rotational vibrational–rotational raman differential absorption lidar for atmospheric ozone...

Rotational vibrational–rotational Ramandifferential absorption lidar for atmosphericozone measurements: methodology and experiment

Jens Reichardt, Scott E. Bisson, Susanne Reichardt, Claus Weitkamp, and Bernd Neidhart

A single-laser Raman differential absorption lidar ~DIAL! for ozone measurements in clouds is proposed.An injection-locked XeCl excimer laser serves as the radiation source. The ozone molecule numberdensity is calculated from the differential absorption of the anti-Stokes rotational Raman return signalsfrom molecular nitrogen and oxygen as the on-resonance wavelength and the vibrational–rotationalRaman backscattering from molecular nitrogen or oxygen as the off-resonance wavelength. Modelcalculations show that the main advantage of the new rotational vibrational–rotational ~RVR! RamanDIAL over conventional Raman DIAL is a 70–85% reduction in the wavelength-dependent effects ofcloud-particle scattering on the measured ozone concentration; furthermore the complexity of the appa-ratus is reduced substantially. We describe a RVR Raman DIAL setup that uses a narrow-bandinterference-filter polychromator as the lidar receiver. Single-laser ozone measurements in the tropo-sphere and lower stratosphere are presented, and it is shown that on further improvement of the receiverperformance, ozone measurements in clouds are attainable with the filter–polychromator approach.© 2000 Optical Society of America

OCIS codes: 010.3640, 280.1910, 290.5860, 010.4950, 290.1090, 290.4210.

1. Introduction

The lidar variant used for ozone measurements inclouds is conventional Raman differential absorptionlidar1,2 ~DIAL!, with two primary wavelengths trans-mitted into the atmosphere and ozone calculatedfrom the differential absorption of the correspondinginelastic molecular return signals of molecular nitro-gen. As the Raman DIAL error analysis shows, thelargest term in the total measurement uncertainty isdue to wavelength-dependent multiple scattering

When this research was performed, J. Reichardt, C. [email protected]!, and B. Neidhart [email protected]! were with the Institut fur Physikalische und ChemischeAnalytik, GKSS-Forschungszentrum Geesthacht GmbH, Postfach1160, D-21494 Geesthacht, Germany. J. Reichardt [email protected]! is now with the Joint Center for Earth Sys-tems Technology, University of Maryland Baltimore County,Laboratory for Atmospheres, Code 916, NASA Goddard Space FlightCenter, Greenbelt, Maryland 20771. S. E. Bisson [email protected]! is with Sandia National Laboratories, Livermore, Cali-fornia 94551-0969. S. Reichardt is at 9709 Evening PrimroseDrive, Laurel, Maryland 20723.

Received 1 December 1999; revised manuscript received 3 July2000.

0003-6935y00y336072-08$15.00y0© 2000 Optical Society of America

6072 APPLIED OPTICS y Vol. 39, No. 33 y 20 November 2000

and extinction by cloud particles. Its magnitude canbe quite substantial,3,4 hampering the study of cloud-related dynamic and chemical processes that affectozone.5,6

To overcome this problem, a novel technique hasbeen developed that reduces these uncertainties andeliminates the need for a second laser or for generat-ing off-resonance laser radiation by Raman shifting.The approach is based on differential absorption byozone of the purely rotational Raman return signalsfrom molecular nitrogen and oxygen as the on-resonance wavelength and the vibrational–rotationalRaman return from molecular nitrogen or oxygen asthe off-resonance wavelength. Because of this com-bination, rotational vibrational–rotational ~RVR! Ra-man DIAL might be an appropriate term for the newmethod.

The methodology of the RVR Raman DIAL is de-veloped, and first experimental results are presented.In Section 2 we outline a formalism that we derivedfor measurements with the RVR Raman DIAL tech-nique. In Section 3 we compare the model calcula-tions of its performance with conventional RamanDIAL for ozone measurements in upper-tropospherecirrus clouds. In Section 4 we provide a detaileddescription of the first experimental RVR RamanDIAL setup at the Combustion Research Facility,

t

a

c

ba

tn

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aD

Sandia National Laboratories, Calif. In Section 5we present and discuss lidar measurements; in Sec-tion 6 we provide a summary.

