Abiotic Source of Reactive Organic
Halogens in the Sub-Arctic
L U C Y J . C A R P E N T E R , *
J A M E S R . H O P K I N S ,
C H A R L O T T E E . J O N E S ,
A L A S T A I R C . L E W I S ,
R A J E N D R A N P A R T H I P A N , A N D
D A V I D J . W E V I L L
Department of Chemistry, University of York,
York YO10 5DD, U.K.
L A U R I E R P O I S S A N T ,
M A R T I N P I L O T E , A N D
P H I L I P P E C O N S T A N T
Service Mete ´orologique du Canada,
Montre ´al, Que ´bec, H2Y 2E7, Canada
Recent theoretical studies indicate that reactive organic
iodocarbons such as CH2I2 would be extremely effective
agents for tropospheric Arctic ozone depletion and that
iodine compounds added to a Br2/BrCl mixture have a
significantly greater ozone (and mercury) depletion effect
than additional Br2and BrCl molecules. Here we report
the first observations of CH2I2, CH2IBr, and CH2ICl in Arctic
air, as well as other reactive halocarbons including
CHBr3, during spring at Kuujjuarapik, Hudson Bay. The
organoiodine compounds were present at the highest levels
yet reported in air. The occurrence of the halocarbons
was associated with northwesterly winds from the frozen
bay, and, in the case of CHBr3, was anticorrelated with
ozone and total gaseous mercury (TGM), suggesting a link
between inorganic and organic halogens. The absence
of local leads coupled with the extremely short atmospheric
lifetime of CH2I2indicates that production occurred in
the surface of the sea-ice/overlying snowpack over the
bay. We propose an abiotic mechanism for the production
of polyhalogenated iodo- and bromocarbons, via reaction
of HOI and/or HOBr with organic material on the quasi-
liquid layer above sea-ice/snowpack, and report laboratory
data to support this mechanism. CH2I2, CH2IBr, and other
organic iodine compounds may therefore be a ubiquitous
component of air above sea ice where they will increase
the efficiency of bromine-initiated ozone and mercury
The role played by bromine during polar sunrise ozone and
mercury depletion events has been described in numerous
studies (1-3 and references therein) and the suggested
in release of gaseous Br2 and BrCl, which are rapidly
the liquid phase has the potential to release two bromine
atoms to the gaseous phase, resulting in an exponential
increase of Br, the so-called “bromine explosion”. In the
marine environment, the presence of highly photolyzable
iodine-containing CH2I2, CH2IBr, and CH2ICl molecules has
been demonstrated (4 and references therein), which leads
to rapid production of iodine atoms whose main fate is to
react with ozone, forming the IO radical. Halogen-related
where X ) Cl, Br, or I) radicals that, rather than photolyzing
to X + O, react to reform chain-carrying halogen atoms
without releasing an oxygen atom. The presence of iodine
depletion effect than additional Br2and BrCl molecules (5,
6). In a theoretical study, Calvert and Lindberg (5, 6) found
that CH2I2 is potentially an extremely effective agent for
tropospheric Arctic ozone and mercury depletion, only
slightly less efficient than I2per molecule and more efficient
is unknown. Certainly, the seasonality of observed aerosol
iodine, the under-ice spring bloom of ice algae that produce
a wealth of organohalogens (8, 9). It is worth noting that,
until now, there are no reported Arctic measurements of
organic I atom producers in coastal environments (4, 10).
To investigate potential organic sources of Arctic iodine,
we made in situ measurements of reactive halocarbons and
collected supporting data in March 2004 at Kuujjuarapik on
the east shore of Hudson Bay (55.30° N, 77.73° W), during
which time the bay was frozen, with a covering of snow.
Halocarbon Measurements. Hourly in situ measurements
of the reactive halocarbons were made using an automated
GC-MS system. Ambient air was sampled through a1/2-in.
metal bellows pump at a flow rate of 20 L min-1. The inlet
was approximately 10 m from any building and 1 m above
ground level. The pump was housed in an aluminum box
located outside the laboratory and connected via 3 m of
1/8-in. stainless steel tubing to the GC-MS instrument.
gas through the entire manifold with sub-sampling into the
instrument. A full description of the instrument and calibra-
tion methods is given in Wevill and Carpenter (11).
Mercury and O3Measurements. Total gaseous mercury
(TGM) was measured with an automatic analyzer (Tekran
2537A). The analytical train of this instrument is based on
the amalgamation of mercury onto a pure gold surface
followed by a thermodesorption and analysis by cold vapor
atomic fluorescence spectrophotometry (CVAFS) (λ ) 253.7
nm) providing analysis of TGM in air at sub-ng m-3levels.
