Extended Experimental Studies of Reactive Halogen Chemistry in the Polar Troposphere (HALOPOLE III)

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Halogen radicals play an important role in the chemistry of the polar boundary layer. Each year during springtime, huge areas, covering several millions of square kilometres, are influenced by strongly enhanced concentrations of reactive bromine, which is emitted from saline surfaces in the Arctic and Antarctic. This phenomenon is referred to as bromine explosion. In addition, both satellite-borne and ground-based spectroscopic measurements show the presence of significant amounts of iodine oxide radicals over the Antarctic, but not over the Arctic. The reason for this asymmetry is yet unknown, but the presence of several ppt of reactive iodine in the Antarctic boundary layer should have a significant impact on the chemical balance of the atmosphere, and is also expected to amplify the ozone destruction by reactive bromine during austral spring. The activities in the framework of HALOPOLE III will focus on the key questions that emerged from our activities during HALOPOLE I and II related to the sources, sinks and transformation processes of reactive halogen species in Polar Regions. Based both on the synergistic analysis of the long-term and field data collected so far and on novel measurements in Antarctica, the main foci will be on:(1) The investigation of a striking discrepancy in the IO observations from active and passive DOAS measurements during HALOPOLE II.(2) A thorough analysis of the long-term MAX-DOAS data collected at Neumayer Station (Antarctica), Arrival Heights (Antarctica) and Alert (Canada) with respect to the influence of meteorology, origin of the air masses, vertical distribution, and the contribution of snow, sea ice and frost flowers to the release of reactive bromine.(3) The investigation of the small-scale spatial and temporal variability of BrO enhancements on the basis of a detailed analysis of the airborne MAX-DOAS measurements collected during the BROMEX 2012 campaign in Barrow/Alaska.(4) The analysis of the recently performed measurements in the marginal ice zone of Antarctica onboard Polarstern with respect to the horizontal and vertical distribution of BrO and IO, as well as the impact of halogen chemistry on ozone and the mercury cycle.(5) Further in-depth studies of the impact of halogen radicals, in particular chlorine and iodine, on the chemical balance of the polar boundary layer on the basis of a measurement campaign at Halley Bay, Antarctica.(6) More detailed long-term measurements of halogen radicals and related compounds at Neumayer Station, Antarctica, using a novel long-path DOAS instrument that will be developed within this project. In addition to the existing MAX-DOAS measurements, it will allow for year-round measurements of the full diurnal cycle and for the investigation not only of bromine- and iodine, but also of chlorine chemistry.