ba

ad

In situ measurements of H2O from a stratosphericalloon by diode laser direct-differentialbsorption spectroscopy at 1.39 mm

Georges Durry and Gerard Megie

A distributed-feedback InGaAs laser diode emitting near 1.393 mm is used in conjunction with an opticalmultipass cell that is open to the atmosphere to yield ambient water-vapor measurements by infraredabsorption spectroscopy. To obtain the high dynamic range for the measurements that is required forcontinuous water-vapor monitoring in the upper troposphere and the lower stratosphere, we used asimple circuit that combined differential and direct detection. Furthermore, the laser emission wave-length was tuned to balance the steep decrease in H2O concentration with altitude by sweeping moleculartransitions of stronger line strengths. The technique was implemented by use of the Spectrometre aDiodes Laser Accordables ~SDLA!, a tunable diode laser spectrometer operated from a stratosphericballoon. Absorption spectra of H2O in the 5–30-km altitude range obtained at 1-s intervals during recentballoon flights are reported. Water-vapor mixing ratios were retrieved from the absorption spectra bya fit to the full molecular line shape in conjunction with in situ pressure and temperature measurements,with a precision error ranging from 5% to 10%. © 2000 Optical Society of America

OCIS codes: 300.6340, 300.6380, 300.6260, 280.3420, 010.1280, 300.1030.

t

FdsmtcOts

1. Introduction

Accurate measurements of water vapor at high tem-poral resolution in both the upper troposphere ~UT!nd the lower stratosphere ~LS! can be used to ad-ress many topics in the science of the atmosphere.1

Water vapor is the most important terrestrial green-house gas.2,3 It is involved in many homogeneousand heterogeneous atmospheric chemical processes,for example, in the removal of nitrogen and chlorinespecies through hydrolysis reactions and in the for-mation of polar stratospheric clouds that are respon-sible for chlorine activation that leads to Arctic andAntarctic ozone depletion. A study of transport andexchange processes that affect water vapor in the UTand the LS is needed for understanding of the globalbudget of terrestrial water vapor and itsevolution.1,4–7 To address such issues in the clima-tology, chemistry, or dynamics of the atmosphere re-

The authors are with the Institut Pierre-Simon-Laplace, Serviced’Aeronomie, Centre National de la Recherche Scientifique, B.P. 3,91371 Verrieres-le-Buisson Cedex, France. The e-mail addressfor G. Durry is georges.durry@aerov.jussieu.fr.

Received 14 February 2000; revised manuscript received 19June 2000.

0003-6935y00y305601-08$15.00y0© 2000 Optical Society of America

quires a precision error of 5–10% in concentrationretrieval and a fast time response of ;1 s for a bal-loonborne or an airborne sensor.1 Furthermore, forcontinuous monitoring of water vapor in the UT andthe LS, a very high dynamic range, of 4 orders ofmagnitude, is required for the measurements. In-deed, ;90% of the terrestrial water vapor lies in theroposphere; consequently the average H2O mixing

ratio varies from ;7000 parts in 106 ~ppm! at groundlevels to ;5 ppm in the stratosphere with steep con-centration gradients between those regions ~tropo-pause!.

Atmospheric water vapor is currently monitoredchiefly with frost-point and Lyman-a hygrometers.

rost-point hygrometers measure the dew point byetecting condensation on a surface. Their time re-ponse is generally a few seconds, and water-vaporeasurements are often inaccurate at low concentra-

ions. Lyman-a spectroscopy consists in photodisso-iating water vapor by discharge of ultraviolet light.H radicals are produced that can be detected as

hey relax radiatively. Several spurious effects,uch as scattering or absorption by O2, must be taken

into account and corrected. Moreover, the measure-ments may be corrupted if the UV discharge is ener-getic enough to produce OH radicals byphotodissociating other chemical species. By com-parison, laser infrared-absorption spectroscopy8–13

20 October 2000 y Vol. 39, No. 30 y APPLIED OPTICS 5601

lfbbp

cfis

bsT~t

12

l

1ptmnmovapc

5

can simultaneously meet all the requirements of ac-curacy, detectivity, time response, and dynamicrange. Indeed, rotation–vibration absorption linesof water vapor can be scanned in the near-infraredspectral range, 1–2 mm, which are sufficiently strongto permit in situ detection from ground levels to 35km with a precision error within 5–10%. The mea-surements obtained by laser probing are not sub-jected to chemical pollution, as H2O rotation–vibration absorption lines can be selected that arefree from interference from other atmospheric spe-cies. With fast and highly sensitive infrared opticaldetectors, measurement times within 10 ms to 1 shave been achieved. In this research we used a sim-ple balanced circuit combining direct and differentialdetection in conjunction with appropriate selection ofthe molecular transitions probed by the diode laser inthe 1.39-mm spectral region to obtain a dynamicrange that was sufficient for continuous H2O moni-toring in the UT and the LS.

