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Photochemical oxidation of atmospheric elemental mercury (Hg0) promotes reactive oxidized Hg (HgII) adsorption on particles and deposition to the polar snowpack. The deposited Hg either returns to the atmosphere via photochemical reduction or remains in the snowpack depending on the strength of adsorption. In this study, we performed ab initio calculations to understand the atomic-level cause of the fate of adsorbed Hg by determining the adsorption affinity for Hg2+, the simplest form of HgII, of barite, halite, muscovite, illite, and ice-Ih as potential adsorbents. The adsorption affinity was estimated by calculating the energy required to dissociate adsorbed Hg2+ from the adsorbents. The results reveal that Hg2+ is stable on the surfaces of the selected adsorbents, except barite, but is prone to photodissociation under solar ultraviolet radiation. This mild adsorption is expected to contribute to the bidirectional exchange of Hg between the atmosphere and the polar snowpack. Thus, this theoretical approach can provide complementary perspectives on polar Hg dynamics beyond the limitations of field and laboratory experiments. Further studies on more complicated and realistic adsorption models with different HgII species and adsorbent surfaces having diverse defective structures are required to better comprehend air−snow Hg cycling in the polar regions.

Despite its important geophysical implications, direct probing of the local electronic structure of mantle minerals, such as MgSiO3 perovskite and post-perovskite is experimentally challenging. Recent advances in ab initio calculations have allowed us to explore the details of the local electronic bonding structure around oxygen in MgSiO3 polymorphs in Earth's interior. Here, we calculate the O K-edge energy-loss near-edge structure (ELNES) spectra for SiO2 and MgSiO3 polymorphs (i.e., a-quartz, stishovite, enstatite, ilmenite-type MgSiO3, MgSiO3 perovskite, and post-perovskite) using ab initio calculations based on the full-potential linearized planewave (FP-LAPW) method. The calculated O K-edge ELNES spectra for SiO2 and MgSiO3 polymorphs show characteristic oxygen K-edge features caused by distinctive local atomic configurations and topology around oxygen, and are in good agreement with previous experimental O K-edge X-ray Raman scattering (XRS) results. The O K-edge ELNES spectra for a-quartz and enstatite show similar edge features at ~538 eV, which is characteristic of corner-sharing oxygen sites ([4]Si-O-4Si). The spectra for stishovite and ilmenite-type MgSiO3 show edge features with double peaks at ~537-538 and ~541-543 eV due to an electronic excitation from an oxygen in edge-sharing topology. The spectrum for MgSiO3 perovskite shows a broad peak spanning from ~538 to ~543 eV, which results from corner-sharing oxygen with two six-coordinated silicon (4Si-O-4Si). The calculated O K-edge ELNES spectrum for MgSiO3 post-perovskite shows a predicted main feature at ~543-545 eV, approximately eV higher than that of MgSiO3 perovskite. These O K-edge features systematically shift to higher energy with increasing degree of densification in atomic arrangement in the polymorphs (from enstatite, ilmenite, perovskite, to post-perovskite), indicating an increase in the energy of unoccupied oxygen 2p-state with pressure. The calculated O K-edge spectra also show the effect of densification on the changes in the edge features for the crystallographically distinct oxygen sites: the features for the corner-sharing oxygen move to higher energy from enstatite, perovskite, to post-perovskite. A drastic peak shift for edge-sharing O atoms in ilmenite-MgSiO3 and post-perovskite is also observed. These results confirm that the oxygen K-edge features at ~540-550 eV for MgSiO3 glass at pressures above ~20 GPa can be due to densification of the atomic configurations around oxygen in melt networks associated with enhanced proximity between oxygen atoms. The current methods also shed light on a unique opportunity to probe the pressure-induced electronic bonding transitions and topology in diverse simple and complex oxides in Earth's interior using ab initio calculations of O K-edge ELNES spectra. [ABSTRACT FROM AUTHOR]

Despite its fundamental importance in condensed matter physics and geophysical implications, establishing the systematic and direct link between the pressure-induced structural changes in crystalline and noncrystalline low-z oxides and their corresponding evolution in O K-edge core-electron excitation features under extreme compression has been challenging. Here we calculated the site-resolved partial density of states and O K-edge x-ray Raman scattering (XRS) spectra for two of the important oxide phases in the Earth's lower mantle, MgSiO3 bridgmanite and post-bridgmanite, up to 120 GPa using ab initio calculations, revealing the electronic origins of the O K-edge features for oxides under compression. The absorption threshold (EA) and band gap increase linearly with a decrease in the O-O distance in diverse SiO2 and MgSiO3 high-pressure phases [EA(eV)≈-10.9dO-O(Å)+34.4], providing a predictive relationship between the EA and the O-O distances in the oxide at high pressure. Despite densification, upon isobaric phase transition from bridgmanite to post-bridgmanite at 120 GPa, a decrease in band gap results in a decrease in edge energy because of an increase in O-O distance. The oxygen proximity is a useful structural proxy of oxide densification upon compression, as it explains the pressure-induced changes in O K-edge XRS features of crystalline and amorphous SiO2 and MgSiO3 at high pressures. These results can be applied to studies of the pressure-bonding transitions in a wide range of oxides under extreme compression. [ABSTRACT FROM AUTHOR]

As oxygen may occupy a major volume of oxides, a densification of amorphous oxides under extreme compression is dominated by reorganization of oxygen during compression. X-ray Raman scattering (XRS) spectra for SiO2 glass up to 1.6 Mbar reveal the evolution of heavily contracted oxygen environments characterized by a decrease in average O-O distance and the potential emergence of quadruply coordinated oxygen (oxygen quadcluster). Our results also reveal that the edge energies at the centers of gravity of the XRS features increase linearly with bulk density, yielding the first predictive relationship between the density and partial density of state of oxides above megabar pressures. The extreme densification paths with densified oxygen in amorphous oxides shed light upon the possible existence of stable melts in the planetary interiors. [ABSTRACT FROM AUTHOR]