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

Fate of Mercury in the Arctic 54
sampling train and heating cap. Sampling was over a 4 hr. sampling period, to minimize the chance
of break through, a possibility because of the relatively high RGM levels expected in the Arctic
during mercury depletion events. Break through is what occurs when the active surface is used up,
or passivated, and can no longer adsorb the gas; thus the gas is said to “break through” the upstream
end of the denuder and lower than actual levels are measured.
RGM collected in the denuders is analysed by thermal desorption following the sampling
period. Manual measurements of reactive gaseous mercury were therefore necessary to obtain the
mercury mass in each annular denuder in the flux measurement system with the RGM concentration
in the sampling period for each denuder subsequently calculated based on sampling time
information and flow rate from the REA system. The difference in concentration found for the up
channel denuder and down channel denuder are multiplied by two factors obtained for the sampling
period as output from the REA control system: the standard deviation of the vertical wind velocity
and the coefficient beta. Flux is then calculated. By dividing with the total ambient RGM
concentration for the sampling period, concurrently measured with the third channel in the flux
machine, or obtained as a sum of the three channels, the depositional velocity was calculated.
For the flux measurements with the REA system, the two collocated denuders acting as
accumulators for collected RGM over the sampling period must provide faithful representations of
the amount of RGM in air masses that are either moving up, or coming down, since differences in
these two denuders are used to calculate the flux.
REA systems that have used denuders as the trace gas accumulators, e.g., Zhu et al., 2000,
generally have three denuders in each of the up and mid channel, so that the difference in
concentration, required for the REA flux calculation, is the difference between the average of the
concentration determined in three denuders in each channel, providing more confidence to the
measurement. The present system was a pilot system, with resources for just one annular denuder