In a previous publication14 we described the Spec-trometre a Diodes Laser Accordables ~SDLA!, a bal-oonborne spectrometer that uses distributedeedback ~DFB! InGaAs telecommunication-type fi-er laser diodes to measure methane and water vapory absorption spectroscopy. The SDLA instrumentrovides in situ absorption spectra of CH4 ~near 1.653

mm! and H2O ~near 1.393 mm! recorded at 1-s inter-vals at the output of a multipass optical cell that isopen to the atmosphere. We discussed the design ofthe SDLA payload to obtain satisfactory behavior ofthe lasers despite the severe environmental condi-tions encountered during a stratospheric flight. ~In-deed, the ambient temperature to which theinstrument is subjected may vary from 130 °C atground level to 270 °C in the tropopause.! We dem-onstrated the operational capability of DFB InGaAslaser diodes for in situ determination of trace speciesin the 1023–1024 absorption range. The mixing-ratio retrieval was based on a nonlinear least-squaresfit to the full molecular line shape in conjunction within situ pressure and temperature measurements.The achievement of the scientific objectives of theSDLA in terms of precision error ~ranging from 5% to10%! and spatial resolution ~;10 m! in the verticaloncentration profiles was discussed in terms of therst set of tropospheric and stratospheric methanepectra yielded by the instrument.In this paper we focus on monitoring water vapor in

oth the UT and the LS. The SDLA balloonbornepectrometer was flown twice in 1999 as part of thehird European Stratospheric Experiment on Ozone

THESEO!. The main objective was to contribute tohe study of the H2O budget in the lower stratosphere

by simultaneously providing CH4 and H2O concen-tration data.4,5 However, the possibility of monitor-ing H2O in the UT was explored as well during bothflights. In Section 2 we describe the detection tech-nique used with the SDLA to achieve continuous de-termination of H2O in the UT and the LS. Thesources of uncertainties in mixing-ratio retrieval arediscussed. In Section 3 we report the results of

602 APPLIED OPTICS y Vol. 39, No. 30 y 20 October 2000

SDLA water-vapor measurements in the UT and theLS.

2. Experimental Methods

A. Near-Infrared Laser Absorption Technique

In the present research, a pigtailed DFB InGaAsPlaser diode purchased from Sensors Unlimited, Inc.,is used at a central emission wavelength of llaser 5.3925 mm to sweep mainly the 3, 0, 3 ~J, Ka, Kc! 3, 0, 2 ~J9, Ka9, Kc9! transition of the n1 1 n3 overtone

band of H2O at 7181.172 cm21.15–18 That room-temperature laser diode exhibits single-mode emis-sion with a linewidth less than 10 MHz. The outputpower is ;1 mW. With the SDLA instrumental ar-rangement the laser beam is transmitted through amonomode silica optical fiber to an optical multipasscell that is open to the atmosphere. The laser beamis absorbed by ambient water vapor as it propagatesinside the optical cell over an absorption path lengthof ;56 m. The in situ absorption spectra obtainedare recorded at 1-s intervals and saved onboard. At-mospheric pressure and temperature are also moni-tored in situ once per second with a Baratron gaugeand several thermistors that are installed in theSDLA gondola. The infrared-absorption techniqueconsists in retrieving the mixing ratio from the ab-sorption spectra by a nonlinear least-squares fit tothe spectral line shape in conjunction with measure-ments of pressure and temperature.

