United States
Environmental Protection
Agency
Environmental Sciences Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S3-83-029 June 1983
Project Summary
Long Path Laser Ozone Monitor
Evaluation
Lucian W. Chaney and Joachim D. Pleil
The purpose of the study reported
here was to evaluate a long pather laser
air pollution monitor developed for the
U.S. Environmental Protection Agency
(EPA) by the General Electric (GE) Com-
pany. The monitor was known as
ILAMS (Infrared Laser Atmospheric
Monitoring System) and designed ex-
plicitly for measuring the ambient ozone
concentration.
The evaluation program was con-
ducted in both the laboratory and under
field conditions. In addition to the
evaluation several system modifica-
tions were carried out; such as, the
addition of a beam steering system, the
addition of a He-No laser alignment
system, and various improvements in
laser alignment techniques.
The field study portion of the evalua-
tion was carried out as a part of the
North-East Regional Oxidant Study
(NEROS) and was conducted during
the month of August 1979. The problem
areas identified during the field test
were the corner cube reflectors and
the irregularity in the laser beam cross-
section. The retro-reflectors returned
six separate lobes all of which were
displaced from the optical center line
to form a wave front approximating a
toroid. The irregularity in the beam
cross-section was previously identified
by G.E. However, it was assumed that
the irregularities were a function of
frequency and closely spaced frequen-
cies would correlate. This assumption
does not appear to be valid.
The experiments conducted, the
modifications made, and the problems
identified are completely described in
the report.
This report was submitted in fulfill-
ment of Grant R80665001 by the Uni-
versity of Michigan under sponsorship
of the U.S. Environmental Protection
Agency. This report covers a period
from Dec. 1, 1978 to Sept 30, 1980
and work was completed as of May 1,
1981.
This Project Summary was developed
by EPA's Environmental Sciences Re-
search Laboratory, Research Triangle
Park, NC, to announce key findings of
the research project that is fully doc-
umented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
The grant effort at the University of
Michigan initially consisted of a program
to evaluate and redesign the ILAMS to
eliminate problems encountered in the
acceptance tests of Oct. 1978. The grant
was later expanded to include the use of
the I LAMS to obtain vertical ozone profiles
as part of an EPA summer field study, the
North East Regional Oxidant Study, de-
signed to examine the fate and transport of
various air pollutants.
The NEROS application required a long
path system capable of measuring over a
kilometer distance with a retro-reflector to
return radiation to the source location, the
type of measurement situation envisioned
fortheILAMS. Thedecisiontocommitthe
ILAMS in the NEROS was made after the
zero drift problem was identified and elim-
inated. Additional field performance tests
were planned prior to NEROS use and
supplemental funds for the field study
were provided. The preparations for the
NEROS, the NEROS testing and the even-
tual transfer of equipment to the EPA at
Research Triangle Park, N.C. concluded the
grant activities.
Results
The ILAMS consists of four main units:
a C02 laser, a mini-computer, a teletype,
and interface electronics.
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V
The laser will be described by referring
to Figure 1. The lasing cavity follows a
zigzag path starting from M1 to M2 to M3
to M4 to the diffraction grating to M4 to
M5 to M6. The lasing lines are selected by
a mask which has four separate apertures
placed in front of the chopper. The chopper
blade has four fixed slots at different radii
such that the apertures are opened in
succession as the chopper rotates. Hence,
each line lases for a time period equal to Va
of the chopper rotation period about 2.5
milli-seconds.
The laser output beam follows a path
from M7 to M8 and through LT and L2 to
the beam splitter. Part of the output
energy passes through the beam splitter
to M9 where it is focused on the reference
detector. The remainder of the output
beam is reflected off the beam splitter to
M-| -|, M-| 2 and the telescope output mirror
M13. The beam traverses the measure-
ment path and is returned by a retro-
reflector to the telescope and back through
the beam splitter to the signal detector.
The chopper position signal and the
signals developed by the two detectors are
fed to the interface electronics for input to
the mini-computer. The first computer
operation is to form a ratio between the
return signal and the reference signal. An
automatic gain control then adjusts the
signal such that the largest of the four
ratios is equal to Vt. The logarithms of the
four ratios are calculated and multiplied by
-1000. This information is printed on the
teletype with each data printout The
computer program also multiplies each
logarithm by a pre-selected weighting factor
and calculates the appropriate differences
which are also printed by the terminal. The
difference of the weighted logarithms of
the ratios should be proportional to the
ozone concentration.
