This invention relates to the reduction of charged particle loss by radially introducing sheath air from the porous wall. The corona-wire unipolar aerosol charger of the present invention includes a charging chamber, at least one aerosol inlet channel, a corona wire, an annular sheath air inlet opening, a porous tube defining a charging chamber, an annular sheath air outlet opening, and an aerosol outlet channel. The sheath air inlet opening is for radially introducing a sheath air flow into the charging chamber. The charged particles will not deposit on the wall surface of the charger if the radial velocity of the introduced sheath air at the wall is higher or comparable to the electrostatic velocity of charged particles. The dilution effect of charged particle due to the use of clean air can be further minimized by redirecting the excess clean air to the outside of the charger.
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