Measurements of photolyzable chlorine and bromine during the Polar sunrise experiment 1995

We report measurements of rapidly photolyzable chlorine (Cl-p; e.g., Cl-2 and HOCl) and bromine (Br-p; e.g., Br-2 and HOBr) in the high Arctic using a newly developed photoactive halogen detector (PHD). Ground level ambient air was sampled daily from mid-February through mid-April in the Canadian Arctic at Alert, Northwest Territories (82.5 degrees N, 62.3 degrees W), as part of the Polar Sunrise Experiment (PSE) 1995. Concentrations of ''total photolyzable chlorine'' varied from <9 to 100 pptv as Cl-2 and that of ''total photolyzable bromine'' from <4 to 38 pptv as Br-2. High concentration episodes of chlorine were observed only prior to sunrise (March 21), while high concentration episodes of bromine were measured throughout the study. The high concentrations of photolyzable chlorine and bromine prior to sunrise suggest a ''dark'' production mechanism that we assume yields Cl-2 and Br-2. An inverse correlation of bromine with ozone is clearly present in one major ozone depletion episode at the end of March. A trajectory analysis, taken with the differences in measured levels of photolyzable chlorine and bromine after sunrise, imply different production mechanisms for these two types of species. A steady state analysis of the data for one ozone depletion episode suggests a [Br]/[Cl] ratio in the range 100-300. The high concentrations of photolyzable bromine after sunrise imply the existence of a precursor other than aerosol bromide.