Absorption technique for OH measurements and calibration

Donovan M. Bakalyar, John V. James, and Charles C. Wang

An absorption technique is described which utilizes a stabilized frequency-doubled tunable dye laser anda long-path White cell with high mirror reflectivities both in the red and UV. In laboratory conditions wehave been able to obtain routinely a detection sensitivity of 3 parts in 106 over absorption paths <1 m inlength and a detection sensitivity of 6 parts in 105 over an absorption path of the order of 1 km. The latternumber corresponds to 3 X 106 OH molecules/cm 3, and therefore the technique should be particularly usefulfor calibration of our fluorescence instrument for OH measurements. However, the presence of atmosphericfluctuations coupled with intensity variation accompanying frequency scanning appears to degrade the de-tection sensitivity in outdoor ambient conditions, thus making it unlikely that this technique can be em-ployed for direct OH monitoring.

1. IntroductionThe hydroxyl radical (OH) is a reactive species which

controls many of the chemical processes operative in theatmosphere. OH is important in ozone chemistry as itrelates to photochemical smog formation in the tropo-sphere and to partitioning of the odd-nitrogen andodd-chlorine compounds in the stratosphere. Variousestimates1 -5 place the global yearly averaged OH con-centration at between 105 and 106 molecules/cm 3 , de-pending to a large extent on the assumed perturbationsto the natural atmosphere. Although OH has beeninvestigated in connection with a number of importantproblems in space and atmospheric physics, attemptsat monitoring OH in the troposphere have so far beenless than satisfactory. For example, the isotope tracingtechniques represents an interesting approach to mea-surements of OH; however, this method requires furtherdevelopment and there are possible errors which needto be investigated. The absorption technique employedby Perner et al.7 should in principle provide accurateOH measurements, but thus far it lacks the sensitivitynecessary for OH measurements in ambient air. Thetechnique of laser-induced fluorescence (LIF)8 currentlyappears to be the most sensitive, but it is an indirect

Donovan M. Bakalyar is with Wayne State University, Departmentof Physics & Astronomy, Detroit, Michigan 48202; the other authorsare with Ford Motor Company, Engineering & Research Staff,Dearborn, Michigan 48121.

Received 1 May 1982.0003-6935/82/162901-05$01.00/0.© 1982 Optical Society of America.

approach which involves many parameters in the datareduction process. It would thus seem desirable todevise a way to calibrate the fluorescence instrumentor perhaps to find an alternate means for measuring OHin the troposphere.

This paper reports progress which we have madesince 1977 in developing a laser-based absorptiontechnique9 for OH measurements as well as for cali-bration of the LIF instrument. The absorption tech-nique is direct in that the signal is proportional to theproduct of the oscillator strength of the absorptiontransition and the concentration of OH in the path ofthe light beam. Since all the proportionality constantsare known or easily measured, the accuracy of themeasurement is essentially determined by the extentto which the small absorption signal can be detected.Since our approach utilizes a coherent source of ultra-violet radiation, the power per bandwidth is orders ofmagnitude greater than that from typical incoherentsources. Therefore, the high shot-noise limit whichplagues conventional absorption experiments in theultraviolet is avoided. Furthermore, our tunable laserprovides the resolution necessary to distinguish ab-sorption by OH from that due to other species. Resultsto date indicate that this technique should be useful forcalibration of our fluorescence instrument. However,as will become clear in later discussions, the sensitivityof this technique may be insufficient for direct obser-vation of ambient OH.

In Sec. II the experimental apparatus used in ourinvestigations will be described and certain novel fea-tures of our method will be pointed out. Relevant signalanalysis and data processing will be presented in Sec.III. Noise and problems of a systematic nature will beconsidered in Sec. IV. Section V illustrates the appli-cation of our technique to OH measurements, and fi-nally, concluding remarks will be given in Sec. VI.

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II. Experimental ApparatusA schematic diagram of the experimental setup is

shown in Fig. 1. The output from a tunable cw dye laseris frequency doubled and the resulting UV beam isamplitude stabilized. To frequency stabilize the laser,a small part of the beam is split off and then detectedafter traversing an OH reference cell. The main partof the beam is again split into two branches, with onepassing through the sampled volume in a White cell.These two beams are separately detected, and the dif-ference signal is averaged and subsequently processedin a microcomputer.

