1997ESASP.397..193A
193
MACSIMS: A NEW BALLOON BORNE MASS SPECTROMETER INSTRUMENT USING ACTIVE CHEMICAL IONIZATION FOR
IN-SITU STRATOSPHERIC TRACE GAS MEASUREMENTS
C. AMELYNCK, E. ARIJS, E. NEEFS, D. NEVEJANS, W. VANDERPOORTEN Belgian Institute for Space Aeronomy, Ringlaan 3, B-1 I 80 Brussels, Belgium E-mail: Crist.Amelynck@bira-iasb.oma.be, Tel.: +32 2 3730391; Fax.: +32 2 3748423
A. BARASSIN, C. GUIMBAUD, D. LABONETTE
Centre Nationale de la Recherche Scientifique, Laboratoire de Physique et Chemie de /'Environnement, Avenue. de la Recherche Scientifique 3A, F-4507 I Orleans, France
H.-P. FINK, E. KOPP, H. REINHARD
Physikalisches Institut, University of Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland
Abstract. A new mass spectrometer instrument based on the active chemical ionization technique was developed and flown to measure the stratospheric trace gases HNO3 and Ni05• In the active chemical ionization method primary ion species are injected into a flow tube with ambient stratospheric air. The primary ions are selected to produce characteristic secondary ions from ion-chemical reactions with the neutral gases to be detected. At the end of the flow tube, after a flow-time of about 30 ms, primary and secondary ions are analyzed with a double focusing mass spectrometer capable of measuring a full spectrum part simultaneously with the use of two electro-optical detectors.
Profiles of HNO3 were determined using the conversion of chlorine cluster ions, generated by a discharge, into NO3• clusters. Additionally the products from ion clustering reactions of
co
3· , produced by a photo- electron source, allowed an independent determination of a nitric acid density profile. In order to detect N2Os (and ClONO2), negative iodine ions were used.Technical aspects of the new instrument and results from a balloon flight in November 1995 launched near Leon, in Spain at mid latitudes and obtained in an altitude range from 32 to 20 km will be presented.
1. Introduction
It is well established that the reservoir gases HNO3, N2O5, ClONO2 and HCl play an important role in the stratospheric ozone chemistry [Ref. I]. Although numerous balloon borne and satellite instruments based on remote sensing techniques allow measurements of nitric acid [Ref. 2], dinitrogen pentoxide [Ref.3], chlorine nitrate [Ref.4, 5], and hydrogen chloride [Ref.6], only HNO3 and HCl out of these four species were also measured in situ by Active Chemical Ionization Mass Spectrometry (ACIMS) [Ref. 7, 8].
It is the aim of the MACSIMS (Measurement of Atmospheric Constituents by Selective Ion Mass Spectrometry) project to extend this technique for the measurement ofN2O5, ClONO2 and possibly HCI.
Method, instrument, results and a possible outlook for future measurements are presented hereafter.
2. Method
As shown schematically in Fig. I, the ACIMS method consists in the production of defined primary ions in specially selected ion sources and the injection of these ions into the flow tube carrying ambient air. The trace gases to be detected form specific product ions in ion- molecule reactions with primary ions. Both primary and product ions are analyzed with a double focusing mass spectrometer coupled to the flow tube.
MACSIMS is using two different types of ion sources, namely the discharge source (DIS) and the photoelectron ion source (PEIS). The DIS is used to form alternately chlorine and iodine ions, whereas the PEIS produces
co
3· cluster ions. Chlorine ions are used to derive nitric acid concentrations and iodine ions I;(where n=I, 2 and 3) are intended to determine the concentration ofNz05 (and CIONO2).
In the chlorine ion mode the DIS mainly produces Cb- ions although some of the primary ions are er,
which are partly converted to Cl;X (X being H2O and HCl). Profiles of HNO3 concentrations were determined either from a scheme using c13· as source ions and NO3-HCI as product or from the transformation of all Cl;X species into NO3-and NO3- cluster ions.
