Frost flowers on sea ice as a source of sea salt and their influence on tropospheric halogen chemistry

Abstract

Frost flowers grow on newly-formed sea ice from a saturated water vapour layer. They provide a large effective surface area and a reservoir of sea salt ions in the liquid phase with triple the ion concentration of sea water. Recently, frost flowers have been recognised as the dominant source of sea salt aerosol in the Antarctic, and it has been speculated that they could be involved in processes causing severe tropospheric ozone depletion events during the polar sunrise. These events can be explained by heterogeneous autocatalytic reactions taking place on salt-laden ice surfaces which exponentially increase the reactive gas phase bromine (``bromine explosion''). We analyzed tropospheric bromine monoxide (BrO) and the sea ice coverage both measured from satellite sensors. Our model based interpretation shows that young ice regions potentially covered with frost flowers seem to be the source of bromine found in bromine explosion events.

Full text

Frost flowers on sea ice as a source of sea salt and their influence on
tropospheric halogen chemistry
L. Kaleschke, A. Richter, J. Burrows, O. Afe, G. Heygster, and J. Notholt
Institute of Environmental Physics, University of Bremen, Germany
A. M. Rankin and H. K. Roscoe
British Antarctic Survey, Natural Environment Research Council, Cambridge, UK
J. Hollwedel and T. Wagner
Institute of Environmental Physics, University of Heidelberg, Germany
H.-W. Jacobi
Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany
Received 2 June 2004; revised 21 July 2004; accepted 2 August 2004; published 25 August 2004.
[1] Frost flowers grow on newly-formed sea ice from a
saturated water vapour layer. They provide a large effective
surface area and a reservoir of sea salt ions in the liquid phase
with triple the ion concentration of sea water. Recently, frost
flowers have been recognised as the dominant source of sea
salt aerosol in the Antarctic, and it has been speculated that
they could be involved in processes causing severe
tropospheric ozone depletion e vents during the polar
sunrise. These events can be explained by heterogeneous
autocatalytic reactions taking place on salt-laden ice surfaces
which exponentially increase the reactive gas phase bromine
(‘‘bromine explosion’’). We analyzed tropospheric bromine
monoxide (BrO) and the sea ice coverage both measured
from satellite sensors. Our model based interpretation shows
that young ice regions potentially covered with frost flowers
seem to be the source of bromine found in bromine explosion
events.
INDEX TERMS: 0322 Atmospheric Composition and
Structure: Constituent sources and sinks; 1640 Global Change:
Remote sensing; 3309 Meteorology and Atmospheric Dynamics:
Climatology (1620); 3339 Meteorology and Atmospheric
Dynamics: Ocean/atmosphere interactions (0312, 4504); 3360
Meteorology and Atmospheric Dynamics: Remote sensing.
Citation: Kaleschke, L., et al. (2004), Frost flowers on sea ice
as a source of sea salt and their influence on tropospheric halogen
chemistry, Geophys. Res. Lett. , 31, L16114, doi:10.102 9/
2004GL020655.