Our light source is a ring dye laser (Spectra-Physicsmodel 380A pumped by an argon laser) which is oper-ated near 616 nm. The second harmonic radiation isgenerated in a crystal of RDP (rubidium dihydrogenphosphate) which is temperature tuned for 900 phasematching. As the frequency of the dye laser is scanned,this second harmonic radiation sweeps across the OHtransition near 308 nm. Since the atmospheric noisedecreases monotonically with scanning frequency, thesweep rate is set as high as possible without disturbingthe single-mode operation of the laser. (This rate istypically 30 Hz in our experimental conditions.)

For amplitude stabilization of the UV beam, negativefeedback is employed. The output signal from a de-tector is compared to a reference voltage, and the am-plified error signal is fed to an electrooptic (EO) mod-ulator placed in the path of the laser beam. The EOmodulator acts as a voltage controlled light attenuatorbecause of the polarization sensitivity of the doublingcrystal. (Note that a polarizer must be inserted in thebeam path for intensity stabilization in those experi-ments not involving second harmonic generation.)

A negative feedback loop is also employed for fre-quency stabilization of the laser. This loop includes anOH reference cell which measures 30 cm in length andcontains 2 Torr of water vapor. Electrical discharge inthe vapor produces an OH concentration of -4 X 1013molecules/cm 3, which results in 50% peak absorptionof the light traversing the cell. This signal is thenmeasured by a lock-in amplifier in order to cancel anyshifts in the center frequency of the laser scan. Properoperation of this feedback loop is verified by monitoringthe OH absorption signal with the signal averager.

The use of a White cell enables measurements to beperformed over a relatively localized region and there-fore facilitates future calibration of our LIF instrument.Our White cell consists of three concave mirrors (10-mradius of curvature) arranged as shown in Fig. 1. Thetwo small mirrors (7.6-cm diam) are placed 10 m awayfrom the large one (17.0-cm diam). All mirrors havefused silica substrates which are dielectric coated forreflectivity exceeding 99.5% at both the fundamental(616-nm) and second harmonic (308-nm) wave-lengths.

About fifty reflection spots can be placed on themirrors in calm indoor air, but for outdoor use thisnumber has to be reduced to avoid clipping of the beamdue to atmospheric turbulence. The number of spotscan be doubled by reflecting the exiting beam so as to

Fig. 1. Schematic block diagram of the absorption experiment usinga white cell: PD, vacuum photodiode; IS, integrating sphere.

retrace its path through the White cell. The returnbeam is then separated from the one entering the Whitecell by a beam splitter as shown in Fig. 1. In additionto doubling the path length, this reinjection scheme iseffective in canceling the beam wander due to atmo-spheric turbulence, as may be seen by comparing themotion of the returned beam with that of the spot on thereinjection mirror. Although this reinjection schemeresults in a 75% loss of intensity at the beam splitter,other methods10'1 ' which avoid this loss are not as ef-fective in reducing beam motion.

Any remaining motion of the beam causes fluctua-tions in the output from the detectors because of thenonuniform sensitivity of their photocathodes. Thisproblem is alleviated by placing integrating spheres12

in front of the detectors. A further advantage of thespheres is that the filters (Schott UG-11) which screenthe visible light can be placed behind the spheres, thusavoiding creation of interference patterns due to re-flections at the filter surfaces. (In general, placementof parallel surfaces anywhere in the path of the beamshould be avoided.)

As shown in Fig. 1, portions of the light entering andexiting the White cell are obtained from the beamsplitter. The difference in the intensity of these twobeams is measured by subtracting the currents from thetwo diode detectors which are wired with opposite po-larity. (Vacuum photodiodes are preferred over solidstate diodes because of the low dark current associatedwith the vacuum devices.) The difference current isthen amplified with a current-to-voltage amplifier andthe signal is fed to a PAR model 4203 signal averager.This method of difference detection avoids errors dueto unequal gain drifts in separate amplifiers.

Initially, the difference current is nulled by adjustingthe separation between one of the detectors and its in-tegrating sphere, which changes the fraction of the lightintercepted by the detector. Then as the laser isscanned, any signal arising from absorption in the Whitecell manifests itself as an imbalance in the difference

2902 APPLIED OPTICS / Vol. 21, No. 16 / 15 August 1982

current. This signal is fed to one channel of the signalaverager, while the second channel monitors the refer-ence cell.

