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

Fate of Mercury in the Arctic 92
denuders by using HgCl2 generated in respectively either a permeation tube or a diffusion tube, as
reported in Schroeder et al., 2003. For the present study, they were not “calibrated” they were
coated and used if a low analytical blank value could be provided prior to exposure, and if there was
no significant Hg (0) signature from the denuder, prior to starting the heating cycle.
The question remains: when we sample RGM using annular denuders in the Arctic, what is it
we are measuring?
We can systematically look at what we know may create problematic measurements. As
pointed out in Schroeder et al., 2003, the use of the inertial impactor in the front of the sampling
chain means that only the gas phase and fine, aerodynamic diameter of 2.5 um or less, particulate
matter are sampled past the active surface of the denuder, with the coarse particulate matter being
impacted on the impact plate. It is not known how much mercury is lost in the inlet or on the
impactor plate. In a heated denuder, it is possible that any mercury on particles on the impactor
plate is degassed. For example, snow and ice crystals will melt in the temperatures in the heated
denuder. The air as it travels through the denuder, becomes warmer and continues to expand. This
might disturb the laminar flow, forcing the gas towards the KCl coated walls. This may actually
improve the RGM trapping efficiency of the annual denuder, but at the same time, possibly induce
measurement artefacts due to fine particle capture on the coated wall surface.
When should denuders be recoated? Recoating of denuders is defined operationally as when
blank values begin to rise, indicating that the surface cannot be “cleaned” well enough through
pyrolization. What is it that is inactivating the surface in the Arctic? When should denuders be
replaced? As long as denuders are providing a good seal, then they are most likely fine for
continued use. In this work, denuders approximately one year old were still functioning well.
With respect to breakthrough under Arctic conditions, Landis et al., 2002 showed that for
automatic sampling up to 2 hours, there was no breakthrough. When sampling over two hours,