Bipolar Charger (Neutralizer)

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Before the aerosol particles enter the DMA, they are brought to a bipolar charge equilibrium using a bipolar diffusion charger. This bipolar charge equilibrium can be theoretically described by the work of Fuchs (1963) and Gunn (1956). Positive and negative ions are produced continuously in this bipolar diffusion charger, for instance by a radioactive source. The radioactive sources used in field observation include 85Kr, 14C, and 63Ni (both beta-radiators), and 241Am and 210Po (both alpha-radiators). To achieve a bipolar charge equilibrium, the particle number concentration should be significantly lower than the equilibrium ion pair concentration, which is believed to be approximately 107 cm−3. Mobility particle size spectrometers can be thus only used for total particle number concentrations up to 106 cm−3. A complication is that alpha-particles emitted from radioactive sources such as 241Am and 210Po lose their energy within 5 cm traveling distance in air and produce beside air ions also ozone leading to the production of unintended nucleation particles.

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