A dual cartridge design allowed alternate sampling and
in the air stream. The analyzer was programmed to sample
the air at a flow of 1.5 L min-1at 5-min intervals. Particulate
µm). Ozone concentration was measured with a TECO 49
analyzer. The ozone monitoring system was designed to
continuously (5 s) measure the concentration with state-
of-the-art instrumentation interfaced with a powerful data
* Corresponding author e-mail: firstname.lastname@example.org.
Environ. Sci. Technol. 2005, 39, 8812-8816
88129ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 22, 200510.1021/es050918w CCC: $30.25
2005 American Chemical Society
Published on Web 10/14/2005
logger. The analyzer was calibrated with zero air and ozone
of known concentration. Ambient sampling was through a
1/4-in. Teflon line at 3 m above ground level.
Laboratory Experiments with I2/HOI and Fulvic Acid.
fulvic acid was investigated using 10 µmol L-1of aqueous I2
of water), prepared by dissolving solid I2(>99.8%, Riedel-de
was injected into an airtight 500-mL round-bottomed flask
containing the I2/HOI solution. The reaction vessel was
laboratory temperatures of between 20 and 25 °C. Samples
(10 mL) were extracted periodically via a gastight syringe
and analyzed for halocarbons by purge-and-trap GC-MS.
Results and Discussion
Figure 1 shows the pronounced diurnal cycles observed in
CH2I2, CH2IBr, and CH2ICl concentrations during the March
5-9 sampling period, when wind speeds were low (<5 m
s-1). The maximum mixing ratios of the two most reactive
halocarbons observed, CH2I2and CH2IBr, were 1.4 pptv and
atmospheric observations; for example, near seaweed beds
at Mace Head where mixing ratios reached only ∼0.3 pptv
Inspection of the diurnal nature of the iodocarbon data
shows that the variability coincided temporally with the
rapidly changing wind direction (Figure 1), indicating that
the halocarbon source was to the north/northwest of the
site, from the direction of the frozen Hudson Bay. A
comparison of air temperatures over Hudson Bay and over
the measurement site during this episode (12) suggests that
the changing wind direction was a “sea breeze” effect due
to a stronger diurnal temperature cycle with higher daytime
values over land. Whether release of the halocarbons from
the bay was due to photochemical reactions is difficult to
tell, although the peak in CH2I2and CH2IBr concentrations
during the night of March 5-6 suggests that this is not the
case. Given the short atmospheric lifetimes of the dihalo-
methanes, CH2I2 in particular (5), it is probable that the
nearest source was within about 1 km, i.e., near the coast of
the bay. However, this does not rule out the possibility that
the halocarbon sources were widespread over the bay, but
we only observed emissions from local sources of CH2I2.
Increased concentrations during north/northwesterly
winds with trajectories over Hudson Bay, and pronounced
diurnal variations during the low wind speed period March
5-9 were also observed for CHBr3, CH2Br2, CHBr2Cl, C2H5I,
1-C3H7I, and to lesser extent CH3I, but not for chloroform.
The latter showed highest concentrations in southeasterly
air, possibly due to anthropogenic sources and/or a marine
source in the North Atlantic. The bromocarbon peaks were
generally anticorrelated with both ozone and total gaseous
period when there was evidence of emission of all these
compounds, or their precursors, from local sea-ice/snow-
pack, as discussed above.
and sub-Arctic boundary layers are strongly correlated with
loss of gaseous elemental mercury (3). These so-called
oxidation of Hg0by BrO (3, 13) and atomic Br (14) into more
soluble forms which, being nonvolatile and water soluble,
elemental mercury. All the high bromoform, low O3 and
the air masses originated from the north of Hudson Bay
(Figure 3), recently highlighted by Kaleschke et al. (16) as a
strong source of inorganic bromine, possibly due to frost
in this region. Frost flowers are ice crystals that grow on
area and enhanced salinities and halide ion concentrations.
The covariance of CHBr3 with the O3 and mercury
depletion events further implies a link between inorganic
and organic bromine, i.e., the same or co-located sources.
Currently, the only known source of polyhalomethanes in
the Arctic is ice algae (8, 9) and it is possible that leads in the
sea-ice of Hudson Bay allowed bromoform and other
halocarbons produced by microalgae underneath the sea-
ice to diffuse into the air above during north/northwesterly
trajectories. However, no leads over Hudson Bay were
observed by eye from Kuujjuarapik until March 14, and
FIGURE 1. CH2I2(red), CH2IBr (green), CH2ICl (blue), net solar radiation (pink - unavailable before March 7) and wind direction (black)
during March 5-10.