2. Theory

The RVR Raman DIAL measurement yields the num-bers N~l, z! of lidar return photons from distance z athe rotational Raman wavelength lR of molecular

oxygen and nitrogen and at the vibrational–rotational Raman wavelength lVR of O2 or N2, if lightof the primary wavelength lL is transmitted. Whenabsorption by particles and trace gases other thanozone is neglected, a condition satisfied for cirrusmeasurements in the free troposphere,3 the RVR Ra-man DIAL ozone molecule number density n~z! in thesingle-scattering approximation is given by

n~z! 5 RVRN 2 RVRM 2 RVRP, (1)

where

RVRN 5~dydz!ln@N~lVR, z!yN~lR, z!#

CO3

abs~lR, T!2CO3

abs~lVR, T!,

RVRM 5amol

sca ~lR, z! 2 amolsca ~lVR, z!

CO3

abs~lR, T! 2 CO3

abs~lVR, T!,

RVRP 5apar

sca~lR, z! 2 aparsca~lVR, z!

CO3

abs~lR, T! 2 CO3

abs~lVR, T!.

Here CO3

abs~l, T! is the ozone absorption cross sectiont temperature T, and amol

sca ~l, z! and aparsca~l, z! are the

Rayleigh extinction and the single-scattering particleextinction coefficient, respectively. Similar expres-sions, RDN, RDM, and RDP, have been derived foronventional Raman DIAL.4

Extinction that is due to Rayleigh scattering andhence RVRM can be calculated easily from the radio-sonde data of weather service stations close to thelidar site. Applying the Raman lidar technique7

to the Rayleigh-extinction-corrected vibrational–rotational Raman return signal at lVR yields

ddz

ln N~lVR, z! 5 @aparsca~lL, z! 1 apar

sca~lVR, z!# 1 n~z!

3 @CO3

abs~lL, T! 1 CO3

abs~lVR, T!#. (2)

On the assumption that the wavelength dependenceof the particle extinction is } lk, Eqs. ~1! and ~2! cane solved for the ozone molecule number density n~z!nd the particle extinction coefficient apar

sca~lL, z! 'apar

sca~lR, z!. A systematic ozone error is introducedby the term RVRP because k is not well known for thedifferent types of atmospheric particulate scatterers.

In optically dense scattering media or in cirrusclouds, the single-scattering approach is not suffi-cient and the influence of multiple scattering must beconsidered. Following the same approach as for theconventional Raman DIAL technique outlined in Ref.4, we can account for multiple scattering in cloudswith particle extinction coefficients *0.1 km21 ~con-

2

ribution of scattering by molecules within the cloudegligible! by rewriting RVRP as

RVRP 5apar

sca,eff~lL, z! 1 aparsca,eff~lVR, z!

CO3

abs~lR, T! 2 CO3

abs~lVR, T!

3 F@FR~lL, z!, FVR~lL, z!, k#, (3)

ith the particle correction function

F@FR~lL, z!, FVR~lL, z!, k# 51 2 FR~lL, z!

1 2 FVR~lL, z!

32

1 1 ~lVRylR!k 2 1. (4)

