An Interhemispheric Difference in Atmospheric Gaseous Elemental Mercury Isotopes Reveals a New Insight in Oceanic Mercury Emissions

Abstract Oceanic emission of gaseous elemental mercury (Hg 0 or GEM) is an important source for atmospheric mercury (Hg), but existing estimates of global gross oceanic Hg 0 emissions are highly variable (800–7,220 Mg yr −1 ). This study measured atmospheric GEM concentrations and isotopic compositions at two coastal sites in Terengganu, Malaysia, a region that receives air masses from both hemispheres, during 2019–2021 to diagnose the amount of oceanic Hg 0 emissions. Significantly lower mean (±1sd) concentration (1.28 ± 0.20 ng m −3 ), Δ 199 Hg (−0.23 ± 0.03‰), and Δ 200 Hg (−0.066 ± 0.018‰) and significantly higher δ 202 Hg (0.43 ± 0.12‰) were observed during the wet season when air masses were predominantly from the Southern Hemisphere, compared with those (mean concentration, Δ 199 Hg, Δ 200 Hg, and δ 202 Hg of 1.77 ± 0.09 ng m −3 , −0.17 ± 0.03‰, −0.045 ± 0.023‰, and 0.25 ± 0.11‰, respectively) during the dry season when air masses were predominantly from the Northern Hemisphere, suggesting interhemispheric difference in GEM concentrations and its isotopic compositions. Using a Δ 200 Hg mass balance model, we estimated that the oceanic Hg 0 emissions from Hg II reduction should be below 2,250 ± 891 Mg yr −1 (±1sd), which is at the low‐end range of the literature reported values. We then used the constrained value as emission input to a three‐dimensional atmospheric Hg isotope model and reproduced well the global distributions and interhemispheric gradient of atmospheric GEM Δ 200 Hg. The findings from the present study will help to better understand Hg 0 emissions from global oceans and their roles in global atmospheric Hg cycling.