SPIRALE Experiment / Preliminary results of theBalloon 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 : gmoreau@cnrs-orleans.fr(2)LPMA Univ. P & M Curie 4 Pl. Jussieu 75252 PARIS 05 (F), Email : camy@ccr.jussieu.fr

ABSTRACT

The balloon-borne SPIRALE instrument contributes to the validation of ENVISAT, through in situ measurements. Sixdiode 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), aspreliminary 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 reachedits completion, that was not the case in the two first flights where only three diode laser were installed instead of sixnow on. The description of the instrument is made in [1], with the detection technique based on tunable diode laserabsorption spectroscopy in the infrared. The first flights helped improving the instrument. Interesting scientific resultshave 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, H2O2and 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 (> 30pptv), fast monitoring (1-s intervals), low pollution by the gondola thanks to the open absorption zone. The scientificobjectives assigned to SPIRALE, through high rate, in-situ and simultaneous monitoring of several species, are thephoto-chemistry and small scale dynamics.

The programme of SPIRALE flight during ENVISAT validation campaign at Aire Sur l’Adour, aims at measuring thespecies : CO, O3, CO2, HCl, N2O, CH4, COF2, HOCl, H2O2, NO, H2O, NO2, HNO3. During the day of flight, (daytime02 Oct. 02), the flight matched poorly with SCHIAMACHY, fairly well with MIPAS and none with GOMOS (due tothe daytime). Unfortunately, MIPAS stopped working in the very beginning of the flight. Nevertheless, since there is nofine matching with any of the instruments on board ENVISAT, back trajectory calculations will be carried out, in orderto 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 orconstraints : 1- TDLAS is implemented, 2 – Absorption signals are increased by a multipass absorption cell makingpossible detection of species whose concentration is as low as a few parts per trillion in volume (or pptv), 3 – Theweight is limited to 500 kg, so that up to 6 diode lasers can be installed on the gondola, 4 - The spectral wavelengthrange extends from 3 µm (the manufacturing lower limit) to 10 µm (limited by the temperature of the liquid nitrogencooler), 5 – The absorption zone is not affected by contamination by gondola degassing. About 40 species aredetectable, 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 belowthe gondola (picture on the right). The lower mirror of the two-mirror HERRIOTT cell is fixed at the top of thedeployable mast. The distance between mirror is about 3.50 m. Given the curvature of the two identical mirrors (5.375m), two stable optical configuration can be used : first 86 reflections and 300 m optical path, second 156 reflections and554 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 rigidmetal frame encompasses it, in order to have a instrument-safe landing. The white thermal insulation on the instrumentmaintains the inner temperature not too low so that the mirrors keep their adjustments (performed in the previous dayson the ground near 20 °C) all flight long. Inside the instrument, three liquid nitrogen cryostats, hold the six diode lasersand 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 themicrospectra generated by the lasers. The measurement signals are converted into 16 bits numerical data. It enables themeasurement of several 10-5 absorption lines from one second spectrum, but consecutive spectra averaging is necessaryto reach 10-5 absorption, considered as the detection limit of the instrument. This limit is not so easy to attain, becausethe signal must be exempt of fringes or signal fluctuations whose spectra coincide with absorption lines. Havingconcentration retrieval every second provides vertical sampling of a few meters, whereas with averaging, the samplingis larger.

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

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 xBrO xClO x

H2CO xNO x x 5 – 10 %O2 x

SO2 xCO2 x x 3 %O4 x

Aerosols x xHCl 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 ofseveral 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 betweenthe 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 fromAire 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 usinginfrared laser diodes, ESA SP-397, 421-426.2. Moreau, G., et al., 1998, Development of tunable diode laser instrument on balloon. Preliminary laboratory studies, VDIBerichte 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 modeand precise concentration measurements of atmospheric air samples, VDI Berichte 1366, 223-232. 5th internationalsymposium 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-471 pp 309-314. 15th ESA Symposium on European Rocket and Balloon Programmes and Related Research, 28-31 May 2001.5. Moreau, G., et al, 2000, Correlation of ozone versus CO as measured in the first flight of SPIRALE, Proc. of theEuropean Symp. On Stratospheric Ozone Published by EC, pp 546-549.

HCl