VALIDATION OF SCIAMACHY WATER VAPOR AND METHANE PROFILES BY BALLON-BORNE IN-SITU MEASUREMENTS WITH THE CHILD SPECTROMETER ONBOARD TRIPLE

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1 VALIDATION OF SCIAMACHY WATER VAPOR AND METHANE PROFILES BY BALLON-BORNE IN-SITU MEASUREMENTS WITH THE CHILD SPECTROMETER ONBOARD TRIPLE Wolfgang Gurlit, Konstantin Gerilowski (*), Carsten Giesemann, Volker Ebert (**), Rainer Zimmermann (***), and John P. Burrows (*) (*) Institute of Environmental Physics, University of Bremen, Otto-Hahn-Alle 1, 801 Bremen (**) Institute of Physical Chemistry, University of Heidelberg, Im Neuenheimer Feld 53, 6910 Heidelberg (***) Institute of Environmental Physics, University of Heidelberg, Im Neuenheimer Feld 9, 6910 Heidelberg ABSTRACT The lightweight near-infrared tunable diode laser spectrometer CHILD (Compact High-altitude In-situ Laser Diode spectrometer) was flown piggyback on the TRIPLE gondola for a SCIAMACHY validation flight during the ASA fall 00 Envisat Validation Campaign to measure stratospheric CH 4 and H O profiles. It incorporates a novel design of a compact optical head and an open-path multi-pass cell, with two individual absorptions path lengths optimised for CH 4 with 74 m (136-pass) and H O with 36 m (66-pass) absorption length. On this flight, in situ spectra were recorded during all phases (ascent, float and descent) from ground level to 3,000 m altitude. Flight parameters were optimized to closely match an ENVISAT overpass. Like all other instruments of TRIPLE, the CHILD spectrometer suffered mechanical damage during the landing, when the gondola hit an electricity pole (concrete) and then was torn across a crop field by its parachutes due to relatively strong ground winds. 1. GENERAL Water vapor and methane are two of the most important greenhouse gases in the atmosphere. Both species are linked together by the fact that the oxidization of methane in the stratosphere is the major source of stratospheric water. However, the ppmv increase of stratospheric water since the 1950s can only partly be explained by the increase of tropospheric methane (0.55 ppmv) during the same period. Due to the coupling of methane and water in the stratosphere, it is an important task to monitor both species simultaneously with a high temporal resolution. in situ measurements of relevant atmospheric trace gases by aircraft or balloon missions are considered extremely important for the validation of the ENVISAT (in this case especially SCIAMACHY) level- products. The general idea behind the CHILD spectrometer was to have a small, automated light-weight instrument, that can be used piggyback on any mission that provides adequate flight trajectories. CHILD is explicitely designed to use existing platforms and to sharply reduce the costs and increase the frequency of highly resolved profile data acquisition in the UTLS region. For this reason, a lot of emphasis was put into a very low mass construction and reliable automation features.. METHOD USED Tunable diode laser absorption spectroscopy (TDLAS) in the near infrared (NIR) spectral range is a well known and accepted method for stratospheric in-situ humidity and tracer measurements. The instrument described here uses two independently tunable diode lasers that emit around 1393 nm and 1648 nm to detect water and methane respectively. The measurement principle of absorption spectroscopy is based upon a spectrally resolved measurement of the losses of the laser beam propagating through the measurement volume. These losses are recovered by wavelength tuning of the laser and are described by Beer`s law I(λ)=I 0 (λ) exp(-s(t) g(λ-λ 0 ) N L) = I 0 (λ) exp(-α m) Proc. of Envisat Validation Workshop, Frascati, Italy, 9 13 December 00 (ESA SP-531, August 003)

