posted on 2006-06-15, 00:00authored byKristen Lohman, Christian Seigneur, Eric Edgerton, John Jansen
Measurements of speciated mercury (Hg) downwind of
coal-fired power plants suggest that the HgII/(Hg0 + HgII)
ratio (where HgII is divalent gaseous Hg and Hg0 is
elemental Hg) decreases significantly between the point
of emission and the downwind ground-level measurement
site, but that the SO2/(Hg0 + HgII) ratio is conserved.
We simulated nine power plant plume events with the
Reactive & Optics Model of Emissions (ROME), a reactive
plume model that includes a comprehensive treatment
of plume dispersion, transformation, and deposition. The
model simulations fail to reproduce such a depletion in HgII.
A sensitivity study of the impact of the HgII dry deposition
velocity shows that a difference in dry deposition alone
cannot explain the disparity. Similarly, a sensitivity study of
the impact of cloud chemistry on results shows that the
effect of clouds on Hg chemistry has only minimal impact.
Possible explanations include HgII reduction to Hg0 in
the plume, rapid reduction of HgII to Hg0 on ground surfaces,
and/or an overestimation of the HgII fraction in the
power plant emissions. We propose that a chemical
reaction not included in current models of atmospheric
mercury reduces HgII to Hg0 in coal-fired power plant plumes.
The incorporation of two possible reduction pathways
for HgII (pseudo-first-order decay and reaction with SO2)
shows better agreement between the model simulations and
the ambient measurements. These potential HgII to Hg0
reactions need to be studied in the laboratory to investigate
this hypothesis. Because the speciation of Hg has a
significant effect on Hg deposition, models of the fate and
transport of atmospheric Hg may need to be modified to
account for the reduction of HgII in coal-fired power plant
plumes if such a reaction is confirmed in further
experimental investigations.