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

Fate of Mercury in the Arctic 104
Depositional velocity and surface resistance for RGM in Barrow
Numerical depositional models require the surface resistance and depositional velocity for
proper model parameterization. Zannetti, 1990, defines an operational definition of dry deposition
velocity as:
1
Vd =
ra + rm + rs
(7)
Where ra is the aerodynamic resistance, depending on the atmospheric turbulent transfer, rm is
the molecular resistance in the atmospheric viscous sub-layer and rs is the surface resistance of the
snow, which depends on the flux into the snow layer.
Zannetti defines the flux operationally as:
F ( z = 0,
t)
= Vd(
z1)
Cg(
z1,
t)
(8)
Where the flux to the surface, when F is negative, or from the surface, when F is positive, at a
reference height z1 as the depositional velocity found at the reference height multiplied by the
concentration of the trace gas found at the reference height. In this study, we experimentally
determine the flux, and the concentration and use this data to solve for depositional velocity; where
our reference height in Barrow is 3 m above the snow pack.
Substituting (7) into (8) and solving for a surface resistance, which can then be modeled, rs
(modeled):
Cg(
z1,
t)
rs(mod eled)
=
− ra − rm
(9)
F(
z = 0,
t)
If ra and rm are taken to be small in relation to the first term, then it can be seen that surface
resistance is the inverse of the depositional velocity. The modeled surface resistance is a necessary
parameterization term for depositional modeling, see Skov et al., 2003, Appendix C, and for this
study, as seen by taking the inverse of the depositional velocities shown in Figure 14., page 77, it is
found to be on average 1 cm -1 s.