2 where I 0 (λ) is the initial laser intensity, I(λ) is the measured intensity after passage of an absorbing medium of thickness L and a number density N of absorbers. The molecular absorption coefficient is characterized by S(T), the temperature dependent line strength, and a function g(λ,λ 0 ) centered at wavelength λ 0 describing the shape of an absorption line. The line profile is measured by a continuous scan of the laser wavelength across the absorption line. By varying the laser temperature, the emission wavelength can be coarsely tuned onto the center of the absorption line. A fast (Hz to khz) periodic modulation of the injection current (saw-tooth or triangle shape) allows a linear tuning across the line. All the properties necessary can be measured prior to the balloonborne data acquisition under controlled lab conditions, so that a direct absorption measurement is essentially calibration free. The availability of narrow bandwidth low-noise laser diodes has made possible minimum detectable absorptions (MDAs) around 10-5 in the field and 10-6 in the laboratory, while the theoretical MDA defined by the laser s shot noise is typically in the 10-8 range. 3. INSTRUMENT DESCRIPTION The CHILD instrument consists (see figure 1) of a dual-species open-path multi-pass Herriott-cell, an extremely compact flange mounted optical assembly and an electronic box, containing the laser controllers, power supply, batteries, current amplifiers and modulation electronics. The mass of the spectrometer optics (excluding electronic components) is only 6.6 kg. The overall dimensions of the optical unit are 5 cm in diameter and 75 cm in length. Figure 1: TDL-Spectrometer with electronic box The main part of the optical setup is the dual-species multi-pass Herriott-type absorption cell shown in figure 1 (with it s gold-coated mirrors covered for protection). It is open to the atmosphere to minimize adsorption effects of water, which can be a major source of uncertainty for airborne in-situ water sensors. In order to simplify the integration of the instrument into existing balloon payloads mass and size had to be minimized. The desired relatively small mirror spacing and the demand for a long absorption path resulted in a high number of optical passes. By use of two independent coupling holes for the two

3 species the absorption path length is independently adapted to the relative magnitude of the expected absorptions for both species. This was achieved by designing a new Herriott cell that allows 136 passes for methane (resulting in 74 m path length) and 66 passes for water (36 m path length) at the same time. This concept eliminates the need for line switching and increases the dynamic range. The cell consists of two gold-coated spherical 5 inch Zerodur mirrors with identical focal lengths of 6 mm. A broad temperature range (30 C to -90 C) has to be expected during the flight. In order to avoid condensation of tropospheric water vapor during balloon launch the mirror temperatures can be controlled with separately attached heaters. As a consequence of the small spot distance of the 136-pass absorption path the performance depends critically on the exact spacing of the two Herriott mirrors. In case of a longitudinal misalignment of the cell the laser beam will not leave the multi-pass cell as calculated and recirculate between the mirrors. The total length tolerance over the full temperature range is below 100 µm. Mechanical design and choice of materials was optimized to get an overall compensation of thermal effects. An electronic auto-calibration and auto-balance feature was designed and implemented to suit the CHILD spectrometer for reliable automated operation. The goal is to maintain the sensitivity and calibration accuracy over time under the harsh conditions of stratospheric balloon measurements. The function of the auto-calibration is to electronically compensate the effects of optical contamination, drift and ageing of optical components and the associated changes in optical transmission of these components. The auto-balance feature ensures the best possible subtraction of background artifacts from the absorption signal. As an additional benefit, the instrument needs less attention and user interaction during a campaign, and can be operated by standard technical staff with reduced requirements for expert attention. 4. DATA ANALYSIS STRATEGY From a theoretical point of view, the relative absorption measured by the CHILD spectrometer can be described as a Voigt line shape with superimposed error contributions due to electronic background, b( ν), detector and electronic noise, n( ν), and interference fringing, f( ν): ν + ν 0 γ l a( ν ) = A0V, + ( ν ) + ( ν ) + ( ν ) γ γ b n f d d Here, A 0 is the integrated absorption, γ d the Doppler line width due to thermal broadening, γ l the Lorentz line width due to collisional broadening, and ν 0 is the offset of the line center position. The Voigt line shape function, V(x,y) is defined as: t = ln y e V ( x, y) dt with V ( x, y) dx = 1 + ( ). 3/ π y x t γ d 0 In the case of methane, the single Voigt line shape function V(x,y) has to be replaced by a superposition of three Voigt functions, as a group of three absorption lines ( cm -1 ) is used for the methane measurement. Interference fringes are caused by unwanted, é talon-like behaviour of optical windows, laser collimation lenses and the Herriott-cell. f( ν) is, theoretically, a superposition of sine-shaped functions of various periods and amplitudes, whose phase may vary strongly between individual spectra. It can be separated into a long-period part (period >> γ d ) and a shortperiod part (period << γ d ). In practice, the contributions of both b( ν) and the long-period part of f( ν) can be represented + by a polynomial background function. The overall short-period fringe amplitude was found to be The noise term n( ν) was found to be negligible in all our measurements. 4 short ( ν ) 10 f. The integral absorption A 0 can then be converted to a volume mixing ratio (VMR) by:

4 VMR = A0 kt, pl S where k is Boltzmann s constant, T the absolute air temperature, p the air pressure, l the absorption length and S the absorption cross-section. Possible sources of systematic errors in the calculated VMR are (1) the pressure measurement error, p/p 1% except for very small pressures; () the temperature measurement error, T K; (3) the amplification error of the subtractor circuit and ADC sensitivity error, both well below 1%; (4) the uncertainty of the absorption cross-section, S. Because of wavelength scaling errors, we must also consider (5) the calibration error of the tuning ramp signal, 1%, and (6) the uncertainty of the laser tuning coefficient, estimated to be %. The cross-sections S, for both CH4 and HO, are taken from the HITRAN database [1], and from [], with a specified errors of better than 5%. Therefore, the final accuracy error of our data is 6%, with S being the leading systematic error term. However, since S is a physical constant, more accurate determinations of S may improve the accuracy of our measurements in the future. 5. FLIGHT RESULTS The trajectory of our flight with TRIPLE on 4 September 00 is shown in fig.. Launch was in the morning of 4 September 00 at 07:00 UT to match a satellite overpass at 10:30 UT. Though measurements were taken during all phases of the flight during launch, ascent, float, and descent, only the data from the descent is shown in the profile in fig. 4. This phase of the flight was timed for the closest possible satellite match during descent at 30 hpa. This is due to the fact that insitu measurements can best be made during a controlled descent (typically -3 meters per second). Tropospherc contaminations mainly from the large surface area of the balloon envelope then have outgassed during the upper part of the ascent and during float. At float, stratospheric balloons with control valve like the one used here tend to perform vertical oscillations on the order of a few hundreds of meters altitude. During these oscillations, water vapour from the balloon s own boundary layer can contaminate the in-situ measurements. These effects were seen in our data (and also in the data of other in-situ instrumentation onboard TRIPLE) as isolated spikes. Fig. : flight trajectory (left) and SCIAMACHY overpass (right) for TRIPLE flight on from ASA. At 10:3, TRIPLE was on controlled descent around 30 hpa, where best measurents can be made.

5 10 [CH 4 ] Fig. 3 (left): Profiles of water vapour and methane recorded with the CHILD spectrometer on board TRIPLE during ENVISAT validation flight on 4 September 00 pressure level (hpa) 100 TP 194hPa 3.6 ppm [H O] [CH 4 ] + [H O] 7.9 ± 0.4 ppm volume mixing ratio (ppm) 6. CONCLUSION The lightweigt CHILD spectrometer was flown piggy-back as an additional sensor on the TRIPLE payload launched with a high-altitude balloon from Aire sour l Adour (France) on 4. September 00. Profiles of water vapor and methane were obtained by in-situ measurements. Flight parameters were chosen as to get a close match with an ENVISAT overpass for the validation of SCIAMACHY. The instrument operated nominal during all phases of the flight and recorded complete profiles of the two trace gases, that are now available to validate the corresponding measurements of SCIAMACHY at the time when they will be available. The CHILD instrument is planned to also take part in future validation missions of SCIAMACHY on board of Envisat within the framework of the ESABC (Envisat Stratospheric Aircraft and Balloon Campaigns). However, activities are offered on a best-effort basis, as to date still no official funding has been provided. Like all other instruments onboard the TRIPLE gondola, the CHILD spectrometer was damaged during a relatively hard landing after the validation flight. It will be back in service ontime for the next validation campaign, which is scheduled for march 003 from Kiruna. 7. ACKNOWLEDGEMENT The authors would like to acknowledge the support of Cornelius Schiller and his group at Forschungszentrum Jülich, and Andreas Engel from University of Frankfurt, Germany, who made possible this flight of our instument piggy-back on the TRIPLE-Gondola, and who continue to give us important support. The work described here was funded in part by the German Government (BMBF) under contract number 01 LA 9835.

6 8. REFERENCES 1) L. S. Rothman, C. P. Rinsland, A. Goldman, S. T. Massie, D. P. Edwards, J-M. Flaud, A. Perrin, C. Camy-Peyret, V. Dana, J.-Y. Mandin, J. Schroeder, A. McCann, R. R. Gamache, R. B. Wattson, K. Yoshino, K. V. Chance, K. W. Jucks, L. R. Brown, V. Nemtchinov and P. Varanasi: The Hitran Molecular Spectroscopic Database And Hawks (Hitran Atmospheric Workstation): 1996 Edition, Journal of Quantitative Spectroscopy and Radiative Transfer, 60, (1998) ) B. Parvitte, V. Zéniari, I. Pouchet, G. Durry, Diode laser spectroscopy of HO in the cm-1 range for atmospheric applications, Journal of Quantitative Spectroscopy and Radiative Transfer, 75, (00)