Indeed, over the entire laser tunability range wecan sweep several water-vapor transitions by operat-ing the laser at different temperatures. The lasertemperature is controlled by a Peltier-cooler heaterlocated beneath the semiconductor. Figure 1 showssimulations of the H2O absorption spectra in the UTand the LS in the laser tunability range. The cor-responding predicted absorption depths and line-widths obtained in the UT and the LS by simulationwith the U.S. Standard 76 Atmosphere concentrationprofiles and the HITRAN molecular database18 areisted in Table 1. The 3, 0, 3 ~J, Ka, Kc!3 2, 0, 2 ~J9,

Ka9, Kc9! transition at 7181.17 cm21 is well suited forwater-vapor monitoring in the stratosphere @Fig.~b!#. This transition is free of interference from theresence of other molecular species. Furthermore,he predicted absorption depth of a few percent can beeasured precisely by the differential-detection tech-ique. In the UT this transition cannot be used any-ore, as 100% of the laser beam is absorbed because

f the presence of a larger amount of ambient waterapor. In addition to saturation, line shapes arelso strongly broadened by collisional effects at tro-ospheric pressures, which results in an overlap oflosed H2O absorption lines, as illustrated in Fig.

1~a!. To overcome these problems, the transitions at7185.6 and 7183.0 cm21 are successively scanned bythe laser during the balloon’s ascent in the tropo-sphere. The line strengths are 1 or 2 orders of mag-nitude weaker, which allows the important increasein water-vapor concentration in that altitude regionto be compensated. The predicted atmospheric ab-

HswtsHp

pth a

sorption signal is within 80–10%, which can be mon-itored accurately by the direct-absorption technique.However, water vapor in the troposphere sufferschanges that depend on season and latitude. There-fore the previous simulation is useful only for obtain-

Fig. 1. Simulation of the water-vapor absorption spectrum overthe laser tunability range. The DFB InGaAs laser diode at 1.393mm can be tuned over several transitions of the n1 1 n3 overtoneband of H2O. ~a! In the troposphere, from ground levels to 10 km,the absorption lines at 7185.6 and 7183.0 cm21 are used to monitor

2O. ~b! In the stratosphere, from 10 to 30 km, the laser scans atronger transition at 7181.17 cm21 to balance the decrease inater-vapor concentration. The mixing ratios, pressures, and

emperatures were obtained from the U.S. Standard 76 Atmo-phere; the molecular parameters, from the HITRAN database.ere and in subsequent figures, L is the optical path length, P isressure, and T is temperature.

Table 1. Relationship of Simulated Absorption De

Altitude~km! P ~hPa! T ~K!

rH2O

~ppm!

Strato~slaser 5 71

Depth~%!

3 701.2 268.7 3182 —4 616.6 262.2 2158 —5 540.5 255.7 1397 —8 356.5 236.2 366.7 —

10 265 223.3 69.96 4912 194 216.7 10.85 10.0115 121.1 216.7 5 4.3520 55.29 216.7 3.9 2.5630 11.97 226.5 4.7 0.98

aThe mixing ratios and the atmospheric pressure and temperamolecular parameters, from the HITRAN database. The optical

ing an overview of the dynamic range required for themeasurements. Practically, we made the decision tochange the H2O transition swept by the laser by ob-serving the in situ absorption spectra that were dis-played in real time during the balloon flight.

B. Direct-Differential-Detection Setup

The differential-detection setup for the SDLA isshown in Fig. 2. The laser’s driving current isramped within 10 ms to scan the emission wave-length over the selected molecular transition. Ap-proximately 20% of the laser beam is passed througha closed 8-cm reference cell filled with 5 Torr of watervapor and focused onto an InGaAs photodetector.The reference spectrum obtained is used for wave-length calibration of the atmospheric spectra and tocontrol laser emission during the flight. The re-maining 80% of the laser beam is transmitted to themultipass cell by an optical fiber. The laser beam isthen split with a beam splitter, and 10% of the re-maining energy is focused onto a 1-mm-diameter In-GaAs photodetector ~from Epitaxx, Inc.! to providereference signal Bo. The remainder of the energy ispassed through the optical cell, where the laser beamis absorbed by water vapor. The direct spectrum,AoT~s!, is then recorded at the output of the opticalcell. T~s! is the molecular transmission that is dueto ambient H2O. Ao is the laser flux in the absenceof an absorber in the optical cell. Furthermore, theanalogical difference AoT~s! 2 Bo is taken, amplified,and digitized to yield differential spectrum SD.Hence in a water-vapor measurement there are foursignals, Sref, AoT~s!, Bo, and SD, which are simulta-neously recorded within 10 ms and digitized over 16bits. Five hundred twelve sampling points aretaken over each spectrum. Then 40 successive spec-tra are coadded. The total measurement time is 400ms. Figure 3 shows the Sref, AoT~s!, Bo, and SD

averaged spectra for a water-vapor measurementtaken at 16 km by the SDLA.

