Modeling Atmospheric Mercury Deposition to the Sounds and Other Water Bodies O. Russell Bullock, Jr. NOAA Air Resources Laboratory (On assignment to the U.S. EPA Office of Research and Development) North Carolina Department of Environment and Natural Resources Mercury and CO 2 Workshop McKimmon Center, Raleigh, NC April 19, 2004

Current State of the Science We know the majority of mercury (Hg) found in aquatic systems comes from atmospheric deposition. We know atmospheric Hg exists in various forms (species) that have widely different behaviors regarding atmospheric transport and deposition. Current atmospheric Hg research is focusing on: Air emission sources (flux rate and speciation) Atmospheric transformations (chemical and physical) Deposition mechanisms (wet and dry) Re-emission (Resting time? Flux rate? What forms?) Simulation models are being used to see how well our understanding of these various processes agrees with observations in the real world.

Simulation models are also being used to identify the sources responsible for atmospheric mercury deposition to specific locations and to entire nations. Lets just say theyre giving mixed signals!

Conceptual Model of Atmospheric Mercury Contemporary to the Mercury Study Report to Congress in 1997

Russ Current Conceptual Model of Atmospheric Hg

Atmospheric Deposition - The First Step As mentioned before, atmospheric deposition does provide the vast majority of the Hg entering aquatic systems. However…………. Hg deposited onto terrestrial watersheds (rather than directly into water bodies) can be bound to soils and vegetation for long periods of time. Much of this bound Hg may be re-emitted to air, never reaching the water body. The Hg that does reach the water body must be converted to MeHg in order to bio-accumulate.

Currently Measurable Forms (Species) of Atmospheric Mercury Elemental Mercury (Hg 0 ): mildly reactive gas (in most cases); sparingly soluble in water; subject to very long range transport throughout the entire atmosphere Reactive Gaseous Mercury (RGM): common term for gaseous Hg compounds that are water soluble and chemically reactive; readily deposited to water, soils and vegetation by wet and dry processes Particulate Mercury (PHg): various condensed Hg compounds and RGM bound to receptive aerosols; its morphology remains rather uncertain

So, what are RGM and PHg really made of? RGM is thought to be primarily HgCl 2 based on vapor pressure and water solubility data, but it could also include minor fractions of other volatile Hg compounds. PHg is thought to be HgO, HgS and other low vapor pressure compounds, plus more volatile compounds (and maybe even Hg 0 ) bound to carbon-rich aerosols. (elemental carbon only?)

So, what are RGM and PHg really made of? No practical air-sampling technology yet exists to measure the specific chemical compounds comprising either of these oxidized Hg species. Therefore, current models can resolve only these generalized species in air.

However, for the aqueous phase, advanced models employ a more resolved mercury speciation.

Models-3 web page

Hg reactions and rate constants in the CMAQ-Hg model

Cloud Chemistry Mechanism for the CMAQ-Hg Model

But, what about dry deposition? It is well known that particulate matter deposits from the atmosphere through dry processes, but gaseous constituents also dry deposit. It appears that dry deposition of RGM is especially important near combustion sources. The Florida everglades experience provides the best example so far of the likely importance of dry deposition of Hg to total ecosystem impacts.

Summary Comments Atmospheric Hg models are under development at a number of institutions around the world, and a concerted effort of model inter-comparison is being made to identify important uncertainties. Sparse and incomplete observational data is hampering model evaluation efforts, especially the lack of closure on the total deposition flux. Atmospheric Hg models will continue to evolve as our understanding of mercury chemistry in air and cloud water evolves.

A Final Message The greatest obstacle to discovery is not ignorance - it is the illusion of knowledge. Daniel J. Boorstin ( )