In a first method (MI), the reaction scheme used to derive [HNO3] is:
c13· + HNO3 ➔ NO3-HCI + Cl2 (I) with a reaction rate k1 = 2.4x10·9 cm3s·1
NO3-HCI + HNO3 ➔ NO3-HNOi + HCI (2) with k2 ~ l.3x!O·IO cm3s·'.
Both reaction rates k1 and k2 were measured in our laboratory [Ref. 9].
In this case the nitric acid concentration is given by the formula:
I
IG-Js [Ncrm]
[HNq]-
('G-Js)xi-ln(l+Tx[ai-] )
(3)where square brackets denote concentrations and ,: is the flow time of ions the flow tube between source and mass spectrometer entrance plate.
Proceedings 13th ESA Symposium on European Rocket and Balloon Programmes and Related Research, Oland, Sweden, 26-29 May 1997, ESA SP-397 (September 1997)
©
European Space Agency • Provided by the NASA Astrophysics Data System
1997ESASP.397..193A
194
In the method M2 the nitric acid concentration 1s derived from:
[HNOJ]=krln(l+R) I (4)
where R is the ratio of product to primary ions. Here the sum of
er,
Ch- and Cl0-X is taken as primary ion term and all NO3- cluster ions are considered as product ions.A common reaction rate k is taken for the conversion of chlorine core ions into NO3--ions. Since Ct3· is by far the most abundant member of the chlorine ion family the value ofk1 is chosen fork.
Another method is based on the reaction of HNOi with the primary
co
3· andco
3· cluster ions produced by the PEIS. This method relies upon the reactions:co
3· + HNO3 + M ➔ CO/HNO3 + M (5) k5 = l.56x I 0·9 cm3s·'CO3"H2O + HNO3 ➔ CO3-HNO3+ H2O (6) k6 = 2.04x 10·9 cm3s·'
CO3-HNO3 + HNO3 ➔ CO3\HNO3)z (7) k7 > 6x!O·IO cm3s·1
Jn this case the nitric acid concentration is inferred from:
[ ] 1 k5 -k7
HNq =
(ks -k1) x rln(l+---;:;-Q) (8)where
[co
3-HN03] (9) Q[co
3-J+[cq-Hp]
Since
co
3· is much more abundant than CO3-H2O in the spectra obtained with the PEIS, the reaction rate ks was taken for reaction (5) as well as for reaction (6) in the derivation of these formulae. The rate constants ks,¾
and k7 have also been measured in the laboratory [Ref. JO, 11].3. Technique
The balloon borne MACSIMS payload which is described in detail elsewhere [Ref. 12] contains the active chemical ionization part, the ion transport system and the ion sensor part.
In the DIS, ions are produced in a glass tube by a DC-discharge between a metal needle and a fine stainless steel tube (2 mm diameter) through which the ions are injected into the flow tube. The DIS can be used either with a mixture of chlorine gas in argon (30-1000 ppm) in which case chlorine ions are produced, or with methyl iodide (CH3I) in argon (1000 ppm) for ~he production of iodine ions. Switching from_ t~e chlonne to the iodine mode can be performed w1thm seconds through changing the nature of the gas flow through the DIS by small electro-pneumatic valves. A permanent flush flow of argon was maintained in between the switching from C'2/Ar to CH3I/Ar source gases in order to remove as much as possible residues from the previous source gas.
The PEIS consists of a UV krypton discharge lamp producing photoelectrons through illumination of the metallic inner walls of the flow tube. These electrons are rapidly attached to molecular oxygen, forming 02· ions which are lost very rapidly and converted to the more stable
co
3· ions.In the ion transport part ambient air is conducted through a stainless steel flow tube of approximately 1 m length and 35 mm in diameter by means of a small turbine. The turbine pump is started close to the ceiling altitude in the ascent phase of the balloon flight sho1tly after opening of both flow tube ends.