The computer software permits the
operator to set a number of parameters
such as: the weighting functions, data
printout rate, number of data points to be
averaged, system time constant and scal-
ing constants. The basic requirement is
that the system measures the difference in
absorption between two lines with known
absorption coefficients over a known optical
path. The difference in absorption is a
measure of the ozone concentration.
Pronounced zero drifting of the ILAMS
was reduced an order of magnitude by
removing the zinc selenide windows on
the reference and signal detectors. The
system was tested in a short pathlength
configuration for a period of 8 hours with a
maximum equivalent noise variation of 5
ppb.
The optical effect which was the source
of the system zero drift or noise is very well
known. In fact, it is common in infrared
systems for two reasons: one, the index of
refraction of the optical materials is usually
quite high which leads to high reflections
and two, the window polishing techniques
are similar to those used for ultra-violet
components, which give surface flatness of
high precision (A/10).
Normally, etalons must be very carefully
made in order to achieve the required
precision. However, if a window normally
intended for use in the visible is used i n the
infrared, the required precision may in-
advertently be obtained. The window as
an etalon is shown in Figure 2.
This sketch assumes that the window is
perfectly flat which, of course, is not the
case. However, it illustrates that a very
small variation in the angle of incidence
Test Retro-Reflector
— c-i:..~.'.in 11. <
MA\
__,— -
L- -1
Grating
48.00
Figure 1. ILAMS Optics—laboratory test.
2
could produce a large modulation of the
output signal. An equivalent noise level of
5 ppb(1 km path) represents an absorption
difference for the two lines at 1056 cm'1
and 1054 crtr1 of 0.6%. This modulation
could be simulated by a phase shift of 0.5°
between the two wavelengths being com-
pared. In the idealized case shown in the
sketch of 0.5° phase shift translates to a
change in the angle of incidence of some-
thing less than 0.001 degrees.
The problem can be completely avoided
in any future designs by simply wedging
the windows.
At about the same time that the zero
drift problem of the ILAMS was solved
plans were being formulated for the sum-
mer 1979 North East Regional Oxidant
Study (NEROS). The overall purpose of
NEROS was to develop models which
could be used for predicting the long
range transport of air pollutants. During
the early planning stages, it was suggested
that a long path laser designed to measure
ozone might be used to determine both
the height, formation time, and break-up
time of the nocturnal inversion. This
assumed that a 1000 foot tower could be
located for mounting the required retro-
reflectors. If the inversion information
were available, one of the current theories
regarding the transport of air pollutants
could be tested.
The essence of the theory is that those
pollutants generated in a metropolitan
area during the daylight hours can be
trapped under a subsidence inversion.
However, shortly after dark, they are cut
off from the ground by the formation of a
nocturnal inversion. Then, during the
night the polluted air mass, caught be-
tween the two inversions, is transported
many miles down wind by the geostrophic
wind until the nocturnal inversion is broken
up the next morning by surface heating.
This could explain the observation of high
pollutant levels in some remote rural areas.
To monitor the vertical ozone distribution
a beam steering mechanism was required
so that slant paths through the atmospheric
from the I LAMS (located in a van) to retro-
reflectors positioned at different locations
on the tower could be defined. A system
was designed and fabricated using a com-
merical package including a gear drive
package and associated microprocessor
purchased from Aerotech. This system
was installed in the GM van that housed
the ILAMS. It met the following design
requirements: (1) azimuth adjustment:
360° ±0.0015°; (2) elevation adjustment
: 2 to 23° above horizon ± 0.001 5°; (3)
beam control of elevation by microproces-'
sor, of azimuth by manual control; (4)
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/,„
1
d= 1000 Aim
|
n = 2.41
vw
Window
Reflectivity R_ =
Phase Angle
\ /out
1 +R
/may 100% -\
'may' 'mm ~ I « _ o
A = 9.6fjm
cosQ = PA/2nd
Fabry-Perot Fringes
=°-41
= 5.71
/mm 18% -4
8S.SS60
'/> = 7
Figure 2. Fabry-Perot Window Effect
beam slew rate: 30 seconds or less between
positions.
I LAMS calibration using the pairs of
C12O162 laser lines P12-P24, Pi2-Pir>and
P2e-P24 in tn's 9 micron band was per-
formed by using a multipass optical cell in
which known ozone concentrations could
be established. The minimum resolvable
ozone concentration change using the
P12-P24 lines and a 179.9 meter folded
path was 2.8 ppb. A set of typical values
including extrapolated values for an optical
path length of 850 meters is shown in
Table 1.
