Speciation and interconversion of atmospheric mercury

Speciation of atmospheric gaseous oxidized mercury (GOM) and particle-bound mercury (PBM) controls the rate of deposition of these two forms to the land and oceans. What chemicals lurk behind the acronyms GOM and PBM? What processes interconvert these forms between each other and gaseous elemental mercury (Hg)? These questions cannot be answered independently of each other, as they are tightly coupled. Even in the case of GEM, in addition to direct emissions from land and oceans it can be formed directly in the atmosphere, e.g., by thermal dissociation of labile Hg(I) intermediates and photochemical dissociation of relatively stable Hg(II) compounds, GOM and PBM, whose chemical makeup is still poorly known.The challenge of chemically speciating Hg(II), in addition to its ultra-trace concentration levels, is compounded by its labile nature. Furthermore, it is highly likely that there are two kinds of GOM in the atmosphere: the first comes directly from the photochemical oxidation of GEM while the second is a result of volatilization of PBM. In essence, GOM of the second kind is formed when the first kind, taken by aerosol particles, undergoes particle-phase processing and then re-enters the gas phase in a different chemical form. For instance, following this scheme, BrHgOH from the photochemical oxidation of GEM mediated by Br, ozone, and volatile organic compounds can be taken by sea-salt aerosol and converted to mercuric chloride, which would re-enter the gas phase to be identified as GOM. A similar uptake-reemission process to form mercuric chloride might be even possible on continental urban aerosols, depending on the values of rate constants and equilibrium constants of relevant reactions, not all of which have been fully identified. This presentation will provide an overview of the above mentioned processes and their roles in controlling the speciation of atmospheric mercury in all of its forms.