The water-vapor mixing ratio is related to molecu-lar transmission T~s! through the Beer–Lambert

nd Linewidth ~FWHM! to Altitude for Water Vapora

ecm21!

Troposphere

slaser 5 7183.0 cm21 slaser 5 7185.6 cm21

width21!

Depth~%!

Linewidth~cm21!

Depth~%!

Linewidth~cm21!

— — — 77.00 0.112— — — 59.4 0.0890— — — 40.73 0.0740— 66.28 0.125 8.90 0.04900836 19.08 0.076 1.29 0.04000560 3.18 0.055 0.16 0.0330398 1.33 0.038 — —0267 0.77 0.025 — —0199 — — — —

profiles were obtained from U.S. Standard Atmosphere 76; thelength is L 5 56 m.

spher81.1

Line~cm

0.0.0.0.0.

turepath

20 October 2000 y Vol. 39, No. 30 y APPLIED OPTICS 5603

fs

Isobtf

twfA

a

wdt

erS

5

law.14 Molecular transmission T~s! @or molecularabsorption 1 2 T~s!# can be extracted from directspectrum AoT~s! or from differential spectrum SD,depending on the absorption range. The detailedretrieval technique is described in Ref. 14. In thetroposphere, strong H2O absorption signals, rangingrom 80% to 10%, are monitored. Direct-absorptionpectrum AoT~s! is processed at that altitude region.

In the stratosphere, the absorption depths for watervapor are only a few percents. The concentrationsare then retrieved from differential spectrum SD.ndeed, the differential technique offers higher sen-itivity than direct detection, because it permits usef the full dynamic range of the measurements ~16its! to the atmospheric absorption signal. Rampinghe laser driving current results in scanning the laserrequency but also produces an amplitude modula-

Fig. 2. Schematic of the SDLA. The SDLA is based on an opticalmultipass cell open to the atmosphere. A near-infrared DFB In-GaAs laser diode is connected by an optical fiber to the multipasscell. The water-vapor measurement uses four spectra, Sref, Bo,AoT~s!, and SD. Reference spectrum Sref is measured at the out-put of an 8-cm closed cell. Signal Bo is taken with a beam splittert the input of the multipass cell. Direct spectrum AoT~s! is

recorded at the output of the optical cell. T~s! is the moleculartransmission that is due to ambient H2O. Ao is what the laser flux

ould be in the absence of an absorber in the cell. The analogicalifference AoT~s! 2 Bo is taken, amplified, and digitized to yieldhe differential spectrum SD. The spectra are recorded within 10

ms by ramping of the laser driving current. G and G1 are variablelectronic gains used to construct the differential signal SD. Zepresents altitude given by a global positioning system (GPS).ee text for more details.

604 APPLIED OPTICS y Vol. 39, No. 30 y 20 October 2000

ion that results in a strongly sloping backgroundith the absorption signal lying on top of it, as shown,

or example, in Fig. 3~b!. Directly digitizing theoT~s! signal for weak absorption results in a loss of

information because of numerical noise. Taking thedifference AoT~s! 2 Bo permits us to remove the slop-ing background and to use the full 16-bit resolution tothe atmospheric absorption signal. We also use dif-ferential detection with the SDLA instrument to takemeasurements of atmospheric methane in the 0.1–0.01% absorption range.14 Once molecular trans-mission T~s! has been extracted from the differentialor direct spectrum, the water-vapor mixing ratio isretrieved by a nonlinear least-squares fit to the fullline shape in conjunction with in situ pressure andtemperature measurements. Figure 4 shows sev-eral water-vapor measurements taken by the SDLAin the UT and the LS. The spectra displayed in Fig.4 are recorded within 400 ms. Noise is of the orderof ;1024 ~expressed in absorption units!. It remainsconstant during the entire flight. Combining directand differential detection with an appropriate selec-

Fig. 3. Atmospheric water-vapor spectrum recorded in the strato-sphere during the SDLA balloon flight in southern France on 10May 1999. ~a! Reference spectrum Sref, ~b! direct spectrumAoT~s! and ~c! differential spectrum SD ~see Fig. 2! are displayed.The spectra feature the 3, 0, 3 ~J, Ka, Kc! 3 2, 0, 2 ~J9, Ka9, Kc9!transition of the n1 1 n3 overtone band of H2O at 7181.172 cm21.At 16.52 km, ;3% of the laser energy is absorbed in the opticalmultipass cell. The spectra are recorded within 10 ms and aver-aged over 40 successive laser scans.

ma

peleapass

f

Table 2. Positions and Line Strengths for Tropospheric Water Vapor

tion of the transitions permits a high signal-to-noiseratio and constant accuracy in the concentration re-trieval to be maintained despite the large fluctua-tions in H2O concentration encountered in the UTand the LS. Indeed, the sensitivity achieved is largeenough that a water-vapor measurement can betaken every millisecond, instead of one second, as iscurrently done by the SDLA. Increasing the tempo-ral resolution of the sensor may be useful for address-ing issues in the dynamics of the atmosphere~turbulence!.

