Experimental determination of the dispersion of ions from a point source in the environment

ABSTRACT

The dispersion of ions from a point source has been extensively modelled but there have been very few attempts to experimentally verify the theoretical findings. The main reason for this has been the difficulty of discriminating between cluster ion and charged particle concentrations in the air. In this paper, we describe a novel technique for the experimental determination of the dispersion of ions from a point source in air. Laboratory experiments showed that the lifetime of cluster ions in an aerosol cloud was of the order of minutes. However, once they attached to aerosols, the particles retained the charge for at least 30 min, suggesting that they may be carried long distances in natural winds. A negative air ionizer was used to produce ions and charged particles in an open field in the presence of a steady horizontal wind. A neutral cluster and air ion spectrometer was used to measure cluster ion and charged particle concentrations as a function of downwind distance from the source. The results are broadly consistent with the Gaussian dispersion model for a continuous point source. We estimate that cluster ions can be carried up to a distance of several hundred metres before they fully attach to particles which can then be carried as far as 3–4 km. Therefore, these observations have important bearing on exposure to cluster ions and charged particles downwind of ion sources such as high voltage power lines and busy roads.

KEYWORDS:

• Cluster ions
• aerosol charging
• charged particle
• air pollution
• atmospheric electricity

Disclosure Statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This project was supported by the Australian Research Council [grant number DP0985726]. The neutral cluster and air ion spectrometer (NAIS) was purchased through funding from the Australian Strategic Technology Program and the Institute of Health and Biomedical Innovation, Queensland University of Technology.