Sea Surface Mercury Fluxes in the Western and Subarctic Regions of the North Pacific Ocean

Air-sea mercury (Hg) exchange is an important process in the natural Hg cycle. Hg evasion from oceans accounts for approximately 40% of the global Hg emissions into the atmosphere, which was estimated to be 8,000 t yr-1. In this study, we determined Hg fluxes through dissolved gaseous Hg (DGM) measurements using a manual method along a 47ºN transect of the North Pacific Ocean and Alaska Bay in the summer of 2017, and conducted continuous monitoring using a gas-liquid equilibrator system in sea areas between 30ºN and 50ºN in the western North Pacific Ocean during the summer of 2022 and 2023. Atmospheric gaseous elemental Hg (GEM) was also continuously observed. The average GEM concentrations in 2017 and 2023 were 1.54 ± 0.22 and 1.25 ± 0.27 ng m-3, respectively, similar to the background levels of the Northern Hemisphere. In contrast, GEM concentrations in 2022 reached approximately 2.00 ng m-3. The GEM and DGM concentrations and Hg fluxes gradually increased from north to south. The Hg fluxes were nearly zero in the region around 45°N. Hg evasion fluxes were greater between 30-35°N because of higher DGM concentrations. In addition, atmospheric GEM and seawater DGM concentrations were positively correlated with surface seawater temperature (SST). From these results, the boundary line of the SST between Hg evasion and invasion in the study areas was estimated at approximately 5 ℃, using a 1.50-ng m-3 GEM background concentration for the Northern Hemisphere.