Electric Current Measurement
The electric currents collected on the outer electrodes are very small, in the range of 1 – 3 fA per electrode in the cluster ion range and even smaller in intermediate ion range (Hõrrak 2001).
The NAIS uses integrating electrometric amplifiers where the fluxes of electric charge are collected on high quality electrical capacitors (Figure). The output voltages of the amplifiers are proportional to the collected electric charge and the change of the voltage is proportional to incoming charge i.e to the aerosol current (figure).
The integrating measurement principle allows for the best possible signal to noise ratio for electric current measurements. Also, the signal is collected continuously almost without any breaks – no signal is missed regardless of measurement frequency.
The voltage from the amplifier outputs is read 10 to 15 times per second. This raw signal is passed through several signal processing steps before the average signal for a time period is calculated and particle distribution deduced (Figure). When measuring high concentrations 1 second average spectra have sufficiently low signal-to-noise ratio to be useful.
Firstly the electric current values are corrected for the offset currents measured periodically during the offset operating mode. Also the estimated noise levels are bundled with the records and all further steps will always consider and operate on the signal and noise together. The offset correction and noise estimates are essential to the data processing.
The offset signal is estimated using linear regression on the current measurements from previous and next offset measurement cycles (Figure). This means that the final measurement result will be available after the next offset measurement cycle has been completed. The noise estimates are calculated from the difference between the regression estimate and actual offset signals.
Often short spikes occur in the electric current signal that can't be the result of actual measured aerosol. Most likely their cause is the random decay of radioactive particles deposited on the electrodes. The frequency of spikes increases as more dirt is collected on the electrodes.
A simple outlier detection algorithm is used to discard the false signal measurements. As long as the instrument is not too dirty, the spikes can be detected reliably. The number of discarded samples is indicated by the measurement parameter "Dropped outlier samples" for the block average records. Each drop concerns only a single electrometer.
The high measurement rate allows to employ optimal signal processing (ARMA filter). The electric current signal is passed through a matched digital filter to whiten the noise distribution. This improves the effectiveness of averaging in case of short periods, i.e. 1 to 10 seconds (Eller 2008)
The collected charge on the capacitors needs to be cleared every once in a while. In NAIS the electrometers will automatically reset when the output signal reaches the upper or lower limit of the integrator (typically below −4 V or above +4 V). Signal from that electrometer is ignored for the duration of the reset and the settling, which takes about ten seconds.
At low concentrations resets can happen about once a day for each electrometer, which practically does not affect the measurements.