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+BbWw (W is H2O and B is mostly acetone (CH3)2CO, and b ≤ 2). Type 2 ions are probably H+BbAaWw (A = H2SO4) 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 AxWy 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.