Aspirated capacitor measurements of air conductivity and ion mobility spectra

Abstract

Measurements of ions in atmospheric air are used to investigate atmospheric electricity and particulate pollution. Commonly studied ion parameters are (1) air conductivity, related to the total ion number concentration, and (2) the ion mobility spectrum, which varies with atmospheric composition. The physical principles of air ion instrumentation are long-established. A recent development is the computerised aspirated capacitor, which measures ions from (a) the current of charged particles at a sensing electrode, and (b) the rate of charge exchange with an electrode at a known initial potential, relaxing to a lower potential. As the voltage decays, only ions of higher and higher mobility are collected by the central electrode and contribute to the further decay of the voltage. This enables extension of the classical theory to calculate ion mobility spectra by inverting voltage decay time series. In indoor air, ion mobility spectra determined from both the novel voltage decay inversion, and an established voltage switching technique, were compared and shown to be of similar shape. Air conductivities calculated by integration were: 5.3 ± 2.5 fSm -1 and 2.7 ± 1.1 fSm -1 respectively, with conductivity determined to be 3 fSm -1 by direct measurement at a constant voltage. Applications of the new Relaxation Potential Inversion Method (RPIM) include air ion mobility spectrum retrieval from historical data, and computation of ion mobility spectra in planetary atmospheres.

Applications of Electrified Dust and Dust Devil Electrodynamics to Martian Atmospheric Electricity

“…Measurements of the conductivity itself can also be made, from determining the charging time to restore the equilibrium potential of the electrode after bringing it to a fixed potential (e.g. Aplin 2005;Bennett and Harrison 2006).…”

Composition and Measurement of Charged Atmospheric Clusters

“…Techniques developed to enable extraction of ion mobility (related to mass: see Sect. 3.1 or Harrison and Tammet (2008)) from the Huygens relaxation probe data (Aplin 2005;Owen et al 2008) could not be used, due to loss of data from the positive channel (Lebreton et al 2005). In general, the Huygens atmospheric electricity data has been more complicated than expected to analyse due to calibration problems and inconsistencies between the results from the two conductivity instruments (described in more detail in Hamelin et al 2007).…”

“…Drift tube spectrometers take a sample of air which is subsequently analysed, but aspiration ion mobility spectrometers continually blow or suck air through a region (often a cylinder) to which an electric field is applied. The current from ions of different mobilities can be determined by either direct measurement (Tammet 1970;Aplin and Harrison 2001;Fews et al 2005) or inferred from the rate of voltage relaxation (Aplin 2005). Disadvantages of cluster-ion mobility measurements are that the carrier gas must be well-known, and that the conversion of mobility to mass is non-trivial (Harrison and Tammet 2008).…”

Atmospheric Electricity at the Ice Giants

“…Mutual impedance probes send a current pulse through the surrounding medium, and the impedance can be determined from the current/voltage characteristic measured by two passive electrodes. A relaxation probe can also measure conductivity, and the spectrum of ion mobilities (Aplin, 2005), from the rate of decay of the potential on an electrode. Given the wide bipolar conductivity range throughout the ice giant atmospheres, and the sensitivity of the negative conductivity to the poorly-known number of electrophiles, a wide-range instrument package is recommended similar to the Pressure Wave Altimetry (PWA) package on the Huygens probe.…”