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 are lamp. In the reaction vessel of the photoactive halogen detector (PHD), photolytically active molecules Cl-p (including Cl-2, HOCl, ClNO, ClNO2, and ClONO2) and Br-p (including Br-2, HORr, 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 Cl-2 and Br-2 in air from commercially available permeation sources. We obtained detection limits of 4 pptv and 9 pptv as Br-2 and Cl-2, respectively, for 36 L samples.