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Abstract

Atmospheric ions formed by cosmic radiation are potentially important in mediating the formation of aerosol particles and cloud condensation nuclei (CCN) and therefore may eventually influence climate in indirect ways. We report on aircraft-based atmospheric measurements of large ions which indicate that aerosol formation via ion-induced nucleation (INU) is operative in the upper troposphere. We have for the first time detected upper tropospheric large negative and positive ions using an advanced air craft -based large ion-mass spectrometer. The observed large ions represent key intermediates involved in INU. From the measured ions we have inferred the total concentration of highly supersaturated trace gases, the INU-rate, and the concentration of very small aerosol particles which are not directly measurable by present techniques. Furthermore we have made model calculations of the evolution of aerosol particles of ionic origin and their transformation to CCN. We conclude that in relatively clean upper tropospheric air INU is more efficient than homogeneous nucleation and that some fraction of the aerosol particles formed via upper tropospheric INU can upon downward transport grow to sufficiently large sizes to act as CCN in the formation of highly climate-relevant lower tropospheric marine stratus clouds.