Characterisation of corona-generated ions used in a Neutral cluster and Air Ion Spectrometer (NAIS)
- 1Department of Physics, P.O. Box 64, 00014 University of Helsinki, Finland
- 2Natural Sciences and Engineering Department, Rezekne Higher Education Institution, Atbrivosanas aleja 90, Rezekne, LV 4601, Latvia
- 3Institute of Physics, University of Tartu, Ülikooli 18, 50090 Tartu, Estonia
- 4Finnish Meteorological Institute, Research and Development, P.O. Box 503, 00101 Helsinki, Finland
- 5Aerodyne Research Inc, Billerica, MA 01821, USA
Abstract. We characterized size and chemical composition of ions generated by a corona-needle charger of a Neutral cluster and Air Ion Spectrometer (NAIS) by using a high resolution differential mobility analyzer and a time-of-flight mass spectrometer. Our study is crucial to verify the role of corona-generated ions in the particle size spectra measured with the NAIS, in which a corona charger is used to charge aerosol particles down to the size range overlapping with the size of generated ions. The size and concentration of ions produced by the corona discharging process depend both on corona voltage and on properties and composition of carrier gas. Negative ions were <1.6 nm (0.8 cm2 V−1 s−1 in mobility) in all tested gas mixtures (nitrogen, air with variable mixing ratios of water vapour), whereas positive ions were <1.7 nm (0.7 cm2 V−1 s−1). Electrical filtering of the corona generated ions and not removing all charged particles plays an important role in determining the lowest detection limit. Based on our experiments, the lowest detection limit for the NAIS in the particle mode is between 2 and 3 nm.