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.