Preliminary calibration results of VEGA 1 and 2 SIGMA-3 gas chromatograph

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Adv. Space Res. Vol. 7, No. 12, pp. (12)329—(12)335, 1987 Printed in Great Britain. All rights reserved.

0273—1177187$O.OO + .50 Copyright © 1987 COSPAR

PRELIMINARY CALIBRATION RESULTS OF VEGA 1 AND 2 SIGMA-3 GAS CHROMATOGRAPH L. M. Mukhin,* D. F. Nenarokov,* N. V. Porschnev,* V. B. Bondarev,* B. G. Gelman,* G. Israèl,** F. Raulin,*** J. Runavotl and R. Thomasl Space Research Institute, Moscow, U.S.S.R. 55Service d’Aéronomie, CNRS, 91370 Verrières-le-Buisson, France * * * Université Paris Va! de Marne, 94000 Créteil, France *

tFrench Space Agency, CNES, 31000 Toulouse, France tLPCE, CNRS 45000 Orleans, France

ABSTRACT

SIGMA

—

3 gas chromatograph on board VEGA 1 and 2 landing probes has been opera-

ted successfully in the”i 60

—

50 km altitude range, providing several in

—

situ

chemical analysis of the gas and the aerosols of Venus cloud layers. Post flight calibration required to derive atmospheric abundancies from gas chromatograms were carried out using the SIGMA

—

3 spare model. A Venus atmospheric aerosol sirnu—

lation chamber was used in which sulfuric acid droplets were generated. Preliminary results of these calibration experiments indicate that the concentration of sulfuric acid in the upper part of the clouds (‘s.’ 60 to 55 kin) is about 1 mg/rn3 and suggest that an additional constituant must be present in noticeable amount in the aerosols. From these experiments the mixing ratio upper limits of SO 2 is 100 ppmV and of H2S and COS is few 10 ppmV.

INTRODUCTION

SIGMA

—

3 gas chromatograph was designed to analyze the atmosphere chemical com-

position and the sulfuric acid content in the cloud layers, from the Venus landing probe of VEGA 1 and 2 (1). During the descent of the probe, the instrument has been operated successfully and has provided several chrornatograms, in both gas and aerosol analysis modes. However quantitative interpretation of these chromatograms requires specific postflight calibration experiments, in particular in order to derive the abundance (or its upper limit) of the minor gaseous constituents and the concentration of sulfuric acid in the aerosols. These calibrations were carried out using the spare model, in cooperation with the soviet team responsible for the SIGMA

—

3 experiment and french scientists

involved in space instrumentation and gas—aerosol analysis. These calibrations

(12)329

(12)330

L. M. Mukhin

were performed in Orleans (France) for the gas—phase analysis and in Toulouse (France) for the aerosol analysis, using a simulation chamber designed to reproduce in laboratory sulfuric acid aerosols in a Venus—like environment. This paper presents the preliminary results of these calibration experiments and their implications for our knowledge of the chemical composition of the Venus atmosphere. Estimates of the abundance of minor constituents in the gas phase, and of sulfuric acid in the aerosols, in the upper part of Venus clouds (“‘60

—

55 km)

are given.

EXPERIMENTAL SIGMA

—

3 Gas chromatograph

Fig. 1 represents the block diagram of the instrument. Samples of Venus atmosphere

—

gas or/and aerosols

—

are pumped from the inlet through three sampling

cells, with the help of a fan.

In the “gas analysis” mode, the atmospheric sample, according to an automatic sequence of operations, is transfered by the carrier gas (N

2 or He) from the cell,

maintained at 80’ C, into the chromatographic columns. Three different columns, packed with Porapak QS

+

N and Porapak T (Waters Assoc.) are used to achieve the

chromatographic separation, with isothermal conditions (70°C). These columns are connected to different and complementary detectors : Helium ionization detector, thermal conductivity detector TCD (carrier gas

He), and electron captu-

re detector ECD (carrier gas : ultra pure N2). With such conditions, the instrument can quantitatively analyse H20 (with a sensitivity better than 100 ppm), C02, O2~ H2S, COS and SO2 (with a sensitivity better thant”#l

—

0.1 ppm).

