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Situ Measurement of Methane and Water Vapor from
Stratospheric Balloons
Georges Durry, Ivan Pouchet
To cite this version:
Georges Durry, Ivan Pouchet. A Near-Infrared Diode Laser Spectrometer for the In Situ Mea-surement of Methane and Water Vapor from Stratospheric Balloons. Journal of Atmospheric and Oceanic Technology, American Meteorological Society, 2001, 18 (9), pp.1485-1494. �10.1175/1520-0426(2001)0182.0.CO;2�. �hal-03124610�
q 2001 American Meteorological Society
A Near-Infrared Diode Laser Spectrometer for the In Situ Measurement of Methane
and Water Vapor from Stratospheric Balloons
GEORGESDURRY AND IVANPOUCHET
Institut Pierre-Simon-Laplace, Service d’Ae´ronomie, Centre National de la Recherche Scientifique, Paris, France (Manuscript received 10 October 2000, in final form 6 February 2001)
ABSTRACT
The Spectrome`tre a` Diodes Laser Accordables (SDLA), a balloonborne near-infrared diode laser spectrometer, was developed to provide simultaneous in situ measurements of methane and water vapor in the troposphere and the lower stratosphere. The instrument was flown several times from stratospheric balloons operated by the Centre National d’Etudes Spatiales within the framework of the Third European Stratospheric Experiment on Ozone in 1998–2000. The SDLA is based on a multipass optical cell open to the atmosphere. Two near-infrared telecommunication-type laser diodes are connected with optical fibers to the cell to take in situ absorption spectra of methane (in the 6047 cm21spectral region) and water vapor (in the 7181 cm21spectral region) at 1-s intervals.
Mixing ratios are obtained, with a precision error of within 5%–10%, from a nonlinear fit to the full molecular line shape in conjunction with in situ pressure and temperature measurements. The SDLA is described, and achieved atmospheric methane and water vapor vertical concentration profiles are reported.
1. Introduction
The Spectrome`tre a` Diodes Laser Accordables (SDLA) is a balloonborne near-infrared diode laser spectrometer devoted to the in situ monitoring of meth-ane (CH4) and water vapor (H2O) in the upper
tropo-sphere (UT) and the lower stratotropo-sphere (LS). The in-strument has been developed since 1997 with the sup-port of the Centre National d’Etudes Spatiales (CNES) and the Centre National de la Recherche Scientifique (CNRS). The science objectives were to contribute to the study of the lower-stratospheric water vapor and ozone budgets (Abbas et al. 1996; Rinsland et al. 1996) within the framework of the Third European Strato-spheric Experiment on Ozone (THESEO); the SDLA was purposely flown several times from stratospheric balloons at mid- and northern latitudes in 1998–2000.
The monitoring capabilities of laser absorption spec-troscopy (Schiff et al. 1994) and the recent progress made in the field of near-infrared semiconductor lasers were a strong impetus for us to start the development of the SDLA. Indeed, to fulfill the science objectives, a good spatial resolution (;50 m) in the vertical con-centration profile and an inaccuracy of less than 5%– 10% in the mixing ratio retrieval were needed. Laser-probing technique can meet these requirement by pro-viding
Corresponding author address: Georges Durry, CNRS/Service d’Aeronomie, Reduit de Verrieres, BP 3, Verrieres-le-Buisson Cedex 91371, France.
E-mail: [email protected]
1) a high temporal resolution ranging from 10 ms to 1 s; 2) a matched inaccuracy of less than a few percent in
the concentration retrieval;
3) a dynamic range in the concentration measurements of four orders of magnitude; and
4) a measurement that is free of pollution by other chemical species, which is achieved by selecting ap-propriate rotation–vibration transitions.
