Calibrated remote measurements of SO2 and O3 using atmospheric backscatterW. B. Grant and R. D. Hake Jr. Citation: Journal of Applied Physics 46, 3019 (1975); doi: 10.1063/1.321992 View online: http://dx.doi.org/10.1063/1.321992 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/46/7?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Rate coefficient measurements for SO2+O=SO+O2 J. Chem. Phys. 73, 987 (1980); 10.1063/1.440748 Measurement of electron affinities of O3, SO2, and SO3 by collisional ionization J. Chem. Phys. 62, 3829 (1975); 10.1063/1.430941 Calibrated remote measurement of NO2 using the differentialabsorption backscatter technique Appl. Phys. Lett. 24, 550 (1974); 10.1063/1.1655049 Use of O2 for ESR Calibration for Quantitative Measurement of Gas Concentrations J. Chem. Phys. 44, 1715 (1966); 10.1063/1.1726918 Upper Atmosphere Temperatures from Remote Sound Measurements Am. J. Phys. 16, 465 (1948); 10.1119/1.1991145
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Calibrated remote measurements of 802 and 0 3 using atmospheric backscatter
W. B. Grant and R. D. Hake, Jr.
Stanford Research Institute, Menlo Park, California 94025 (Received 3 March 1975)
Remote measurements of calibrated samples of S02 and 0 3 have been achieved with a lidar using ultraviolet signals produced by a tunable dye laser and a nonlinear crystal. The operating wavelengths for these measurements were 292.3 and 293.3 nm for S02 and 292.3 and 294.0 nm for 0 3, The atmosphere in front of and behind the chamber acted as a distributed reflector to send laser light back through the chamber to a receiver near the laser. The laser measurements agreed well with in situ measurements. Integration of eight laser pulses at each of two wavelengths allowed the determination of S02 concentration with an uncertainty equivalent to ±0.6 ppm in 100 m for low concentrations. For 0 3, the corresponding uncertainty limit was ± 1.2 ppm in 100 m. The measurement errors are primarily attributable to variations in atmospheric backscattering intensity during the experiment, since the different wavelengths were radiated sequentially rather than simultaneously. The sensitivity of a system transmitting more favorable wavelengths at intervals separated by less than 1 min is estimated to be near ±O.I ppm in 100 m for both S02 and 0 3,
PACS numbers: S7.60.P, 89.60., 42.6O.Q, 42.68.M
Recent experimental results have shown that it is possible to make remote measurements of N02 at concentrations typical of urban environments. 1,2 The measurements in Refs. 1 and 2 were made USing mono static pulsed-laser radars employing the differential-absorption-Udar (DIAL) technique. This technique uses two wavelengths for which the gas of interest has differing absorption coefficients. The measured difference in attenuation for the two wavelengths, determined by making time-resolved observations of the energy backscattered by the atmosphere, can be combined with the known difference in absorption coefficients to provide a range-resolved measurement of the absorbing species.
The N02 measurements were made using pulsed dye lasers operating in the visible near 450 nm. It has been suggested that a similar device operating in the uv could be used to monitor S02. 3,4 Total COlumn-content measurements of S02 have, in fact, been made using a retroreflector of uv energy to provide the return signal. 5 This paper reports results of active remote measurements of S02 and 0 3 using atmospheric backscatter; it demonstrates the feasibility of using the DIAL
r - - --
5.3 x BEAM
technique for range-resolved measurement of S02 and 0 3 at uv wavelengths with an eye-safe lidar.