The sampling cells contain a carbonized fiber—glass filter, designed to absorb collected aerosols. In the “pyrolytic” mode, after flushing with the carrier gas, the cell is heated up to 350°C. At this temperature, H2S04 absorbed on the carbonized filter is catalytically decomposed, according to the chemical reaction

C+2H2S04

)C02+2H20+2S02

with trace amounts of COS and H2S. Quantitative analysis of the pyrolysis products and in particular of SO2 obtained after pyrolysis allows to estimate the quantity of sulfuric acid deposited on the filter. Then, knowing the flow—rate of the fan and the time duration of aerosol sampling, one can calculate the concentration of sulfuric acid in Venus clouds. Calibration experiments will show that the limit of sensitivity of the instrument for quantitative analysis 3. of H2S04 in aerosols is less than 1 mg / in Venus Aerosols Simulation Chamber (EVE)

The chamber “EVE” was designed to simulate Venus atmosphere with its aerosol content. It has been built at the department of Studies and Researches in Space Technology (DERTS) of CERT—ONERA, in Toulouse (France), under CNES grant.

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diagram To the instrument under test

Fig. 2 VENUS AEROSOLS SIMULATION CHAMBER EVE

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SIGMA-3 Gas Chromatograph

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The block diagram of EVE is shown on fig. 2. The aerosol generator uses a pneumatic nebuliser generating sulfuric acid aerosols by bubbling a CO

2 (96.5%)

—

N2 (3.5%) gas mixture through concentrated H2S04. A timing system allows the regulation and flow adjustment of the mixture of gas and aerosols into the chamber, using electro—valves, pressure regulators and controllers.

The chamber is a 220 liters cylinder ; it is made in stainless steel, covered with PTFE (l0O~m). A pressure less than i~—~ torr can be obtained in the chamber, the temperature of which can be regulated within the range 10 to 80°C, with an accuracy of 0.5°C. Pressure can also be controlled, between 0 and 1 bar, with an accuracy of 5 rnbar. This chamber can deliver a controlled flux of sulfuric acid aerosols in CO2 N2 droplet gas mixture, with concentration 3, —and size distribution rangingranging from 1 from 50 5flm to 500inparticulates to diameter. The/ Hcm 2SO4 aerosol concentration during calibration test is controlled by collecting with a known flow—rate the aerosols on reference filters. GAS CALIBRATION STAND The main purpose of this gas calibration stand located at LPCE

—

CNES Orleans

was to deliver the gas mixture in a very well known concentration ratio at controlled pressure. The concentration range for all gases must be between 1 ppm and l0~ppm. The pressure range is 8

—

15 bars. The used gases are either noble gases (Ar,

Kr, Xe) or sulfured compounds such as CS2, COS... For these last species, the effects due to surface reactions (absorption or des— orption) have to be avoided. It was then necessary to increase the flow speed in all tubes in order to reduce the relative influence of such phenomena. The device uses electromagnetic valves monitored by mass flowmeters.

RESULTS AND DISCUSSION

Fig. 3a shows a typical gas chromatogram obtained in the “gas analysis” mode, from a sampling of Venus atmosphere at altitude varying from about 60 km down to 55 km. The only detectable compounds are CO2 and H2O. The absence of H2S, COS and SO2 peaks on this chromatogram shows that there is no contamination of the atmospheric sample coming from the carbonized filter present in the cell. In addition, it allows, from gas calibration performed in LPCE Orleans, to get upper limits for these non detected compounds. Post flight quantitative calibration of the instrument for gaseous constituents gives the following upper limits in Venus atmosphere at the altitude of sampling (60 10 ppmV, SO2 K 100 ppmV.

