The role of bromine and chlorine chemistry for arctic ozone depletion events in Ny-AÊ lesund and comparison with model calculations

During the Arctic Tropospheric Ozone Chemistry (ARCTOC) campaigns at Ny-AÊ lesund, Spitsbergen, the role of halogens in the depletion of boundary layer ozone was investigated. In spring 1995 and 1996 up to 30 ppt bromine monoxide were found whenever ozone decreased from normal levels of about 40 ppb. Those main trace gases and others were speci®cally followed in the UV-VIS spectral region by di€erential optical absorption spectroscopy (DOAS) along light paths running between 20 and 475 m a.s.l.. The daily variation of peroxy radicals closely followed the ozone photolysis rate J(O3(O D)) in the absence of ozone depletion most of the time. However, during low ozone events this close correlation was no longer found because the measurement of radicals by chemical ampli®cation (CA) turned out to be sensitive to peroxy radicals and ClOx. Large CA signals at night can sometimes de®nitely be assigned to ClOx and reached up to 2 ppt. Total bromine and iodine were both stripped quantitatively from air by active charcoal traps and measured after neutron activation of the samples. Total bromine increased from background levels of about 15 ppt to a maximum of 90 ppt during an event of complete ozone depletion. For the spring season a strong source of bromine is identi®ed in the pack ice region according to back trajectories. Though biogenic emission sources cannot be completely ruled out, a primary activation of halogenides by various oxidants seems to initiate an ecient autocatalytic process, mainly driven by ozone and light, on ice and perhaps on aerosols. Halogenides residing on pack ice surfaces are continuously oxidised by hypohalogenous acids releasing bromine and chlorine into the air. During transport and especially above open water this air mixes with upper layer pristine air. As large quantities of bromine, often in the form of BrO, have been observed at polar sunrise also around Antarctica, its release seems to be a natural phenomenon. The source strength of bromine from halogen activation on the pack ice, as based on the measured inorganic bromine levels, averages about 10 Br-atoms mÿ2 sÿ1 during sunlit periods in Arctic spring. The total source strength of inorganic bromine from sunlit polar regions may therefore amount to 30 kt yÿ1.