Trace Metal Chemistry in Atmosphere and Atmosphere-Ice-Water-Snow Interfaces

Kinetic and mechanistic study of oxidation-reduction mechanisms of mercury

Mercury is a persistent, toxic and bio-accumulative pollutant of global interest. Its main mass in the troposphere is in the form of elemental gas phase mercury. Rapid, near-complete depletion of mercury has been observed during Springtime in the atmospheric boundary layer of frozen marine areas in the Arctic, sub-Arctic and Antarctic locations. It is strongly correlated with ozone depletion. Evidence has indicated that the chemistry involving halogen gases from surface sea-salt is the mechanism of this destruction. Precisely which halogen gases are the main players remained unresolved. Our novel kinetic and mechanistic data on several reaction schemes and multi-scale modeling showed that Br atoms and to lesser extent BrO radicals are the most effective halogens driving mercury oxidation, even by changing the rate coefficients by two folds, accounting for the wide range of discrepancy of the existing literature data on Hg kinetics. The reduction of oxidized mercury deposited in the snow pack back to Hg0, and subsequent diffusion to the atmosphere has also been observed. However, it cannot compensate for the total deposition and thus a net accumulation occurs.

Simplified schematic of mercury depletion in the arctic
Image by Parisa Ariya.

We used a unique global atmospheric mercury model to show that halogen driven mercury depletion events resulting in an increase in the net deposition of mercury to the Arctic relative to scenarios that do not include halogen chemistry. We have also shown evidence for the formation of mercury aerosols upon the oxidation reactions in the course of laboratory experiments and field measurements in the high Arctic. This may impact the extent of bioaccumulation of oxidized mercury. The detailed homogeneous and heterogeneous production and transformation routes for stable Hg(II) and Hg(I) have been investigated in our labs, as well as the impact of phase partitioning of mercury species under different environmental conditions, and the impact of organic mediated reactions. We use several complementary instruments to investigate a given system ranging from high resolution FTIR-Raman, various mass spectrometer techniques (chemical ionization, electron impact, MALDI-TOF, GC/MS), cold-vapour fluoresencence spectroscopy, atomic absorption spectroscopy, to high-resolution transmission electron microscopy. In this manner, we can follow the kinetic and mechanistic study in the gas phase, as well as reactions on aerosols and reactions walls.

Mercury reactions on surfaces:

A picture of the femto-second spectroscopy setup in Parisa Ariya's lab
Image by Parisa Ariya.

Current scientific understanding of either gas or aqueous phase chemical reactions cannot completely predict the fate and transformation of chemical species in the atmosphere. Given the omnipresence of interfaces in nature, it is apparent that reactions occurring at the surfaces need to be taken into account. To this end, surface-selective spectroscopic methods of second harmonic and sum frequency generation will be utilized to investigate both the equilibrium and the dynamic processes occurring at various atmospherically relevant surfaces. Our ongoing research focuses on fundamental studies of reactions on adsorption processes of mercury on model surfaces to provide ultimately the understanding of the impact of surfaces such as snow and ice on mercury transformation and also the understanding of the organic assisted photo-redox mechanisms of oxidized mercury in aerosols or at air/water, and air/snow/ice interfaces.

Selected related publications in this domain:

  • L. Si and P. A. Ariya, The kinetics and mechanistic studies of photoreduction of selected natural thiols, Chemosphere, in press (2011)
  • M. Subir, P. A. Ariya, and A. Dastoor, , Fundamental limitations and the importance of trace metal heterogeneous chemistry, Atmospheric Environment, in press (2011)
  • M. Subir, P. A. Ariya, and A. Dastoor, A Review of the Sources of Uncertainties in Atmospheric Mercury Modeling I. Uncertainties in existing kinetic parameters, Atmospheric Environment, in press (2011).
  • Parisa A. Ariya, Atmospheric science: Mid-latitude mercury loss, Nature Geoscience, 4, 2011 (doi:10.1038/ngeo1048) (2011)
  • Graydon Snider and Parisa Ariya, Photo-catalytic oxidation reaction of gaseous mercury over titanium dioxide nanoparticle surfaces, Chemical Physics Letters, 491, 23-28 (2010)
  • Parisa A. Ariya, Kirk Peterson, Graydon Snider and Marc Amyot, Mercury Chemical transformation in the gas, aqueous and heterogeneous phases: State-of-the art science and uncertainties, book chapter 15, Mercury fate and transport in the global atmosphere, Pirrone and Mason editors,Springer, pp, 459-501, ISBN: gtt v 987-0-387-93957-5 (2009)
  • G. Snider, F. Raofie, and P. A. Ariya, "Effects of relative humidity and CO(g) on the O-3 initiated oxidation reaction of Hg-O(g): kinetic & product studies". Physical Chemistry-Chemical Physics, 10(36): 5616-5623 (2008)
  • F. Raofie, G. Snider, and P. A. Ariya, "Reaction of gaseous mercury with molecular iodine, atomic iodine, and iodine oxide radicals - Kinetics, product studies, and atmospheric implications", Canadian Journal of Chemistry, 86(8): 811-820 (2008)
  • Lin Si and P. A. Ariya, "Reduction of oxidized mercury species by dicarboxylic acids (C2-C4): Kinetics and products studies". Environ. Science and Technology, 42: 14, 5150-5155 (2008)
  • A. J. Poulain, E. Garcia, M. Amyot, P. G. C. Cambell, F. Raofie, and P. A. Ariya, Mercury speciation and reactivity in the high Arcitc on Cornwallis Island, / Geochemica Cosmochimica Acta/, 71, 3419-3431 (2007)
  • Poulain A.J., Ni Chadhain S. M., Ariya P. A., Amyot M., Garcia E., Campbell P. G. C., Zylstra G., and Barkay T. 2007. A potential for mercury reduction by microbes in the high Arctic. /Appl. Environ. Microbiol.,/ 73(7): 2230-2238 (2007)
  • Poulain, A. J.; Garcia, E.; Amyot, M.; Campbell, P. G. C.; Raofie, F.; Ariya, P., Biological and Chemical Redox Transformations of Mercury in Fresh and Salt Waters of the High Arctic during Spring and Summer /A., Environ. Sci. Technol., /41(6); 1883-1888 (2007)
  • M. A. Engle, M. Sexauer Gustin, S. E. Lindberg, A. W. Gertler. P. A. Ariya, The influence of ozone on atmospheric emissions of gaseous elemental mercury and reactive gaseous mercury from substrates, Atmospheric Environment 39, 7506-7517 (2005)
  • P. A. Ariya and K. Peterson, Atmospheric Chemical Transformation of Elemental Mercury, Mercury in Environment, chapter 13, Kluwer, Nicolas Pirrone (editor) (2005)
  • E. Garcia, M. Amyot and P.A. Ariya, "The relationship between DOC photochemistry and mercury redox transformations in aquatic systems of varying humic content", Geochimica et Cosmochimica Acta, 69, 8,  1917-1924 (2005)
  • E. Garcia, A.J. Poulain, M. Amyot and P.A. Ariya, "Diel variations in photo-induced oxidation of Hg0 in freshwater", Chemosphere, 59(7), 977-982 (2005)
  • P.A. Ariya, A P. Dastoor, M. Amyot, W.H. Schroeder, L. Barrie, K. Anlauf, F. Raofie, A. Ryzkhov, D. Davignon; J. Lalonde, A. Steffen, The Arctic: A sink for mercury, Tellus B, 56, 5, 397-403 (2004)
  • M. Amyot, F. Morel and P.A. Ariya, "The effect of surfaces on Hg droplets in environmental compartments", Environmental Science and Technolog, 39, 1, 110-114 (2004)
  • F. Raofie and P.A. Ariya, "First evidence of stable Hg+1 in aerosols", Environmental Science and Technology, 38(16); 4319-4326 (2004)
  • B. Paul and P.A. Ariya, "Studies of O3-initiated reaction of gaseous mercury Hg0: kinetics, product studies, and atmospheric implications atmosphere", Physical Chemistry-Chemical Physics, 6, 572-579 (2004)
  • J.D. Lalonde, M. Amyot, J. Orvoine, F. Morel, J.-C. Auclair and P.A. Ariya, "Photoinduced oxidation of Hg(aq) in the waters from the St. Lawrence estuary", Environmental Science and Technology, 38(2), 508-514 (2004)
  • F. Raofie and P.A. Ariya, "Mercury aerosols in atmosphere", RMZ- Materials and Geoenvironment, 51: 774-778, (2004)
  • B. Pal and P.A. Ariya, "Gaseous reactions of atmospheric oxidants with elemental mercury: kinetics, mechanistic, and atmospheric implications", RMZ- Materials and Geoenvironment, 51: 724-28 (2004)
  • P. A. Ariya, A. F. Khalizov, and A. Gidas, "Reaction of Gaseous Mercury with Atomic and Molecular Halogens: Kinetics, Product Studies, and Atmospheric Implications", Journal of Physical Chemistry A, 106(32), 7310-7320 (2002)
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