1. Introduction
[2] The discovery of events of low ozone concentration
in the atmospheric boundary layer at measurement stations
located at high latitudes in the Northern Hemisphere has
prompted much research into their origin. These events
were found to be associated with enhanced amounts of
inorganic bromine compounds [Barrie et al., 1988;
McConnell et al., 1992; Fan and Jacob, 1992; Foster et
al., 2001] . Similar epi sodes have been observed in the
Southern Hemisphere at high latitudes [Wessel et al.,
1998; Frieß et al., 2004]. The advent of the measureme nt
of tropospheric trace gases from space by the Global Ozone
Monitoring Experiment, GOME, led to the discovery of
enhanced amounts of BrO close to regions of sea ice in the
Northern and the Southern Hemisphere [Richter et al.,
1998; Wagner and Platt, 1998]. GOME measures the light
scattered from the atmosphere and reflected by the ground
between 240 and 790 nm wavelength with a horizontal
ground resolution of about 320  40 km
2
[Burrows et al. ,
1999]. A retrieval algorithm, based on differential optical
absorption spectroscopy (DOAS) and stratospheric BrO
modeled by a three-dimensional radiative- dynamical-
chemical model, yields the tropospheric f raction of the
column density of BrO [Richter et al., 1998; Wagner and
Platt, 1998; Chipperfield, 1999]. Bromine destroys ozone
very efficiently in two interlinked catalytic cycles which
produce BrO and HOBr in the gas-phase [Foster et al.,
2001]. Gaseous HOBr reacts with Br ions in a slightly acidic
sea salt solution and releases Br
2
and BrCl into the gas phase
[Fickert et al., 1999; Adams et al., 2002]. The photolabile
Br
2
molecule is subsequently photo-dissociated into atomic
Br [Foster et al., 2001]. Therefore, every Br atom of the
HOBr molecule entering the liquid phase has the potential to
release two Br atoms to the gas phase. The above gives a
simplified description of the heterogeneous autocatal ytic
reaction that causes an exponential increase of gaseous Br
radicals, the so-called bromine explosion. The main source
of bromine over the open oceans in the marine boundary
layer outside the polar regions was identified to be sea salt
aerosol generated by breaking waves on the ocean surface
[Tang and McConnell, 1996; Vogt et al., 1996; Sander et al.,
2003]. The processes and sources unique to the polar ocean
surfaces still remained unidentified, though the highest BrO
amounts have been observed over the sea ice during the
polar sunrise [Ridley et al., 2003 ; Zeng et al., 2003; Frieß et
al., 2004]. Recently, the potential role of frost flowers
(Figure 1) in this processes has been raised [Rankin et al.,
2002]. Frost flo wers are ice crystals which grow on frozen
leads (linear breaks in the sea ice cover) an d polynyas
(openings between drift ice and fast ice or the coast). Frost
flowers exhibit enhanced salinities and bromide ion concen-
trations of about three times of that of bulk seawater
[Perovich and Richter-Menge, 1994; Rankin et al., 2002].
GEOPHYSICAL RESEARCH LETTERS, VOL. 31, L16114, doi:10.1029/2004GL020655, 2004
Copyright 2004 by the American Geophysical Union.
0094-8276/04/2004GL020655$05.00
L16114 1of4

Frost flowers only last for a few days until they are blown
away by strong winds or covered by drifting snow [Perovich
and Richter-Menge, 1994]. The aerosol produced by frost
flowers was identified in Antarctic ice cores and at coastal
stations due to its depleted sulfate to sodium ratio compared
to the aerosol originating from the open ocean [Wagenbach
et al., 1998; Rankin et al., 2002; Rankin and Wolff, 2003].
2. Methods and Data Sets
[3] It is assumed, that the open water area (which is given
by one minus the sea ice concentration) will be covered
soon with thin new ice on which frost flowers can grow.
The sea ice concentration is determined from analyses of the
thermal microwave emission me asured by the Special
Sensor Microwave Imager (SSM/I) aboard the Defense
Meteorological Satellite Program (DMSP) platform using
the ASI algorit hm [Kaleschke et al., 2001; Maslanik and
Stroeve, 2003]. Briefly this algorithm takes advantage of the
higher polarisability of a specularly reflecting water surface
compared to the more diffusely reflecting sea ice surface
[Kaleschke et al., 2001; Kern et al., 2003].
[
4] A one dimensional thermodynamic model has been
developed to calculate the frost flower coverage (Figure 2).
