Atmospheric Chemistry Program Seminar
Chemical intuition on the oxidation mechanism of Hg(0) in the gaseous atmosphere by Prof. Theodore Dibble, SUNY-ESF
"Mercury harms ecosystems and harms human health on account of consumption of fish near the top of the food chain. The atmosphere transports mercury around the globe from emission sources (e.g., power plants, re- emission from ecosystems). These emissions are mostly in the form of Hg(0) (atomic mercury), but deposition of Hg(II) compounds dominates transfer from the atmosphere to the Earth.
Investigations of the kinetics and mechanism of Hg(0) oxidation in the atmosphere present many challenges to experiment. Field studies are still plagued by interferences and issues of quantification. In the past several years my group has made great strides in understanding gas- phase oxidation of Hg(0). These advances have come from chemical intuition and quantum chemistry calculations. The first critical advance came in recognizing that BrHg radical was much more likely to react with atmospherically abundant radicals, •Y, such as NO2 and HOO, (see Figure 1) than with OH or Br, which had been the only two reactants included in models of BrHg• chemistry. Theory suggests very large rate constants for reactions of BrHg with Y. Experiments on BrHg• chemistry and kinetics are starting, but have yet to produce results.
The second major point was to realize that BrHgONO, like HONO, would photolyze at tropospherically relevant wavelengths to yield BrHgO•. Subsequent studies revealed that BrHgO• behaved a lot like OH (left half of Figure 1), although BrHgO• is better at hydrogen abstraction than OH.
Finally, we have used theory to quantify the equilibrium constant for OH + Hg <-> HOHg and show that bond energies for HOHg-Y are essentially the same as those for BrHg-Y. With a few assumptions, this lets us build a mechanism and set of rate constants for atmospheric modeling of OH-initiated oxidation of Hg(0) to Hg(II)."