The equipment arrangement for this experiment is similar to that for N02 in Ref. 2. The principal changes are the use of a nonlinear crystal [ammonium dihydrogen arsenate (ADA)]6 after the dye laser to generate uv wavelengths by nonlinear doubling, and the replacement of the Pyrex windows in the sample chamber with commercial-grade quartz windows (see Fig. 1). Other details of the laser and receiver are given in Table L The ADA cr'ystal is temperature tuned to achieve 900 phase matching at the different wavelengths. The wavelengths employed were selected to give the highest available differential in absorption coefficients for S02 and 0 3 that could be achieved with operation of the ADA crystal in a reasonable temperature range. Use of a single ADA crystal prevented taking the measurements alternately at the two wavelengths at 8-s intervals, as was the case for N02 in Ref. 2. Instead, data were accumulated at one wavelength for about ~-h, with at least a i-h delay between different wavelengths required for changing and stabiliZing the crystal temperature. Thus, there was
~L EXPANDER QUARTZ PMT j _ WINDOWS
r=~ ~ fr~~§ ) FIG,!. Block diagram of the experimental apparatus used in this experiment. The equipment enclosed in the dashed line was not used in these measurements but could be added to provide nearly simultaneous transmission at two wavelengths (see text for discussion) .
3019
MAGNETIC ,-----2.5 m----j TELESCOPE 12~
8-BIT TAPE 9.6 MHz SIGNAL AEROSOLS
DIGITIZER rMI AND ~ MOLECULES
REAL-TIME DISPLAY
Journal of Applied Physics, Vol. 46, No.7, July 1975 Copyright © 1975 American Institute of Physics 3019
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TABLE I. Experiment parameters.
Laser (Refs. 7 and 8)
Energy Dye Beam diameter Beam divergence (far field) Pulse length Grating Linewidth Repetition rate
""15 mJ Rhodamine 6G (3.5 Xl(l"·5 Mil in MeOR) ""2 mm (3 dB) 0.7 mrad (3 dB)
250 ns (3 dB) 316 l/mm, used in ninth order "'II,. 0.017Hz
Second-harmonic-generating crystal (Ref. 6)
Type Ammonium dihydrogen arsenate Length 2 cm Index matching fluid Fluorocarbon 77 Conversion efficien-cy for 15-mJ funda- 5% mental Pulse length '" 2 00 ns (3 dB)
Aperture
Filter
PMT ~tical efficiency from ADA crystal through PMT
Lamp
Filter
PMT Accuracy
Wavelength (rIm)
292.3 293.3 294.0
Receiver
0.056 m2
293 nm, 13-nm half-width, 27% peak transmission RCA 7200
0.01
Transmissometer
Deuterium 285.7 nm, 3-nm half-width, 17% peak transmission RCA IP 28 ± 7 ppm
Spectral data
Absorption coefficient (cm-1 atm-1)
(base e)
S02 0 3 (Ref. 9) (Ref. 10)
26 28 14
22
Atmospheric conditions
Time Visibility Molecular scattering coefficient (calculated) Aerosol scattering (extinction) coefficient (observed) Ambient S02' N02, and 0 3 concentrations (Ref. 11)
19 Dec. 1974, midnight to 5:00 a. m. 10-20 km
"" 0.14 km-1 atm-!
"" O. 5-0. 7 km-!
< 0.03 ppm
at least a l-h delay between collection of comparable data at different wavelengths.
The current range-resolution limit is 70 m, determined by the amplifier bandpass that was fixed at 2. 5 MHz to oversample these test returns. The resolution of a field system would be limited to 35 m by the 200-ns uv pulse duration.
3020 J. Appl. Phys., Vol. 46, No.7, July 1975
The 802 was injected by syringe and had a l/e residence time of about 14 min. The 0 3 was generated using a Welsbach ozonator. 12 It could fill the O. 9_m3 sample chamber with 30 ppm of 0 3 in 2 min. The gas content of the chamber was monitored by a dual-beam transmissometer.
