GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L10810, doi:10.1029/2006GL025809, 2006
Frost flower influence on springtime boundary-layer ozone depletion
events and atmospheric bromine levels
Lars E. Kalnajs1 and Linnea M. Avallone1
Received 19 January 2006; revised 10 April 2006; accepted 20 April 2006; published 25 May 2006.
[1] Springtime boundary layer ozone depletion events
have been observed in both the Arctic and Antarctic. It has
been suggested that these bromine-catalyzed depletion
events may be caused by bromine released from frost
flowers. We have measured the pH and ion concentrations
of snow and frost flowers in the Ross Island, Antarctica area
and find that if these are representative, frost flowers are
unlikely to be a direct source of atmospheric bromine. The
pH of the sampled frost flowers is too alkaline to support
the release of molecular bromine resulting from the
heterogeneous reaction of HOBr with Br ions. It is more
likely that frost flowers are a source of aerosol bromine
from which gas-phase bromine can be released through
subsequent reactions in aerosol form or on the snow pack.
example [Fan and Jacob, 1992; Tang and McConnell,
1996]:
Citation: Kalnajs, L. E., and L. M. Avallone (2006), Frost
flower influence on springtime boundary-layer ozone depletion
events and atmospheric bromine levels, Geophys. Res. Lett., 33,
L10810, doi:10.1029/2006GL025809.
1. Introduction
[2] Boundary layer ozone depletion events are a common occurrence in the springtime polar maritime environment [Oltmans and Kohmyr, 1986; Bottenheim et al.,
1986]. It is generally accepted that these events are largely
the result of bromine-catalyzed reactions occurring either in
the gas phase or heterogeneously on snow and ice surfaces
[Foster et al., 2001]. Observations in the Arctic have
shown a rapid onset of elevated levels of BrO associated
with the depletion events, a phenomenon known as the
‘‘bromine explosion’’ [Wennberg, 1999]. It has been suggested that the origin of the increased bromine is frost
flowers growing on recently frozen sea ice [Rankin et al.,
2002]. Frost flowers are intricate brine crystals that grow in
the saturated environment over leads and polynyas in sea
ice. Frost flowers have elevated salinity and bromide
content compared to seawater by a factor of approximately
three, making them a potent source of bromine to the
environment. Analysis of satellite data has shown a possible correlation between elevated tropospheric BrO (some or
all of which may be in the boundary layer) and atmospheric
and ice conditions that are likely to produce frost flowers
[Kaleschke et al., 2004].
[3] The reaction of gas-phase bromine non-radical species (HOBr, HBr, BrNO3) with aqueous bromide is a likely
mechanism for the release of molecular bromine. For
1
Laboratory for Atmospheric and Space Physics, University of
Colorado, Boulder, Colorado, USA.
Copyright 2006 by the American Geophysical Union.
0094-8276/06/2006GL025809$05.00
ðR1Þ
HOBrðaqÞ þ Hþ
ðaqÞ þ Br ðaqÞ ! Br2ðgÞ þ H2 OðaqÞ
ðR2Þ
Br2 þ hv ! 2Br
ðR3Þ
Br þ O3 ! BrO þ O2
ðR4Þ
BrO þ BrO ! Br2 þ O2
ðR5Þ
BrO þ HO2 ! HOBr þ O2
[4] The molecular bromine produced by R1 is quickly
released into the gas phase where it is photo-disassociated to
Br (R2), which can destroy ozone through reactions R3 and
R4. BrO is eventually converted to the non-radical HOBr
through reaction R5, which can then be re-deposited on an
ice surface. Via this mechanism, two gas-phase bromine
atoms are released for the uptake of each bromine atom,
leading to an exponential growth in gas-phase bromine
drawing on an aqueous sea salt reservoir of bromide. This
particular mechanism is widely believed to be responsible
for the ‘‘bromine explosion’’ as its exponential production
of gas-phase bromine could explain the rapid onset of
elevated BrO events.
[5] Frost flowers have the potential to release gas-phase
bromine [Kaleschke et al., 2004] as well as to generate sea
salt aerosols rich in bromide [Rankin et al., 2000]. Sea salt
aerosols released from frost flowers may themselves be a
source of gas-phase bromine in addition to depositing
bromide on the snow pack where it may subsequently be
released into the gas phase. There is a growing body of
evidence that indicates that the latter may be a more
significant source of gaseous bromine than direct release
from frost flowers [Simpson et al., 2005].