Table 1. Minimum Resolvable Ozone
Concentration Line Pair vs. Range
Line Pair 179.9 meters 850 meters
Angle of Incidence (Q)
These variations were significant but repro-
ducible so that off-set errors could be
measured when no ozone was present,
and the corrections applied in the field
measurements. The designed positioning
accuracy of 0.25 arc minutes essentially
eliminated positioning errors.
The I LAMS was transported to Hallam,
Pennsylvania, arriving on location July 30,
1979. The system was arranged as
shown in Figure 3 using the WGAL-TV
tower for positioning the retro-reflectors.
With the system in place, experiments
were begun to perform the long path
monitoring. The main objective of provid-
ing useful information on the vertical dis-
tribution of ozone was not achieved due to
several system problems: (1) breakage of
the laser tube during transport; (2) in-
adequacy of the optical system to collect
the return laser signal; (3) substitution of a
low capacity vacuum pump for use with
the laser. Evaluation of the optical system
was carried out at the field site using a
He-Ne laser. The return signal consisted
of a six-sided lobe pattern the spreading of
which depended on the particular retro-
reflector being used. The result of this fact
was that the return optics only collected a
fraction of the return energy. This problem
could be solved by using retro-reflectors
with more accurate joining of the constit-
uent intersecting plane sides or by using
larger collection optics. The remaining
two problems require minor corrective
efforts.
The I LAMS was transferred to the U.S.
EPA for further evaluation.
Conclusions
The conclusions for the grant can be
grouped into four categories: (1) redesign
of the ILAMS; (2) controlled testing results;
(3) NEROS results; (4) final system status.
Redesign
A crucial factor in the ILAMS operation
is the maintenance of a stable relationship
(ratio) of power returned from the remotely
placed reflector, ls, to the output from the
C02 laser, lr, under short pathlength con-
ditions (no ozone or atmospheric burden).
Using a short pathlength configuration,
the drift in ls /lr for the wavelengths was
reduced by an order of magnitude by
WGAL Tower
P12'P24
P12~P10
P26TP24
2.8 ppb
4.4 ppb
6.3 ppb
0.59 ppb
0.93 ppb
1.34 ppb
In preparation for field tests in the
NEROS, the ILAMS was set up with an
ambient air path. Experiments were per-
formed to measure the variation in appa-
rent ozone concentration (due to energy
partitioning) with movement of the laser
intensity pattern on the retro-reflectors.
Laser Van
Figure 3. Measured Optical Path Locations on WGAL Tower
-------
removing the windows covering the pyro-
electric detectors and thereby eliminating
temperature dependent Fabry-Perot inter-
ference effects.
Controlled Testing
The ILAMS was successfully modified
to locate distant targets in a vertical plane
by installing an Aerotech ARS 304 Eleva-
tion Drive. Additional modifications were
incorporated which permitted a He-Ne
laser to be used for all optical alignments,
i.e., for aligning optical components in the
system and for sighting in on the remotely
placed retro-reflectors. Prior to testing in
the NEROS the ILAMS was tested along a
210m path at the University of Michigan.
The path was defined by the CC>2 laser
source mounted in a sheltered area and a
retro-reflector placed on a tripod on the
ground outside. Test conclusions were:
the automatic steering would be capable
of positioning the beam on targets located
at 850m, the NEROS field distance, and
the expected noise equivalent signal at the
same distance would be 26.5 ppb of
ozone with a 34 second integration time.
NEROS Results
In the process of placement of the
ILAMS in the EPA van, the ILAMS C02
laser tube was cracked. This effectively
prevented participation in the NEROS since
the system could not be returned to opera-
tional status by efforts made in the field.
However, some tests were made to examine
the quality of signal return from retro-
reflectors placed, as originally intended,
on a television antenna at various heights.
A He-Ne alignment laser was used to give
a visual display of the pattern of energy
returned to the ILAMS. These patterns
showed a significant variation among re-
flectors in the six lobe pattern of the return
beam. Each of the six return lobes was
displaced from the geometric centerline
so that the return pattern in the best case
had a torus shape and low energy near the
center of the beam. In the worst case
virtually no signal would have been re-
turned inside the collecting aperture.