C. Sources of Uncertainties

Laser diodes could exhibit a degradation of their spec-tral emission over the several hours of a balloon flightin a severe temperature and pressure environment.

Fig. 4. Water-vapor absorption measurements obtained by theSDLA during the balloon flight in southern France on 10 May1999. ~a! In the troposphere, the laser scans the H2O transition at7185.6 cm21 ~TLaser ; 26 °C!. The molecular absorption is re-trieved from the direct-absorption spectrum. At 8.2 km, ;35% ofthe laser energy is absorbed in the optical cell by ambient watervapor. ~b! At tropopause altitude levels, the laser sweeps the H2Otransition at 7183.0 cm21 ~TLaser ; 32 °C!. ~c! In the strato-sphere, the laser is tuned over a stronger transition at 7181.17cm21 ~TLaser ; 36 °C!. The molecular absorption was extractedrom differential spectrum SD of Fig. 3~c!. At 16.5 km, only ;3%

of the laser energy is absorbed in the optical cell. The mixingratios were retrieved by a nonlinear least-squares fit to the full lineshape ~with a Voigt modeling! in conjunction with the in situpressure and temperature measurements. See text for more de-tails.

Therefore, to permit diagnosis of a possible laser fail-ure, spectra are saved onboard and processed afterthe flight. Thereby we make sure that no temporaldrifts or unexpected noise in the laser emission oc-curs by checking the residuals obtained by fitting thesuccessive reference and atmospheric spectra.14

Furthermore, no unexpected optical fringing causedby exposure of the multipass optical cell to the severeatmospheric environment was found in the spectra inthe 1023 to 1024 absorption range. Regardless ofother sources of uncertainties discussed below, thesignal-to-noise ratio achieved in the absorption spec-tra ~Fig. 4! is large enough to permit the retrieval of

ixing ratios with a precision error of 1% for the UTnd the LS.Fitting the molecular line shape involves many

rocedures that contribute to the overall precisionrror: estimation of the baseline ~1–2%!, wave-ength calibration and correction of the laser nonlin-arities ~0.5%!, detector dark-current subtraction,ccuracy in determining the electronic gains, andrecision error in monitoring the in situ pressuresnd temperatures of not more than 1%. With re-pect to the molecular parameters, the strong tran-ition at 7181.17 cm21 in the stratosphere was

modeled by means of the HITRAN database. Nev-ertheless, for some of the weaker transitions used inthe troposphere, discrepancies in position and linestrength were observed with the HITRAN database,essentially with respect to the components at 7185.6cm21. Indeed, the H2O spectra recorded at 7185.6cm21 feature mainly two H2O transitions that are;0.2 cm21 apart @Fig. 4~a!#; a distance of ;0.1 cm21

between the spectral components was predicted byHITRAN. Moreover, the absorption of the weakesttransition is predicted to be approximately eighttimes as much as what is measured. With respect tothe H2O spectra at 7183.0 cm21 @Fig. 4~b!#, a discrep-ancy of ;0.03 cm21 in the distance between the twoH2O transitions was observed. The values used inthis paper to process the corresponding spectra in thetroposphere are given in Table 2. Taking into ac-count the signal-to-noise ratio achieved in the spectraand all other sources of error that are due to theretrieval process, the mixing ratios are determinedwithin a 5–10% precision error.

3. Flight Results and Discussion

As part of the Third European Stratospheric Exper-iment on Ozone, the SDLA spectrometer was flowntwice in 1999 from a stratospheric balloon operated

Position ~cm21!Line Strength ~cm2

molecule21 cm21!

