An ionizing arrangement for electrically charging particles in an air flow in an air duct, said air duct including a section B extending along an axis A. The arrangement includes: a shielding electrode intended to be arranged within the air duct, said shielding electrode including: a portion for arrangement in a plane substantially transverse to the longitudinal axis A, said member has a shape corresponding to the cross sectional shape of the duct. The arrangement furthermore includes at least one corona electrode arranged in the center of the portion, wherein the shielding electrode is further arranged to provide shielding of an electromagnetic field emanating from the corona electrode.