Surface-Atmosphere Mercury Exchange: Stable Isotope Fractionation and Bidirectional Processes (Invited Presentation)

Bidirectional gaseous elemental mercury (Hg⁰) exchanges between surface and the atmosphere are important sources (or sinks) of atmospheric Hg and play an important role in global Hg cycling. However, the fractionation of stable Hg isotopes and the bidirectional Hg⁰ exchange fluxes during surface-atmospheric exchange remain largely unknown. In this study, we used a dynamic flux chamber technique coupled with the measurement of atmospheric Hg⁰ concentration and isotope composition to investigate the fractionation of Hg isotopes and bidirectional Hg⁰ exchange processes during surface-atmosphere exchange. Our results overall showed net Hg⁰ emissions from various types of soils and waters, but with occasional net Hg⁰ deposition that mainly observed at night. Hg⁰ emissions overall showed negative dependent fractionation signatures of odd Hg isotopes (odd-MDF, mean δ²⁰²Hg: -2.16 to 1.33‰, n = 12), slightly negative odd-mass independent fractionation signature (odd-MIF, mean Δ¹⁹⁹Hg: -0.34 to -0.01‰), and even-Hg MIF signatures that are similar to those of soil and water matrix. On the other hand, Hg⁰ deposition did not generate measurable odd- and even-Hg MIF, and produce positive and negative MDF of Hg isotopes over soils and water, respectively. Our results therefore suggest that the odd-MIF of Hg isotopes is overall dominated by the photochemical reduction of Hg(II) associated with surfurless ligands. Based on the Hg isotope mixing model, we estimate that the Hg⁰ emission and deposition fluxes were in the range of from 11.2 to 1282 ng m^-2 h^-1 and 1.1 to 360 ng m^-2 h^-1 over soils, and from 2.1 to 4.1 ng m^-2 h^-1 and 1.2 to 2.3 ng m^-2 h^-1 over water, respectively.