Here aparsca,eff is the measured effective particle extinc-

tion, and FR and FVR are the multiple-scattering pa-rameters4,8 of the rotational and the vibrational–rotational Raman return signals. F depends on thecattering properties of the atmospheric particles.Because a promising field of application of the RVR

aman DIAL technique is the simultaneous observa-ion of cirrus clouds and ozone concentrations in theree troposphere,9–12 we chose a RVR Raman DIAL

laser wavelength lL of approximately 308 nm withinthe excitation spectrum of XeCl because this is theon-resonance wavelength of most upper-troposphereand stratosphere conventional ozone Raman DIAL’s.Note, however, that in contrast to the RVR RamanDIAL the former must emit laser light at a secondwavelength, usually between 350 and 355 nm, to gen-erate the off-resonance Raman signal. Whereas dif-ferential absorption by ozone is approximately thesame for both Raman DIAL and RVR Raman DIAL,the wavelength separation between the on-line andthe off-line signals is much smaller in RVR RamanDIAL ~Table 1!. The new technique is therefore farless sensitive to differential Rayleigh scattering and towavelength-dependent cloud effects such as multiplescattering and particle extinction. This reasoning issubstantiated in Section 3. Note that, if ozone andcloud measurements in the boundary layer ratherthan in the free troposphere are of primary interest, itcould be advantageous to use lidar wavelengths thatare shorter than 308 nm. Then single-laser RamanDIAL measurements could also be made by exploita-tion of the differential absorption by ozone at thevibrational–rotational Raman wavelengths of molec-ular oxygen and nitrogen @vibrational–rotationalvibrational–rotational ~VRVR! Raman DIAL#. Theheory formulated here applies to the VRVR RamanIAL as well.

3. Model Calculations

Table 1 lists a summary of the lidar parameters thatare assumed for the comparison of O2 RVR RamanDIAL, N2 RVR Raman DIAL, and conventional Ra-man DIAL. First, the case of negligible multiple-scattering effects is considered; in this case themultiple-scattering parameters in RVRP @Eq. ~3!#nd in the corresponding term RDP of the RamanIAL equation4 are zero. This condition is met

0 November 2000 y Vol. 39, No. 33 y APPLIED OPTICS 6073

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cm

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Table 1. Parameters of the Conventional Raman DIAL and the RVR Raman DIAL’sa

6

when small particles of relatively low number densitydominate atmospheric particulate scattering such asin tropospheric and stratospheric aerosol layers.The relationship between systematic ozone uncer-tainty, k uncertainty, and the particle extinction co-efficient is shown in Fig. 1. For the calculations weset the ozone molecule number density to 0.7 3 1018

m23. As expected, owing to the smaller wavelengtheparation between the on- and off-resonance signalsTable 1!, the performance of the RVR Raman DIALs superior to that of conventional Raman DIAL, and

2 RVR Raman DIAL performs better than N2 RVRRaman DIAL. The results are easily extrapolated toother ozone concentrations because it can be shownthat for given uncertainties in ozone and k the ac-eptable particle extinction coefficient is linear in theolecule number density of the ambient ozone.Reference 4 provides a detailed discussion of theultiple-scattering effect on ozone measurementsith conventional Raman DIAL. To quantify the ad-antage of the RVR Raman DIAL over conventionalaman DIAL, it is sufficient for one to compare both

echniques under some simplifying assumptions:

Fig. 1. Relation between the particle extinction coefficient anduncertainty in k for a given ozone systematic uncertainty: RD,Raman DIAL.

Lidar

On-Resonance

RamanWavelength

~nm!lL

~nm!

O2 RVR Raman DIAL 307 308N2 RVR Raman DIAL 307 308Raman DIAL 332 308

aDl, maximum spectral separation of the signals used for the mcalculated from Ref. 13 ~T 5 226 K!. Wavelength values are rou


nly the limiting case of optically thick clouds ~negli-gible molecular scattering! is studied, no dependenceson lidar system parameters are discussed, and themodel cloud considered is homogeneous in particleproperties and particle number density throughout itsvertical extent. Additionally, the cloud-particle ex-tinction coefficient is supposed to be wavelength inde-pendent ~k 5 0!; particle correction functionscalculated with this assumption are denoted F0.