The ion mass analyzer consists of a cryogenically pumped double focusing mass spectrometer using a slightly modified Mattauch-Herzog geometry combined with a ion optical detector system capable of fast simultaneous spectrum read-out [Ref. 13].
4
6
1
8 \ 2
Figure 1: Schematic representation of MACSIMS instrument.
I: ambient air inlet side ; 2: flow tube; 3: ion injection point for discharge ion source (DIS); 4: discharge ion source; 5: argon/chlorine gas mixture; 6: argon/methyliodide mixture; 7: switching electro- pneumatic valves; 8: Krypton discharge lamp; 9: turbine maintaining gas flow in flow tube; 10: cryogenically pumped mass spectrometer;
4. Measurements and Results
The data shown and discussed here were obtained during the descent phase of a balloon flight from Virgen de! Camino near Le6n in Spain (42°35' N, 5°38' W), the launching of which took place on 23 Nov. 1995, at 20:18 UT. The instrument was carried by a 100,000 m3 balloon to a ceiling altitude of 32.1 km, where it was kept for 50 minutes to tune the instrument. Hereafter a slow descent was started at 1.2 m/s down to about 18 km and spectra were recorded in an automatic mode controlled by an on-board microprocessor.
During each spectrometer cycle 15 mass spectra were recorded in groups of five different ion acceleration voltages (238V, 348V, 513V, 750V and 950V). The
© European Space Agency • Provided by the NASA Astrophysics Data System
1997ESASP.397..193A
first five spectra were obtained with the DIS in the iodine mode, the next five spectra were resulting from the DIS producing chlorine ions and the last five are recorded with the PEIS producing eo3- ions. A spectrum is a sum of 500 detector read-outs of 22 ms ion integration time each (two detectors with 512 pixels each). All spectra are corrected by subtracting the background spectrum (regularly recorded in flight) and by using a detector correction function taking into account the different analogue sensitivity on each pixel.
The intensity of the individual mass peaks is determined either with the use of a special Gauss-fit on each peak or by a numerical integration between the limits of a mass peak.
At regular time intervals, the flow time , of the ions was measured in flight with a fast electrometer by pulsing the DIS high voltage. A typical value of 35 ms was obtained, slightly dependent on altitude.
When using a eli(Ar gas mixture in the DIS between 32 and 24 km altitude, the major source ion was e13·,
representing about 95% of all ions in the chlorine family (consisting of er, erHel and e13-), and the most abundant product ion was NO3-Hel. Below 24 km the signal of 1e1z-, which was a minor product ion at higher altitudes, increased and eventually became larger than that of e13-_ It is believed to be formed in the DIS at higher pressures through reaction of e10 - ions with residues of eH31, in spite of permanent argon flush through the DIS.
The major ions observed from the PEIS in the altitude range 32 to 18 km were eoJ·, eo1-(HNO3)1.2, and eo3·H2O.
The nitric acid profiles as derived from the DIS in the chlorine mode and the PEIS are shown in figure 2. In this figure the HNO3 concentrations derived with method MI are shown as full circles and those obtained with method M2 as full inverted triangles. Laboratory measurements [Ref. 14] have shown that some of the e13• ions could have been produced outside of the source in the flow tube by the reactions er + e(z + M ➔
e1i· +Mand erH 20 + e12 ➔ e1i- + H2O. Since er is also produced by the DIS as source ion (and subsequently converted to erH 20 in the flow tube) and since chlorine gas is injected in the flow tube through the DIS in a non negligible concentration, a supplementary production of e13- in the flow tube cannot be neglected. The results of HNO3 derived by the method MI should thus be considered as lower limit values.
The concentration of HNO3 as derived with the method M2 takes into account all e10 - and erH2O as source ions and as product ions all NO3-X cluster ions (X being H2O, Hel, HNO3 or 2HNO3).The most abundant product ion being e11-, the value of k1 was taken fork in formula (4). As the rate constants for the other conversions (such as er + HNO3) are somewhat higher, the nitric acid profile derived with method M2 should probably be considered as an upper limit.
