Halogen activity in the coastal boundary layer at Neumayer III/Antarctica – results from two and a half years of observations.

The role of Reactive Halogen Species (RHS - IO, BrO, ClO, etc.) in the lower polartroposphere has been subject of intense research for over three decades. Elevated BrO mixingratios have been identified as the cause of Ozone Depletion Events (ODEs) and thecoincidental oxidation of gaseous elemental mercury frequently occurring in polar springtimein Arctic and Antarctic. An autocatalytic release mechanism from saline surfaces (sea ice oraerosols), the so-called bromine explosion, has been identified as the likely cause of the rapidincrease of inorganic bromine mixing ratios over large areas. Laboratory studies and recentfield observations suggest however, that other release pathways might exist. The role ofhalogens like iodine and chlorine species remain unclear – for a large part due to a lack ofobservations. From January 2016 until August 2018, we operated an automated Long-Path DifferentialOptical Absorption Spectroscopy (LP-DOAS) instrument at the German research stationNeumayer III (NMIII) in coastal Antarctica. Over the 31 months observation period atemporal coverage of about 60% was reached with interruptions mainly due toblowing snow during strong storms. The instrument’s coverage of the UV and visiblespectral range allows the detection of a wide range of trace gases including ClO,BrO, OClO, IO, O3 and NO2 at temporal resolutions of 5-30 minutes. Co-locatedinstruments (e.g. MAX-DOAS, ozone monitor, CPC, SMPS, nephelometer) and extensivemeteorological observations at NMIII station allow detailed interpretations of the dataset. In addition to the well-known springtime events, our analysis unexpectedly revealshalogen activity throughout large parts of the year, which seems to be modulated by thesemi-annual oscillation over the Southern Ocean and the associated occurrence and strengthof cyclones that transport salty aerosols to snow surfaces on the ice shelf at NMIII.Most of the non-springtime events occur during surface temperature inversions inshallow layers close to the ground and seem to be the result of local chemistryrather than transport. BrO mixing ratios frequently exceed 20 ppt and peak up tounprecedented 110 ppt. Furthermore, ClO mixing ratios up to 90 ppt could be detectedand the influence of inter-halogen reactions on ozone mixing ratios was observed.The source mechanism for reactive chlorine, however, remains unclear. IO most ofthe time was below the detection limit of 1-2 ppt and sporadically reached 3-4ppt. [ABSTRACT FROM AUTHOR]