BGC-01 Mercury biogeochemical cycling in the ocean
Photoproduction and air-sea exchange of dissolved gaseous mercury in the Arctic Ocean  (Invited)
Seunghee Han* , School of Earth Sciences and Environmental Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju 61005, Republic of Korea
Sangwoo Eom, School of Earth Sciences and Environmental Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju 61005, Republic of Korea
Hakwon Jeong, School of Earth Sciences and Environmental Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju 61005, Republic of Korea
Ju-Hyeong Chae, School of Earth Sciences and Environmental Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju 61005, Republic of Korea

Dissolved gaseous mercury (DGM) in surface seawater is regarded as an important source of mercury (Hg) in the atmosphere due to its rapid evasion to the atmosphere subsequent to photoproduction. Although decrease of sea ice extent and increase of river discharge stimulated by climate change in the Arctic Ocean have been known for decades, their effects on the air-sea exchange of DGM are largely unexplored. In this study, we aim to investigate the distributions of DGM and photoreduction rate constants of Hg(II) (kr) in relation to the environmental changes in the East Siberian Sea and Beaufort Sea. The surface seawater of the East Siberian Sea was collected with a latitudinal variation in the marginal ice zone and contiguous sea-ice zone on the RV Araon in August 2021. Diverse Hg species, such as total Hg, DGM, and photochemically reducible Hg, were measured using cold vapor atomic fluorescence spectroscopy. The kr was also analyzed using the photolysis box equipped with UV-A and UV-B lamps. The highest DGM concentrations were found in the contiguous ice zone, suggesting that sea ice could act as a barrier to Hg evasion. A higher kr was observed under UV-B than under UV-A with the same seawater, attributed to the higher quantum yield of the photoreducible Hg under UV-B. However, kr values normalized to the light intensity of UV-A (7.6–20 E m-2) were considerably higher than those of UV-B (0.44–1.3 E m-2). Concerning the site distribution of kr, the highest kr normalized to the UV-A intensity was found in the marginal sea-ice zone with the lowest salinity (27.1). Freshwater released through sea ice melting could effectively increase kr at the surface of the East Siberian Sea. Similar measurements were performed in the Beaufort Sea on the RV Araon from August to September 2022. The distribution of kr normalized to the UV-A intensity is currently being analyzed in comparison with the seawater properties to understand how freshwater and organic matter discharge from Makenzie River affects kr at the surface of the Beaufort Sea.