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  • Ionisation as a precursor to Aerosol Formation (also known as IONS) was a NERC Polluted Troposphere Research Programme project (Round 1 - NER/T/S/2002/00150 - Duration 2002 - 2005) and was led by Dr RG Harrison, University of Reading. Formation of ultrafine particles from molecular cluster ions in the atmosphere has been shown to occur in urban air, from both experimental findings and theoretical considerations. Ion-induced (or mediated) aerosol formation is currently neglected, despite the considerable variability known in atmospheric ions. Ionisation in the atmosphere is ubiquitous, arising from cosmic rays and natural radioactivity, with a further contribution from nuclear reprocessing. The efficiency of ion to particle conversion was sensitive to gas composition and condensable vapour supply. The work measured ion-induced ultrafine particle production in surface air, combining ion and aerosol data. Existing ion-aerosol theory will be extended to include particle production from ions, to assess the significance for clouds of additional condensation nuclei. This dataset collection contains surface level atmospheric measurements of background ion production rates and small ion concentrations. Data from the TORCH2 campaign are also included as used by the project participants. The Polluted Troposphere Programme was a 5-year NERC thematic research programme which was centred upon the study of polluted boundary layer air and its transport to the free troposphere. The programme focussed on the regional scale, defined as intermediate between urban and hemispheric.