SIDAMS & MACSIMS

1991: SIDAMS and MACSIMS are balloon borne mass spectrometers for the in-situ measurement of ions and neutrals in the stratosphere.

Simultaneous Ion Detection in Atmospheric Mass Spectrometry (SIDAMS) and Measurement of Atmospheric Constituents by Selective Ion Mass Spectrometry (MACSIMS) are balloon borne mass spectrometers. SIDAMS is a passive instrument, sampling simply the stratospheric ion content. MACSIMS is more evolved, and contains a chemical ionization section allowing to measure also neutral in-situ stratospheric trace gases. These instruments were developed in the nineties and were used during several successful balloon flights.

Delivery year: 1991

MACSIMS instrument ready for launch in Gap-Tallard (France) in 1995

SIDAMS measured positive and negative stratospheric ion composition

The aim of the Simultaneous Ion Detection with an Atmospheric trace gas Mass Spectrometer (SIDAMS) instrument was to measure simultaneously in-situ the positive and negative stratospheric ion composition with a new second generation balloon borne ion mass spectrometer offering higher mass and spatial resolution, an increase in absolute sensitivity and reduced fragmentation of complex ions.

This technique has the capability to determine in-situ the concentration of various stratospheric trace gases by using passive chemical ionization methods. These methods are based upon the analysis of the abundances of the natural ions as measured with an ion mass spectrometer in the stratosphere. From the knowledge of the ion chemistry which gives rise to the formation of these ions, the concentration of the trace gases playing an active role in the stratospheric ion chemistry can be derived.

Available stratospheric ion mass spectra exhibit a great variety of species. The goal of SIDAMS was to allow the exploitation of this wealth of information through sensitivity increase.

MACSIMS used active chemical ionization method

MACSIMS was a SIDAMS-like instrument that used the active chemical ionization method to detect N₂O₅ and HNO₃, trace gases playing an important role in the ozone problem.

In the active chemical ionization method, an ion source is mounted in front of the mass spectrometer and the artificially created ions react selectively with some trace gases to form product ions, which are together with the precursor ions measured by the mass spectrometer. From the abundance ratio of precursor to product ions, the reaction rates of the conversion reactions and the flight time of the ions from the ion source to the mass spectrometer, the number density of the reactive trace gases can be inferred. This method had been used by other research groups to determine HNO₃ vapor concentrations in the stratosphere with an ion source producing mainly CO₃ cluster ions and exploiting the reaction of these ions with nitric acid vapor.

In MACSIMS an ion source was installed in front of the ion mass spectrometer, producing alternatively Cl^- and I^- ions in flight. The use of such an ion source allowed the simultaneous measurement of HNO₃ and N₂O₅ in the lower stratosphere, because Cl^- reacts with both HNO₃ and N₂O₅ whereas I^- only reacts with N₂O₅. In a first phase MACSIMS was used for measuring positive and negative ambient ions and trace gases such as H₂O, CH₃CN, HNO₃ and H₂SO₄ and trace gases such as HCl and others. The method allowed measurements at any time of the day.

SIDAMS and MACSIMS ion mass spectrometer

To achieve the above objectives, a new type of ion mass spectrometer has been developed capable of detecting both positive and negative ions.

This instrument consisted of a double focussing magnetic mass analyzer followed by a simultaneous ion detection system. The mass spectrometer had a much higher sensitivity than quadrupole instruments used for similar applications, which resulted in several benefits, such as:

• for measurements of natural ions in the stratosphere, lower ion focussing fields can be used and the problem of cluster break up or collisional induced dissociation can be avoided. In this way, concentrations of trace gases, such as H₂O, CH₃CN, HNO₃ and H₂SO₄, as derived from natural ion composition data are more reliable and accurate;
• for measuring artificially created ions using a suitable external ion source and allowing the detection of other trace gases (active chemical ionization), the observation of mass peaks due to natural isotopes is very helpful in identifying the ions and the higher sensitivity allows a much better altitude resolution of the inferred altitude profiles.

Balloon launch

The ion mass spectrometer for natural positive ions (SIDAMS) has been flown for the first time into the stratosphere by means of a stratospheric balloon from the balloon launching center of the CNES in Aire-sur-l'Adour (S. France) on May 21^st, 1991. This flight, which was of merely technological nature showed that the instrument worked well from the technical point of view. High resolution spectra of positive ions were obtained between 20 and 32 km altitude.

SIDAMS and MACSIMS have been flown several times during the nineties from launching sites in:

• Aire-sur-l'Adour (France)
• Gap-Tallard (France)
• Léon (Spain).

 Publications

• Amelynck, C.; Arijs, E.; Neefs, E.; Nevejans, D.; Vanderpoorten, W.; Barassin, A.; Guimbaud, C.; Labonette, D.; Fink, H.-P.; Kopp, E.; Reinhard, H. (1997), “MACSIMS: A new balloon borne mass spectrometer instrument using active chemical ionization for in-situ stratospheric trace gas measurements”, ESA-SP 397: Proceedings of the 13th ESA Symposium on European Rocket and Balloon Programmes and Related Research, Vol. 397, 193-196
  
  
• Amelynck, C.; Arijs, E.; Neefs, E.; Nevejans, D.; Vanderpoorten, W.; Barassin, A.; Guimbaud, C.; Labonnette, D.; Fink, H.P.; Kopp, E.; Reinhard, H. (1997), “MACSIMS: A new balloon borne active chemical ionization mass spectrometer to derive the concentration of stratospheric trace gases”, (Bojkov, R.D., Ed.), Atmospheric Ozone: Proceedings of the XVIII Quadrennial Ozone Symposium, L' Aquila, Italy, 12-21 September 1996, Vol. 2, 453-456
  
  
• Arijs, E.; Nevejans, D.; Fussen, D.; Neefs, E.; Amelynck, C. (1994), “MACSIMS: Measurement of atmospheric constituents by selective ion mass spectrometry”, (Vanderborght, O., Ed.), IGBP and 'Global Change' related research in Belgium II, 92-93