The multiple-scattering model4,8 that is used forthe calculations accounts for the wavelength shiftbetween lL and lVR. The model input parameter,which describes the scattering behavior of the cloudparticle ensemble, is the scattering phase function p.For cirrus, the type of cloud under investigation here,p must be determined in the geometrical-optics ap-proximation. At the time this paper was written nocrystal phase functions at the RVR Raman DIALoff-resonance wavelengths ~323 or 332 nm! wereavailable to the authors. Instead, phase functions ofensembles of ice spheres that are projection-areaequivalent to randomly oriented imperfect hexagonalcrystals14 are used for the multiple-scattering com-putations, because these phase functions can be de-termined with Mie theory. This approach isjustified by the fact that multiple-scattering param-eters obtained with phase functions of the two parti-cle classes are almost identical.8

Figure 2 shows particle correction functions in cir-rus clouds consisting of imperfect hexagonal columnsor plates. The RVR Raman DIAL substantially re-duces the influence of multiple scattering on ozonemeasurements. Compared with conventional Ra-man DIAL, F0 is a factor of ;3.5 and 5.5 smaller for,respectively, N2 and O2 RVR Raman DIAL, relativelyindependently of particle shape and penetrationdepth. These factors are close to the ratios of the Dlvalue of the conventional Raman DIAL to the corre-sponding Dl values of N2 and O2 RVR Raman DIAL~Table 1!. From Eq. ~3! and from the fact that thedifferential ozone absorption cross sections of allthree lidars are approximately the same, it followsthat reductions in F0 translate directly into decreasesin the multiple-scattering effect on ozone measure-ments; e. g., given a cirrus extinction coefficient of 0.1km21 and a Raman DIAL F0 of 20.04 ~Fig. 2!, RDPbecomes 20.65 3 1018 m23 for the conventional Ra-man DIAL, and RVRP is 20.19 3 1018 and 20.12 3

Off-Resonance

Dl~nm!

DCO3abs

~10224 m2!

RamanWavelength

~nm!lL

~nm!

323 308 16 11.7332 308 25 12.3387 355 79 12.4

ements. Differential ozone absorption cross sections ~DCO3abs! are

to full nanometers.
easurnded

Cca

sw

ns~naTpcLrTctT71mttas

ct

1018 m23 for the N2 and O2 RVR Raman DIAL, re-spectively.

4. Experimental Setup

The first experimental RVR Raman DIAL was in-stalled in the trailer of the water-vapor Raman li-dar16 of Sandia National Laboratories, Livermore,

alif. A Lambda Physik LPX 150 T MSC XeCl ex-imer laser served as the tunable narrow-band radi-tion source, injection locked at a wavelength of lL 5

307.94 nm. Optionally a Lambda Physik LPX 220iXeCl excimer laser was used to amplify the outputpower of typically 12 W to more than 40 W. Afterbeam expansion by a factor of 5 the laser pulses weretransmitted into the atmosphere. The spectral pu-rity of the outgoing light was monitored on line. Wedetermined that the laser system can be consideredan ideal narrow-band transmitter for the ozone mea-surements for two reasons. First, during hours ofseeded laser operation, we did not observe the buildupof scavenging sidebands or amplified spontaneousemission. Second, tuning the laser wavelength to307.94 nm ~close to the short-wavelength boundary ofthe broadband excitation spectrum of XeCl! and us-ing the anti-Stokes branch of molecular nitrogen andoxygen rotational Raman scattering as the on-resonance signal for the lidar measurements furtherreduce considerably the influence of possible un-seeded laser pulses or amplified spontaneous emis-sion.

A schematic diagram of the receiving system isshown in Fig. 3. The atmospheric return signal iscollected with a 760-mm-diameter Cassegrain tele-scope ~ fy4.5! and focused on an adjustable dia-phragm serving as the field stop. Apertures of 1–3mm and corresponding divergences of the collimatedlight inside the receiver of 6.3–19.0 mrad are usuallychosen for the experiments. In this setup, radiationbackscattered from N2 vibrational–rotational Ramancattering is chosen as the off-resonance signal,hich is separated from the return signals at shorter

Fig. 2. Particle correction functions F0 of imperfect hexagonalcolumns ~left! and plates ~center! for O2 RVR Raman DIAL ~curveswith squares!, N2 RVR Raman DIAL ~curves with circles! andonventional Raman DIAL ~solid curves!, and ratios of conven-ional Raman DIAL F0 to corresponding RVR Raman DIAL F0

~right! in cirrus. Index 0 indicates the presupposed wavelengthindependence of the particle extinction coefficient. Cirrus-crystaldiameters range between 20 and 200 mm. A size distribution fromRef. 15 was used. The receiver field of view is 0.6 mrad.