Ill. Signal AnalysisThe intensity of the light returning from the White

cell in an absorption experiment can be expressed as

I(v) = Io exp(-l/1o)RI/d exp [-n () o(v)l]

ex n U) (1

whereIo = intensity of light entering the White cell,1 = absorption path length in the White cell,

I/o = coefficient of nonresonant scattering andabsorption,

R = mirror reflectivity,d = base length of the White cell,n = concentration of the absorbing species,

(IAn)/n = fraction of population residing in the rota-tion level from which the absorption tran-sition originates, and

a(v) = cross section for the absorption transi-tion.

For weak absorption, Eq. (1) can be written asI(v) = I -A(v), (2)

whereAI(v) = Ifn (Ž) o(v)1. (3)

I is the transmitted laser intensity in the absence ofabsorption by OH, which is equal to the average inten-sity of the light returning from the White cell in thisapproximation. AI(v) is the ac signal registered on thesignal averager under our scheme of difference detec-tion.

The cross section can be conveniently written as o(v)= -0g(v), where o- is the integrated cross section, andg(v) is the line shape function. To achieve better sen-sitivity for detecting small absorption signals, thecomputer is programmed to perform a correlation of theaveraged signal from the detectors with a slightlymodified form of this line shape function, g'(v) g(v)- g, where g is the average value of g(v) over the fre-quency interval used for correlation. This insures thatthe value of the correlation is insensitive to any dc offsetin the electronics. The OH concentration deduced inthis manner is given by

n =4 ((v)g(v))crr)

Io ( ) lUo(g'()g'(p))corr

For the absorption experiments described in thispaper, the Qi(2) line in the 211(v" = 0) - 22;+(V' = 0)band of OH is used because it occurs far from otherabsorbing transitions and has a large value of theproduct [(An)/n] 00. From Refs.13 and 14 the value of0 is calculated to be 3.5 X 10-16 cm. In ambient con-

ditions, (An)/n = 0.2 for the initial state.The line shape function is generated in the computer

by a closed-form approximation to the Voight profile.15

The two parameters characterizing this function are theDoppler and Lorentzian widths, which have values of0.1- and 0.13-cm-1 (FWHM), respectively, in ambientconditions. 16 17 This gives a total Voight width15 of 0.18cm-1, which is to be compared to the scanning range of0.3 cm-1 used in our experiments.

IV. Noise and SystematicsIn considering the sources of error, a distinction must

be made between those effects which can be reduced bytime averaging (noise) and those which cannot (sys-tematics). The former category includes shot noiseassociated with the photocurrent, background or in-strument noise, laser amplitude fluctuations, and at-mospheric turbulence. The systematics refer to effectswhich are synchronous with the scanning of the laserfrequency.

Each source of noise listed above has a different pathlength dependence. Therefore, the path length whichgives optimal signal-to-noise ratio (SNR) depends onthe relative magnitudes of these contributions to thenoise. The signal-to-noise ratio is given by

SNR= (5)VNl + N2+ N32

where AI is given by Eq. (3),N1 = shot noise proportional to V,N 2 = background or instrument noise,N 3 = laser amplitude fluctuations proportional

to I', andN 4 = atmospheric turbulence proportional to

I"0.In the limit that one term dominates in the summation,the optimum path length (pt) can be found for thefollowing four cases: (1) When the shot noise associatedwith the detector current dominates, the SNR is pro-portional to 1 exp(-l/10)Rl/2 d. For this case

21o

1 - (N/d) lnR(6)

(2) In conditions of dominant background noise, theSNR is proportional to 1 exp(-l/lo)Rl/d. The formulafor lopt is similar to Eq. (6):

lo

op- 1- (10/d) lnR(3) Lack of stability in the UV beam due to laser fluc-tuations gives noise which is proportional to laser in-tensity but which is independent of path length. In thiscase, the SNR is proportional to path length. (4) At-mospheric turbulence causes fluctuations which becomeworse as path length is increased. Tests conductedboth indoors and outdoors indicate that the noise in-creases approximately linearly with 1; therefore the SNRis independent of path length.

