A supersonic jet ionizer (10) comprises dielectric structure defining a chamber (30) for receiving pressurized gas and a convergent-divergent nozzle (31) for causing the gas to issue from the chamber (30) in a jet at supersonic velocity. A high-voltage electrode tip (51) and a grounded metallic ring (60) are mounted in a diverging region (41) of the nozzle (31) so that a corona can be maintained in the diverging region (41). Ions formed in the corona are swept out of the nozzle (31) by the supersonic gas jet. The supersonic jet ionizer (10) can be utilized as a particulate charging means in a system (75) for filtering charged particulates from a particulate-laden gas stream.