2

wavelengths by a dichroic beam splitter. After theoff-resonance signal passes through an interferencefilter @center wavelength ~CWL!, 332 nm; FWHM, 2

m#, it is detected in channel VR. The atmosphericignal around 307.94 nm is divided by a beam splitter92% transmission! for the detection of elastic ~chan-el L; interference filter, 308-nm CWL, 2-nm FWHM!nd rotational Raman backscatter light ~channel R!.he channel-R interference filter is mounted on arecision rotary stage to permit fine tuning of theenter wavelength by tilting. Both channels R and

can be protected from the intense short-distanceeturn signals by optional neutral-density filters.he photomultipliers are operated in the photon-ounting mode; the data-acquisition electronics isriggered by the stray light of the transmitted pulses.he range bins of the multichannel scalers are set to5 m; 1000 range bins are stored on a hard disk every20 s of measurement time. Note that the RVR Ra-an DIAL is a single-laser ozone lidar. Therefore

his system is much less complex than free-roposphere lower-stratosphere conventional DIAL’snd Raman DIAL’s that all require two radiationources.For on-resonance wavelength lR, calculations were

carried out to optimize the channel-R filter band-width and center wavelength for minimum depen-dence of the return signal on atmospherictemperature and for maximum signal intensity.17

An important constraint on these considerations isthe filter performance at wavelength lL because thehigh suppression of elastically backscattered radia-tion is crucial for the ozone measurement. It turnsout that filters with a center wavelength of 307.4 nmand a bandwidth between 0.3 and 0.5 nm are the bestchoice. Here temperature effects are well below0.5% for temperature differences of 50 K, and 60–80% of the intensity of the anti-Stokes branch lieswithin the bandpass of the filter. This is equivalentto 10–12 times the vibrational–rotational signal in-tensity of molecular nitrogen.

The channel-R interference filter used for the mea-

Fig. 3. RVR Raman DIAL receiver: D, diaphragm; L, lens; DBS,dichroic beam splitter; BS, beam splitter; ND, neutral-density fil-ter; IF, interference filter; S, flexible shield. Atmospheric rota-tional Raman, elastic, and N2 vibrational–rotational Ramanbackscattering signals are detected in channels R, L, and VR.The center wavelength of the channel-R interference filter can betuned by rotating the filter around the vertical axis.


afw

tomlEsmatm8oH

ttumtDssswn

dgairs

6

surements presented here has a CWL at normal in-cidence ~CWL0! of 307.81 nm and 0.4-nm FWHM.The measured dependence of the CWL on the angle ofincidence and thus on the filter tilt angle ~FTA! a isshown in Fig. 4. CWL~a! 5 CWL0 @1 2 ~sin

yne!2#1y2, which holds for small a and which is used

or discussion of the results in Section 5, is plotted asell. Here ne represents the effective refractive in-

dex of the filter ~ne 5 1.62 according to the manufac-turer!. With the measured channel-R filterransmission curve, the intensity and the dependencen the atmospheric temperature of the rotational Ra-an signal impinging on the detector can be calcu-

ated as a function of the filter CWL ~Fig. 5!.vidently a CWL between 307.33 and 307.39 nmhould be chosen for the measurements; this opti-um wavelength interval corresponds to a filter tilt

ngle of ;5° ~see Fig. 4!. At this angle of incidencehe ratio of transmissivity at the filter CWL to trans-issivity at the laser wavelength, lL 5 307.94 nm, is

30. Mainly because of a poor CWL transmissivityf 5%, the detectable intensity is relatively low.owever, 2% of the intensity of atmospheric rota-

Fig. 4. Dependence of channel-R interference filter center wave-length on the FTA.

