Measurement technique for the determination of photolyzable chlorine and bromine in the atmosphere

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 Cl_p (including Cl₂, HOCl, ClNO, ClNO₂, and ClONO₂) and Br_p (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.