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

Fate of Mercury in the Arctic 106
values found are within reported ranges for a comparatively highly reactive gaseous species.
Though RGM Vd may be better compared with NO2 Vd over snow.
Karlsson and Nyholm note that at temperatures under –2 0 C the depositional velocity appears to
be controlled by the surface resistance. The surface resistance decreases as the temperature warms
due to increasing amounts of available water. Measurements with this system need to be made
around 0 0 C to see if this is also the case for RGM.
Slinn et. al., 1978, show that if dry deposition is the only removal mechanism for a substance
not affected by chemical transformation, that the atmospheric residence time for the substance can
be estimated as the deposition velocity multiplied by the height. If it assumed that Hg, once
converted within the MBL is deposited almost as quickly as converted, as shown by the averaged
measured depositional velocity of 1 cm s -1 then RGM would have a lifetime in Nord of around 10
hrs. This would correspond to an MBL height of 360 m which is reasonable. One sees from the
above relationship, that as the height of the MBL decreases, the atmospheric residence time will
exponentially decrease. The measured values may reflect upper limits of surface resistance over the
snow.
Comparison of RGM flux with RGM ambient concentrations
Figure 23., page 107, are the monitored results provided by NOAA, for RGM concentrations
during the Barrow 2001 flux measurement campaign. Comparing with Figure 14., page 77, or the
values in Table 3., page 76, show that when there is a deposition recorded by the REA machine,
there are correspondingly low RGM ambient values. The air has been apparently depleted of RGM
due to deposition. Trends of RGM rising in the air on the 8 th , 9 th and 12 th of April are recorded by
the REA system as reemission, perhaps indicating that RGM can be re-volatized from the snow
surface.