7185.5969 7.810 3 10222

7185.4385 4.046 3 10224

7185.3738 6.353 3 10223

7183.0620 6.118 3 10222

7182.9950 5.300 3 10221

20 October 2000 y Vol. 39, No. 30 y APPLIED OPTICS 5605

m

Tia

5

by the Centre National d’Etudes Spatiales, theFrench space agency. The first flight took place atnorthern latitudes from Kiruna ~in Sweden! on 23February 1999 in the late Arctic vortex, followed by asecond flight at mid-latitudes on 10 May 1999 fromAire-sur-l’Adour ~in southern France!. The SDLAinstrument is flown under a medium-sized strato-spheric balloon ~35 000 m3! inflated with hydrogen.Each flight duration was 3–4 h with a float altitude at;30 km. The SDLA instrument was equipped withtwo Sensors, Inc., laser devices, one emitting at 1.653mm to monitor CH4 and a second emitting at 1.393

m to measure H2O. For both flights, measure-ments at 1-s intervals started at ground levels, andapproximately 10,000 spectra were obtained for bothspecies during ascent and descent of the payload.

A. Stratospheric Water Vapor

Water vapor outgassed by the balloon envelope iscritical for monitoring H2O in the stratosphere be-cause of the low concentration involved ~;5 ppm at 10hPa!. Measurements in the stratosphere are thustaken during the descent of the payload to avoid pol-lution by the water vapor outgassed from the balloon

Fig. 5. Water-vapor vertical concentration profiles of H2O in theSweden! in the Arctic Vortex and at mid-latitudes ~in southern Frathe H2O transitions at 7185.6 cm21 and at 7183.0 cm21. In the sThe altitudes at which the laser sweeps the transitions at 7125.6,

he H2O profile yielded by a balloonborne P, T, U ~humidity! metes also displayed. The complete processing of the 10,000 absorptio

more comprehensive discussion.

606 APPLIED OPTICS y Vol. 39, No. 30 y 20 October 2000

envelope. The balloon is equipped with a valve topermit slow descent by adjustment of the balloon’shydrogen content. Furthermore, the materials usedin the design of the payload were chosen to preventadditional pollution; the gondola is entirely based ona tubular metallic structure. Moreover, the cell islocated 1 m under the main part of the spectrometerand is open to the atmosphere.

Despite these precautions, the stratospheric datawere corrupted at the beginning of the descent in the30–24-km altitude range. This corruption was dueto the presence of some tropospheric air trapped inthe mechanical mounting of the optics that was usedto couple the optical fiber with the multipass cell.Indeed, unexpectedly weak water-vapor absorptionwas recorded in reference spectrum Bo that wastaken at the input of the optical cell ~Fig. 2!. Themeasured absorption was consistent with tropo-spheric air absorption over a path length of a fewcentimeters. The resultant interfering absorption~1000–2000 ppm! is of the order of that which is dueto stratospheric water ~5 ppm over a distance of 5600cm! and produces 25% pollution at 30 km. Succes-sive Bo reference spectra were processed during the

nd the LS obtained by the SDLA in 1999 at northern latitudes ~inn springtime. In the troposphere, the laser diode is scanned oversphere the laser is tuned over the absorption line at 7181.1 cm21.0, and 7181.1 cm21 are indicated, respectively, by [1], [2], and [3].gical sonde that was launched from Kiruna on the day of the flightectra obtained during both flights is still under way. See text for

UT ance! itrato7183.orolon sp

bscww

~to

flight; we found that the interfering absorption di-minished with time as the parasitic water vapor outdiffusing from the mechanical trap. Approximately1.5 h after the launch ~corresponding to an altitude of;24 km during the descent! it vanished, and strato-spheric measurements become consistent, of the or-der of 5–3 ppm, and in good agreement with thatyielded by other hygrometers in the same atmo-spheric conditions. Mechanical upgrades have beenmade to avoid having tropospheric air trapped in fu-ture flights. Figure 5 shows the vertical concentra-tion profiles obtained in the stratosphere at mid-latitudes and northern latitudes. Approximately400 differential spectra were processed for each pro-file from the ;5000 recorded during the descent ofthe payload from ;30 to ;10 km. Mixing ratioswere retrieved from the differential spectra by a non-linear least-squares fit to the full molecular lineshape ~Voigt modeling! by use of the in situ pressureand temperatures measurements and the HITRANdatabase. Concentrations measured at northernlatitudes are slightly larger because of the diabaticdescent of upper stratospheric humid air in the latevortex. The precision error is within 5% for alti-tudes lower than 24 km, where measurements can betaken without outgassing.