In designing the SDLA we have taken advantage of the recent developments made in the field of near-infrared semiconductor lasers that were chiefly driven by the need for telecommunication. Indeed, distributed-feed-back indium–gallium–arsenide (InGaAs) laser diodes with emitting wave numbers that range from 5000 to 10 000 cm21are now available from various suppliers,
with laser emission properties well-suited for gas mon-itoring. Such devices exhibit a reliable single-mode emission with no mode hops over the tunability range. Therefore, the laser emission wavenumber can be scanned continuously over a molecular transition with no troublesome artifacts to measure a gas concentration. The achieved output power is a few milliwatts, sufficient to monitor gases by absorption spectroscopy with var-ious industrial or laboratory applications (Modugno et al. 1998; Mihalcea et al. 1997; Silver et al. 1995). Unlike cryogenically cooled midinfrared lead–salt laser diodes that are usually used for atmospheric sensing (Harris et al. 1989; Wienhold et al. 1994; Toci et al. 1999; Scott et al. 1999), InGaAs lasers can be operated at room temperature, which makes it very convenient for field instruments. A Peltier cooler–heater located beneath the
indium–gallium–arsenide–phosphide (InGaAsP) semi-conductor is sufficient to obtain a stable laser emission. Moreover, to detect the laser beams, highly linear and sensitive room-temperature InGaAs photodiodes are available in the near-infrared spectral range, and optical fibers can be used to ensure convenient light connections with the purpose of designing compact sensors.
Methane and water vapor exhibit several rotation– vibration transitions in the near-infrared spectral range that are free of interference from other atmospheric spe-cies and therefore suitable for laser probing. With the SDLA, the 2n3, R(3) rotation–vibration transition of
CH4at 6046.96 cm21, and then11n3, (J, Ka, Kc) (3,
0, 3)→(J9, , ) (2, 0, 2) transition of HK9 K9a c 2O at 7181.17
cm21 are used. The monitoring technique consists in
tuning the laser diode in frequency by ramping of its driving current to sweep the selected transitions. The laser flux is then absorbed by the molecular species (e.g., methane), and the detected signal may be expressed as
AoT(sLaser). Here, Aois what would be the laser flux in
the absence of methane. Following the usual spectro-scopic notation,sLaseris the inverse of the laser emission
wavelength; it is expressed in reciprocal centimeters. Here, T(sLaser) is the molecular transmission due to
am-bient methane that is related to the methane mixing ratio through the Beer–Lambert law:
T(sLaser)5 exp 2r N(T , P )L
[
CH4 atm atmN
3
O
k (Tn atm)F(T , P ,atm atm sLaser) ,]
(1)lines
is the methane mixing ratio, N(T, P) is the total
rCH4
molecular density determined by the Mariotte law, L is the absorption pathlength (cm), and kn(T ) is the molec-ular line strength of the selected rotation–vibration tran-sitions (cm2molecule21cm21). Here,F(T, P,s) is the
molecular normalized profile, a standard Voigt model that takes into account both Doppler and collisional broadening effects to reconstruct the molecular line shape over a large atmospheric pressure and temperature range. The effect in Eq. (1) of the spectrometer appa-ratus function is neglected, because the laser line width (;10 MHz) is much less than the methane average line width (;1 GHz) at atmospheric pressures. The molec-ular parameters (the line strengths and the various pres-sure-broadening coefficients needed to compute the Voigt model) are extracted from the ‘‘HITRAN’’ da-tabase (Rothman et al. 1992). The atmospheric pressure
Patmand temperature Tatmare provided by onboard
sen-sors. The used laser-probing technique consists then of extracting the mixing ratio from the in situ absorption spectra AoT(s) using Eq. (1) and by means of a nonlinear
fit to the full molecular transmission T(s) in conjunction with the in situ atmospheric pressure and temperature measurements.
To monitor methane and water vapor in the UT–LS,
an absorption pathlength of a few tens of meters is need-ed to reach an appropriate sensitivity; hence a multipass optical cell providing an L 5 56-m absorption path-length is used with the SDLA. If one takes into account the average atmospheric concentration of methane (;1 ppm) and water vapor (;5 ppm) in the lower strato-sphere, predicted absorption 1 2 T(s) at 6046.96 and 7181.17 cm21is of within 1022 to 1024 [which means
that the amounts of absorbed laser energy range from 1% to 0.01%, assuming a 56-m absorption pathlength and according to Eq. (1)]. A direct-differential detection setup was implemented in combination with a 16-bit digitizer to reach the needed sensitivity and permit an error in the mixing ratio retrieval of within 5%–10% and a temporal resolution of 1 s. Furthermore, to monitor the much larger amounts of water vapor in the tropo-sphere (ranging from about 10 000 ppm at ground levels to a few tens of parts per million in the tropopause), we developed a technique consisting of sweeping succes-sive H2O transitions of increasing line strengths to
match the strong decrease in the water vapor concen-tration with altitude. Thereby, we obtained a dynamic range for the measurements of 4 orders of magnitude that is sufficient to monitor water vapor continuously in the UT–LS.