The conclusion of case (1) is relevant to long-pathabsorption experiments using incoherent light sources,where the detectability is often shot-noise limited.Equation (6) can be modified to describe single-pathexperiments by setting R = 1, thus yielding the simpleresult lopt = 210. The largest value of lo is realized in thelimit of Rayleigh scattering; lo = 7 km for radiation near

15 August 1982 / Vol. 21, No. 16 / APPLIED OPTICS 2903

300 nm.18 Since scattering is typically much worse thanthe Rayleigh limit, it is apparent that path lengthsconsiderably shorter than 14 km should be used forabsorption experiments which are shot-noise limited.

In our experiments, shot noise is negligible becauseof the ample UV power generated in the doublingcrystal (typically 20 ,uW or more). Even though sub-stantial losses of intensity occur in the White cell andthe integrating spheres, the photocurrent is >10 nA.For an averaging time of a half hour, the fractional shotnoise on this current is calculated to be <10-7, wellbelow the level of noise from other sources. Theamount of UV power available also makes it easy toreduce background light to a comparatively insignifi-cant level.

The noise caused by laser amplitude instability isessentially eliminated in our experiments by the nega-tive feedback circuitry and by the use of difference de-tection, both of which have been discussed in Sec. II.The remaining source of noise is atmospheric turbu-lence, which dominates in our experiments in spite ofthe use of integrating spheres and reinjection. Notethat this noise could be reduced if the sweep frequencyof the laser could be increased.

An example of the systematic error mentioned earlieris the large change (10%) in doubling efficiency as thelaser is scanned. This is alleviated by the same tech-niques of light stabilization and difference detectionused to overcome the problem of random amplitudefluctuations. However, the lack of complete cancella-tion of amplitude variations is potentially more seriousin this case, because synchronous effects appear asstructure in the absorption spectrum and cannot bereduced by averaging. This structure is typically 1 partin 103 of the light amplitude and does not appear to varywith path length. Therefore, the problem is analogousto case (3) considered above, where the SNR increaseswith path length. At the longest path length obtainablein our White cell, however, this synchronous effect isstill more serious than the atmospheric noise mentionedearlier.

To the extent that the synchronous effect is repro-ducible, it can be measured separately and subtractedfrom the overall signal if the absorbing species can beremoved periodically from the beam path. Our meth-ods for accomplishing this are discussed in Sec. V.After several cycles of this measurement/subtractionprocess, the remaining noise is usually not discernibleover the total noise from other sources.

V. Application to OHThe techniques discussed previously were used in the

following experiments relevant to the study of atmo-spheric OH. These include (1) measurements of theminimum detectable absorption signal in OH dischargecells, (2) a determination of the OH detection limit inthe laboratory using the White cell to obtain a long pathlength, (3) a determination of the OH detection limit inoutdoor air using the White cell, and (4) measurementsof the SO 2 absorption spectrum in the neighborhood of308 nm.

5xl10r

0

0I-

0InM

w40W

4

3

2

0' I I0 10 20 30

TIME (MINUTES)

Fig. 2. Temporal variations of the absorption of OH in a dischargecell. The solid line is a least-squares fit of the data to a straight line.

The detection limits obtained in the discharge cellsillustrate the potentially high sensitivity of the tech-niques described in this paper. With an electrical dis-charge operating in nitrogen (2 Torr) containing onlyresidual amounts of water vapor, a small OH absorptionsignal was impressed on the sampling laser beam.Subtraction of the synchronous effect was obtained byalternately starting and stopping the discharge. As canbe seen in Fig. 2, the absorption increased linearlyduring the half-hour measurement period, and thestandard deviation of the moving average was 3 X 10-6.This increase is attributed to a rising OH concentrationin the cell associated with changing conditions in thedischarge tube.

This experiment was repeated with no dischargeduring both halves of the measurement cycle. In thisnull test the mean was zero to within its standard de-viation of 3 X 10-6. Taking this standard deviation asa measure of our detection sensitivity, absorption by OHof 3 parts in 106 of the beam intensity would be detect-able with a SNR of one.

We also performed null tests over a path of 840 mobtained with the White cell indoors. In these condi-tions, a detection sensitivity of 6 X 10-5 was obtainedover a period of a half hour. This sensitivity corre-sponds to a minimum detectable OH concentration of3 X 106 molecules/cm 3. This sensitivity should besufficient for calibration of our fluorescence instrumentin an enclosed volume.

