NAIS - Neutral cluster and Air Ion Spectrometer
The NAIS is a multichannel aerosol instrument for measurement of size and mobility distributions of aerosol particles and air ions in the atmosphere.
The NAIS consists of two multichannel electrical mobility analyzer columns operating in parallel. The columns differ by the polarity of the ions measured, but are otherwise identical. The aerosol is mobility-classified in the mobility analyzers and measured in parallel with an array of 21 electrometers in both columns.
Both columns have a software controlled sample preconditioning unit in front of the analyzers, which allows the instrument to switch between detecting either naturally charged particles (ions) or all particles (including the uncharged fraction) by using unipolar charging.
The two similar measurement columns with opposite polarity in parallel allow the NAIS to detect variations of natural electric charge balance in the atmosphere and possible effects of electric charge polarity on charging of nanometer size aerosol.
|Particle distribution||from ~2 to 40 nm|
|Ion distribution||from 3.2 to 0.0013 cm2/V/s|
(from 0.8 to 40 nm size equivalent)
|Sample flowrate||54 l/min|
|Time resolution||1 second|
1 - 5 minute averages typically used during long-term monitoring
|Power consumption||70 W, AC 110/240 V|
|Dimensions||L 580 mm, W 305 mm, H 810 mm|
|Servicing frequency||1 to 6 months|
The primary components of the NAIS are the two mobility analyzer columns: one for negative one for positive particle polarity. In front of each analyzer is a sample preconditioning unit which performs aerosol charge neutralization and controlled charging.
The sample preconditioning unit consists of a discharger, an electric filter, a charger and another electric filter (called the post-filter). The charger and post-filter correspond to the electrical measurement principle (Flagan, 1998) where the instrument consists of aerosol charging followed by mobility analysis and data acquisition.
In ions mode all components of the preconditioning unit are switched off, so the aerosol sample is left unmodified and only naturally charged particles are sensed by the electric mobility analyzer. In this case the NAIS operates just like the AIS.
In particles mode the main charger is switched on and the instrument detects all particles. The post-filter is used to remove traces of charger ions. In this operating mode the NAIS is similar to the Electrical Aerosol Spectrometer (EAS, Tammet et al., 2002 1). To improve the instrument performance when measuring aerosol with non-steady state charge distribution, the discharger may be switched on (alternative charging mode). This provides some neutralization of the aerosol sample and so reduces the effect of the particle initial charge on the measurement result.
When only the discharger and the adjacent filter are switched on then no detectable particles can enter the analyzer. This is called the offset mode and it is used to periodically verify the instrument operation (e.g. evaluate noise levels and measure parasitic currents).
The mobility analyzer sheath air is taken from the analyzer exhaust air that has been cleaned by electric filters.
The NAIS does not use or produce any harmful materials. The aerosol only treated electrically – electric charging and filtering are natural processes which happen to particles in the atmosphere as well.
The size and mobility distributions for either polarity are derived by a mathematical deconvolution procedure from the 21 electrometers of the corresponding analyzer.
The measurement process is completely automatic and well controlled. The airflows are driven by four blowers that are software controlled to compensate for possible obstructions and to take into account the effect air pressure on particle mobility. Particle charging and filtering are also continuosly monitored and adjusted by software.
All parts of the instrument are contained in the single enclosure. The NAIS is controlled by an external computer via a RS-232 interface.
Every once in a while the NAIS requires cleaning to remove deposited particles from the mobility analyzer and corona needle tips. The cleaning frequency depends on the pollution level and may exceed three months.
- Tammet H., Mirme A. & Tamm E. 2002. Electrical aerosol spectrometer of Tartu University. Atmos. Res. 62: 315–324. [return]