B. Tropospheric Water Vapor

Measurements of water vapor in the UT were takenduring the ascent of the gondola. A medium-sizedballoon cannot probe the UT during the descent be-cause of interference from the civil air traffic. Usu-ally the flight terminates at ;12 km, with aseparation of the payload from the flight chain. It isfollowed by a descent under parachute. The pollu-tion observed in the stratosphere can be neglectedwhen the much larger concentrations from the UTare processed, but it prevents complete coverage ofthe tropopause altitudes in the transition region be-tween the UT and the LS. The processing of the UTdata was stopped for both flights at altitudes at whichthe retrieval of H2O concentration could be preventedy the estimated interference from lower tropo-pheric water vapor. Figure 5 shows the verticaloncentration profiles obtained in the UT. Theater-vapor profile obtained at northern latitudesith a balloonborne P, T, U ~humidity!-sonde is also

displayed. Indeed, the meteorological sondes thatwere used are devoted to measuring the lower tropo-sphere and are known to be inaccurate at low tem-perature and concentrations. As the sonde wasoperated in a cold and dry atmosphere in the Arcticwinter, the data for the UT must be considered care-fully, as humidity measurements degrade dramati-cally in that altitude range. Nevertheless, it is ofinterest to confirm the tropopause height and dia-batic descent of air in the Arctic Vortex measured bythe SDLA by comparison with the standard mid-latitudes profile. Approximately 200 direct absorp-tion spectra for the flight of the SDLA at northernlatitudes and 400 for the flight at mid-latitudes fromthe ;1000 spectra recorded during the ascent of the

payload in the troposphere were processed. Mixingratios were retrieved from the direct absorption spec-tra at 7185.6 and 7183.0 cm21 by a nonlinear fit to themolecular line shape by means of the in situ pressureand temperature measurements and by use of thepositions and line strengths listed in Table 2. Theprecision error was within 5–10%.

4. Conclusion

A simple circuit combining direct and differential de-tection was used in conjunction with a commercialnear-infrared laser diode at 1.393 mm to providewater-vapor measurements at 1-s intervals in theupper troposphere ~UT! and the lower stratosphereLS! with a precision error within 5–10%. We ob-ained a dynamic range sufficient to permit continu-us H2O probing in the UT and the LS by combining

direct and differential detection in conjunction withappropriate selection of the molecular transitionsprobed by the laser. The achieved time response,accuracy, and dynamic range for the measurementfully match the requirements set for the science ob-jectives. Indeed, one could enhance temporal reso-lution by taking a H2O measurement once permillisecond. This short measuring time could be ofinterest for addressing new topics in the dynamics ofthe atmosphere.

We plan to construct a lightweight balloonbornesensor, capable of continuous H2O monitoring in theUT and the LS during ascent of the payload, to fly atmany places and times. To overcome the problemsthat are due to interfering outgassed water vapor inthe stratosphere, we intend to reduce the balloon sizeby reducing the overall weight of the gondola. Fur-thermore, the optical cell will be located a few metersunder the main part of the spectrometer and fed withthe laser beam by means of an optical fiber.

Several members of the Institut National des Sci-ences de l’Univers, Division Technique of the CentreNational de la Recherche Scientifique ~CNRS! and ofthe Service d’Aeronomie: A. Abchiche, N. Am-arouche, B. Brient, T. Danguy, H. Poncet, J. C. Sa-make, P. Schibler, and F. Semelin, were intenselyinvolved in the design and field operations of theSDLA and are thanked for their very valuable assis-tance. The research described in this paper wassupported by the Centre National d’Etudes Spatialesand the Centre National de la Recherch Scientifique.

References1. World Meteorological Organization, “Stratospheric processes

and their role in climate,” World Climate Research ProgrammeRep. WCRP-105 ~World Meteorological Organization, Geneva,1998!.

2. K. P. Sinha and A. Sinha, “Sensitivity of the Earth’s climate toheight-dependent changes in water vapour mixing ratio,” Na-ture ~London! 354, 382–384 ~1991!.

3. A. Sinha and J. E. Harries, “Water vapour and greenhousetrapping: the role of far-infrared absorption,” Geophys. Res.Lett. 22, 2147–2150 ~1995!.

4. C. P. Rinsland, M. R. Gunson, R. J. Salawitch, M. J. New-church, R. Zander, M. M. Abbas, M. C. Abrams, G. L. Manney,

20 October 2000 y Vol. 39, No. 30 y APPLIED OPTICS 5607

H. A. Michelsen, A. Y. Chang, and A. Goldman, “ATMOS mea- surements of CO, CO , OH, and H O in room-temperature and

1

1

5

surements of H2O 1 2CH4 and total reactive nitrogen in theNovember 1994 Antarctic stratosphere: dehydration anddenitrification in the vortex,” Geophys. Res. Lett. 23, 2397–2400 ~1996!.