Typical return signals with air and with 105 ppm 802 in the sample chamber are shown for 292.3 and 293.3 nm in Fig. 2. Each signal point is an average of eight consecutive return pulses. The initial peak near 100 m arises from the coaxial nature of the transmit/ receive geometry. The effect of the shadow created by the central obscuration decreases with distance and is negligible by about 150 ro. The peak near 300 m is the return signal from the quartz windows due to scattering caused by bubbles and surface reflections. Both peaks are artifically flattened by the data-acquisition system to extend the dynamic range of the 8-bit digitizer to low signal levels. Lidar data similar to those shown in Fig. 2 were obtained during four nights of operation. Minor system improvements were made between the various runs.
In order to make a comparison of the lidar data with the transmissometer data, a number must be extracted from the lidar return signal that is related to gas concentrations in the sample chamber. The easiest way to accomplish this is to form a ratio of the lidar signal
'§ >
~
~ M cO en '" >-f-in z w f-
"= ..J <! z
'" in
1000
100
o AIR
o • S02 - 105 ppm
I.., • 0 I ... ---------~I A'
293.3 nm
o AIR
o •
• o
00
• S02 - 105 ppm o
.... 'f0 0. 00 ••• ~ CbO oO
• • 0 ".0 •••
• lL-~ __ ~ __ _L __ ~ __ ~_L_~L_~ __ -L __ ~
1000
100
10
a 100 200 300 400 500 600 700 800 900 1000
RANGE - meters
-
> . ~
M
'" ~
>->:::
"' ~ f-
"= ..J
'" Z
'" in
FIG. 2. Plot of received signal vs range with air or 105 ppm S02 in the sample chamber. Each point is an average of eight laser pulses. The regions used for taking the ratios for calculation of the gas concentration are indicated as A, A', and B.
W.B. Grant and R.D. Hake, Jr. 3020
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11,---------,---------,--------,---------,
10 °0 0 S02
0.9 ~ 0 0 (a) 292.3 nm 10
08 20
0.7 0
30 ~ 40
Z
50 0
>=
06 0
0
0:
60 a: ... 70 ~
u 80
z 0
90 u
100 ~
110 g
05
0
04 0
00 0
03
02 5l 110 Z
iii 70 80 90 100
12 ~ t;;
0 '" a: 0 w
0 >-10 ~ 20 ~
11 0 5°2
10 (b) 293.3 nm
0
30 ~ z 09
- 40 '" ;:
=150
60 70
0 0 00
0 0 0 0 08 00
0
07 00 80
90 100
0.6 0
110 OS
140 150 160 170 180 T1ME AFTER 00:35 PST - minutes
FIG. 3. Plots of lidar ratio B/(A +A') as a function of time for injection of 110 ppm 802 into the sample chamber. The ratio is normalized to 1 for air in the chamber at 100 min. The ppm scale on the right-hand side of each plot was calculated using Beer's law and the ratio for air in the sample chamber after the 802 was exhausted. The solid line is the transmissometerdetermined value of the concentration; the points are ratios from an eight-pulse lidar averages.
from behind the chamber to the signal in front of the chamber. The ratio will decrease as the amount of gas in the chamber increases, in accordance with the expression
RL ex: exp(- 2kpl), (1 )
where RL is the lidar ratio, k is the absorption coefficient, p is the concentration, l is the length of the chamber, and the constant of proportionality includes range and atmospheric factors that are independent of wavelength and gas concentration in the test chamber. We have formed this ratio using the average signal in a 560-m segment starting 150 m behind the receiver, and the average signal in 50- and 75-m segments in front of the sample chamber. The segments used in front of and behind the chamber are denoted by A and A', and B, respectively, in Fig. 2. Lidar ratios, RL =B/(A+A'), obtained on 19 December 1974, were used to generate the data points shown in Figs. 3 and 4.
