The influence of sea ice variability on bromine monoxide in coastal Antarctica

Abstract:

Sea ice plays an important role in the production and distribution of bromine in the polar troposphere. Bromine molecules are released from the sea ice via heterogenous reactions on sea salt surfaces, both in the sea ice and on blowing snow. This can initiate a chemical reaction, known as a bromine explosion, depleting ozone and oxidising gaseous elemental mercury, leading to the deposition of toxic mercury to the polar environment. Sea ice formation helps to concentrate sea salts, resulting in freshly formed first-year sea ice (and snow on top) having a higher salinity and therefore being particularly important for bromine production. Given the remoteness of Antarctica, the distribution and variability of reactive bromine, along with environmental mechanisms that control bromine cycling, are still poorly characterised.

Presented here are MAX-DOAS observations of bromine monoxide (BrO) carried out onboard the Australian icebreaker, the RSV Aurora Australis between Oct 2017 and Mar 2019 to three Antarctic locations: Davis (68.6°S, 78.0°E), Casey (66.3°S, 110.5°E) and Mawson (67.6°S, 62.9°E). The temporal, vertical and geographical distribution of BrO in the lower troposphere is described along with the mechanisms that drive BrO cycling in the sea ice environment.

Results show that BrO concentrations are largely uniform between Antarctic locations in the lower troposphere, with median BrO ranging from 3.32 ± 0.98 pptv at Casey to 4.00 ± 0.99 pptv at Davis. At Davis, BrO concentrations decreased between spring and summer, likely due to a reduction in sea ice. Vertical profiles show that BrO concentrations decrease with altitude, with the BrO maxima always located below 1 km altitude. Additionally, observations show that high BrO events fall into two distinct types. The first type involves high BrO concentrations in the surface layer (below 0.1 km) that peak around midday and correlate with low-wind speeds (<7m/s), allowing bromine to accumulate above the sea ice. The second type involves high BrO concentrations in the lower troposphere (~0.3 to 0.7 km), forming a double maxima (morning/evening) and correlate with high wind speeds (>7m/s), potentially as a result of blowing snow. Implications of these BrO events for mercury as well as uncertainties in the distribution of BrO over coastal Antarctica will also be discussed.