A Novel Technique to Determine Atmospheric Ion Mobility Spectra

Detailed tropospheric ion measurements are needed to improve understanding of the electrical microphysics affecting clouds. Additionally, atmospheric ion mobility spectra can be used to identify ion growth processes leading to condensation nucleus formation. However these measurements are rare, particularly in the troposphere where the majority of clouds form. Developments in the operating theory of the classical instrument for ion measurement, the aspirated cylindrical capacitor, are described, which enable ion mobility spectrum information to be extracted from the rate of voltage decay of the aspirated capacitor in air. In this paper, data from historical balloon-borne ion counter ascents will be reanalysed to extract new ion mobility spectra from simple voltage time series. Such data recovery will increase the amount of atmospheric ion spectra available for analysis. INTRODUCTION Atmospheric ions are one source of atmospheric aerosol particles. If, as has been proposed, particles formed from ions can influence cloud formation under some conditions [Yu and Turco, 2001], atmospheric ion measurements may reveal unique data on atmospheric electrical influences on clouds and climate. Although long-term observations of ion spectra exist at the surface [Hõrrak et al, 2000], data is very limited in cloudforming regions. Free tropospheric ion spectra usually require a dedicated mass spectrometer, flown on a plane [Arnold et al, 1998]. The ventilated cylindrical capacitor or Gerdien condenser [Gerdien, 1905] is a long-established instrument for atmospheric ion measurements. It consists of a cylindrical outer electrode containing a coaxial central electrode, with a fan to draw air through the electrodes. With an appropriate bias voltage applied across the electrodes, a current flows in proportion to the ion concentration. The conductivity of the air can be inferred from the rate of decay of voltage across the electrodes. For surface measurements, this technique has generally been superseded by direct measurements of the current at the central electrode. Contemporary instruments based on this principle deploy modern electronics and computer control to use both the Current Measurement and Voltage Decay modes for self-calibration [Aplin and Harrison, 2000; 2001]. However the Voltage Decay mode still remains preferable for balloon-borne atmospheric ion instrumentation due to its simplicity in operation, measurement and telemetry. Surface measurements with modern instrumentation showed that although generally comparable, the two modes were not always completely consistent [Aplin and Harrison, 2001]. This motivated reconsideration of the classical principles of the Voltage Decay mode. The modified approach enables ion mobility spectra to be retrieved from voltage decay measurements. In this paper, the new technique is applied to calculate ion mobility spectra from historic conductivity-sonde data. CLASSICAL THEORY Atmospheric conductivity σ is related to both the atmospheric ion concentration n, and the average mobility μ of the air ion population. Molecular ions with μ > 0.5 cmVs are defined as “small ions” [Hõrrak et al, 2000]. σ can be written as