Measurements of photolyzable halogen compounds and bromine radicals during the Polar Sunrise Experiment 1997

As part of the Polar Sunrise Experiment (PSE) 1997, concentrations of halogen species thought to be involved in ground level Arctic ozone depletion were made at Alert, NWT, Canada (82.5°N, 62.3°W) during the months of March and April, 1997. Measurements were made of photolyzable chlorine (Cl₂ and HOCl) and bromine (Br₂ and HOBr) using the Photoactive Halogen Detector (PHD), and bromine radicals (BrO(x)) using a modified radical amplifier. During the sampling period between Julian Day 86 (March 27) and Day 102 (April 12), two ozone depletion episodes occurred, the most notable being on days 96-99, when ozone levels were below detectable limits (≃ 1 ppbv). Concentrations of BrO(x) above the 4 pptv detection limit were found for a significant part of the study, both during and outside of depletion events. The highest BrO(x) concentrations were observed at the end of the depletion event, when the concentration reached 15 pptv. We found substantial amounts of Br₂ in the absence of O₃, indicating that O₃ is not a necessary requirement for production of Br₂. There is also Br₂ present when winds are from the south, implying local scale (e.g. from the snowpack) production. During the principal O₃ depletion event, the HOBr concentration rose to ≃ 260 pptv, coincident with the BrO(x) maximum. This implies a steady state HO₂ concentration of 6 pptv. During a partial O₃ depletion event, we estimate that the flux of Br₂ from the surface is about 10 times greater than that for Cl₂.