-------
TABLE 4-1
RESPONSE VS ORIENTATION OF ACTINOMETER RELATIVE TO THE SUN
Angle Qf &V"
(degrees)
0 0.0111
45 0.0109
90 0.0104
135 0.0099
180 0.0098
0
RESPONSE VS NO CONCENTRATION
Although the quantity of primary interest is AV", it is also of interest
to know the approximate concentration of NO in the test bulb at any time.
Figure 4-4 shows a curve with which the NO concentration can be related to
the instantaneous velue of V". The data on which the curve is based were
obtained with mixtures of NO + N, in a sample cell in place of the test bulb.
The absorber thickness of NO is equal to the partial pressure of NO in atmos-
pheres times the optical path length of the sample cell. Since the optical
path of the test bulb is approximately 21 cm, the NO concentration is (1/21)
times the value of absorber thickness determined from Fig. 4-4 and the observed
value of V". As an example, if V" = 0.02, the absorber thickness of NO is
approximately 0.014 atm cm. This corresponds to a partial pressure of
0.014/21 = 0.67 x 10 atm, or 0.9 torr of NO..
Because the absorption by a given absorber thickness of NO depends on
the widths of the absorption lines, it depends on the collision frequency
o£ the absorbing molecules and thus upon the total pressure of a mixture
of NO with other gases such as N02, N2, or 0- that do not absorb in the
spectral region where the NO is measured. The NO + N« mixtures used to
obtain the data in Fig. 4-4 were adjusted to a total pressure of 120 torr,
the same as that in the test bulb. Differences in the broadening abilities
oC N» and 0_ + N0« are not accounted for; however, they are expected to
produce only slight errors when using Fig. 4-4 in the described manner to
determine the concentration of NO in the test bulb.
4-7
-------
0.1
V"
0.01
03
0.001
0.001 0.01
ABSORBER THICKNESS OF NO (atm cm)
0.1
FIG. 4-4. Response of actinometer to different absorber thicknesses of
NO in a mixture of NO + ^ at a total pressure of 120 torr.
By comparing the value of V" observed with the actinometer
in normal use to this curve, it is possible to determine
the NO concentration in the test bulb.
-------
SECTION 5
ELECTRICAL DIAGRAMS AND DETAILS OF COMPONENTS
ELECTRICAL AND MECHANICAL
Figures 5-1 through 5-8 show schematic diagrams of the electrical
components. These figures should be adequate for any trouble shooting,
modifying or replacing of parts. The phase-lock amplifier has not been
modified and is used in its normal manner. An input plug with a 1 K ohm
resistor connected across the input leads has been added to the recorder.
Suppliers of the electrical and mechanical components and their
specifications are listed below. Manufacturers' manuals are provided
separately for the recorder and amplifier.
Recorder: Hewlett-Packard Model 7123A with
remote controls for the pen-lift and
chart drive. Chart speeds 3.75 and
15 cm/min.
Amplifier: Princeton Applied Research Model 122.
Power Supply: Zeltec Model Z15AT100 DP. For the preamp
and reference circuits.
Power Supply: Powertec Model 2B5-3. For thermo-electric
cooler for the detector.
Source Transformer: Triad F-3X, 2.5 V ac @ 10 amp.
Timer Assembly: Minarik Electric Company, multi-cam timer
6C-120S, recyling.
5-1
-------
Chopper Motor:
Shutter Drive Motor:
Clutch:
Globe Industries, 75A-121-1.
Hurst Model GA, 5 rpm, synchronous,
stock #171-01-043.
On shutter drive. PIC #R3-3-50.
OPTICAL COMPONENTS
Listed below are the specifications and the suppliers of the
optical components:
Source:
Detector:
Test Bulb:
Lens:
Mirrors Ml and M3
Perkin-Elmer Part No. 457-0244. Operated
in this instrument at approximately
2.5 V ac and 10 amps.
InSb vith 2 mm x 2 mm sensitive element
cooled to approximately -25°C by a
thermoelectric cooler.
Part Wo. OTC-3M1.
Optoelectronics
Quartz body with 1" diam Ca?2 windows.
Windows and the line to the valve are
bonded to the bulb with RTV, a silicone
cement. Additional strength to the
bonds is provided by epoxy cement.
Because of a chemical interaction between
epoxy cement and NO™» the silicone is
used to separate the epoxy from the NOn
in the test bulb. A Monel valve connected
to the test bulb makes it possible to
refill the test bulb. The quartz body
was provided by Quartz General, 12440 Exline
Street, El Monte, Ca., 91732, who origi-
nally made it as a round bottom 500 ml
flask. Cat. #U 106 Q.
CaF,, 1.37 cm diam, 1.39 cm effective
focal length at 5.3 n,m. Unique Optical,
P.O. Box 585, Farmingdale, New York.
5.0 cm diam, 10 cm focal length, spherical.
Ealing Corporation, 2225 Massachusetts Ave.,
Cambr id ge, Mas s., 021400.
5-2
-------
M6:
Mirror M7:
Mirror M9:
Mirrors M2, M4,
M5, M8:
Mirror Chopper:
Fixed Attenuator:
5.0 cm diam, 10 cm focal length,
spherical. Baling Corporation.
Cut from a spherical mirror of 7.5 cm
focal length provided by Baling Corporation.
Cut to approximately 2.6 cm diam from a
7.5 cm focal length mirror provided by
Baling Corporation.
Flat mirrors cut to the desired dimensions
Machined from stainless steel, chemically
coated with aluminum and polished to pro-
vide a smooth finish. Contains 6 blades
and revolves at approximately 3500 rpm
producing a chopping frequency of 350 Hz.
