Corona discharge effects on aerosol sampling efficiency

Experiments were carried out to investigate the effects of probe electrification on the efficiency of the sampling of particles from air moving at relatively high wind speeds ranging from about 10 to 30 m s^-1 The aim was to gain physical insight which might be extended to the case of the sampling of atmospheric aerosol from aircrafts. Polydisperse NaCl aerosol particles with number median diameter of 0.05 μm were sampled using a metallic thin-walled probe aligned with the air stream. Penetration of aspirated particles through tubing of different materials was measured when a potential of -30 kV DC was applied to the probe. Penetration was found to increase from 0.16 to 0.87 as the external air speed increased, and the results were interpreted as indicating that sampling efficiency was influenced by the electrification of the probe itself. Experiments were also performed with nearly monodisperse liquid DOP aerosols with number median diameters from 0.05 to 0.5 μm and the results were used to examine the charge transfer of ions (from the corona discharge which occurred at the sharp-edged probe tip) to the aerosol particles. In this part of the study, negative and positive corona discharges around the sampling inlet were created by applying potentials of up to ± 20 kV to the probe. It was found that, for given potential, the negative corona was much more effective in charging the aerosol, probably due to the presence of unattached free electrons. Such charge transfer process may be attributed to combined field and diffusion charging mechanisms.