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Measurements of ions in atmospheric air are used to investigate atmosphericelectricity and particulate pollution. Commonly studied ion parameters are (1)air conductivity, related to the total ion number concentration, and (2) theion mobility spectrum, which varies with atmospheric composition. The physicalprinciples of air ion instrumentation are long-established. A recentdevelopment is the computerised aspirated capacitor, which measures ions from(a) the current of charged particles at a sensing electrode, and (b) the rateof charge exchange with an electrode at a known initial potential, relaxing toa lower potential. As the voltage decays, only ions of higher and highermobility are collected by the central electrode and contribute to the furtherdecay of the voltage. This enables extension of the classical theory tocalculate ion mobility spectra by inverting voltage decay time series. Inindoor air, ion mobility spectra determined from both the novel voltage decayinversion, and an established voltage switching technique, were compared andshown to be of similar shape. Air conductivities calculated by integrationwere: 5.3 +- 2.5 fS/m and 2.7 +- 1.1 fS/m respectively, with conductivitydetermined to be 3 fS/m by direct measurement at a constant voltage.Applications of the new Relaxation Potential Inversion Method (RPIM) includeair ion mobility spectrum retrieval from historical data, and computation ofion mobility spectra in planetary atmospheres.Comment: To be published in Review of Scientific Instrument

Aspirated capacitor measurements of air conductivity and ion mobility spectra