Molecular halogens before and during ozone depletion events in the Arctic at polar sunrise: concentrations and sources

The molecular halogens Br2, BrCl and Cl2 were monitored from 9 February to 13 March 2000 as part of the ALERT 2000 campaign to investigate the causes of ozone depletion at polar sunrise. The measurements were performed over the transition period from winter to spring in the high Arctic, at Alert, on northern Ellesmere Island in Nunavut, Canada. The measurement campaign for these species covered the period from 24-h darkness, at the beginning of the campaign, to several hours of direct sunlight per day at the end of the campaign. The halogen measurements were made by atmospheric pressure ionization tandem mass spectrometry, using multiple isotopes for each species, and reporting a 20-s average for each species every 2 min. Bromine was observed above the 0.2 ppt detection limit throughout the campaign at mixing ratios up to 27 ppt. BrCl was not observed above its 2 ppt detection limit until mid-way through the campaign, but was present almost continuously thereafter, and reached levels of 35 ppt. Molecular chlorine was not observed above its 2 ppt detection limit. During periods of ozone depletion, there was a very strong inverse relationship between O3 and Br2, and a moderately strong inverse relationship between O3 and BrCl. The slopes of linear regressions of Br2 and BrCl vs. O3 indicate ≈1 ppb decrease in O3 mixing ratio for every ppt of either of the molecular halogens. In some cases, O3 depletion events seemed to be triggered by bursts of the halogen species initiated by photochemical processes, even in very weak “twilight”. In other cases, ozone depletion observed at Alert appeared to result from transport of O3-depleted, halogen-enriched air from other locations.