In the next section, the SDLA is described. We lay emphasis upon the arrangements made to allow the SDLA to be operated in a severe atmospheric environ-ment (the temperature to which the instruenviron-ment is sub-mitted may be ranging from 1308C at ground levels down to 2708C in the tropopause). Then we report achieved atmospheric methane and water vapor mea-surements from recent balloon flights of the SDLA.
2. The SDLA instrument
The SDLA is based on an optical multipass cell open to the atmosphere (see Fig. 1). We designed a standard Herriott-type multipass cell made of two gold-coated spherical mirrors at a distance of 1 m (Altmann et al. 1981). It provides a 56-m absorption pathlength. In-sights on the optical arrangement of the cell are found in Durry and Megie (1999). The optical cell is operated open to the atmosphere, and thereby it is submitted to severe temperature gradients during the flight, which could cause optical misadjustment by thermal expansion of the cell mechanical mounting. Therefore, we have taken several precautions in designing the optical cell: 1) titanium and cryogenic Invar alloy are used with the cell mechanical mounting to minimize thermal ex-pansion (less than 1 mm for a 1-m cell length) within a temperature range from1308 to 2808C;
2) resistor heaters located beneath the mirrors are used to avoid the formation of ice during the flight; and 3) motorized shutters help to prevent the degradation of the mirror surfaces at launching and landing or,
FIG. 1. Overview of the SDLA balloonborne gondola.
in case of very cloudy weather, during the ascent in the lower troposphere.
Two near-infrared distributed-feedback InGaAsP la-ser diodes purchased from Sensors Unlimited, Inc., emitting in the 6047 and 7181 cm21regions to monitor
CH4 and H2O are connected with standard
telecom-munications optical fibers to the cell. The optical fibers are protected with polyurethane and Mylar sheets to avoid troublesome behaviors of the fibers caused by the temperature of the environment. Polyurethane and My-lar are also used as a thermal protection for the laser modules containing the laser diodes and its driving elec-tronics. The temperature of the laser modules is main-tained during the complete flight within ;108 and
;408C by means of resistor heaters; the laser Peltier
thermoelement can then work properly and ensure a laser temperature stabilization within the 1 mK that is required for a stable laser emission (the CH4 device
works at;268C, the H2O device at;368C).
Neverthe-less, the laser’s diodes and its driving electronics are installed in unpressurized mechanical mountings to re-duce the mass of the laser modules (;1 kg). Both laser beams propagate between the mirrors of the cell and are absorbed in situ by ambient methane and water vapor over a 56-m pathlength. In situ absorption spectra for both species are recorded simultaneously at 1-s intervals with 1-mm-diameter InGaAs photodiodes (from Epi-taxx, Inc., and Sensors Unlimited, Inc.) at the output of
the optical cell. The detection signals are then sampled with a 16-bit digitizer (ALPHI, Inc.) and are stored on a flash disk with a storage capacity of 300 Mbyte (M-System, Inc.) located in the data acquisition and control command module on the upper part of the gondola (see Fig. 1).
The instrument is fitted with telemetry–telecommand equipment at 1.5 GHz provided by CNES. The range of the telemetry and telecommand is about 500 km. The data are downlinked with various instrument house-keeping signals and are displayed in real time once per second in the ground station during the complete flight. Indeed, the spectra are processed after the flight to re-trieve the mixing ratios. In situ pressure and temperature are also taken synchronously with the spectra acquisi-tion once per second with dedicated sensors (purchased from Paroscientific, Inc., and Viz Manufacturing Com-pany) installed in the gondola providing an accuracy in the P and T measurements of ;0.01 hPa and ;0.18C in the altitude region from ground levels to 30 km. Information on the gondola position is obtained from two global positioning systems. The data handling, the control and command of the SDLA, and the dialog with the telemetry–telecommand equipment are ensured by three central processing units (Motorola 68K) installed in the data acquisition and control command module.