The results achieved in the laboratory led us to at-tempt measurement of OH with the White cell posi-tioned outside. During these experiments, the entirepath of the beam was enclosed in 25-cm diam tubesduring half of each measurement/subtraction cycle.This was done to block the solar radiation responsiblefor the generation of ambient OH. Because of atmo-spheric turbulence, the detection limit outside wassignificantly worse than that obtained in the calm in-door environment. The best detection limit obtainedoutside was 1.5 X 107 molecules/cm 3 for a half-hourperiod of measurement, but often it was worse by asmuch as an order of magnitude. With this detectionlimit, no evidence of ambient OH was seen.

2904 APPLIED OPTICS / Vol. 21, No. 16 / 15 August 1982

Additional experiments were performed to seewhether the presence of SO2 in the atmosphere couldinterfere significantly with measurements of ambientOH. For this purpose, a 30-cm long absorption cell wasfilled with SO 2 at various pressures ranging from 0.1 to10 Torr. In these conditions,. no self-broadening in theabsorption profile was observed; however, pronouncedpressure broadening became evident when air at 1 atmwas introduced into the cell. In this environment, theSO2 absorption cross section near 308 nm has an averagevalue of 1.2 X 10-19 cm2, with a maximum variation inthe cross section of -5 X 10-21 cm2 over the frequencyrange used in our experiments. This latter numberimplies that, at a concentration of 5 X 1012 molecules/cm3 (200 ppbv), SO 2 will produce absorption structurecomparable in magnitude with the peak absorptionfrom 108 OH molecules/cm 3. However, we have dem-onstrated that the use of correlation reduces this latternumber to 3 X 106 OH molecules/cm 3. Further re-duction should also be realized through the proceduredescribed in the preceding paragraph.

VI. DiscussionsThe absorption technique described in this paper

offers certain advantages over other absorption tech-niques. The principal advantages include the lowfractional shot noise associated with the high UV powerand the convenience with which narrowband tunableradiation may be obtained. Another advantage is theuse of a White cell, since it permits essentially localizedmeasurements and is adaptable for the calibration ofour fluorescence instrument for OH measurements.Our best detection sensitivity of 3 X 10-6 is comparablewith the best obtained in the visible region.19 Our workthus extend the high sensitivity associated with laser-based absorption techniques from the visible to theultraviolet region.

With regard to OH measurements in ambient air, itis clear from our results that the technique suffers fromeffects which occur in synchronism with the scanningof the laser frequency as well as from noise caused byatmospheric fluctuations. The former appears to bea shortcoming which is practically inevitable whenscanning a laser, whereas the latter could be reduced ifthe scan frequency could be increased further. The

presence of these effects limits our detectability to -4X 107 molecules/cm 3 for OH measurements in ambientconditions. However, the technique offers much im-proved detectability when employed in an enclosedvolume where absorbing species can be generated andremoved in a controlled manner. This situation is mostappropriate for calibration of our fluorescence instru-ment, whereby artificially generated OH is measuredsimultaneously using both the fluorescence and ab-sorption techniques. This calibration work is currentlyunder way, and the results will be published else-where.

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Geophys. Res. 86, 7210 (1981).2. W. L. Chameides, Geophys. Res. Lett. 5, 17 (1978).3. B. Weinstock and H. Niki, Science 176, 290 (1972).4. S. C. Liu, Geophys. Res. Lett. 4, 325 (1977).5. S. Hameed, J. P. Pinto, and R. W. Stewart, J. Geophys. Res. 84,

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(1977).10. D. Horn and G. C. Pimentel, Appl. Opt. 10, 1892 (1971).11. J. U. White, J. Opt. Soc. Am. 66, 411 (1976).12. D. J. Lovell, Integrating Sphere Performance (Labsphere, P.O.

Box 70, North Road, North Sutton, N.H. 03260, 1981). Thistechnical report is available from Labsphere.

13. C. C. Wang and C. M. Huang, Phys. Rev. A 21, 1235 (1980).14. G. H. Dieke and H. M. Crosswhite, J. Quant. Spectrosc. Radiat.

Transfer 2, 97 (1962).15. E. E. Whiting, J. Quant. Spectrosc. Radiat. Transfer 8, 1379

(1968).16. C. C. Wang, D. K. Killinger, and C. M. Huang, Phys. Rev. A 22,

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This research has been supported in part by the At-mospheric Sciences Division, National Science Foun-dation, by the National Aeronautics and Space Ad-ministration, by the U.S. Department of Energy, andby the Wayne State University Research Award Pro-gram.

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