The invention provides an apparatus for charging or altering the charge of gas-entrained particles in an aerosol, the apparatus comprising:(a) an ion generating chamber (1) containing a first electrode (2) for generating a corona discharge, the first electrode (2) being connected to a power supply of sufficiently high voltage to create the corona discharge; the ion generating chamber (1) having an ion outlet (10) through which ions generated by the corona discharge can leave the chamber (1); (b) a particle charging chamber (5) in which charging or altering the charge of gas-entrained particles in an aerosol takes place, the particle charging chamber (5) being in fluid communication with the ion generation chamber (1) and having an inlet and an aerosol outlet; and (c) an electrically non-conductive interface body (7) positioned between the aerosol particle charging chamber (5) and the ion generating chamber (1), the interface body (7) having a hollow interior which is in fluid communication with the ion generating chamber (1) and the aerosol particle charging chamber, and having a gas inlet (8) through which a stream of gas can be introduced into the hollow interior of the interface body (7).