Gas-Phase Photolysis of Hg(I) Radical Species: A New Atmospheric Mercury Reduction Process

The efficient gas-phase photoreduction of Hg­(II) has recently been shown to change mercury cycling significantly in the atmosphere and its deposition to the Earth’s surface. However, the photolysis of key Hg­(I) species within that cycle is currently not considered. Here we present ultraviolet–visible absorption spectra and cross-sections of HgCl, HgBr, HgI, and HgOH radicals, computed by high-level quantum-chemical methods, and show for the first time that gas-phase Hg­(I) photoreduction can occur at time scales that eventually would influence the mercury chemistry in the atmosphere. These results provide new fundamental understanding of the photobehavior of Hg­(I) radicals and show that the photolysis of HgBr increases atmospheric mercury lifetime, contributing to its global distribution in a significant way.