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

Fate of Mercury in the Arctic 105
A more exact determination of the surface resistance was not possible with the available output
data from the pilot REA system. However, the new NOAA system produces the necessary output
factors to allow modeled calculation of ra and rm in accordance with the Karlsson and Nyholm dry
deposition and desorption model (Karlsson and Nyholm, 1998); i.e., friction velocity, Obukov’s
length and the stratification function as long as a diffusion coefficient for RGM is known. Therefore
campaigns with the new system will provide a better approximation of depositional velocity and
surface resistance.
As seen in Figure 14., page 77, the depositional velocities noted for depositional events are
fast, around 2 cm s -1 this is what would be expected for a very reactive gaseous species such as
HNO3. On average, the depositional velocity is approximately 1 cm s -1 . Comparing with measured
dry deposition velocities over snow for HNO3, reviewed in Karlson and Nyholm, 1998, show that
the measured dry depositional velocities for RGM may be an order of magnitude higher than that
for HNO3, given the snow surface temperature of < 2 0 C. However, there are also reported values
that fall within the range found for RGM in this study. For example, Cress et al.,1995, as cited in
Karlsson and Nyholm, found the depositional velocity about 0 0 C to be in the range of 0.88-3.79,
though the air concentration is not given. It is seen that as ambient concentrations of HNO3
decreased, depositional velocities apparently increased: for a concentration in the air of 6-15 µg m -3
Johansson and Granath, 1986, find a depositional velocity for air temperatures < -2 0 C of HNO3 to
be 0.02 – 0.1 cm s -1 ; near 0 0 C the deposition velocity increases, to 0.6 cm s -1 . Cadle et al., 1985,
as cited in Karlsson and Nyholm, report a depositional velocity averaged for all temperatures, with
an air concentration of 6 µg m -3 to be 1.4 cm s -1 .
Comparing the depositional velocities found in this study with the experimental data reported
for HNO3 shows that while the expected values could have been an order of magnitude lower, the