Cosmic ray‐induced aerosol‐formation: First observational evidence from aircraft‐based ion mass spectrometer measurements in the upper troposphere

Large positive cluster ions with atomic mass numbers up to 2500 have for the first time been detected in the upper troposphere using an aircraft‐based large ion mass spectrometer (LIOMAS). Three ion families seem to be present (types 1, 2 and 3). Type 1 ions probably are H + B b W w (W is H 2 O and B is mostly acetone (CH 3 ) 2 CO, and b ≤ 2). Type 2 ions are probably H + B b A a W w (A = H 2 SO 4 ) formed by A‐attachment to type 1 ions. Type 3 ions are probably very small charged aerosols formed preferably by type 1 ion attachment to stable electrically neutral A x W y clusters. The latter are preferably formed by ion‐ion recombination. Hence our observations provide strong evidence for the ion‐mediated formation and growth of aerosol particles in the upper troposphere.