Fig. 5. Intensity and dependence on the atmospheric tempera-ture of the rotational Raman signal impinging on the detector as afunction of the channel-R interference filter CWL. The intensityis given relative to the integrated anti-Stokes and Stokes rota-tional Raman backscattering cross section of N2 and O2 with at-mospheric mixing ratios. The temperature dependence isobtained from computations carried out for 200- and 250-K atmo-spheric temperature. For the calculations we used the measuredtransmission curve of the channel-R filter.


ional Raman backscattering is slightly more thanhe signal strength that can be expected from molec-lar nitrogen, or oxygen, vibrational–rotational Ra-an scattering. Therefore, even with this low filter

ransmissivity, statistical errors of RVR RamanIAL measurements are smaller than those of mea-

urements with the conventional Raman DIAL, as-uming the same lidar parameters. This RVRystem advantage will be particularly importanthen combined with future advances in filter tech-ology.

5. Measurements

In Fig. 6 the ratio of the signals detected in channelsR and L are given as a function of the tilt angle of thechannel-R interference filter. During this experi-ment no clouds were present. The field stop diame-ter and the corresponding receiver field of view~RFOV! were 1.5 mm and 0.3 mrad, respectively.

As expected, the measurement curves are symmet-ric with respect to normal incidence. The data of theintegration height intervals from 5 to 9.5 km andfrom 14 to 18.5 km ~not plotted! are the same withinthe statistical error; the implication is that even for anarrow RFOV the overlap functions of the receiverchannels are identical. The angular dependence ofthe signal ratios follows theoretical predictionsclosely if channel transmissivities, the shift of theCWL of the channel-R filter with tilt angle, the con-tribution of elastic light scattering to the channel-Rsignal, and the backscattering cross-sectional ratio ofRayleigh and rotational Raman scattering are takeninto account. The departure of the calculated curvefrom the measured data in Fig. 6 is due to an over-estimation of the suppression of Rayleigh scatteringin channel R at small FTA’s ~see Fig. 8!. The depen-

ence of the signal ratio on intrareceiver light diver-ence has been studied for fixed FTA’s, and for tiltngles ,5° a slight influence of the divergence isndeed measured. For larger tilt angles the signalatio is not affected by the divergence of the returnignals.Figure 7 displays a nighttime measurement of a

Fig. 6. Ratio of channel-R and channel-L signals as a function ofthe tilt angle of the channel-R interference filter. Normal inci-dence corresponds to 0°. The measured values are integrals overthe height interval from 9.5 to 14 km. During the measurementno clouds were present.

T

bIa

r

F

titAtclU

ka

s

cirrus cloud that was taken on 17 September 1997.Obviously, tuning the interference-filter CWL ofchannel R away from the laser wavelength by in-creasing the tilt angle reduces drastically the contri-bution of the elastically backscattered light to thesignal of channel R. The suppression, i.e., the quo-tient of U ~the integral of the N2-rotational-Ramanintensity-weighted filter transmission curve over therotational Raman spectrum! and the transmissivity

~lL! of the channel-R interference filter, can be com-puted according to

UT~lL!

5S~zci!R~z0! 2 R~zci!

@R~zci! 2 R~z0!#V, (5)

where R~z0! and R~zci! are the ratios of the channel-Rsignal to the vibrational–rotational Raman signal be-low and in the cirrus, respectively, S~zci! is the elasticackscatter ratio, and V is constant ~Appendix A!.n Fig. 8 ~left! the experimental results are presentednd compared with theoretical UyT~lL! values de-

rived from the actual, measured transmission curveof the rotational-Raman channel filter. Channel-R

Fig. 7. Time series of RyR0 ~the normalized ratio of the channel-Rignal and the channel-VR signal! and of the elastic backscatter

ratio S measured during a cirrus event on 17 September 1997.From left to right, the tilt angle of the channel-R interference filteris 3°, 5°, and 7°. The temporal resolution of the profiles is 480 s;the lidar signals are smoothed with a sliding average windowlength of 600 m. Error bars indicate statistical noise.

