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Although ion chemistry models1,2have predicted proton hydrates (PH) that is ions of the form H+(H2O)n, as major ions in the stratosphere, the first in situ mass spectrometric measurements3–5revealed another ion family, called non-proton hydrates (NPH). The fractional abundance of these NPH, represented by H+Xl(H2O)m, increases from 1 to 90% between 55 and 23 km (refs 1, 5–7). Several proposals5,8,9have been made for the identity of the molecule X, but high resolution spectra10and ion abundance measurements11,12suggest that X should be acetonitrile (CH3CN). This suggestion has been reinforced by laboratory measurements13,14and in situ data between 20 and 42 km (refs 6, 7), allowing a determination of the concentration profile of X in this altitude region. Here we report the first positive ion composition data obtained using a balloon-borne instrument between 42 and 46 km altitude. These data extend the density profile of X and give supplementary indications about its identity.

There has been recent considerable interest in the stratospheric ion composition and ion chemistry, especially in view of its possible impact on aerosol formation1,2and the possibility of detection of minor constituents with very low concentrations3,4. The first composition measurements of positive ions in the stratosphere conducted by a rocket-borne instrument5, revealed the existence of the theoretically predicted proton hydrates (PH)6and also of some unexpected non-proton hydrates (NPH). (Proton hydrates are primarily ions of the form H+(H2O)n.) Because low altitude rocket data may be biased by shock wave effects, such as ion fragmentation, the need for balloon flights was strongly felt. In 1978, data obtained with balloon-borne instruments7–9partly confirmed and extended the rocket data. There have been several explanations1,5,7,8,10of the presence of the observed NPH, but the lack of resolution of the data means an ambiguity still exists about the exact masses of these ions. We report here the first high resolution mass spectra of positive ions in the stratosphere, at 34-km altitude, where the major mass peaks can be identified unambiguously.

THE ion composition of the upper atmosphere has been the subject of many experiments. Early mass spectrometer measurements have been primarily concerned with the ion composition at altitudes between 60 and 250 km (refs 1–6). Few data, however, are available on the charged particle composition of the stratosphere. Global ion density measurements using electric probes7–11indicate number densities of the order of 100–10,000 cm−3for positive ions in this region. Arnold et al.12have recently published the first mass spectrometric data of the stratospheric ion composition, obtained on the downleg portion of three rocket flights. Above 40 km they observe the proton hydrates as being the most abundant ions. Below 40 km a change in the predominant ion species is seen, which is explained by Arnold et al. by ion molecule reactions between water cluster ions and formaldehyde. One of the possible dangers, inherent to rocket flights, however, is the alteration of the ion composition, due to shock wave-induced fragmentation. Mass spectrometric measurements on this region using a balloon-borne instrument were suggested13as early as 1969. Here we give the first preliminary results of a successful flight, performed with a balloon-borne quadrupole mass spectrometer on 30 September 1977. A 100,000 m3Zodiac balloon was flown at mid-latitude (CNES lauching site at Aire sur l'Adour, France, 44 °N) at 18.27 UT and reached the altitude of 35 km at 20.00 UT. The measurements were started at 20.14 UT, which is after sunset.