MEASUREMENTS OF PHOTOLYZABLE CHLORINE AND BROMINE DURING THE POLAR SUNRISE EXPERIMENT 1995

We report measurements of rapidly photolyzable chlorine (Cl-p; e.g., C l-2 and HOCl) and bromine (Br-p; e.g., Br-2 and HOBr) in the high Arct ic using a newly developed photoactive halogen detector (PHD). Ground level ambient air was sampled daily from mid-February through mid-Apri l in the Canadian Arctic at Alert, Northwest Territories (82.5 degrees N, 62.3 degrees W), as part of the Polar Sunrise Experiment (PSE) 199 5. Concentrations of ''total photolyzable chlorine'' varied from <9 to 100 pptv as Cl-2 and that of ''total photolyzable bromine'' from <4 t o 38 pptv as Br-2. High concentration episodes of chlorine were observ ed only prior to sunrise (March 21), while high concentration episodes of bromine were measured throughout the study. The high concentration s of photolyzable chlorine and bromine prior to sunrise suggest a ''da rk'' production mechanism that we assume yields Cl-2 and Br-2. An inve rse correlation of bromine with ozone is clearly present in one major ozone depletion episode at the end of March. A trajectory analysis, ta ken with the differences in measured levels of photolyzable chlorine a nd bromine after sunrise, imply different production mechanisms for th ese two types of species. A steady state analysis of the data for one ozone depletion episode suggests a [Br]/[Cl] ratio in the range 100-30 0. The high concentrations of photolyzable bromine after sunrise imply the existence of a precursor other than aerosol bromide.