Atmospheric cluster ion concentrations cause charging of aerosol particles, which can influence cloud properties through enhanced aerosol scavenging[i]. Production of condensation nuclei from ions has also been proposed[ii], and infra-red absorption by positive cluster-ions is known to occur[iii]. Studying physical properties of cluster ions in the atmosphere constrains the processes involved, but measurements for investigating these hypotheses at the microphysical level are relatively rare.
Electrical mobility m describes the speed attained by an ion in a unit electric field. It is inversely related to ion mass and is strongly influenced by temperature and pressure[iv]. Differences in cluster chemistry cause negative ions to have a higher mobility (lower mass) than positive ions in terrestrial air. The negative to positive ion mobility ratio was measured almost a century ago in Scotland to be 1.1[v]. Modern measurements at a rural site in Estonia show the ratio has remained similar, with the mean mobility m+~1.4cm2V-1s-1 and m–~1.5cm2V-1s-1[[vi]]. The ion mobility spectrum in urban air is more variable with m+~1.1cm2V-1s-1 and m–~1.8cm2V-1s-1[[vii]]. Mobility measurements are important for identifying conditions in which ions can grow into aerosol particles. Ion concentrations, mobility spectra and bipolar air conductivity can be measured using the Programmable Ion Mobility Spectrometer (PIMS)[viii]. The operating, retrieval and validation procedures for the ion measurements are described, and the ion properties obtained based on 7 days of data from the Reading University Atmospheric Observatory (RUAO) (51.442°N, 0.938°W) are compared with local meteorological and atmospheric electrical measurements.