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

Fate of Mercury in the Arctic 95
Cx/Co = 0.82 exp ( - 22.53 (πDL/4Q)(d1+d2/d2-d1) (1)
As seen from the equation, it is valid for collection efficiencies of 100% where the value of the
mass concentration of the gas at a distance, Cx, over the mass concentration of the gas at the
entrance, Co is less than 0.82. D is the diffusion coefficient of the gas in the air, and Q is the
volumetric flow rate, d1 and d2 are the internal and external diameters of the annulus. Important
assumptions for the above therefore are: 1., laminar flow must be obtained in the tube; 2., all the
molecules reaching the wall must be trapped and should not diffuse chromatographically further
into the tube; 3., The capacity of the trapping medium limits collection time, since eventually the
active surface will become saturated and the gases will no longer be trapped on the wall, being
carried through the denuder instead. This is what is known as breakthrough (Bemgård et al., 1996).
These conditions must be satisfied for annular denuders to work as the RGM reservoir for the REA
sampling system.
Considering the results from the denuder tests, with the previously mentioned considerations
and uncertainties, when sampling at 1 Hz, the above requirements are met. The denuders were
simply not able to sample RGM at a sampling flow rate of 10 Hz. To do so, their geometry would
need to be redesigned in accordance with the Gormley Kennedy equation. This would mean that
they would need to be shorter and thicker. This would limit the development of laminar flow and is
not rudimentary redesign work.
One of the ideas for the future could then be to indirectly measure RGM using KCl coated
beads in a quartz denuder tube, of the same dimension of the annular denuders in Landis et al., 2002
and thereby facilitating, using the same analytical sampling chain and heating caps. This system
would collect TPM + RGM. TPM could be measured alone with a co-located TPM collector and
subtracted from the RGM+TPM, giving an indirect measurement of RGM. A TPM sampler could
also be modified to collect RGM. This would take advantage of there being greater confidence in