Cl and Br atom concentrations during a surface boundary layer ozone depletion event in the Canadian high Arctic

C-2-C-5 hydrocarbons were measured in situ at Alert, Nunavut,Canada from April 14 to May 10, 1998 at a minimum frequency of 12 samples per day. During the sampling period, an event occurred where the O-3 mixing ratio fell from about 15 ppbv to the instrumental detection limit of less than 1 ppbv in a timespan of about 24 hours. Mixing ratios of alkanes and ethyne decreased concurrently with the O-3 decrease. Ancillary data suggested that this was essentially due to chemical reactions only, most probably involving halogen atoms. The hydrocarbon concentrations from the Onset of O-3 decrease until its virtually complete depletion are used in a first order kinetics analysis to calculate the concentrations of CI and Br atoms that are required to explain these observations. The Cl atom concentration is calculated to be 7.5 x 10(4) molec.cm(-3). Such a concentration is near the upper limit of earlier estimates, but cannot explain the observed decay of ethyne. Assuming the deficit to be due to bromine atom chemistry, the estimated Br atom concentration is 1.4 x 10(7) molec.cm(-3). The ethyne mixing ratio continued to decrease after virtual total O-3 depletion suggesting that Br atoms persisted when O-3 was depleted. Hence it is postulated that the depletion of total gaseous mercury (TGM) which terminates upon virtual complete O-3 depletion is due to a reaction between gaseous mercury and the BrO molecule.