Final System Status
Due to the limited funds remaining after
the NEROS, the ILAMS was returned to
EPA for transfer to EPA's inhouse con-
tractor Northrop Services, Inc. (NSI). The
system was placed in operational status
and a demonstration test was performed
over a 350 m optical path. The beam
steering performed satisfactorily, but the
laser output power was low. The responsi-
bility for correcting this problem was as-
sumed by NSI.
Recommendations
There are two areas of effort which
deserve additional attention in dealing
with the ILAMS. One is the testing of the
system in its present configuration with
the possible replacement of the C02 laser
tube which appears to be slightly bowed
and is likely responsible for the present
low power output from the system. The
testing should be a dedicated attempt to
run the system over a period of time with
no pressure to perform in a field test. This
is the only way to objectively judge the
feasibility of the laser-based long path
approach to atmospheric monitoring. The
second area of effort is the updating of the
system with state-of-the-art components
which would improve the system perform-
ance, e.g., a waveguide laser of much
smaller size and alternative signal pro-
cessing procedures to correspond to sim-
pler wavelength modulation techniques.
This second area of effort should only be
pursued if the advantages of a long path
monitor are actually needed to pursue
EPA's current objectives.
Lucian W. Chaney is with the University of Michigan. Ann Arbor. Ml 48109, and
Joachim D. Pleil is with Northrop Services, Inc., Research Triangle Park, NC
27711.
W. A. McClenny is the EPA Project Officer (see below).
The complete report, entitled "Long Path Laser Ozone Monitor Evaluation,"
(Order No. PB 83-196 006; Cost: $10.00, subject to change) will be available
only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Postage and
Fees Paid
Environmental
Protection
Agency
EPA 335
Official Business
Penalty for Private Use $300
PS 0000329
U S ENVIR PROTECTION AGENCY
REGION 5 LIBRARY
330 S DEARBORN STREET
CHICAGO IL 60604
-------
United States
Environmental Protection
Agency
Environmental Sciences Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S3-83-030 Aug. 1983
Project Summary
Measurements of Formaldehyde
and Hydrogen Peroxide in the
California South Coast Air Basin
Greg Kok
The chemiluminescent reaction be-
tween formaldehyde (HCHO) and gallic
acid has been adapted for the determi-
nation of HCHO in ambient systems.
The system has a detection limit of 1
ppbv for gas-phase HCHO, based on a
one-hour integrated sample. An un-
known negative interference is present
when ambient air is sampled under
conditions of severe photochemical
smog.
Measurements of HCHO and hydro-
gen peroxide (H2O2) have been made
in rainwater collected in Claremont,
California, during the winter storm sea-
sons 1979-1981. HCHO levels show a
regular pattern, with the initial rainfall
containing 500-800 ppbm. These levels
decrease rapidly as the storm pro-
gresses. H2O2 levels show wide vari-
ations during the course of a storm,
ranging from a few ppbm to over 800
ppbm.
Measurements of gas-phase HCHO
have been made at several locations in
the California South Coast Air Basin.
Under low to moderate levels of photo-
chemical smog, primary sources appear
to dominate HCHO levels and concen-
trations of 5-10 ppbv have been mea-
sured. Under conditions of more in-
tense photochemical smog, HCHO
levels of up to 40 ppbv have been
measured. Strong diurnal patterns are
noted when high levels of HCHO are
present.
This Project Summary was developed
by EPA's Environmental Sciences Re-
search Laboratory. Research Triangle
Park, NC, to announce key findings of
the research project that is fully doc-
umented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
The objective of this program was to
develop the chemiluminescent reaction
between gallic acid and formaldehyde
(HCHO) into an analytical technique for
HCHO in ambient systems. In addition,
rainwater and gas-phase measurements
of HCHO and other species were made in
the California South Coast Air Basin.
Measurements of HCHO in ambient
systems are important to an understanding
of atmospheric chemistry for the following
reasons:
• HCHO can be easily photolyzed to
form free radicals. If substantial
quantities of HCHO are present in
the early morning, photolysis of it
will accelerate photochemical smog
formation.
• HCHO is a strong eye irritant Accurate
measurements of HCHO can help to
define sources of eye irritation.
• Sinks for HCHO are poorly defined.
Measurements of HCHO in rainwater
can help to provide information on
this removal pathway.
Analytical Instrument
Development
The chemiluminescent reaction between
HCHO and gallic acid (3,4,5, trihydroxy-
benzoic acid) has been developed into a
technique for quantitatmg HCHO in aqueous
solution. The technique is applied in order
to determine HCHO in ambient air and
rainwater.
