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

Fate of Mercury in the Arctic 90
The annular denuders are subjected to numerous analytical steps to inspect, clean and coat
them, expose them and afterwards analyse them. The physical principle of these denuders is so
simple, that it should make them ideal for use. However, in practice they are so difficult to employ
that of the measurements for GEM, particulate mercury and RGM; RGM is the one that scientists
working with atmospheric mercury in the Arctic have least confidence in (Schroeder et al., 2003).
Why is this? As shown in Figure 6., page 64, in general, parallel measurements in these
experiments are reproducible within the 15% documented by Landis et al., 2002, except for the first
measured replicate. In Lindberg et al., 2002, Appendix C., manual denuders were compared with
the automated system and 5 of 6 replicates gave excellent results. However, a single outlier is
enough to document the need for continued improvement. Especially since the only proper
validation so far, is to make multiple replicate measurements, as there is at present no standard
method of the calibrating the annular denuders prior to use. This is especially important since only
two denuders are used as accumulators in the REA RGM system. In principle, if these two denuders
cannot measure the same amount of RGM, then they will necessarily produce a measurement
artefact that will result in an erroneous flux measurement. For example if the two denuders in the
first parallel measurement shown in Figure 6., page 64, represented denuders that were hung on the
up and down channel, then this would show as a significant difference in concentration, and thus a
difference in flux. This single observation reinforces the need for a calibration system, especially if
the system is to be deployed for further REA flux measurements.
Figure 7., page 65 confirms that the physical geometry of the denuder sampling train as well as
the fact that the parts are made out of quartz make the sampling train only negligibly exposed to
passive uptake. Thus fulfilling a pre-requisite for using the annular denuders as RGM REA
accumulators. With the passive uptake rate found for the observed total concentration, this means
that over a typical 4 hour REA sampling period that the RGM due to passive uptake does not