—

55 km)

COS and H2S K few

These values are consistent with the other reported esti-

mates of the abundance of these constituents in Venus atmosphere, at the cloud level (2). Fig. 3b shows the flight gas chromatogram obtained in “pyrolytic” mode, from sampling at the same altitude. It is clear that SO2, COS and H2S are only due to

L. M. Mukhin

(12)334

the pyrolysis of aerosols absorbed on the carbonized filter of the cell. Several questions arise from this chromatogram. First, what concentration of 11

2S04 in

Venus aerosols can be derived from the SO2 peak 7 Second, can the relative intensities of the H2S and COS GC peaks be explained only by the pyrolysis (on carbon) of H2S04 7 Calibration experiments using the spare model of SIGMA

—

3,

connected to the EVE simulation chamber were performed, in particular to answer these questions. Fig. 3c shows a typical gas chrornatogram obtained from such simulation experiments, using SIGMA — 3 instrument in the “pyrolytic” mode. The total quantity of H2S04 introduced through the inlet

of3, the gas chromatograph was aboutchannel 10 mg, of as measured by the reference with a concentration of about 60 mg/rn quantitatively chromatograms 3a and 3c, the calibration chamber. By comparing and taking into account the flow rate of SIGMA

—

3 fan

(“-‘

12 1/mm) and the ti-

me durations of sampling, we derive the following concentration of H 2S04 in 3 for the 60 — 55 km altitude range. This value Venus atmosphere : about 1 mg/rn is consistent with the values which have been reported elsewhere using other techniques

(3).

But comparison of these chrornotagrams shows also that the relative quantity of H 2S obtained in the flight experiment is much more important that the quantity obtained in the calibration experiment. Since H2S is only detected after pyroly— sis, this suggests that another condensed or dissolved constituent, able to produce after pyrolysis with H2S04 on the carbonized filter an important quantity of H2S must be present in the collected Venus aerosols. Several candidates, including elemental sulfur can be proposed, but have to be experimentally studied.

CONCLUSIONS Preliminary results of these calibration experiments of SIGMA

—

3 instrument

provide quantitative information on the chemical composition of Venus atmosphere in the upper cloud region. From these data, SO2 concentration must be less than 100 ppm, H2S and COS concentration must be less than few 10 ppm. The mass 3. In concentration H2S04 in the aerosols carried appear out to be of the the order of 1 mg/rn addition, the of calibration experiments with “pyrolytic” mode, and using the sulfuric acid aerosol chamber suggest that an additional constituent must be present in Venus aerosols. Further calibration experiments in the same analytical mode, using mixture of aerosols of sulfuric acid and, for instance, elemental sulfur are needed to elucidate this question.

ACLKNOWLEDGMENT

We wish to thank C. Sable and H. Oscar, of the Department d’Etudes et de Recherches en Technologie Spatiale, at the CERT (France) for their technical assistance.

—

ONERA Center in Toulouse

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SIGMA-3 Gas Chromatograph

(12)335

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Ii\:Jt RETENTION

TIME

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Gf~SCHROMATOGRAMS

REFERENCES

1. V.M. Balebanov, C.A. Skuridin, E.V. Vorontzova and V.S. Bassolo, Venus—Halley mission, International Scientific and Technical Committee (MNTK) Report, 181

—

185 (1985).

2. U. Von Zahrs “Composition of the Venus Atmosphere” p 299 “VENUS” (Edited by DM. Piunten

—

University of Arizona Press

431 issued from

—

—

Copyright 1983).

3. Chemical Analysis of Aerosol in the Venusian cloud layer by a reactionary gas chromatography on board the Vega landers, by B.C. Gel’man, Yu.V. Drozdov, V.V. Mel’nikov, V.A. Rotin, V.N. Khokhlov, V.B. Bondarev, G.G. Dol’nikov, A.V. D’yachkov, D.F. Nenarokov, L.M. Mukhin, N.y. Porshnev and A.A. Fursov. Letter of Astronomical Journal, part 12 n°2 “Naouka” 1986.