By Daniel O’Sullivan
Introduction
Over the past 25yrs, researchers have begun to realize
the role that gaseous halogen species have on our
atmosphere
 Whilst we now understand the impact
of halogens in our stratosphere
quite well…..
 recent observations in the polar
troposphere remain unexplained
Introduction
 Sudden increases in BrO have been observed in the polar
tropospheres
 At the same time,
ozone depletion events occur
(ODE’s)
 Observed primarily in springtime
 Ozone concentrations drop from ppbv to ~ zero
Introduction
•Could seawater-snow interaction be triggering these
events?
Introduction
Wind
Wind
At least 3 known sources of halide salts in snowpack:
Sea-salt migration
through
sea-ice up known
to the
Shattering
of dendritic
briny
structures
Sea-spray aerosol
surface
offlowers
snowpack
as frost
Why Should We Care?
1. In the troposphere, Ozone is a greenhouse gas
Decrease in
ODE
frequency
Increase in
[O3]
Decrease in
frozen water
Increase of
Surface
Temperature
Why Should We Care?
Further effects of Halogen atoms……
 Sudden drops in the concentration of
Gaseous
Elemental Mercury (GEM)
 Halogen species (Br and BrO) much more reactive with
GEM than Ozone
 Species formed such as HgBr are deposited quickly on
snowpack
Frozen State Chemistry
 Novel changes can occur when dilute solutions of
ions are frozen
 New products being formed, or rates of reactions
increased
 Believed to occur as a result of a “freezeconcentration mechanism”
Frozen State Chemistry
One such freezing reaction……..
 Occurs if iodide, chloride and nitrite are frozen together
(pH<3)
I- + Cl-
NO2-,
H+,
FREEZE
ICl2-
 Why are these conditions (NO2-, H+ and freezing)
necessary?
 Could similar chemistry be involved in releasing ozone
destroying bromine at the poles?
Frozen State Chemistry
 In acidic environments, nitrite becomes doubly
protonated.........
 Yields the nitroacidium ion, H2ONO+
H+
NO2-
H+
HONO
H2ONO+
1.2
Fraction
1
H2ONO+
0.8
0.6
HONO
0.4
NO2-
0.2
0
0
1
2
3
pH
4
5
6
7
Laboratory Results…
What happens if we use a bromide salt in place of
chloride?
UV-Vis spectrum of:
1. 0.5mM NaI and NaNO2, with pH=2.8
2. 50mM KBr added to solution
I2254nm,
Br-
IBr2-
Freeze
NO2-, pH<3
271nm, I2Br-
IBr2-
Laboratory Results
0.5
Concentration of Species Present Vs. pH
0.45
Nitroacidium
Concentration (mM)
0.4
0.35
Dibromoiodide
0.3
0.25
0.2
0.15
0.1
0.05
0
0
1
2
3
pH
4
5
6
•Acidified w/ H2SO4
•Dibromoiodide formed only after freezing
•At pH close to 3, reaction goes almost to completion
7
Laboratory Results
 More moles dibromoiodide formed than moles
H2ONO+ present
 Appears that H2ONO+ is acting as a catalyst…….
 But why is freezing the solution necessary?
I
-
Initial Hypothesis
H2ONO+
I
I
Br
Br-
I2
Keq= 10.5M-1
Br -
I2Br-
INO + H2O + IBr
Keq=480M-1
IBr 2-
I2 + BrNO + H2O
Br
Br - + H 2ONO+
I
Br
Laboratory Results
Same ratios of starting reactants, but at higher
concentrations, forms dibromoiodide without
freezing…….
I2 +
Br-
I2 + 2Br -
H2ONO+
H2ONO+
I2Br-
Low Conc
I- + IBr 2-
High Conc
Increase Conc. uniformly = Increase all rates in the cycle
‘Kinetic barriers’ to formation of IBr2- overcome
Further Results
 Concentration IBr2- decreases if exposed to light
 I2Br- does not reform in the reaction vessel
IBr2-
0 min
2 hrs
•Nitroacidium is not the only oxidant that works
•H2O2 also works at pH < 4
Bromine ‘Explosion’
 Previous studies haven’t shown how Bromine atoms
arise from the interaction of snow-seawater
 But once bromine gas is present in the atmosphere……
Br2
+
Br-
Br + Br
Br + O3
BrO + O2
BrO + HO2
HOBr + O2
HOBr + Br- +H+
H+
h
+ HO2 + O3
Gas Phase
Reactions
Ice-Vapour
Interface
Br2 + H2
Br + H2O + 2O2
NET
Reaction
Further Work
 Next step is to analyse the decompostion of IBr2-
 Low concentrations of short lived halogen species
being lost to gas phase
 Undetectable using traditional UV-Vis
Solutions
High Sensitivity
Spectroscopy
Use [O3] with
time as a probe for
halogen species
Conclusions
 ‘Proof of concept’ that freezing reactions may play
crucial role is Polar chemistry
 Mechanism proceeds via relatively simple
freeze-concentration effect
 Release of gaseous halogen species viable
 Can alter chemistry to mimic natural more accurately
Thanks to……
Prof. JR Sodeau