Sapphire window, 2.54 cm diam by 1 mm
thick.
5-3
-------
_s t- US(L
• A/ , g/«je. ofe*j F«)o
fig. 5-1. Simplified schematic diagram for ac power. The shutter drive motor and its limit
switches are located on the optics assembly; all other switches shown are on the
control assembly. The power supplies, the transformer for the source and the
chopper are on the optics assembly. The cycle-timer motor and the cams driven by
it are in the control assembly.
-------
Optics
PAR
CAbJc
i.y j I $vf~f>L-f [
Fig. 5-3. Wiring diagram of electrical components on the optics assembly.
The 115 V ac inputs for the power suppliers are shown; the output
lines, not shown, go to the corresponding components.
•5-5
-------
-rs-/
Fig. 5-4. Wiring diagram of timer and control assembly. One
revolution of Che cycle timer motor corresponds to one
period of the shutter. Two interchangeable motors are
provided; one runs at 1 rpm and the other at 0.5 rpm
giving shutter cycles of 1 and 2 minutes, respectively.
Cams 2 and 3 can be adjusted to change the period of
time that the recorder chart switch is in the automatic
position. Similarly, cams A and 5 control the pen lift.
When the corresponding switches on the control assembly
are in the manual position, the recorder chart drive and
the pen lift are controlled by the switches on the recorder.
5-6
-------
+ ISV
Fig. 5-5. Schematic diagram of preamplifier used with the InSb detector.
loo-n.
5/OJT.
- L
_T^L3§L
<
/dK-t- <
«
<
J
r
v_
?
>
>
>
Fig. 5-6. Schematic diagram of the 350 Hz reference-pickup.
-5-7
-------
Fig. 5-7. Power supply for thermo-electric cooler for the detector.
•^rr1 m^d
<)•*£!
— DC ."jtutrf
I s^^pjy
tcfe
Fig. 5-8. Power supply for preamp and reference circuits,
5-8
-------
SECTION 6
REFERENCES
1. Hanst, P. L., Environmental Protection Agency (private communication).
2. Leighton, Philip A., Photochemistry of Air Pollution, Academic Press,
New York (1961).
3. Giauque, W. F. and Kemp, J.D. "The Entropies of Nitrogen Tetroxide and
Nitrogen Dioxide. The Heat Capacity from 15 °K to the Boiling Point.
The Heat of Vaporization and Vapor Pressure. The Equilibria N20, =
2N02 = 2ND + 02." J. Chem. Phys. 6, 40 (1938).
4. Burch, D. E. and Pembrook, J. D. "Instrument to Monitor CH^, CO, and
C02 in Auto Exhaust." Report No. EPA-650/2-73-030. Prepared by Philco-
Ford Corp., Aeronutronic Division for the Environmental Protection Agency.
October 1973.
5. Hanst, P. L. "Spectroscopic Methods for Air Pollution Measurements,"
in Advances in Environmental Science and Technology. (New York: John Wiley
and Sons, 1971).
6. GuLtman, A. "Absolute Infrared Intensity Measurements on Nitrogen Dioxide
and Dinitrogen Tetroxide." J. Quant. Spectrosc. Radiat. Transfer 2, 1 (1962)
6-1
-------
TECHNICAL REPORT DATA
(Please read luamcnons on the reierse before completing)
I RfcPORT NO
l-.PA-650/4-74-036
3 RECIPIENTS ACCESSION NO
4 TITLE AND SUBTITLE
N02 Actinometer for Field Use
5 REPORT DATE
August 1974
6 PERFORMING ORGANIZATION CODE
7 AUTHORIS)
Darrell E. Burch, Ross C. Bean and Francis J. Gates
8 PERFORMING ORGANIZATION REPORT NO
U-6074
9 PERFORMING ORG '\NIZATION NAME ANO ADDRESS
Philco-Ford, Aeronutronic Division, Ford Road
Newport Beach, California 92663
1O PROGRAM ELEMENT NO
1A1003
11 CONTRACT/GRANT NO
68-02-0798
11 SPONSORING AGENCY NAME ANO ADDRESS
National Environmental Research Center
Research Triangle Park, N. C. 27711
13 TYPE OF REPORT ANO PERIOD COVLREU
14 SPONSORING AGENCY CODE
15 SUPPLEMENTARY NOTES
16 ABSTRACT
Solar radiant energy in the ultraviolet and short-wavelength visible dissociate
NOp in the atmosphere to produce NO and 0?. This photolytic reaction plays an
important role in the formation of photochemical smog, and information about the
amount of actinic energy available in the lower atmosphere is required for the
development of mathematical models of the atmospheric processes. This report
describes the development and testing of an actinometer designed to measure the
actinic energy available for the photolytic dissociation of N0?. <\ spherical test
bulb contains a mixture of NCu and 0~ when it has been in the aark for several
minutes. When the bulb is exposed to solar energy NO is formed; its concentration
is monitored by gas-cell correlation methods involving the infrared absorntion by
.iO. A shutter periodically shades the test bulb from the sun for a one-minute
period each two minutes. During the shaded period, part of the NO recombines with
0? to form NO,. The cyclic change in the NO concentration is related to the
actinic irradlance.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
Actinometer
Nitric Oxide
liitrogcn Dioxide
I) IDENTIFIERS/OPEN ENDED TERMS '- COSATI I U'lci'(.nill|i
3 DISTRIBUTION STATEMENT
Unlimited
19 SECURITY CLASS f Till': Hcpnrtl
Unclassified
21 NO OF PAGES
5S
20 SECURITY
Unclassified
22 PRICE
EPA Form 2220 1 (9 73)
6-2
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