The gondola structure is made of aluminum tubes with diameters ranging from 1 to 2 cm that are combined to maintain the optical cell and the various modules. A flight certification is needed from the CNES to permit the instrument to be flown, which involved a finite-elements modeling of the gondola. Therefore the struc-ture was computed to ensure total security, particularly at flight termination as the parachutes are opened to control the descent of the instrument; the structure can resist 10g vertical accelerations, where g is the gravi-tational acceleration. The metallic tubular structure also plays the role of shock absorber at landing and is con-sequently renewed for each flight. The mass of the com-plete aluminum tubular structure is of about 15 kg. The overall design of the gondola was chiefly driven by the objective of measuring water vapor in the lower strato-sphere; indeed, the very weak amounts of water vapor involved in that altitude region (;5 ppm at 10 hPa) make the measurement very sensitive to any corruption due to the water vapor outgassed from the instrument. Several precautions were taken, particularly in selecting the materials used with the gondola and regarding the disposition of the different modules, as follows. 1) The complete gondola structure is based on
alumi-num tubes; no other materials are used in the vicinity of the optical cell open to the atmosphere.
2) The optical cell is located;1 m below the main part of the spectrometer, which was made possible by means of optical fibers to ensure a convenient light connection.
3) In a previous paper discussing preliminary results
(Durry and Megie 2000), we reported tropospheric water vapor being trapped over a few centimeters in the mechanical mounting of the optics used to couple the optical fibers with the optical cell. The resultant interfering absorption (1000–2000 ppm) was on the order of that due to stratospheric water vapor (;5 ppm over a distance of 5600 cm). Appropriate me-chanical upgradings were undertaken to prevent such artifacts all along the optical path followed by the H2O laser beam.
4) The stratospheric water vapor concentrations are ob-tained from the spectra recorded during the descent of the instrument in the lower stratosphere to avoid corruption of the measurements by the water vapor outgassed from the balloon envelope.
5) The SDLA balloon flights took place at night to avoid amplification of water vapor outgassing pro-cess from the instrument due to sun radiation. For the ability to measure stratospheric water vapor at daylight, a dedicated test flight would be required here, because it is indeed very difficult to evaluate the contribution of solar radiation to the outgassing. The gondola is about ;3 m high because of the con-siderations exposed above. The overall mass of the gon-dola is of about 100 kg, including the lithium battery packs that offer a 6-h flight capability for a total power consumption of 100 W and the telemetry–telecommand equipment (30 kg). The SDLA can also work as a lab-oratory instrument by adapting the optical multipass cell in a metallic cylindrical envelope coupled with a vac-uum pumping system and fitted with appropriate MKS Instruments, Inc., Baratron gauges and thermistors to test the validity of the retrieval algorithm on known mixtures of gases with the same instrumental apparatus function as the balloonborne version.
3. Concentration retrieval and results
To monitor methane (;1 ppm) in the UT–LS or water vapor (;5 ppm) in the LS, a sensitivity of within 1022
to 1024 is needed, assuming an absorption pathlength
of about 56 m. To reach that sensitivity, we use direct-differential absorption spectroscopy. The detection prin-ciple is described in Fig. 2. Indeed, contrary to deriv-ative or high-frequency modulation spectroscopy (Hen-ningsen and Simonsen 2000), the direct-differential de-tection gives direct access to the full molecular line shape without any distorting filtering or derivative ef-fects in the molecular profile. Therefore, the modeling of the detected signal needed for the concentration re-trieval is drastically eased. Furthermore, when the full molecular line shape is reconstructed with a nonlinear least squares fit in combination with in situ pressure and temperature measurements, it is possible to monitor at-mospheric species over many orders of magnitude in concentration, pressure, and temperature. Moreover, the determination of the baseline (the zero absorption level)
FIG. 2. Detection principle of the SDLA tunable diode laser spectrometer.
in the spectra, which remains a major source of uncer-tainty in the mixing ratio retrieval, can be obtained pre-cisely (Durry and Megie 1999). The needed sensitivity of 1024is reachable with a very simple and inexpensive
detection electronics, removing the need for complex high-frequency laser modulations that are potential sources of electromagnetic interferences in the field in-strument. To that purpose, a balanced dual-beam detec-tor was designed with two highly linear InGaAs pho-todiodes to provide simultaneously the following. 1) A direct absorption spectrum AoT(s) is obtained by
recording the laser signal at the output of the optical
cell. It is used, for instance, to process water vapor (in the UT) in the 1021 absorption range.
2) A differential absorption spectrum AoT(s) 2 Bois
obtained by taking the balanced analogical difference between the signals recorded at the input Bo and
output AoT(s) of the optical cell. It is used to process
methane (in UT and LS) and water vapor (in the LS) in the 1022to 1024 absorption range.
