A technique has been developed to enable measurement of photolyzable chlorine and bromine at trace levels in the troposphere. In this method, ambient air is drawn through a cylindrical flow cell, which is irradiated with a Xe arc lamp. In the reaction vessel of the photoactive halogen detector (PHD), photolytically active molecules Clp (including Cl2, HOCl, ClNO, ClNO2, and ClONO2) and Brp (including Br2, HOBr, BrNO, BrNO2, and BrONO2) are photolyzed, and the halogen atoms produced react with propene to form stable halogenated products. These products are then sampled and subsequently separated and detected by gas chromatography. The system is calibrated using low concentration mixtures of Cl2 and Br2 in air from commercially available permeation sources. We obtained detection limits of 4 pptv and 9 pptv as Br2 and Cl2, respectively, for 36 L samples.¿ 1997 American Geophysical Union