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 photoreduction 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 longrange 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 the Ottawa 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.