3) A reference spectrum is recorded synchronously with the atmospheric spectra from an 8-cm closed cell filled with 40 torr of CH4(5 torr of H2O). The
ref-erence cells are located in the laser modules and are fed with 20% of the laser beam taken with a fused
fiber coupler (Figs. 1 and 2). The stability of the water vapor contained in the closed cell is not a problem, because the temperature inside the modules is maintained above 108C with resistor heaters, as mentioned before, to ensure a proper laser stability. The temperature of the reference closed cells is also monitored by means of dedicated thermistors once per second. The obtained reference methane and wa-ter vapor spectra are used to carry out the wavelength calibration of the atmospheric spectra. The reference spectra are also used to check the laser emission during flight and to ensure that there are no incidental spectral drifts in the laser emission.
The differential detection offers higher sensitivity than direct detection because it permits use of the full dy-namic range of the measurements (16 digits) to the ab-sorption information by removing the sloping back-ground in the spectra due to the laser amplitude mod-ulation. Indeed, to provoke the wavelength scanning of the laser, its driving current is ramped, which also pro-duces an amplitude modulation that results in a sloping background with weak absorption information lying on top of it. Taking the balanced analogical difference be-tween reference Boand signal AoT(s) permits removal
of that sloping background and application of the full dynamic range of the measurements to absorption in-formation, drastically reducing the effects of numerical noise. The performances and the complete analogical design of the shot-noise limited dual-beam detector are found in Durry et al. (2000). The direct absorption tech-nique is well adapted to measure the strong absorption signals (of a few tens of percent) obtained when probing the large amounts of water vapor in the troposphere (of a few hundreds of parts per million). Differential de-tection takes over in the stratosphere to monitor the few parts per million of water vapor lying in that region of the atmosphere. Nevertheless, in the troposphere to avoid saturation (100% of the laser beam is absorbed) and overlapping between close H2O transitions due to
collisional broadening at tropospheric pressures (that makes false the zero-absorption-level determination), we selected in the laser tunability range appropriate H2O
molecular transitions with line strengths 1 or 2 orders of magnitude weaker than the transition at 7181.17 cm21
used in the stratosphere (see Durry and Megie 2000 for a more comprehensive discussion of the H2O
spectros-copy). Thereby, combining both direct and differential detections in conjunction with an appropriate selection of the H2O molecular transitions, we can monitor in situ
water vapor continuously in UT–LS over a concentra-tion range of 4 orders of magnitude with a constant accuracy. In practice, the decision to change the H2O
transitions swept by the laser is made by observing the in situ absorption spectra during the ascent of the bal-loon in the troposphere and by control of the laser work-ing temperature. For methane, the differential detection
is used in troposphere and stratosphere to record the absorption signals in the 1023-to-1024absorption range.
The spectra are recorded within 10 ms by ramping the laser diode driving current. Forty successive ele-mentary spectra are coadded to improve signal-to-noise ratio. The total measurement time is 400 ms. A spectrum consists of 520 sampling points taken with a 16-bit dig-itizer. The technique used to retrieve the mixing ratio is standard in absorption spectroscopy. It includes a first step of determining the spectral range swept by the laser by fitting of the reference spectrum. Once the wave-length calibration is done, the atmospheric mixing ratio is extracted from the in situ direct or differential spectra by fitting of the full molecular transmission T(s) in combination with the in situ pressure and temperature measurements and by means of Eq. (1). The molecular profile is obtained from a Voigt model computed with the standard Humlicek approximation. A Levenberg– Marquardt algorithm was implemented on a Pentium personal computer by means of the proprietary Matlab software; it takes a few seconds to perform a nonlinear fit and to retrieve the mixing ratio. The complete set of data is processed after the flight with automatic retrieval programs; indeed, assuming a 3–4-h flight and a tem-poral resolution of 1 s, about 10 000 absorption spectra are obtained for each species at ascent and descent of the gondola. However, before starting the iterative tech-nique, we make sure that in the absorption spectra there are no unexpected artifacts (Fabry–Perot fringes, dis-tortions as a result of misalignments of the cell, etc). Furthermore, to ensure that there are no unexpected changes in the laser emission properties during flight, we compare the residuals obtained by fitting the suc-cessive reference spectra. The detailed retrieval tech-nique for direct-differential spectroscopy is given in Durry and Megie (1999). Figure 3 shows CH4and H2O
in situ absorption spectra taken with the SDLA in UT– LS during recent balloon flights in southern France in 1999 and 2000.
