Our team originally consisted of P. A. Ariya at McGill and S. Scott at Ottawa. After departure of S. Scott for a new appointment at University of California - Santa Barbara (end of Year 2), David Lean joined our group, while S. Scott continued close collaboration with our team. For the third fiscal year, we have amalgamated projects 3113 and 3114 to increase interactions between researchers and the total efficiency of the project.

Atmosphere provides the fastest moving fluid amongst various compartments of our ecosystem. Chemical and physical transformation in atmosphere can on the one hand, affect processes such as aerosol formation and implication on climate change, and on the other hand, influences with the input (via deposition) to aquatic system, as well as lithosphere. The global Hg cycle is intimately linked to other atmospheric cycles of redox-active trace constituents. Since the major air-borne form, Hg0, is volatile and has very low water-solubility, wet deposition requires prior chemical transformation. Oxidation by photochemical oxidants such as ozone is thought likely, but whether this oxidation occurs in the gas phase, where most of the Hg exists, or in the aqueous environment of clouds, is uncertain. In addition, the air contains reducing agents such as SO2, which are capable of reversing the oxidation of Hg0 and thus prolonging its atmospheric lifetime. The precise balance between oxidation and reduction rates is a key factor in the speciation of atmospheric Hg. These reactions are precisely the ones for which crucial information is missing, according to the 1997 USEPA Hg Study Report to Congress. In our project we address fundamentals of these key oxidation processes in the gas-phase, redox reaction in aquatic media and interface, and evaluate their atmospheric importance using chemical (box) modeling,

In our team, we provide

  • Thermo-chemical and kinetic Inputs to understand and perform field projects on photo-reduction processes
  • Atmospheric aqueous phase chemistry is equally valid for surface waters (aquatic chemistry).
  • New laboratory experiments on the chemistry of DOC/Hg

Specifically our contributions are:

  • Important quantitative data (equilibrium constants, rate constants, activation barriers) for the redox reactions of atmospheric mercury.
  • Input for modeling of deposition patterns for atmospheric mercury.
  • Demonstration that mercury chemistry is strongly coupled to other atmospheric gases, especially SO2, and remediation efforts must take into account the effect on the whole atmospheric chemistry.
  • Parameterization of kinetics and mechanistic data using box models
  • Implementation of data in Global Circulation Model of Environment Canada to understand biogeochemical cycle of mercury.

(I)Halogen and halogen oxide reactions of elemental mercury in atmosphere: Laboratory and Modeling studies (project 3113 – McGill)

Elemental mercury plays a key role in the cycle of mercury in the atmosphere and oceans. It represents nearly 98 % of the atmospheric mercury and 10 to 30 % of dissolved mercury in aquatic systems. Until recently, it was estimated that the mean global tropospheric residence time of this compound is approximately 1 to 2 years and hence it is subject to long-range transport. However, recently, a rapid and unusual trend in gaseous mercury has been observed in several locations Arctic, Antarctic, and sub-Arctic, mostly coinciding with ozone depletion and peaks of reactive halogens. The field observations lead to the assumption that atmospheric mercury depletion can be not only a local phenomenon, but occurring in regional or global scale. During the last couple of years, several group members at McGill atmospheric chemistry laboratories were involved with understanding of the oxidation reactions of Hg, both from a laboratory (kinetics/mechanistic) and box modelling approaches. Our contributions were:

  • Development of methodology to perform the gas-phase kinetics of elemental mercury under near atmospheric conditions
  • To provide a self-consistent kinetics and product studies for the gas-phase reactions X2/X+ Hg0 (X = Cl & Br)
  • Provide the first kinetics data set for X/XO + Hg0
  • To identify potential mechanism & evidence for photo-reduction processes
  • To develop a novel MS technique to identify HgO
  • To provide evidence that Br radical can account for observed rapid mercury depletion
  • To develop modelling scheme for Hg chemistry in environment and parameterization of halogen chemistry for global circulation models of mercury of environment Canada using an atmospheric chemistry box model (MOCCA).

Future research direction for the McGill research group:

April 2003 to April 2006

  • Kinetics and product studies of reactions of BrO and ClO + Hg0
  • Thermochemical calculations of reactions of BrO and ClO + Hg0
  • Speciation of mercury upon halides on air/water interface using MS technique
  • Box modelling of the chemical reactions involving BrO
  • Paramerization of snow physics for incorporation into the global circulation model (GCM) of Environment Canada (Ashu Dastoor).
  • Measurement of mercury speciation in atmospheric/ocean/lake water using MS developed in this lab

May 2003 to April 2006

  • GCM modelling of atmospheric particles, scavenging, adsorption, condensation of mercury containing aerosols
  • Paramerization of surface physics/chemistry for incorporation into the global circulation model (GCM) of Environment Canada (Ashu Dastoor).

June 2003 to June 2007

  • Kinetics and mechanistic pathways of DOC reactions with elemental mercury

(II) Chemical reactions of elemental mercury in atmospheric water (former project 3114 - Ottawa)

Oxidation of volatile, poorly soluble Hg(0) in the atmosphere is presumed to precede wet or dry deposition. Redox reactions are accelerated in clouds, where the the aqueous environment of water droplets serves to concentrate the reagents. HOCl, which is formed by the action of photochemical oxidants on sea-salt aerosol, has been proposed in modeling studies as the dominant oxidant for Hg(0) in the marine troposphere (accounting for up to 90% of the sink), and has also been identified as a major oxidant for Hg(0) in coal-derived exhausts.

We investigated the reaction of Hg(0) with HOCl in order to determine its mechanism and measure its rate as a function of temperature and pH. We developed a kinetic probe method for this purpose, using the bleaching of methylene blue by HOCl to indicate the extent of reaction. The measured rate constant is 500 times lower than values used in the recent models. The magnitude of the pH effect (small) is interpreted in terms of an oxygen-transfer mechanism with chloride as the leaving group from either HOCl or OCl-. Finally, our investigation of the reduction of cloud Hg(II) by sulfite derived by dissolution of atmospheric SO2 has revealed the role of Hg(0) in causing the transformation of sulfite to thiosulfate and eventually to sulfide. Subsequent formation and precipitation of HgS is observed. These reactions are not currently incorporated in any atmospheric models but should alter predictions about the susceptiblity of Hg(II) for wet deposition.

Future research direction for theOttawa research group

  • We will continue to study redox reactions which impact the speciation of mercury in precipitation, including hydrogen peroxide and hypobromite.
  • We will search for mercury(0) in rain solubilized by sulfur dioxide, as predicted by our laboratory experiments. We will also look for HgS, in precipitation, derived from the reaction between Hg(0) and sulfur dioxide.
  • We will develop techniques for measuring and speciating mercury emitted in stack gases. This information and knowledge of the other chemical constituents present will determine appropriate recovery strategies. In collaboration with NRCanada, we will be able to assess ways of reducing mercury in stack gases.