195
The HNO3 mixing ratios derived from the data of the PEIS ion source, based on the reactions (5), (6) and (7) as described previously, are shown as open squares. in figure 2.
The MAeSIMS results can be compared to HNO3 concentrations measured from the LIMS experiment in November 1979 at the same latitude [Ref. 15], shown as a full line in figure 2. Within the estimated errors on the MAeSIMS data (about 40%), the agreement between MAeSIMS results and LIMS measurements 1s reasonably good.
The DIS used with eHN Ar produced mainly
r
above 27 km and 13· below 24 km. The major product ions were NO3-, NO3-H2O and NO3-HNO3• This ion source was originally meant to produce NO3-from the reaction ofr
with N2O5. Recently however, Huey et al. [Ref. 16]showed that
r
also reacts rapidly with elONO2. This result implies that only the weighted concentration of N2O5 + elONO2 can be obtained by using this method.The height profile of N2O5 + eIONO2 as derived from the data obtained with the DIS in the iodine mode and using a method similar to method M2, corresponds roughly in form to the values obtained from previous measurements. However, the values are in general too high by a factor of 3 or more. The reason of this discrepancy is not yet known and requires further investigations and laboratory measurements.
30
,. ,.
" ,.
a
• ,. ,. '
,..._
E □•
~
•
a'""
• •
a ,."
w
•
a,.
O 25
• ,.
□:J
•
~I-
;. ,.
a
,.
I-
•
□-'
•
<{
• •
□,.
20 □
15 ~~-~-~~-~-~-~~~~-~
0 10
HN03 MIXING RATIO (ppbv)
Figure 2: Nitric acid volume mixing ratios versus altitude as derived from balloon flight of 23 November 199S. Full circles: results obtained with DIS and method MI (based upon conversion of Cl3 · ions into N03"HCI). Full inverted triangles: results obtained with DIS and method M2 (based upon conversion of all er cluster ions). Open squares: results from PEIS. Full line: LIMS data ofNov. 1979 at same latitude.
© European Space Agency • Provided by the NASA Astrophysics Data System
1997ESASP.397..193A
196
5. Conclusions and outlook
A new chemical ionization mass spectrometer based upon the use of different types of ion sources was developed and flown. The photo-electron ion source producing as source ions CO3- core ions allows the derivation of nitric acid profiles using previously used reaction schemes [Ref. 7].
A new type of discharge ion source, allowing the production of source ions of different nature through switching the source gas flow in the discharge, was tested. This switching discharge ion source has worked to our full satisfaction, although the fact that a large variety of ions is produced in one single mode (such as er, crHCl and Cl3-in the chlorine mode and
r
as wellas 13• in the iodine mode) complicates spectra interpretation and data reduction.
Within the accuracy of the method, however, a good agreement with previously published nitric acid profiles was reached.
To derive the height profile of dinitrogen pentoxide and chlorine nitrate concentrations, which should be possible using the appropriate gas mixture in the discharge source, further ion-molecule reaction studies in the lab have to be undertaken or new reaction schemes have to be applied.
Within the next balloon flight (June 1997 in Gap- Tallard) a fourth ion source will be implemented in the instrument. This source will be used for the production of CF30" ions. Recent laboratory measurements [Ref.
17] have shown that CF30" gives selective product ions for a series of trace constituents in the stratosphere, in particular for HNO3 , ClONO2 and HCI.
Acknowledgements
The authors are grateful to the Commission of the European Communities (contract numbers EV5V-CT92-0062 and ENV4- CT95-0042), the FKFO/MI (Fonds voor Kollektief Fundamenteel Onderzoek - Ministerieel lnitiatief), the CNRS (Centre National de Recherches Scientifiques, France), the CNES (Centre Nationale d'Etudes Spatiales, France) and the SNF (Schweizerischer Nationalfonds zur Ftirderung der wissenschaftlichen Forschung) for their support.
