Key Factors Controlling Number Concentrations of Non-charged Particles: An Experimental Study

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Abstract
The charging state of submicron particles strongly affects the number of particles deposited on the inner surface of human airways during inhalation, and an increase in this number leads to adverse effects on human health. However, the key factors controlling the charging state of particles are poorly understood because of the limited measurements of the number concentrations of charged submicron particles. In order to experimentally investigate the key factors controlling the number concentration of charged particles, the technique for accurately measuring the number concentration of non-charged particles (Nnon-charge) was improved based on a system consisting of optical particle counters and a parallel-plate particle separator to which a high voltage (1.0–1.5 kV) was applied. The ratio of the Nnon-charge to the total number concentration of particles (Ntotal), with diameters 0.3–0.5 μm, was used as a metric of the number fraction of non-charged particles. A decrease in the Nnon-charge/Ntotal ratio with an increase in the negative ion concentration (nion−) and the nion−/Ntotal ratio under stable meteorological conditions showed an increase in the number of charged particles by collisions between aerosols and ions. Under fluctuating humidity conditions, the Nnon-charge/Ntotal ratio also decreased with an increase in the nion− and the nion−/Ntotal ratio. Particularly, the decrease rate in the Nnon-charge/Ntotal ratio was greater at higher nion− and nion−/Ntotal ratio values than that under stable meteorological conditions. This suggests that the generation of water vapor drives an increase in ion concentration, leading to frequent collisions between excessive negative ions and aerosols. A quantitative understanding of the changes in the number fraction of charged particles with increasing humidity and nion− can assist in estimating the number of particles deposited on the inner surface of human airways.

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