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  • SPIRALE Experiment / Preliminary results of the Balloon Flight : 02 Oct. 2002

    G. Moreau(1), C. Robert(1), F. Goffinon(1), M. Pirre(1), C. Camy-Peyret(2)

    (1)LPCE/CNRS 3A Avenue de la Recherche Scientifique 45071 Orléans cedex 02 (F),Email : [email protected] (2)LPMA Univ. P & M Curie 4 Pl. Jussieu 75252 PARIS 05 (F), Email : [email protected]

    ABSTRACT

    The balloon-borne SPIRALE instrument contributes to the validation of ENVISAT, through in situ measurements. Six diode lasers allow the absorption of up to 13 species, including several species involved in the validation, namely : O3, NO2, NO, N2O, HNO3, CO, CH4, HCl. The paper reports the vertical profiles of four species (NO2, N2O, CH4, HCl), as preliminary results of the flight which took place in Aire Sur L’Adour on October 2nd 2002.

    1 INTRODUCTION

    The present flight devoted to ENVISAT validation is the third flight of SPIRALE gondola. SPIRALE has now reached its completion, that was not the case in the two first flights where only three diode laser were installed instead of six now on. The description of the instrument is made in [1], with the detection technique based on tunable diode laser absorption spectroscopy in the infrared. The first flights helped improving the instrument. Interesting scientific results have been obtained yet, with the vertical profiles of O3, CO, NO2, HNO3, N2O, COF2 [4] [5]. A long absorption length, of either 300 m, or 554 m permits the detection of very low concentrations. In addition to the species listed above, H2O2 and HOCl were detected above 25 km, where the spectral signature is clear, i. e. above the absorption limit of ~10-5. The first flights have demonstrated the valuable concept of the instrument : low concentration measurements (> 30 pptv), fast monitoring (1-s intervals), low pollution by the gondola thanks to the open absorption zone. The scientific objectives assigned to SPIRALE, through high rate, in-situ and simultaneous monitoring of several species, are the photo-chemistry and small scale dynamics.

    The programme of SPIRALE flight during ENVISAT validation campaign at Aire Sur l’Adour, aims at measuring the species : CO, O3, CO2, HCl, N2O, CH4, COF2, HOCl, H2O2, NO, H2O, NO2, HNO3. During the day of flight, (daytime 02 Oct. 02), the flight matched poorly with SCHIAMACHY, fairly well with MIPAS and none with GOMOS (due to the daytime). Unfortunately, MIPAS stopped working in the very beginning of the flight. Nevertheless, since there is no fine matching with any of the instruments on board ENVISAT, back trajectory calculations will be carried out, in order to retrieve the air mass visited by both SPIRALE and ENVISAT instruments.

    Lacking of time up to now to process all flight data, we present here striking results with the profiles of NO2, N2O, CH4, and HCl.

    2 SPIRALE INSTRUMENT

    SPIRALE has been designed to be a multi species instrument by itself, and to fit with several main characteristics or constraints : 1- TDLAS is implemented, 2 – Absorption signals are increased by a multipass absorption cell making possible detection of species whose concentration is as low as a few parts per trillion in volume (or pptv), 3 – The weight is limited to 500 kg, so that up to 6 diode lasers can be installed on the gondola, 4 - The spectral wavelength range extends from 3 µm (the manufacturing lower limit) to 10 µm (limited by the temperature of the liquid nitrogen cooler), 5 – The absorption zone is not affected by contamination by gondola degassing. About 40 species are detectable, when accounting for the upper specifications.

    Having a look on SPIRALE in the two pictures of Fig. 1. The 6 laser beams circulate in a multipass cell located below the gondola (picture on the right). The lower mirror of the two-mirror HERRIOTT cell is fixed at the top of the deployable mast. The distance between mirror is about 3.50 m. Given the curvature of the two identical mirrors (5.375 m), two stable optical configuration can be used : first 86 reflections and 300 m optical path, second 156 reflections and 554 m optical path, by moving the lower mirror 5 mm up. In the picture on the left, the gondola is ready to be launched

    __________________________________________________________________________________________________________ Proc. of Envisat Validation Workshop, Frascati, Italy, 9 – 13 December 2002 (ESA SP-531, August 2003)