The points in Fig. 3 show a time history of the lidar ratio for S02' using sets of eight pulses at each of the two wavelengths. Gas injected into the sample chamber at a measured level of 110 ppm was allowed to leak out slowly for about 25 min, then an exhaust fan was turned on. The solid line in Fig. 3 gives the transmissometermeasured concentration of S02' referenced to the lidar ratio for air in the chamber after the S02 has been exhausted. The position and magnitude of the transmis-
3021 J. Appl. Phys., Vol. 46, No.7, July 1975
someter concentration scale were adjusted so that the solid line indicates the actual material concentration and the expected lidar ratio as functions of time. Although there is fairly good agreement between the lidar ratios and the transmissometer concentration values, it is apparent that the return signals with air in the sample chamber (after the S02 had been exhausted) are slightly higher than before the S02 was injected. Also, there are significant deviations of the data trends from the transmissometer-determined S02 concentrations • Both effects arise from changes in atmospheriC conditions during the ~ h of data taking.
Time histories for the return ratios with 0 3 in the sample chamber are shown in Fig. 4. After the first few shots with air in the chamber, there are a few shots with 5-10 ppm of 0 3 in the chamber. Then the concentration was rapidly increased to about 30 ppm and allowed to decay slowly for about 25 min, after which time the exhaust fan was turned on. The uncertainty in the data is similar to that for S02.
Two kinds of fluctuations in the data are evident in Figs. 3 and 4: (i) long-term fluctuations [such as the difference between the clear-air shots at 5 and 45 min in Fig. 4(a)], which provide about a ± 15% uncertainty in the ratio; and (ii) short-term (point-to-point) fluctuations, which provide closer to a ± 3% uncertainty in the ratio. The long-term fluctuations in the lidar ratio are ascribed to variations in aerosol content along the lidar path as the local winds move different parcels of air through the scattering region. The slow changes in the amount of scattering in front of and behind the sample chamber cause the long-term excursions of the lidar ratio from the expected value determined from the clear-air calibration and the transmissometer values of gas concentration.
In a differential-absorption (DIAL) measurement, lidar ratios at two wavelengths are used to determine material concentration p as follows:
p == (1/26.kl )Q.nR Lv -lnRLP )' (2)
where 6.k is the differential-absorption coefficient, l is
E 1.0 0 8:
0 0 I 0
0 0 0_9 0 00 °0 z 00 10 0
~ DB >= " 03 20 a::
i 0.7 ... ~ 101 30 <)
0.6 z
I 40 8
0.5 b' -.. 0 30 40 50 . iil 1.2 z
~ i ., 1.1 0 ~
g' t;; 1.0 10 '" .. ~ 0: 0
0: 0.9 20 t;;
" OJ :l
0 DB 30 ~ :;
I~I _0- i 0.7 40 !2 0.8
:: I-
210 230 240 250 :zeo TIME AFTER 00:36 PST - minut.
FIG. 4. Plots of lidar ratio vs time with 0 3 introduced into the sample chamber. Time, ratio, and transmissometer concentration determined as for 802 in Fig. 3.
W.B. Grant and R.D. Hake, Jr. 3021
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the chamber length, and R Lv and RLJ> are the lidar ratios taken at wavelengths of the chosen minimum (valley) and maximum (peak) absorption coefficients.
Since lidar measurements for the valley and peak wavelengths were separated by at least 1 h, it was necessary to use the transmissometer readings to identify the lidar ratios at both wavelengths taken with similar S02 concentrations. Equation (2) was then used to generate a DIAL-determined S02 concentration from these two lidar ratios. The measurement uncertainty was estimated by plotting the DIAL-determined S02 concentrations against the transmissometer-determined S02 concentrations. The average uncertainty over four nights of operations and for the range of concentrations displayed in Fig. 3 was found to be ± 25 ppm in the 2. 5-m sample chamber, or ± O. 6 ppm in 100 m. Using Eq. (2), this ± 25 ppm uncertainty in p can be related to a ± 10% uncertainty for each of the lidar ratios, R L •
The ± 10% average for four nights is somewhat smaller than the ± 15% observed on 19 December.
