Article open access publication

Measurement of the charging state of 4–70nm aerosols

Journal of Aerosol Science, Elsevier, ISSN 0021-8502

Volume 114, 2017

DOI:10.1016/j.jaerosci.2017.08.009, Dimensions: pub.1091409713,


* Corresponding author



  1. (1) Technical University of Denmark, grid.5170.3, DTU
  2. (2) University of Copenhagen, grid.5254.6, KU






The charging state of aerosols in an 8m3 reaction chamber was measured using an electrostatic classifier with a condensation particle counter at different levels of ionization in the chamber. By replacing the Kr-85 neutralizer in the classifier with a radioactively neutral dummy we were able to measure only the aerosols that were charged inside our reaction chamber. These measurements were then compared with measurements using the neutralizer to get the charging state of the aerosols, which refers to the charged fraction of the aerosols compared to an equilibrium charge distribution. Charging states were measured for both positively and negatively charged aerosols while the ionization in the chamber was varied using external gamma sources. We find that the negatively charged aerosols were overcharged (relative to the equilibrium) by up to about a factor of 10 below 10nm and at 16±2% from 10 to 70nm. At higher levels of radiation on the chamber the smaller aerosols were less overcharged while the large aerosols were more overcharged (23±2%). For the positively charged aerosols only the smallest aerosols were overcharged while those over 10nm were undercharged (relative to the equilibrium) by 21±3%. Increasing the radiation on the chamber increased the undercharge above 10nm to 25±2% while the overcharge below 10nm disappeared. The split between positive and negative charges above 10nm can be explained by differences in mobility of small negative and positive ions. The overcharge below 10nm can be explained by ions participating in the formation of aerosols of both signs, while the reduction in this overcharge at higher levels of ionization may be explained by faster recombination.


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