5.2  Sample Preconditioning

Figure 1: Schematic of the sample preconditioning unit

5.2.1  Chargers

Both analyzer columns of the NAIS are preceded by a charger and a discharger. All the chargers and dischargers are unipolar corona chargers.

Ions from the tip of the needle travel across the aerosol sample flow and attach to particles mainly by thermal diffusion.

It is important to keep the electric field strength in the charger volume as low as possible to minimize particle loss and simultaneously to keep the charger ion concentration as high as possible to maximize charging efficiency. So the charger is operated quite close to the corona ignition limit.

Corona discharge is inherently unstable. The NAIS uses a feedback system (Figure 2) to adjust the voltage of the corona needle based on the electric current of charger ions to the outer electrode. This is shown as the “Charger current” or “Discharger current” parameter in the measurement software.

Figure 2: Charger control principle

The NAIS utilizes a software PID controller to adjust the charger voltage. The charger current may be different for the different operating modes of the instrument.

If the corona needle becomes too dirty, the automatic control will be unable to keep the corona stable. The charger current will start to fluctuate rapidly. The fluctuation is very fast and may not be visible in 1 second average data. See Charger fluctuation troubleshooting for checking the issue and Charger cleaning for cleaning instructions.

The charger currents in particle mode are typically about 20nA. The discharger currents in offset mode are about 25nA and in alternating charging mode about 10nA.

5.2.2  Filter

The main filter is situated between the discharger and the charger. It is simply an electric filter. The filter is activated in the offset measurement mode when no particles must deposit on the collecting electrodes in the analyzer. The discharger charges particles to the opposite polarity than what is detected. The filter removes the majority of the charged particles, so that they will not induce currents in the analyzer.

The main filter electrode resembles a pulley wheel. The filter voltage when switched on is about 500V.

5.2.3  Postfilter

Ions created by the corona discharge are in the same size range as the smallest particles measured by the instrument. If they are allowed to pass into the analyzer then they would saturate the fist measurement channels as well as cause invalid signal in later channels. The postfilter is used to remove the particles before the analyzer.

The choice of the postfilter voltage is a compromise between charger ion penetration and removal of small particles. The mobility of the corona ions depends on air temperature, relative humidity and the concentration of different gaseous impurities in air. Hence it may change over time.

In the current NAIS instruments, the postfilter voltage is adjusted by a software algorithm. The principal idea is to keep the average current measured by the first channels of the analyzers at a constant low value. The method has proven to work well for long term atmospheric monitoring. For laboratory experiments with rapidly changing or unusual aerosol distributions, it is possible to switch the automatic adjustment off.

The automatic adjustment of the postfilters happens at the end of particle measurement cycles. So the adjustment is quite slow. When the instrument is fist started up it may take 30minutes to a few hours before the algorithm has settled on good voltages. The state of the filters will be stored in the measurement folder on the computer, so next measurement runs will start with the values from the previous run.

The postfilter is active in particles and alternating charging measurement modes – it is always switched on when the main charger is active and switched off otherwise. The postfilter voltage is typically around 50V.

The postfilter is a small ring in the lower part of the main charger space at about the same level as the top of the mobility analyzer central column.