5  Principles of Operation

5.1  Overview

The NAIS is a multichannel aerosol instrument for measurement of size and mobility distributions of aerosol particles and air ions in the atmosphere. The instrument is capable of measuring the distribution of ions (charged particles and cluster ions) of both polarities in the electric mobility range from 3.2 to 0.0013 cm2/V/s and the distribution of aerosol particles in the size range from 2.0 to 40 nm. The NAIS is a successor to the Air Ion Spectrometer (AIS) (A. Mirme et al. 2007).

The NAIS has been specifically designed for monitoring of atmospheric nanometer aerosol. It can operate for long periods in a wide range of ambient conditions from polluted downtown to remote forest to measure the size distributions of naturally charged particles (ions) of both polarities as well as uncharged particles.

5.1.1  Specifications

Measurement range Particle distribution from ~2 to 40 nm
Ion distribution from 3.2 to 0.0013 cm2/V/s
(equivalent size range: 0.8 to 40 nm)
Sample flow rate 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
Weight 60 kg
Consumables none
Servicing frequency 1 to 6 months

5.1.2  Description

The NAIS utilizes the principle of electrical aerosol spectrometry. The instrument 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 synchronously mobility-classified in the mobility analyzers and measured with an array of 25s electrometers per column.

The two similar measurement columns with opposite polarities 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.

Figure 1: The NAIS primary component and airflow scheme.

Both columns have a software controlled sample preconditioning unit in front of the analyzers comprising unipolar corona chargers and electric filters. The operating mode of the preconditioner determines what the instrument is measuring. By changing the operating mode of the preconditioning, the instrument can detect either only ions or all particles including the uncharged fraction:

The measurement process of the NAIS is completely automatic and well monitored. The airflows are driven by three or four blowers that are automatically adjusted to keep a constant sampling volume flow rate and to compensate for effect of air pressure variation on particle mobility. Particle charging and filtering are also constantly monitored and adjusted by software.

All parts of the instrument are contained in a single enclosure. The NAIS requires an external computer connected via a RS-232 interface to run the measurements. Windows and Linux operating systems are supported.

The NAIS does not use or produce any harmful materials. The sampled aerosol is only treated electrically, but electric charging and filtering are just natural processes that particles are always exposed to in the atmosphere.

The NAIS requires periodic cleaning to remove deposited particles from the mobility analyzer and corona needle tips. The cleaning frequency depends on the pollution level. In case of forest measurement the NAIS can very well operate without maintenance or any human interaction for over three months.

For details on the instrument operation see (Sander Mirme 2011) and (S. Mirme and Mirme 2013).

Flagan, R. C. 1998. “History of Electrical Aerosol Measurements.” Aerosol Science and Technology 28: 301–80.
Mirme, A., E. Tamm, G. Mordas, M. Vana, J. Uin, S. Mirme, T. Bernotas, L. Laakso, A. Hirsikko, and M. Kulmala. 2007. “A Wide Range Multi-Channel Air Ion Spectrometer.” Boreal Env. Res. 12: 247–64.
Mirme, Sander. 2011. “Development of Nanometer Aerosol Measurement Technology.” PhD thesis, University of Tartu. http://dspace.utlib.ee/dspace/bitstream/handle/10062/18094/mirme_sander.pdf?sequence=1.
Mirme, S., and A. Mirme. 2013. “The Mathematical Principles and Design of the NAIS – a Spectrometer for the Measurement of Cluster Ion and Nanometer Aerosol Size Distributions.” Atmospheric Measurement Techniques 6 (4): 1061–71. https://doi.org/10.5194/amt-6-1061-2013.
Tammet, H., A. Mirme, and E. Tamm. 2002. “Electrical Aerosol Spectrometer of Tartu University.” Atmospheric Research 62 (3-4): 315–24.