The model combines a frost flower growth parameterization
obtained from laboratory experiments with the equations of
sea ice heat balance which are described in more detail by
Martin et al. [1996] and Maykut [1986]. We assume that
the growth of frost flowers depends only on two basic
prerequisites, the existence of new ice which is formed in
leads or polynyas and of a strong negative temperature
gradient above the ice surface. The two model input
parameters, the surface air temperature T
a
and the open
water (OW) fraction, are taken from numerical weather
prediction reanalysis data (NCEP/NCAR) and from satellite
passive microwave measurements, respectively. The ice
thickness H = 1.33Q
0.53
[cm] is calculated from the cumu-
lative freezing days Q =
R
(T
f
 T
a
)dt with air temperature
T
a
and the freezing point of sea water T
f
= 1.9C. The sea
ice surface temperature T
0
=
15:96 þ HT
a
8:4 þ H
is approximated for
thin ice using an averaged heat transfer coefficient which
describes both sensible and latent heat exch ange [Maykut,
1986]. Assuming that the influence of a varying wind field
and the insulating effect of frost flowers on the heat flux are
negligible, then the frost flower growth rate g =
0.000785e
0.478(T
0
T
a
)
is readily calculated using coefficients
from laboratory experiments [Martin et al., 1996; Maykut,
1986]. The area coverage F
t
is calculated using the recursive
expression F
t
= F
tdt
+ g(1  F
tdt
)dt for a time step dt.
Because the growth rate decreases rapidly as the ice
thickness increases, the model yields a maximum percent-
age area F
max
(T
a
) covered by frost flowers for a given
surface air temperature. This is defined as the re lative
potential frost flower (PFF) area. The total PFF area is
obtained by weighting the total area with the new ice
fraction. The predicted relative PFF areas for different
integration times and air temperatures are presented in
Figure 2. The height and hence the volume of the frost
flowers cannot be calculated as the parameterization of the
growth rate was derived from video ima ges which measured
the area coverage. As there is currently insufficient micro-
physical understanding about the initial nucleation and
growth of frost flowers as well as about the decay processes,
a minimum frost flower area for a given set of conditions
cannot be estimated [Perovich and Richter-Menge, 1994;
Martin et al., 1995, 1996]. The original aim of the above
described method was to identify dates and regions worth to
be analysed using costly very high resolution satellite
images for the future development of a more direct frost
flower retrieval algorithm [Kaleschke and Heygster, 2004].
3. Comparison of Model Results and BrO Data
[5] One typical example of the resulting PFF and the BrO
data for the Antarctic is shown in Figure 3. The overall
mechanism requires the release of bromine atom precursors,
either directly on the frost flower surface or within its
aerosol. We calculated forward air trajectories starting at
regions with a high probability of frost flowers from the
NCEP/NCAR Reanalysis surface wind field in order to
account for the atmospheric transport of the aerosol or
BrO. Some regions with a high probability of frost flower
occurrence for example at the Ronne-Filchner Ice Shelf in
the Weddell Sea show no corresponding BrO plumes
Figure 1. Frost flowers on sea ice covering a lead: Stellar
dendrites of about 1 to 2 cm height on young sea ice
(Courtesy of Stefan Kern, University of Hamburg). The
photograph was taken at 7558
0
N2534
0
E, 24 March 2003.
The air temperature was about 18C.
Figure 2. Frost flower model and frost flower coverage as
a function of air temperature for different integration times.
The three curves represent the integration times of one, five
and ten days, respectively. The theoretical upper limit of
frost flower coverage F
max
(T
a
) is approximately given by
the upper curve.
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because sunlight is needed for the photochemical reactions.
The polynya at the Ross Ice Shelf that occurs frequently due
to strong katabatic winds is shown to be a strong source of
frost flower aerosol responsible for the enhanced BrO
concentration farther north in the illum inated area. The
trajectories are only a rough approximation as errors could
occur in convective regions which are not well represented
in the atmospheric boundary layer of the NCEP/NCAR
model over sea ice [Kaleschke et al. , 2001]. Nevertheless,
more than ninety percent of the trajectories hit the enhanced
BrO areas in Figure 3. One key area of frequently occurring
ozone depletion events in the Arctic is shown in Figure 4.
This typical example shows the huge (almost 500  50 km
2
)
recurring shore polynya in the northwest of the Hudson Bay
which is potentially covered with frost flowers. This polynya
frequently appears under offshore wind conditions.
