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

Fate of Mercury in the Arctic 109
REA measurements rely on the transport of mass near the surface of the Earth where
turbulence in the air is the main transport and mixing mechanism that eventually causes deposition.
Fluid dynamics show that at any rough boundary, the friction causes velocity of a fluid (e.g. air) to
go to zero. As this occurs, the air velocity vectors straighten out and are laminar in a plane a few
millimeters thick over the Earth. Once gas molecules get into this plane they will either diffuse
towards the surface, or evade towards the atmosphere, depending on the concentration gradient
between this layer, known as the quasi-laminar boundary layer. Under higher turbulent conditions,
the amount of mass transported greatly exceeds the amount of mass diffused, and generally
turbulent transport is the most important physical method for deposition. A REA system samples
this mass, as it is transported up from emission, or deposited. A valid flux determination relies on
the principle that the mass of the trace gas measured, in this case RGM, is unique to that air mass.
This means that it wasn’t produced or destroyed in the air mass.
From the GEM concentration results from the Station Nord campaign, it can however be seen
in the high Arctic, that under stable weather conditions that gaseous diffusion may be important.
This is an area requiring further investigation in the future.
Shear stress from surface friction is one of the main driving forces of turbulence; the other
driving force is the change in buoyancy with the change in air temperature and thus density with
height. This force may be important over the Arctic snow surface, however the cold surface cause
stable cold air, that is difficult to move, and results in calm conditions, with wind speed under 2 m
per second, as witnessed at Station Nord in 2002.
These stable weather conditions are the greatest draw back to deploying the RGM system in the
Arctic for investigating mercury depletion events. Lu et al., 2001, summarize that the environmental
conditions favouring mercury depletion events at high latitudes are: 1, marine/maritime location; 2,
calm weather, low wind speeds, non-turbulent air flow; 3, the existence of a temperature inversion;