2. Methods and Results
[6] Frost flowers were observed growing in a recently
refrozen sea ice crack at the terminus of the Barne Glacier in
McMurdo Sound, Antarctica (166.30E 77.65S), on the 28th
of September 2005. The frost flowers were found on 20 cmthick sea ice in a crack protected from wind and were
estimated to be less than 24 hours old based on observations
of the area on 3 consecutive days. The ambient temperature
as recorded by a nearby automatic weather station (Pegasus
North) averaged 37C during the period of frost flower
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KALNAJS AND AVALLONE: FROST FLOWER PH AND ATMOSPHERIC BROMINE
Table 1. Measured pH and Halogen Ratios
Source
pH
Cl/Br
Sea water
Frost flowers, measured
Surface snow, measured
7.6 – 8.2
8.1 – 8.7
5.4 – 7.3
650
269 – 367
13 – 980
formation. A second area of frost flowers was observed on
the 30th of September, growing on a seawater pool formed
by sea ice folding 5 km south of the Barne Glacier, after 24
hour period of temperatures averaging 33C. Samples of
frost flowers from both locations were collected in supercleaned sterile centrifuge tubes and returned, frozen, to the
Crary Laboratory at McMurdo Station for analysis. Analysis
was performed within a week of collection, immediately
after melting and volumetric dilution, using Ion Chromatography (Dionex DX-120, AS14 column, AS14G guard
column, conductivity detection). Analyzed ions included
Br, Cl, SO
4 and NO3 . For each analyte a six-point
calibration was performed over ion concentrations ranging
from the detection limit to one-tenth seawater concentrations. Samples with high ionic concentrations such as frost
flower samples were diluted by a factor of 20 with deionized water. In each batch of samples two IC check standards
were analyzed (SPEX IC Check Standard 2) and duplicate
samples were run for every tenth sample. From these
calibrations, the accuracy is estimated to be 10% for both
Br and Cl, with a precision better than 5%. The detection
limit for Br was 5 ppb, and several snow samples had Br
below detection limits. All analyzed samples had detectable
levels of Cl. The pH of the samples was determined by
melting the samples and analyzing them with a pH probe
(Orion 720A) calibrated with buffer solutions at pH 4, 7,
and 10. In addition to frost flower samples, routine sampling
and analysis of surface snow and snow pit profiles were
performed from August 22nd to November 1st at various
locations in McMurdo Sound.
[7] Analysis of the frost flowers samples showed a ratio
of Cl to Br ranging from 269 to 367, indicating an
enhancement of bromide over chloride by a factor of 2 to
3 compared to the bulk seawater ratio of 650 [Lide, 2004].
The pH of the frost flower samples averaged 8.47 (s = 0.2).
The bromide enhancement factor in non-frost flower snow
samples collected in the same region in 2004 ranged from
near seawater ratios (no enhancement) to an enhancement
factor of 50. The average pH for non-frost flower snow
samples was 6.0 with a standard deviation of 0.4 units (see
Table 1). These results are similar to those observed by
Morin in the Arctic (frost flower pH 7.3– 8.6, surface snow
pH 4.95 – 6.99) during the Out On The Ice campaign (S.
Morin and F. Domine, personal communication, 2006). The
measured pH is close to the average value for surface
seawater of 8.2, indicating that there is little acidification
of frost flowers by titration against gas-phase acids. The
absence of bromide depletion in frost flowers is also in
agreement with that seen by Simpson et al. [2005] in the
Arctic.
3. Discussion
[8] Frost flowers are rich in aqueous bromide and a
mechanism exists for the release of gas-phase bromine from
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aqueous bromide (R1). However it is unlikely, if the frost
flowers sampled are representative, that they are a significant direct source of BrO. The efficiency of reaction R1 is
highly dependent on the availability of protons and hence
on the pH of the aqueous surface layer (see Figure 1). For
pH 6 and below, the efficiency of the conversion approaches
100%, while for pH 8.5 and above the reaction ceases
[Fickert et al., 1999]. From the measured pH of frost
flowers (8.47) this reaction chain cannot be a significant
source of gas-phase bromine.
[9] Only two other possible mechanisms that might
explain the release of gas-phase bromine from frost flowers
have been published. The first - radical chemistry - converts
bromide to molecular bromine. This mechanism, as proposed by Mozurkewich [1995], relies on the uptake of gasphase free radicals to ice surfaces which then oxidize
dissolved SO2 to peroxymonosulfuric acid, which can, in
turn, oxidize bromide to HOBr. While the reaction up to this
point can occur at a pH close to that of seawater, the HOBr
must react with Br in order to produce Br2. As shown in
R1, this will not occur at pH 8.4, suggesting that the
Mozurkewich mechanism cannot explain the release of
Br2 from frost flowers. In addition, the acid-base equilibria
of S(IV) buffers the pH of the aqueous phase toward 7, so
the measured pH of frost flowers is inconsistent with the
presence of a significant amount of S(IV).
