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

Oceanic emission of gaseous elemental mercury (Hg⁰ or GEM) is an important source for atmospheric mercury (Hg), but existing estimates of global gross oceanic Hg⁰ 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 Hg⁰ emissions. Significantly lower mean (±1sd) concentration (1.28 ± 0.20 ng m⁻³), Δ¹⁹⁹Hg (−0.23 ± 0.03‰), and Δ²⁰⁰Hg (−0.066 ± 0.018‰) and significantly higher δ²⁰²Hg (0.43 ± 0.12‰) were observed during the wet season when air masses were predominantly from the Southern Hemisphere, compared with those (mean concentration, Δ¹⁹⁹Hg, Δ²⁰⁰Hg, and δ²⁰²Hg 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 Δ²⁰⁰Hg mass balance model, we estimated that the oceanic Hg⁰ 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 Δ²⁰⁰Hg. The findings from the present study will help to better understand Hg⁰ emissions from global oceans and their roles in global atmospheric Hg cycling.