  • from Aire Sur L’Adour, the lower mirror lifted up in a safety position for the rather fragile mirrors (350 mm diameter). The mast is deployed during the flight to have the first measurements at the tropopause. Around the instrument, a rigid metal frame encompasses it, in order to have a instrument-safe landing. The white thermal insulation on the instrument maintains the inner temperature not too low so that the mirrors keep their adjustments (performed in the previous days on the ground near 20 °C) all flight long. Inside the instrument, three liquid nitrogen cryostats, hold the six diode lasers and the 12 detectors. Each laser is split in two beams : 1 - measurement beam travelling inside the multipass cell, 2 – reference beam passing through a reference cell containing species able to provide reference absorption lines in the microspectra generated by the lasers. The measurement signals are converted into 16 bits numerical data. It enables the measurement of several 10-5 absorption lines from one second spectrum, but consecutive spectra averaging is necessary to reach 10-5 absorption, considered as the detection limit of the instrument. This limit is not so easy to attain, because the signal must be exempt of fringes or signal fluctuations whose spectra coincide with absorption lines. Having concentration retrieval every second provides vertical sampling of a few meters, whereas with averaging, the sampling is larger.

    Concentration retrieval is achieved first by the usual method [3]. This method consists in reconstructing the signal without the absorption lines. An alternative and more reliable method arises then by using a sort of second derivative of the signal, like the well known frequency modulation [2]. The main advantage of the latter is that the signal has minima and maxima, whereas in the former, there is only one minimum. Quantitatively, the latter is more precise since normalisation process depends on the signal amplitude, opposed to the former where the amplitude of the signal depends on the reconstructed signal without absorption lines, which can be affected of severe bias. Fig. 2 presents graphically both methods.

    Fig. 1 : Pictures of SPIRALE. On the left, just before taking off on Oct. 2nd 02 6:41 UT. On the right, the mast supporting the lower mirror or the multipass cell is deployed under the gondola, 3. 5 m beneath.

    3 OBJECTIVES OF THE VALIDATION

    Table 1 presents the species expected to be measured by ENVISAT and SPIRALE. The lists deals with agreement achieved several years ago. It may be subject to corrections due the present status of ENVISAT instruments.

  • Fig. 2 : Example of concentration retrieval process with a microspectrum exhibiting absorption lines of O3 and CO. Upper : absorption signal (black), and reconstructed signal without absorption (blue). Middle : Absorption curve,

    experimental (dashed black), synthetic spectrum (red), residue (blue). Lower : 2nd derivative, same meanings as for Middle, Geophysical data plus retrieved concentrations. (The abscissa is the linearized frequency with sampling every

    5.03 MHz)

    Table 1 : Species measured by SPIRALE during the flight, in accordance with species provided by ENVISAT instruments.

    Species MIPAS SCHIAMA- -CHY

    GOMOS SPIRALE Accuracy

    O3 x x x x 3 % H2O x x x x 5 % CH4 x x x 3 % N2O x x x 3 %

    HNO3 x x 5 % CO x x 3 % NO2 x x x 5 % NO3 x x

    OClO x BrO x ClO x

    H2CO x NO x x 5 – 10 % O2 x

    SO2 x CO2 x x 3 % O4 x

    Aerosols x x HCl x x 5 %

  • 4 RESULTS

    The SPIRALE flight took place daytime, mostly in the morning Oct. 2nd. In fig. 3 to fig. 5 are presented profiles of several species, either in ascent or in descent.

    Fig. 3 : Vertical profile of NO2 obtained during ascent. Temperature is also displayed.

    Fig. 4 : Vertical profiles of nitrous oxide and methane obtained during ascent. Let us note the strong correlation between the two species.

    N2O

    CH4

    NO2

  • Fig. 6 : Vertical profile of HCl obtained during descent. Temperature is also displayed.

    5 CONCLUSION

    This paper presents a part of the profiles obtained with SPIRALE in the frame of ENVISAT validation campaign from Aire Sur L’Adour (Oct. 2nd 2002 flight). Since the match between SPIRALE and ENVISAT instruments is not good, validation will have to be achieved with the help of trajectory calculations.

    6 REFERENCES

    1. Moreau, G., et al., 1997, A new balloon-borne instrument for in situ measurements of stratospheric trace species using infrared laser diodes, ESA SP-397, 421-426. 2. Moreau, G., et al., 1998, Development of tunable diode laser instrument on balloon. Preliminary laboratory studies, VDI Berichte 1366, 121-124. 5th international symposium of gas analyses by tunable diode lasers, Freiburg, 25-26 Feb. 1998. 3. Henry, A., et al., 1998, Analysis of line profiles taking into account the intensity distribution within a TDL emission mode and precise concentration measurements of atmospheric air samples, VDI Berichte 1366, 223-232. 5th international symposium of gas analyses by tunable diode lasers, Freiburg, 25-26 Feb. 1998. 4. Moreau, G., et al, 2001, Results and goals of SPIRALE after the first flight from GAP in June 1999, ESA SP-4

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