[10] The second mechanism for the release of gas-phase
bromine was suggested by Oum et al. [1998], namely the
dark reaction of ozone with bromide. Experimentally it has
been shown that this mechanism can produce gas-phase
bromine at a pH of 8, similar to that measured in frost
flowers. While we cannot discount the occurrence of this
mechanism in frost flowers, it is improbable that it is the
dominant mechanism responsible for the springtime bromine explosion. The reaction does not require sunlight and
hence, if it were occurring over the winter, there would be a
build up of atmospheric Br2 prior to the initial sunrise.
Measurements prior to sunrise in the Arctic show low levels
of molecular Br2, averaging less than 5 ppt, with an increase
Figure 1. Efficiency of reaction R1 with respect to pH.
The region labeled ‘Surface Snow’ indicates the measured
pH range of surface snow samples. The region labeled
‘Frost Flowers’ is the measured pH range for frost flowers.
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KALNAJS AND AVALLONE: FROST FLOWER PH AND ATMOSPHERIC BROMINE
at sunrise [Foster et al., 2001]. Secondly this mechanism
does not produce the geometric growth that would be
exhibited by the reaction of HOBr with bromide. While
neither of these mechanisms can be the dominant source of
active bromine, both may play an important role in the
observed bromine explosion by providing the ‘‘seed’’ of
gas-phase bromine to initiate the recycling mechanism
described in R1 – R5.
[11] The measured ratio of chloride to bromide could also
indicate that frost flowers are not a significant direct source
of gas-phase bromine. With the molarity of bromide (2.5 103M) and chloride (1.7 M) found in frost flowers
[Kaleschke et al., 2004], the product from the reaction of
HOBr with aqueous halogens favors Br2 over BrCl by a
factor of 20 [Fickert et al., 1999]. Hence if there were a
significant reactive uptake of HOBr onto frost flowers and
subsequent release of Br2 we would expect to see a
depletion of Br relative to Cl. Instead, there is an
observed enhancement of Br relative to Cl compared to
the seawater ratio. This may be due to the lower eutectic
point of NaBr than NaCl, allowing NaCl to crystallize into
the solid phase preferentially, leaving a surplus of Br in the
liquid phase from which the frost flowers grow. The extent
of the enhancement of Br through this mechanism is
unknown, so this analysis alone cannot rule out that frost
flowers play a minor role in gas-phase bromine release.
[12] Although they seem not to be a direct source of
bromine by any known chemical mechanism, frost flowers
may still be important as a source of bromine in the polar
boundary layer through the production of sea salt aerosol.
Studies in the maritime Antarctic [Rankin and Wolff, 2003]
have suggested that greater than 60% of sea salt aerosols are
attributable to frost flowers. Both the aerosol and the snow
pack provide an environment more favorable to the production of gas-phase bromine through heterogeneous reaction
R1 because their pH is lower and more conducive to the
release of gas-phase bromine. Studies of midlatitude sea salt
aerosol have demonstrated that sea salt alkalinity is rapidly
titrated by gas-phase acids in a matter of minutes [Pszenny
et al., 2004]. Calculated sea salt aerosol pH varies between
4.5 and 5.9 [Pszenny et al., 2004], which would support
100% conversion of HOBr to Br2. Analysis of surface snow
shows a significant concentration of bromide and chloride
(presumably deposited from sea salt aerosol) as well as a pH
conducive to the recycling of HOBr to Br2. While the
concentration of halogens in surface snow is 3 to 5 orders
of magnitude smaller than that found in frost flowers, the
relative area of the snow pack compared to frost flowers
makes it an equally likely site for bromide activation. A
recent study by Domine et al. [2005] has shown that the
specific surface area of frost flowers is considerably smaller
than originally thought, and is now estimated to be 1.4 m2
per square meter of frost flower-covered ice.
4. Conclusion
L10810
tic frost flowers, it is unlikely that they are a direct source of
bromine in the atmosphere. Frost flowers sampled in
Antarctica are too basic to allow for release of sea saltderived bromide into the atmosphere as bromine gas. Even
though the direct role of frost flowers may not be as
significant as once thought, they still have the potential to
be important as a source of sea salt aerosol which may
undergo further chemistry, resulting in the release of gasphase bromine.
[14] Acknowledgments. This work was carried out with funding
from the National Science Foundation under grant number ATM-9875829.
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[13] While the role of frost flowers in polar boundary
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L. M. Avallone and L. E. Kalnajs, Laboratory for Atmospheric and Space
Physics, University of Colorado, Boulder, 1234 Innovation Drive, Boulder,
CO 80309, USA. (kalnajs@colorado.edu)
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