Fitting the molecular line shape involves many com-mon procedures that contribute to the overall precision error: estimation of the baseline, the zero-absorption level (1%–2%), wavelength calibration and correction of the laser nonlinearities (0.5%), detector dark current subtraction, accuracy on the electronic gains, and achieved precision in monitoring the in situ pressures and temperatures (1%). Furthermore, achieved noise in the in situ spectra is within 1025, expressed in
absorp-tion units (See Fig. 3c); therefore the contribuabsorp-tion of the signal-to-noise ratio to the inaccuracy in the mixing ratio retrieval is less than 1%. The molecular parameters were revisited in the laboratory to ensure a contribution of within about 1% to the accuracy and were found to be in good agreement with the HITRAN database, but for the very weak H2O transitions used to probe water
vapor in the troposphere. The H2O molecular parameters
used in this paper to process water vapor in the tro-posphere are found in Durry and Megie (2000). The
FIG. 3. Atmospheric CH4and H2O spectra taken with the SDLA in the troposphere and in the
lower stratosphere. (a) H2O absorption spectrum in the troposphere obtained from the flight of
the SDLA in Gap on 20 Jun 2000. The mixing ratio was retrieved by fitting the direct spectrum in conjunction with the in situ pressure and temperature measurements and by means of the molecular parameters given in Durry and Megie (2000). The calculated line shape is also displayed that was reconstructed with a Voigt modeling. (b) H2O and (c) CH4in situ absorption spectra
yielded by the SDLA in the lower stratosphere during a balloon flight in southern France (Aire sur l’Adour) on 10 May 1999. To illustrate the noise achieved in the spectrum, the spectral region between 6046.7 and 6046.8 cm21was expanded and displayed in the right corner of (c). The
noise is 1025, expressed in absorption units. The mixing ratios were obtained from a nonlinear
fit to the differential spectra in conjunction with the in situ pressure and temperature measurements and by means of the HITRAN database. Spectra are recorded in situ within 403 10 ms.
HITRAN database was used in this paper to process methane and stratospheric water vapor. Taking into ac-count the achieved signal-to-noise ratio in the spectra and all other sources of errors due to the retrieval pro-cess, mixing ratios are determined within 5%–10%
pre-cision error. Indeed, the achieved signal-to-noise ratio in the absorption spectra makes it possible to increase the temporal resolution from 1 s to 10 ms and with a fairly constant accuracy in the concentration retrieval; it could be of interest to address new topics in dynamics
FIG. 4. Methane vertical concentration profiles obtained by the SDLA during two recent flights at northern latitude (in the polar Arctic vortex from Kiruna) and at midlatitude within the framework of the THESEO campaign. For the flight at midlatitude, profiles obtained at both ascent and descent of the gondola are displayed. For the sake of clarity, a constant offset was introduced in the measurements made during the descent. See text for more details.
of the atmosphere (turbulence or measurement of fluxes; see Zahniser et al. 1995).
The SDLA was flown four times in 1998–2000 from a 35 000-m3(hydrogen) stratospheric balloon operated
by CNES from Kiruna, Sweden, and Aire-sur-l’Adour and Gap in France. A typical duration of a flight is 3– 4 h. The float altitude is approximately 30 km. The velocity of the balloon at ascent and descent is a few meters per second. The balloon is equipped with a valve to control the descent by adapting the hydrogen capac-ity. Once the flight is terminated, the gondola is recov-ered from a descent under parachutes. The recovery of the gondola is ensured by CNES. For the flights of the SDLA, the gondola was recovered within one day at a distance of a few hundred kilometers from the launching site and showed limited damage. The SDLA is capable of flying several times during a campaign. After a flight, the change of the metallic tubular structure of the gon-dola and its fitting with new thermal protections (Mylar sheets and polyurethane) takes a few hours. New bat-teries are connected in two hours. Then, both mirrors of the optical multipass cell are replaced with the spare ones, which is done within one hour. For each flight, we use mirrors with a new coating to get the highest reflection coefficients and thereby avoid incidental loss of laser energy. The cost of the refurbishing is roughly $10,000, the most expensive part being the metallic tu-bular structure (roughly $6000) because of the complex design of the shock absorbers. Nevertheless, even if the time required for refurbishing between flights is
ap-proximately one day, there are still many flight proce-dures to follow before being allowed by CNES to fly again, which delays the launching opportunities. In es-sence, the telemetry–telecommand system must be thor-oughly tested and the mechanical structure of the gon-dola is also carefully checked. The mass of the gongon-dola must be determined precisely to prepare the flight chain. The last determining factors consist of the meteorolog-ical conditions. The rain or a ground wind speed above 5 m s21prevent the launching of a medium-size balloon.
