Unusually strong diurnal variability of ozone (O3) above summer snow in East Antarctica – a discussion of pre-cursor snow emissions and atmospheric transport

Field studies in the high and mid latitudes have demonstrated that snowpack emissions of reactive trace gases driven by photolysis alter regional atmospheric composition, the fate of pollutants and the polar ice core archive of past environmental change. Of particular interest are reactive nitrogen and halogen species released by surface snow, which in turn influence atmospheric levels of O₃ and hydroxyl radicals (OH and HO₂). Previous field campaigns at South Pole and Dome C showed that surface-near air on the high East Antarctic Plateau in summer is highly oxidising due to the interplay of photolytic snow emissions, a shallow boundary layer and cold temperatures.  However, open questions remain regarding the atmospheric oxidant budget above polar snow. Here we present recent observations carried out as part of the ISOL-ICE project at Kohnen Station (75ºS 0ºW) in austral summer 2017, located at a similar latitude as Dome C. Concurrent measurements of nitrogen oxides (NO and NO₂), atmospheric particulate nitrate collected on filters, O₃, slant-column bromine oxide (BrO), actinic flux and atmospheric turbulence were carried out for the first time at Kohnen. The bulk ion composition of in surface snow and shallow pits was measured as well.

While diurnal cycles of NO_x and turbulent diffusivity were similar to previous observations at Dome C, a distinct and strong diurnal cycle of surface O₃ with an amplitude of more than 10 nmol mol^-1 was detected. O₃ showed also a negative correlation with BrO in the lower atmosphere. These observations may imply O₃ photochemical source/sink processes, which are stronger than seen previously on the East Antarctic Plateau. We discuss the role of O₃ precursor emissions from the sunlit snowpack and vertical mixing with a view of the implications for our understanding of O₃ above polar snow.