5. M. M. Abbas, H. A. Michelsen, M. R. Gunson, M. C. Abrams,M. J. Newchurch, R. J. Salawitch, A. Y. Chang, A. Goldman,F. W. Irion, G. L. Manney, E. J. Moyer, R. Nagaraju, C. P.Rinsland, G. P. Stiller, and R. Zander, “Seasonal variations ofwater vapor in the lower stratosphere inferred from ATMOSyATLAS-3 measurements of H2O and CH4,” Geophys. Res. Lett.23, 2401–2404 ~1996!.

6. H. Teitelbaum, J. Ovarlez, H. Kelder, and F. Lott, “Some ob-servations of gravity-wave-induced structure in ozone and wa-ter vapour during EASOE,” Geophys. Res. Lett. 21, 1483–1486~1994!.

7. J. Ovarlez, P. Velthoven, and H. Schlager, “Water vapor mea-surements from the troposphere to the lowermost strato-sphere: some signatures of troposphere to stratosphereexchanges,” J. Geophys. Res. 104, 16,973–16,978 ~1999!.

8. H. I. Schiff, G. I. Mackay, and J. Bechara, “The use of tunablediode laser absorption spectroscopy for atmospheric measure-ments,” in Air Monitoring by Spectroscopic Techniques, M. W.Sigrist, ed., Vol. 127 of Chemical Analysis Series ~Wiley, NewYork, 1994!, pp. 239–318.

9. K. Uehara and H. Tai, “Remote detection of methane with a1.66-mm diode laser,” Appl. Opt. 31, 809–814 ~1992!.

10. D. C. Hovde and C. A. Parsons, “Wavelength modulation de-tection of water vapor using a vertical cavity emitting laser,”Appl. Opt. 36, 1135–1138 ~1997!.

11. B. L. Upschulte, D. M. Sonnenfroh, and M. G. Allen, “Mea-

608 APPLIED OPTICS y Vol. 39, No. 30 y 20 October 2000

2 2

combustion gases by use of a broadly current-tuned multisec-tion InGaAsP diode laser,” Appl. Opt. 38, 1506–1512 ~1999!.

2. R. M. Mihalcea, D. S. Baer, and R. K. Hanson, “Diode lasersensor for measurements of CO, CO2, and CH4 in combustionflows,” Appl. Opt. 36, 8745–8752 ~1997!.

13. P. Cancio, C. Corsi, F. S. Pavone, R. U. Martinelli, and R. J.Menna, “Sensitive detection of ammonia absorption by using a1.65 mm distributed feedback InGaAsP diode laser,” InfraredPhys. Technol. 36, 987–993 ~1995!.

14. G. Durry and G. Megie, “Atmospheric CH4 and H2O monitor-ing with near-infrared InGaAs laser diodes by the SDLA, aballoonborne spectrometer for tropospheric and stratosphericin situ measurements,” Appl. Opt. 38, 7342–7354 ~1999!.

15. J.-Y. Mandin, J.-P. Chevillard, C. Camy-Peyret, and J.-M.Flaud, “Line intensities in the n1 1 2n2, 2n2 1 n3, 2n1, n1 1 n3,2n3, and n1 1 n2 1 n3 2 n2 band of H2

16O between 6300 and7900 cm21,” J. Mol. Spectrosc. 118, 96–102 ~1986!.

16. R. A. Toth, “Extensive measurements of H216O line frequencies

and strengths: 5750 to 7965 cm21,” Appl. Opt. 33, 4851–4867~1994!.

7. R. A. Toth and J. S. Margolis, “Line positions of H2O in the1.33 to 1.45 micron region,” J. Mol. Spectrosc. 55, 229–251~1975!.

18. L. S. Rothman, R. R. Gamache, R. H. Tipping, C. P. Rinsland,M. A. H. Smith, D. Chris Benner, V. Malathy Devi, J.-M.Flaud, C. Camy-Peyret, A. Perrin, A. Goldman, S. T. Massie,L. R. Brown, and R. A. Toth, “The HITRAN molecular data-base: editions of 1991 and 1992,” J. Quant. Spectrosc. Radiat.Transfer 48, 469–507 ~1992!.