For the determination of HCHO in the
gas phase, HCHO is collected and concen-
trated in aqueous solution using a Saltzman
bubbler. The aqueous solution is then
analyzed directly for HCHO using the
chemilumm'escent technique. The detec-
tion limit is better than one part per billion
-------
by volume (1 ppbv), based on a one-hour
integrated sample. An automated system
for gas-phase HCHO has been developed
that collects and analyzes samples on an
hourly basis without operator intervention.
The chemilummescent analytical tech-
nique has been tested with a wide variety
of potentially interfering substances com-
monly found in ambient samples. No
interferences were observed. Testing of
this technique in parallel with other HCHO
analytical techniques under low to moder-
ate smog conditions in the California South
Coast Air Basin indicated reasonable agree-
ment between the chemiluminescent tech-
nique and other methods. Under severe
smog conditions, as evidenced by ozone
levels greater than 350 ppbv, parallel
testing indicated that the chemiluminescent
technique was influenced by a negative
interference that reduced the signal output
by approximately 50%. The identity of the
interference is unknown.
Rainwater Measurements of
Formaldehyde and Hydrogen
Peroxide
Measurements of HCHO and hydrogen
peroxide (H202) were made in Claremont,
California during seven precipitation events
in the winter months of 1980 and 1981.
The precipitation was collected in fractions
of typically 0.8 - 1.1 mm. The rate of
rainfall was also recorded to observe
changes in chemical concentration as a
function of rainfall rate.
Levels of HCHO in the rainwater sampled
generally show a regular trend, with the
initial precipitation containing an HCHO
level of 500-800 parts per billion by mass
(ppbm). The level drops off rapidly as the
precipitation continues; typically, after6-8
mm of rainfall the HCHO reaches a relatively
constant level of 50-100 ppbm. It does
not appear that the concentration of HCHO
in precipitation is influenced by the rainfall
rate.
In contrast to HCHO levels, H202 levels
varied considerably during the course of
precipitation, with no discernable patterns.
Measurements of H202 in rainwater gave
concentrations ranging from a few ppbm
to over 800 ppbm. During some precipita-
tion events it appeared that an increase in
the concentration of H202 was accom-
panied by increase in precipitation intensity.
No explanation is readily apparent for the
widely varying concentrations of H2O2 in
rainwater.
Gas-Phase Measurements of
HCHO and H2O2
Gas-phase measurements of HCHO and
H202 have been made at several locations
in the California South Coast Air Basin
during the summer months of 1 979 and
1980. Since gas-phase HCHO can be
present from both primary and secondary
sources, efforts were made to determine
the contribution of each source to HCHO
levels in the Basin. Measurements of
HCHO made under conditions of low photo-
chemical smog or at sites in the western
part of the California South Coast Air Basin
generally gave HCHO levels of 5-10 ppbv.
These concentrations are presumably in-
dicative of HCHO levels from primary
sources. Measurements of HCHO made at
Claremont, in the eastern part of the Cali-
fornia South Coast Air Basin, yielded a
range of HCHO levels from 5 ppbv for light
smog conditions up to 40 ppbv for intense
smog conditions (ozone >350 ppbv). An
examination of the 03 and HCHO data
taken at Claremont over a three-year period
1978-80 indicates that a good correlation
can be obtained between maximum ozone
levels on a given day and maximum HCHO
levels.
The validity of the gas-phase H202 data
collected in parts of this study is in question
due to a variable ozone reaction that can
produce either a positive or negative inter-
ference in the analysis. Hydrogen peroxide
measurements made in the absence of
ozone, generally at night, show slowly
varying levels of H202 between 2 and 5
ppbv. Since the only known source of gas-
phase H202 is via photochemical reaction,
it was not expected that night time levels
of H202would be that high. Measurements
of gas-phase H202 made in the presence
of photochemical smog are widely varying,
ranging from 2 to 12 ppbv, with no
apparent correlation with smog intensity.
Gregory L Kok is with Harvey Mudd College, Claremont. CA 91711.
Bruce W. Gay. Jr. is the EPA Project Officer (see below).
The complete report, entitled "Measurements of Formaldehyde and Hydrogen
Peroxide in the California South Coast Air Basin," (Order No. PB 83-196 725;
Cost: $14.50, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield. VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
A-US GOVERNMENT PRINTING OfFICE 1983-659-017/7162
Postage and
Fees Paid
Environmental
Protection
Agency
EPA 335
Official Business
Penalty for Private Use $300
PS 000032JROTŁCTION
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