Fig. 8. Suppression UyT~lL! ~left! and transmissivity T~lL! ~right!of the channel-R interference filter as a function of the FTA.

2

filter transmissivities at primary wavelength lL asderived from the atmospheric and the laboratorymeasurements are also shown ~Fig. 8, right!. Exper-imental and calculated data agree well; yet minordifferences can be seen. For tilt angles ,3.5° themeasured suppression UyT~lL! is higher and the cor-esponding T~lL! is lower than the theoretical values.

In view of the statistical uncertainties of the cirrusmeasurement, this may be considered an artifact be-cause S and RyR0 profiles do not differ significantlyfor measurements with small FTA’s of, e.g., 3° ~see

ig. 7!. However, the differences in UyT~lL! andT~lL! between experiment and theory appear to besystematic since the measured data do not fluctuatearound the theoretical curve but are consistentlylower and apparently level off toward the small an-gles. So another possible error source might bechannel-R signal nonlinearities during the cirrusmeasurement that was used for determination ofUyT~lL! and T~lL!. Given the high throughput ofhe beam splitter ~BS, Fig. 3!, the return signal thatmpinges on the detector cathode of channel R is in-ense for small tilt angles of the channel-R filter.lthough neutral-density filters were used to reduce

he signal strength, it is quite likely that at the cloudenter the signal was still too intense to be detectedinearly. In contrast to the case discussed above,

yT~lL! measured for tilt angles between 4° and 5.5°is smaller and T~lL! is larger than the theoreticalvalues. The deviations may be explained by the di-vergence of the return signal. Within the tilt angleinterval of steepest change in the transmissivity ofelastically backscattered light, photons of wave-length lL that hit the interference filter with anglessmaller than the mean angle of incidence can contrib-ute significantly to the detected signal. Althoughthe UyT~lL! data presented are far from being suffi-cient for measurements of ozone in cirrus, the mea-surement clearly demonstrates that interferencefilters can be used to separate rotational Raman fromelastically backscattered light in the UV. With twointerference filters of slightly improved performance,i.e., higher peak transmission and blocking of pri-mary radiation, in a row, UyT~lL! values of 106 andhence RVR Raman DIAL ozone measurements inclouds should be feasible.

Finally, a single-laser ozone observation with theRVR Raman DIAL is shown in Fig. 9. Clear-skyconditions permitted the choice of a small FTA of 2.8°.Because of problems with the laser the mean outputpower was only ;5 W during the measurement.Hence the integration time needed for ozone profilingwas quite long ~3.3 h!. Between heights of 3 and 9

m the measured ozone distribution exhibits valuesround 0.5 3 1018 m23; in the upper troposphere,

smaller ozone molecule number densities are ob-served. The profile of the ozone mixing ratio, how-ever, is height independent throughout thetroposphere with values between 40 and 50 ppbv~parts per billion by volume!. The steep onset of thestratospheric ozone layer at 16 km coincides with thetropopause measured with a local radiosonde.


cfR

R

d

R

6

6. Summary

RVR Raman DIAL for ozone measurements in cloudsis a new approach that may well develop into a pow-erful technique for studies of particle–ozone interac-tions in the troposphere and lower stratosphere.From model calculations on ozone measurements inupper-troposphere cirrus clouds it is concluded that,independently of the shape and the size of the cirrusparticles, RVR Raman DIAL promises to perform in away far superior to conventional Raman DIAL.Compared with a conventional Raman DIAL, the ef-fects of both wavelength-dependent particle opticalextinction and of particle multiple scattering are re-duced by ;70% ~N2 RVR Raman DIAL! and ;85%~O2 RVR Raman DIAL!. The first experimentalRVR Raman DIAL uses a narrow-band interference-filter polychromator as the lidar receiver. Thissingle-laser ozone lidar setup is much less complexthan the free-troposphere and lower-stratosphereconventional DIAL and Raman DIAL systems, whichboth require two radiation sources.

