FATE OF MERCURY IN THE ARCTIC Michael Evan ... - COGCI

Fate of Mercury in the Arctic 25
The two reviews suggest that much like other trace constituents in the atmosphere, the
atmospheric chemistry of mercury involves 5 probable families of reaction mechanisms/pathways:
1. Gas phase reactions; 2. Aqueous phase reactions, in cloud and fog droplets and deliquesced
aerosol particles; 3. Partitioning of elemental and oxidised mercury species between the gas and
solid phases; 4. Partitioning between the gas and aqueous phases; 5. Partitioning between the solid
and aqueous phases for insoluble particulate matter scavenged by fog or cloud droplets.
Since the high Arctic is so dry, the predominating reactions must occur in the gas phase and the
predominating deposition in the spring will be dry deposition. Dry depositional removal of gases
from the atmosphere to a surface is one of the main processes by which pollutants are removed
from the atmosphere. Dry deposition refers to the combined effects of three transfers: 1. Transfer
through the turbulent layer of the atmosphere, 2. Molecular transfer through the viscous layer and 3.
Transfer to the surface as a result of contact adsorption, dissolution and other contact phenomena
(Karlsson and Nyholm, 1998). With respect to dry deposition on snow, Valdez et al. (1987) found
that depositional velocity, Vd, most importantly is influenced by the liquid water content of the
snow, i.e. the liquid water-to-air ratio in the snow, and lesser influenced by the density i.e. age of,
and the degree of metamorphism of the snow pack, as well as sun light and temperature. Vd
increases for increased values of snow water content, decreased values of Henry’s law Constant, H,
and increased diffusion coefficient. The corollary is that surface resistance, rs, decreases. Vd
decreases, i.e. rs increases, with decreased snow temperature, or increased time. The increased time
exposure reflect that one expects desorption to occur as atmospheric concentrations are depleted
and the concentrations in the snow come into equilibrium with the atmosphere.