Modeling of Atmospheric mercury
The concentration of mercury (Hg) in water has become
an important issue since high methylmercury levels have been found in fish. The
methylmercury is neurotoxin, which forms from deposited atmospheric mercury and
then bioaccumulates in the aquatic food chain. Each year, about 7700 tons of
mercury is emitted into the atmosphere. Human activities are responsible for
about one third is it, and the remaining emissions come from various natural terrestrial
surfaces including soil, water and vegetation.
In the atmosphere Hg chemically inter-converts between
various forms, namely elemental mercury (GEM), reactive oxidized bivalent
mercury (RGM), and particulate mercury (TPM). GEM constitutes more that 90% of
total mercury in the atmosphere. The
atmospheric chemical processes, which interconvert different mercury species,
strongly influence the transport and deposition of the mercury. Modeling
efforts to assess global cycling of mercury require an in-depth knowledge of
atmospheric mercury chemistry.
Mercury is deposited from the atmosphere by wet and
dry processes, with lifetime up to 2 years. The mercury states vary in chemical
and physical characteristics, and these strongly influence their deposition
rates. Deposited Hg undergoes chemical and biochemical processes, which include
transformation to methylmercury and reductions to GEM, which is reemitted back
to the atmosphere. Therefore, understanding
of exchange processes between atmosphere and Earth surfaces is very important
in mercury cycling and transport.
The bidirectional mercury surface exchange is based on
assumption that the emission of volatile elemental mercury deposited in land
and water surfaces follow Fick’s law. The exchange
velocity of the mercury between the surface media and the atmosphere are
parameterized using a resistance analogy and partitioning coefficients. The
change in surface media concentrations is parameterized by a system of ordinary
differential equations.
Scheme
of mercury exchange processes between the atmosphere and Earth surfaces
Accurate
descriptions of gas phase as well as aqueous phase chemistry of mercury are
necessary because there is a complex relationship between a number of species
and the aqueous Hg concentrations. A complete model describing the gas and
aqueous phase chemistry of mercury contains as large as 40 gas phase species in
80 reactions and 30 aqueous species in 100 reactions. Such model requires
special algorithms for solving the corresponding system of differential equations.
Scheme of chemical transformations of mercury in the atmosphere
Quantum-chemical modelling of mercury
Potential energy surface (PES) of the Mercury-Water CS complex structure