VOL. 39, NO. 22, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY98813
satellite images (e.g., Figure 4, albeit at 1 × 1 km resolution)
of the ice cover over Hudson Bay show that there was no
significant open water over Hudson Bay until the end of
The opening of a lead a few km off Kuujjuarapik after
March 14 did not result in increased levels of halocarbons,
after March 10. As already discussed, the short lifetimes of
close to the site. We conclude therefore that ice algae were
not a major source of the polyhalogenated iodine and
bromine compounds at Kuujjuarapik. This is consistent not
only with our data but also a 2-year study of halocarbons at
Alert, Canada, where bromocarbons showed no link to
blooms in marine biota (18). We propose instead an abiotic
ice, possibly enhanced within frost flowers located to the
northwest of the site (the latter only observable in CHBr3at
Kuujjuarapik due to its relatively long lifetime). Arctic
of trace gases including the alkyl halides CH3Br, CH3I, and
C2H5I (19). Swanson et al. (19) suggested a photochemical
source of the alkyl halides from reaction of alkyl radicals
produced from photolysis of carbonyl compounds, which
found in sea-ice (20, 21), with iodine atoms produced from
in surface snow (22, 23). However, this mechanism cannot
in this study. Rather, we propose that HOBr and HOI in the
quasi-liquid component of snow or sea-ice can react with
available TOC to produce such compounds via haloform-
type reactions, e.g.
High levels of dissolved HOBr and HOI in the quasi-liquid
layer containing enhanced I-, Br-, and Cl-on the sea-ice
FIGURE 2. Bromoform (CHBr3), ozone, total gaseous mercury (TGM), and wind direction.
FIGURE 3. NOAA HYSPLIT model 3-day back trajectory for March
16, 2004, ending at Kuujjuarapik on the east coast of Hudson Bay.
HOBr + humic material f
brominated humic material + CHBr3+ CH2Br2+
HOI + humic material f
iodinated humic material + CHI3+ CH2I2+ CH3I
88149ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 22, 2005
surface, may result from either atmospheric deposition or
from reactions in the liquid phase
The formation of trihalomethanes in natural waters via
haloform reactions of HOI with dissolved organic material
is well-known (e.g., 24). Halogenation of compounds con-
taining hydrogens R- to keto groups is both acid and base
catalyzed (25) and leads to production of CH3X, CH2X2, and
CHX3, with relative amounts depending upon the reaction
conditions. We carried out laboratory experiments using
fulvic acid as a proxy for the organic matter content of sea
ice (26), which behaves as an organic matter “tank” wherein
the humification process proceeds as phytoplanktonic
material is trapped during its formation. We observed that
CHI3 and CH2I2 were the major products of reaction with
HOI at pH 6.5 (Figure 5). The reason for the depletion of the
halocarbons with time is currently being investigated. We
and other polyhalomethanes observed at Kuujjuarapik fol-
lows our laboratory experiments, then it is likely that CHI3
and/or its decomposition product I2were also present. The
lack of evidence of CHCl3emission from the frozen bay is
consistent with other reports of abiotic halocarbon produc-
A haloform-type reaction is widely believed to be the
source of volatile polyhalogenated compounds from mac-
roalgae (28) but so far has not been invoked to explain
halocarbon production in sea-ice/snowpack. This mecha-
and iodine and is consistent with our and other published
observations that CHBr3 is inversely correlated to O3 and
explain the so-far puzzling enrichment in CHBr3observed
where rapid release of Br2 and BrCl occurs from sea salt
particles on the snowpack, sea-ice and frost flowers, release
may also occur. Our observed total organic reactive iodine
are of the same order of magnitude as BrCl and Br2mixing
ratios of 0-35 pptv and 0-25 pptv, respectively, observed
during polar sunrise at Alert, Canada (2, note many of the
observations were below the detection limit). Since Calvert
and Lindberg’s study (5, 6) using currently available kinetic
data found that, per molecule, CH2I2 and other iodine
greater ozone and mercury depletion effect than additional
Br2 and BrCl molecules, halogen chemistry in the polar
boundary layer may be more efficient than previously
(grant NER/A/S/2001/01064) for funding this project. We
are also grateful to Claude Tremblay (Centre d’ E Ätudes
field campaign, and to Christina Peters (University of
of Canada, Toronto, now deceased) for useful discussions.
We gratefully acknowledge the NOAA Air Resources Labora-
model. D.J.W. thanks the EPSRC for award of a studentship.
FIGURE 4. MODIS/Terra Sea Ice Extent Daily L3 Global 1 km EASE-Grid Day (MOD29P1D) satellite image of water/sea ice extent over
red ) sea ice determined by both reflectance and IST. The light blue star on the right plot denotes the location of Kuujjuarapik.
FIGURE 5. Reaction profile of CH3I (dashed line), CH2I2(gray line),
and CHI3 (black line) production following addition of 10 µL of
fulvic acid to 0.5 L of 10 µmol L-1I2/HOI solution.
HOBr + I-f HOI + Br-
VOL. 39, NO. 22, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY98815
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Received for review May 13, 2005. Revised manuscript re-
ceived August 5, 2005. Accepted September 15, 2005.
88169ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 22, 2005