Acetonitrile in the stratosphere—implications from laboratory studies

Abstract

The presence of acetonitrile (CH₃CN) in the stratosphere has been tentatively inferred¹ from measurements of stratospheric positive ions around 36 km. As well as the expected proton hydrates H⁺(H₂O)_n′ another ion family was observed to which was assigned the general form, H⁺X_l(H₂O)_m′ where X is a molecule of mass 41 ± 1 AMU. If these ions are formed from proton hydrates by reactions involving the unknown trace gas X, the latter must have a proton affinity of ≥ 175 kcal mol⁻¹ and a number density of the order of 10⁵ cm⁻³ around an altitude of 36 km. Two previously proposed identifications for X—NaOH and MgOH—have recently been excluded by new stratospheric ion composition data^3,4 which unambiguously determined the mass of X to be 41 AMU and failed to show H⁺X_l(H₂O)_m ions containing the Mg isotopes 25 and 26. Another suggestion¹ that X might be acetonitrile (CH₃CN) has been criticized on the grounds that it is difficult to explain its presence in the stratosphere². We now report laboratory studies of clustering equilibria for H⁺(CH₃CN)_l(H₂O)_m ions, which further support identification of X as acetonitrile.