New positive ion species in the stratosphere

Abstract

Our understanding of atmospheric ions in the ionospheric region is far better than of those in the denser regions, the stratosphere and the troposphere. Mass spectrometric measurements conducted on rockets¹ and balloons^2–4 at altitudes above ∼35 km recently revealed that the major ions are proton hydrates (H⁺(H₂O)_n and heteromolecular cluster ions identified as H⁺X_L(H₂O)_M (refs 2, 5) (X-ions), which probably are formed from proton hydrates by reactions involving a stratospheric trace gas X (ref. 2). The most likely candidate for X seems to be acetonitrile (CH₃CN) (refs 2, 6). The abundance ratio of X ions and proton hydrates increases with decreasing altitude from ∼0.02 around 50 km to ∼2 around 35 km. It has only been possible to detect ion species with number densities larger than about 6 cm⁻³, corresponding to a fractional ion abundance of 3% around 35 km. We report here new balloon-borne positive-ion composition measurements around 34 km using improved mass spectrometers which allowed for an ion detection limit of ∼0.02 cm⁻³ or 0.1%.