The tropospheric and stratospheric winds must also comply with the desired trajectory to prevent a landing of the gondola in a region where it could be dangerous (urban zones, roads, mountains, lakes, etc.). The pre-dicted ground wind speed on the land site is also of high importance; a strong wind can cause the gondola to be dragged on the ground by the parachutes and se-verely damaged. Taking into account the time needed for the recovery of the gondola, the time required for the refurbishing of the instrument, the various flight procedures, and the meteorological constraints, the in-strument can be flown at most 2 times per week.
For each flight, we started the measurements at ground levels and kept the acquisition running during both ascent and descent. On average, 10 000 in situ spectra were recorded at 1-s intervals for both species during ascent and descent. Nevertheless, as mentioned before, stratospheric water vapor is obtained chiefly by processing the spectra that were taken during the descent of the balloon in the lower stratosphere to avoid
cor-FIG. 5. Water vapor and methane vertical concentration profiles obtained simultaneously with the SDLA during a flight at midlatitude from southern France on 20 Jun 2000. The water vapor measurements yielded by a balloonborne meteorological P, T, U (humidity)-sonde are also displayed to confirm the H2O measurements made in the troposphere with the SDLA. See text for more
details.
ruption of the measurement by the water vapor out-gassed from the balloon envelope. Figure 4 shows a methane vertical concentration profile obtained from the flight of the SDLA during the Arctic winter in 1999 from Kiruna; it consists of about 3500 measurement points taken during the ascent of the instrument in UT– LS. On the same figure, the methane vertical profile obtained a few months later from a balloon flight at midlatitudes is represented. About 3300 in situ absorp-tion spectra were processed from the measurement made at ascent and about 4000 from the descent. The simul-taneous CH4and H2O concentration profiles achieved
during a recent midlatitude flight in June of 2000 from Gap is shown in Fig. 5. About 1500 in situ spectra taken during the ascent of the SDLA in the troposphere and 2500 during descent in the lower stratosphere have been processed to yield the water vapor profile. The methane profile consists of about 2800 measurements. Several spatial structures are apparent in the profiles that we are currently trying to relate to filamentary structures from planetary Rossby-wave breaking in the late Arctic vor-tex; this process is suspected to contribute to ozone depletion in the Northern Hemisphere midlatitudes by permitting transport of chemically perturbated air from within the polar vortex to midlatitudes (Waugh et al. 1994; Tuck et al. 1992).
4. Conclusions
We described the development of a balloonborne near-infrared diode laser spectrometer and reported the
first set of methane and water vapor simultaneous in situ measurements achieved in the upper troposphere and lower stratosphere, within the framework of the THESEO campaign. The operational capability of near-infrared distributed-feedback InGaAs telecommunica-tion diode lasers in combinatelecommunica-tion with optical fibers was demonstrated for the in situ sensing of methane and water vapor from stratospheric balloon platforms. Ob-tained time resolution (1 s), accuracy (within 5%–10%), and sensitivity (in the 1021 to 1024 absorption range)
fully meet the assigned science objectives. Nevertheless, to reinforce the scientific interpretation, a light-weight remote CH4–H2O sensor capable of flying at many times
from various platforms such as pilotless and strato-spheric aircraft or balloons is now needed, which we plan to realize within the two coming years by taking advantage of the experience gathered during the SDLA project.
Acknowledgments. The authors thank Professor G.
Megie who has been very helpful in setting up the SDLA project. Furthermore, several members of the technical department of CNRS (Institut National des Sciences de l’Univers—Division Technique) and of the Service d’Ae´ronomie were strongly involved in the design and field operations of the SDLA and are thanked for their assistence: A. Abchiche, N. Amarouche, B. Brient, T. Danguy, H. Poncet, J. C. Samake´, P. Schibler, and F. Semelin. The work described in this paper has been
supported by CNES, CNRS, and the European Com-mission.
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