[
6] We investigated the entire 1996 to 2002 dataset and
found commonly more than two third of the PFF trajectories
hitting the enhanced BrO areas for the Arctic and Antarctic
during polar sunrise. Almost all cases of enhanced BrO
amounts were associated with a high probability of frost
flowers on the previous days. Occasionally enhanced PFF
values appear without enhanced BrO amounts. However,
this does not reject the hypothesis of frost flowers causing
BrO production as the BrO retrieval could be hampered
by clouds [Richter et al., 1998; Wagner and Platt, 1998;
Frieß et al., 2004]. Furthermore, the PFF is a potential
theoretical upper limit. Specific meteorological conditions
could have prevented the actual growth of frost flowers. The
influence of the wind is ambiguous: the dynamical opening
of leads and polynyas is a wind driven effect and a
prerequisite for thin ice production, but persisting strong
winds could prevent the growth of frost flowers [Perovich
and Richter-Menge, 1994]. Changing wind fields s uch as
passing cyclones probably support the growth of frost
flowers.
4. Conclusion
[7] It can be summarized that the sea salt and associated
halogen flux from the ocean to the atmosphere is governed
by different processes inside and outside the polar regions:
Outside the polar regions , the sea salt is injected into
the atmosphere by breaking waves on the ocean surface
dominated by the wind [Sander et al., 2003]. Whereas the
process inside the sea ice covered re gions is mainly
modulated by the air temperature [Zeng et al., 2003; Frieß
et al., 2004]. Previous work has been insufficient to localize
the potential bromine sources [Richter et al., 1998; Wagner
Figure 3. Comparison: Example total PFF coverage
(green stars) and enhanced BrO amounts (red isolines) over
the Antarctic ocean. The maximum values of two
consecutive days are shown: from 28 to 29 and 29 to
30 August 1997 for PFF and BrO, respectively. The green
stars mark the endpoints of 24h air trajectories starting at a
frost flower area coverage of greater than 0.4%, sampled
every 187 km. The PFF field was smoothed to 300 km
spatial re solution to approximately match the GOME
resolution. The total PFF coverage is proportional to the
size of the stars. Two stars of 1% and 10% area coverage are
shown in the lower right corner for comparison. The
red isoline corresponds to enhanced BrO amounts of 3.6 
10
13
molec/cm
2
which is the mean (1.4) plus one standard
deviation (2.2). The black circle indicates the almost dark
latitudes (solar zenith angle > 80). Sea ice covered regions
are presented in white and the open ocean is coloured in
blue.
Figure 4. Example total PFF coverage (green stars) and
enhanced BrO amounts (red isolines) over the Hudson
Bay for 11 March 2001. Symbols and lines are similar to
Figure 3, but the PFF coverage is displayed color coded.
The 24h air trajectories start at PFF greater than 1% sampled
every 62.5 km. The red isoline corresponds to enhanced
BrO amounts of 8.8  10
13
molec/cm
2
which is the mean
(6.2) plus two standard deviations (1.3). The isoline is
broken due to the data gaps of the GOME coverage.
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and Platt, 1998; Zeng et al., 2003; Ridley et al., 2003; Frieß
et al., 2004]. We provided a method to localize the young
ice regions potentially covered with frost flowers that seem
to be a prerequisite for the bromine explosion. This provides
a crucial step for a better understanding of the exchange
processes between ocean, sea ice and atmosphere that are of
great importance for the Earth’s climate system [Shepson et
al., 2003].
[
8] Acknowledgments. L. K. and HW. J. gratefully acknowledge the
German Research Foundation (DFG) for funding. L. K. thanks Gunnar
Spreen, Stefan Kern, Roland von Glasow, Eric W. Wolff, Mark Drinkwater,
Robert Ezraty, Wolfgang Dierking, Christof Lu¨pkes, Thomas Busche and
Christian Haas for discussions.
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
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