One of us (C.A.) was supported by the Belgische Staal, Diensten van de Eerste Minister - Federate Diensten voor Wetenschappelijke, Technische en Culturele aangelegenheden and an other one (C. G.) by the Conseil Regional du Centre.
The authors also thank the CNES launching team and the INT A representatives for their excellent collaboration during the flight campaigns.
References
I. World Meteorological Organization, Scientific assessment of ozone depletion: 1994, Global Ozone Research. and Monitoring Project., WMO Rep. 37, Geneva, 1995
2. Zhou D.K. et al. 1997, Stratospheric CH4, N2O, H2O, NO2, N2O5, and ClONO2 profiles retrieved from Cryogenic Infrared Radiance Instrumentations for Shuttle(CIRRIS IA)/STS 39 measurements, J. Geophys.
Res. 102, pp. 3559-3573.
3. Nevison C.D. et al. 1996, Nighttime formation of N2O5 inferred from the Halogen Occultation Experiment sunset/sunrise NOx ratios, J. Geophys. Res. IOI, pp.
6741-6748.
4. Rinsland C. P. et al. 1994, Profiles of stratospheric chlorine nitrate (ClONO2) from atmospheric trace molecule spectroscopy. ATLAS I inferred solar occultation spectra, J. Geophys. Res., 99 pp. 18895- 18900.
5. Fischer H. and H. Oelhaf, 1996, Remote sensing of vertical profiles of atmospheric trace constituents with MIPAS limb-emission spectrometers, Appl. Opt., 35 pp.
2787-2796.
6. De Valk, J.P.J.M.M. et al. 1997, Airborne heterodyne measurements of stratospheric CIO, HCI, 03, and N2O during SESAME I over northern Europe, J. Geophys.
Res., 102, pp. 1391-1398.
7. Arnold F. and G. Knop 1987, Stratospheric trace gas detection using a new balloon-borne ACSIMS method., Jnt. J. Mass Spectr. Jon Proc., 81, pp. 33-44.
8. Arnold, F. and S. Spreng 1994, Balloon-borne mass- spectrometer measurements of HCI and HF in the winter arctic stratosphere, Geophys. Res. Lett., 21, pp. 1255-
1258.
9. Amelynck, C. et al. 1994 Reactions of nitric acid with di- and tri-chloride ions, di- and tri-iodide ions and with
CO4" in the gas phase, Int. J. Mass Spectr. Jon Proc.,
133, pp. 13-28.
10. Mohler, 0. and F. Arnold 1991, Flow reactor and triple quadrupole mass spectrometer investigations of negative ion reactions involving nitric acid: implications for atmospheric HNO3 detection by chemical ionization mass spectrometry., J. Atmos. Chem., 13, pp. 33 - 61.
11. Huey, L. G. 1996, The kinetics of the reactions of er, 0-, and 02- with HNO3• Implications for measurement of HNO3 in the atmosphere., Int J. Mass Spectr. Ion Proc., 153, pp. 145-150.
12. Arijs et al., to be published
13. Rytz Ch. Et al. 1994, Simultaneous ion detection mass spectrometer for the measurement of stratospheric trace gases, !nt. J. Mass Spee. Ion Proc., 137, pp. 55-66.
14. Amelynck et al., to be published
15. Gille, P. L. et al. 1996, Revised reference model for nitric acid., Adv. Space Res., 18, pp. 125-138.
16. Huey L.G. et al. 1995, Reactions of SF6-and
r
withatmospheric trace gases. J. Phys. Chem., 99, pp.5001- 5008.
17. Huey L.G. et al. 1996, The reactions ofCF30" with atmospheric trace gases. J. Phys. Chem, 100, pp. 190- 194.