Since the differential-absorption coefficient for ozone10 for the wavelengths employed is one-half that for sulfur dioxide9 (6 cm-1 atm-1 versus 12 cm-1 atm-1
),
and since the ratio uncertainty is equivalent for the two gases, the material uncertainty inferred from the measurements is about ± 50 ppm in the sample chamber for ozone (± 1.2 ppm in 100 m).
The major sources of error in this type of measurement are photoncounting fluctuation, digital-processing errors, and scintillation and other atmospheric variations. Minor errors arise from wavelength uncertainty, laser-mode stability, timing jitter, and absorptioncoefficient uncertainty.
To use the data presented here to deSign an improved system for field use for S02 and 0 3 measurement, it is necessary to determine the relative importance of the various major error sources. For the lidar system employed here, and to within 50% accuracy, 100 photons are detected at 500 m with air in the sample chamber. The voltage of the PMT was set so that each digital unit of data represented about eight photons. The photon fluctuation plus digital proceSSing errors then contribute ± 6% to the uncertainty of a single lidar pulse ratio, B/ (A + A'). Since eight pulses are averaged, the uncertainty for the data in Figs. 3 and 4 would be slightly in excess of ± 2%. Two ratios combined would give ± 3%, the same as the average short-term fluctuations noted in the data. At the present system sensitivity, signal averaging thus appears to reduce the short-term scintillation error below an observable leveL This shortterm scintillation error is about one-third as large as the average long-term error of ± 10% determined above. Thus, using the two wavelengths in alternate groups, each within 1 min of each other, should reduce the average uncertainty in determined material concentration by a factor of 3.
In general, for a field-measurement system, the time interval required to cycle between the two wavelengths should be short compared to the time constants for fluctuations in both S02 concentration and atmospheric properties. In practice, the appropriate atmospheric
3022 J. Appl. Phys., Vol. 46, No.7, July 1975
time constant depends on what type of atmospheriC fluctuations (e. g., clouds, dust, or scintillation) are most important to the measurement situation. For instance, the measurements here have shown that a 1 min cycle time gives a concentration uncertainty of ± 25 ppm + 3 = ± 8 ppm (in 2. 5 m) for a lidar path through relatively clear air.
Examination of atmospheric effects on one-way transmission13 has shown that the atmosphere does not change significantly on a time scale less than 10-3s, so that transmission of the two wavelengths within that time interval would permit obtaining a noise level governed only by photon counting and digitization errors. The simplest method that could be employed to obtain two wavelenths within 1O-3s of each other is to use a second laser and crystal, as shown in Fig. 1. They could be combined without loss of intensity using a Glan airspaced prism. The time delay could be adjusted so that one Signal disappears as the next pulse is fired.
A further improvement would be to select wavelengths for which the differential-absorption coefficient is larger. For S02' the use of the wavelengths 299.5 and 300.05 nm 14 would give a differential-absorption coefficient of 25.6 cm-1 atm-1 , a factor of 2 greater than for the wavelengths used here. For 03' the use of 292.3 and 301.0 nm would give a differential-absorption coefficient of 18 cm-1 atm-1
, compared to 6 cm-1 atm-1 used here (an improvement of a factor of 3). An eight-pulse average at each of two wavelengths, with the data for the two wavelengths being taken alternately at intervals of less than 1 min, and using the better wavelengths, should then improve the sensitivity to ±0.1 ppm in 100 m for S02 and ± 0.13 ppm in 100 m for 0 3 for a lidar system with the laser, receiver, and digitizing capability employed here.
Additional improvements could be obtained by increasing the transmitted energy, by increaSing the laser repetition rate, and by increasing the receiver sensitivity (a five-fold improvement is easily possible by increaSing the telescope diameter, the mirror reflectivities, the filter transmissivity, the PMT quantum efficiency, and the signal digitization accuracy).
If S02 and 0 3 are both present, additional care must be given to the selection of wavelengths. Either two wavelengths must be chosen so that the absorption coefficient for one gas is identical at each wavelength while the other varies, or more than two wavelengths must be employed to permit measurement of both bases simultaneously.
