CIC - Cluster Ion Counter

The Cluster Ion Counter (CIC) is an instrument for measuring the total concentration of both positive and negative cluster ions. The instrument is designed to be simple and robust to provide reliable cluster ion measurements for both long term field montoring as well as laboratory experiments.

Specifications

Measurement range:positive and negative cluster ions z > ±0.5 cm2/V/s
Sample flow rate:10 – 60 l/min per polarity
Noise level:20 #/cm3 total concentration at 40 l/min sample flow
Time resolution:1 Hz or better, depending on signal level
Dimensions:20 x 20 x 40 cm
Power consumption:DC 15 V, 0.5 A
Interface:RS-232

Principles

The CIC uses two independent first order cylindrical differential mobility analyzers (Tammet 1970 1) to measure the ions of positive and negative polarities in parallel

The measured ions entering the analyzers are repelled by a central electrode which is held at a steady but software adjustable voltage. The ions deposit on outer wall of the analyser which is divided into three separate collecting electrodes. The electric current produced by the deposited ions is measured using high precision integrating electrometers.

The principal schematic of the CIC.

The sampling rate of the electrometer signals is 30 measurements per second to allow high time resolution. The zero level currents and noise level estimates of the electrometers are periodically measured by switching on a high voltage electrical inlet filter which prevents ions from entering the analyzer.

The outlets of the analyzers are internally connected to flow meters followed by software controlled blowers. This allows sample flow rates of both analyzers separately to be freely specified in the range from 10 l/min to 60 l/min depending on the rquiremens of the experiment and available signal level. The central electrode voltages are automatically adjusted to keep the detected ion mobility range constant.

The instrument includes air pressure sensors to take into account the relation between particle mobility and air pressure. This allows the instrument to operate on-board aircrafts and in chamber experiments.

In contrast to second order dfferenctial mobility analyzers, e.g. as used in the Neutral cluster and Air Ion Spectrometer (NAIS), the CIC does not use a separate sheath air flow. This significantly simplifies the construction of the instrument and allows for low inlet losses and high time resolution. This also results in a very low size resolution and makes the instrument primarily useful as an integral counter.

However the three separate electrodes allow the instrument to acquire additional information about the ion mobility distribution.

The limiting mobilities of the collecting sections are chosen so that the the signals from the first and second electrodes can be used to estimate roughly the average cluster ion mobility and air conductivity. The third electrode is used to find the signal of larger ions beyond the cluster ion band and subtract it from the total concentration.

The schematic of one of the analysers in the CIC.
The theoretical transfer functions of the three collecting sections without considering ion diffusion losses. The colors corresponding to sections match the pervious figure.

  1. Tammet, H. (1970). The Aspiration Method fo the Determination of Atmospheric-Ion Spectra. (Israel Program for Scientific Translations, Jerusalem). [return]