Ozone profiles were measured with the RVR Ra-man DIAL in cloud-free atmospheric conditions. Al-though the suppression of elastically backscattered

Fig. 9. Ozone molecule number density ~solid curve! and back-scatter ratio ~dashed curve! measured above Livermore, Calif.,~37.7 °N, 121.8 °W! on 30 September 1997 between 2154 and 0112local time. The transmitted laser power was 5 W, and the tiltangle of the channel-R filter was 2.8°. For the ozone calculationthe rotational and vibrational–rotational Raman signals aresmoothed with sliding average window lengths of 1500 and 2475 mbelow and above 9.8 km, respectively; values are calculated with astep width of 75 m. Error bars indicate statistical noise. Tem-perature ~dotted curve! and tropopause height ~thin dotted line!measured around midnight with a local radiosonde are shown aswell.


light in the on-resonance rotational Raman returnsignal is not yet sufficient for measurements of ozonein cirrus, the measurements show that interferencefilters can be used to separate the two components ofthe backscatter spectrum. With two slightly im-proved interference filters in series, RVR RamanDIAL ozone measurements in clouds should be fea-sible with the filter–polychromator approach.

Appendix A

The suppression of elastically backscattered radia-tion by the filter in channel R is defined as the ratioof the integral of the N2-rotational-Raman intensity-weighted filter transmission curve over the full rota-tional Raman spectrum ~U! to the transmissivity ofthe filter at the laser wavelength @T~lL!#. UyT~lL!an be calculated from the measured lidar signals asollows. The number of detected photons in channel

is given by

NR~z! 5 K9z22@UbN2

R ~z! 1 T~lL!S~z!bairL ~z!#

3 exp@2t~lL, z!#, (A1)

where K9 is a height-independent constant; bN2

R ~z!and bair

L ~z! are the volume backscatter coefficients ofrotational Raman backscattering from molecular ni-trogen and of elastic backscattering from air mole-cules, respectively; S~z! is the elastic backscatterratio; and t~lL, z! is the optical depth of the atmo-sphere ~full overlap between the lidar beam and the

FOV assumed!. Introducing

V 5 cN2~dCN2

b,RydV!y~dCairb,LydV!,

where cN2is the atmospheric N2 mixing ratio and

CN2

b,RydV and dCairb,LydV are the cross sections of the

backscattering processes, we obtain

NR~z! 5 K9z22@UV 1 T~lL!S~z!#

3 bairL ~z!exp@2t~lL, z!#. (A2)

R~z!, the ratio of NR~z! to the vibrational–rotationalaman signal

NVR~z! 5 K0z22bairL ~z!exp@2t~lL, lVR, z!#,

where K0 is a constant, can then be written as

R~z! 5 K@UV 1 T~lL!S~z!#

3 exp2@t~lL, z! 2 t~lL, lVR, z!#, (A3)

where K 5 K9yK0. Let zci and z0 be heights withinand slightly below the cirrus cloud. Then we canderive the equation

R~zci!

R~z0!5

UV 1 T~lL!S~zci!

UV 1 T~lL!. (A4)

scattering parameters of cirrus particle ensembles determined
Here the differential transmission between zci and z0has been neglected. From Eq. ~A4! we determinethat
UT~lL!

5S~zci!R~z0! 2 R~zci!

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We gratefully acknowledge the assistance of PhilPaul, Combustion Research Facility, Sandia Na-tional Laboratories, with the alignment of the exci-mer laser. We also thank Rudolf Baumgart of theInstitut fur Physikalische und Chemische Analytikat GKSS-Forschungszentrum Geesthacht for thespectral measurements of the rotational-Raman-channel interference filter.

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rotational vibrational–rotational raman differential absorption lidar for atmospheric ozone...

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