The lower limit to the amount of S02 (and 03) that could be measured is set by the system sensitivity (± O. 6 ppm of S02 in 100 m for the single-crystal system, 1-h wavelength cycle time, and the two wavelengths used here); the upper limit is reached when the signal return through the penetrated material becomes too low for accurate measurement. Thus, a certain dynamic range of material concentration is appropriate to a specific system. However, this dynamiC range may be shifted to much higher material concentration by selection of operating wavelengths with lower absorption coefficients. For both S02 and 03' the absorption co-
W.B. Grant and R.D. Hake, Jr. 3022
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efficient decreases with increasing wavelength while retaining the structure necessary for the DIAL technique. It appears quite feasible, therefore, to select a wavelength pair that would allow measurement of the very high 802 concentrations in smokestack plumes.
The power levels used here are low enough that even a modest beam expander is sufficient to make the lidar eye safe at the transmitter output. A O. 75-mJ pulse at 292 nm requires a 2-in-diam transmitter to meet the ANSI (1973) acceptable-exposure-Ievel standard at all ranges.
In summary, this experiment demonstrated the feasibility of making remote range-resolved measurements of 802 and 0 3 using an eye-safe lidar operating in the uv spectral region. The agreement of the lidar data with the transmissometer measurements was fairly good; the main source of error was temporal fluctuations in atmospheric aerosol content and distribution. Several improvements in the method can easily be made using current technology that would qualify this technique for both moderate ambient concentrations and stack emissions at distances in excess of 1 km.
The authors thank S.D. Evitt, J.G. Hawley, J. van der Laan, E.M. Liston, R.G. March, E.R. Murray, and D.A. Richardson for aid in performing the experiment; J. B. Marling for advice and the temporary use of
3023 J. Appl. Phys., Vol. 46, No.7, July 1975
an ADA crystal; and D.A. Johnson, R.L. Byer, J.H. Clark, E. K. Proctor, Jr., and R. W. Wallace for helpful discussions. The research was sponsored by NSFRANN Grant No. GI-38986.
lK. W. Rothe, V. Brinkman, and H. Walther, Appl, Phys. 3, 115 (1974); Appl, Phys. 4, 181 (1974).
2W.B. Grant, R.D. Hake, Jr., E.M. Liston, R.C. Robbins, and E.K. Proctor, Jr., Appl, Phys. Lett. 24, 550 (1974).
3R.M. Measures and G. Pilon, ~to-Electron. 4, 141 (1972)., 4T. Igarashi, Fifth Conference on Laser Radar Studies of the Atmosphere, Williamsburg, Va., 1973, p. 57 (unpublished),
5J. Kuhl and H. Spitschan, Opt. Commun. 13, 6 (1975). 6R.S. Adhav and R.W. Wallace, IEEE J. Quantum Electron. QE-9, 855 (1973),
TJ.B. Marling, J.G. Hawley, E.M. Liston, and W.B. Grant, Appl, ~t. 13, 2317 (1974).
BJ. Kuhl and H. Spitschan, Opt. Commun. 5, 382 (1972), 9p. Warneck, F.F. Marmo, and J.O. Sullivan, J. Chern. Phys. 40, 1132 (1964).
1~. Griggs, J. Chern. Phys. 49, 857 (1968). l1These values were recorded by the Bay Area Air Pollution
Control District in Redwood City, California, 4 km northwest, using in situ techniques.
12Welsbach Corp., Ozone Processes Div., Westmoreland and Stokely Streets, Philadelphia, Pa. 19140.
13Handbook of Military Infrared Technology, edited by W. L. Wolfe (Office of Naval Research, Washington D.C., 1965), p. 215.
14R. T. Thompson, Jr., J.M. Hoell, Jr., and W.R. Wade, J. Appl, Phys. (to be published).
W.B. Grant and R.D. Hake, Jr. 3023
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