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ACPD
6, 11051–11066, 2006
Sea ice, frost flowers
and halogen
activation
W. R. Simpson et al.
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Atmos. Chem. Phys. Discuss., 6, 11051–11066, 2006
www.atmos-chem-phys-discuss.net/6/11051/2006/
© Author(s) 2006. This work is licensed
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Atmospheric
Chemistry
and Physics
Discussions
First-year sea-ice contact predicts
bromine monoxide (BrO) levels better
than potential frost flower contact
W. R. Simpson
1
, D. Carlson
1
, G. Hoenninger
1,2,†
, T. A. Douglas
3
, M. Sturm
3
,
D. Perovich
3
, and U. Platt
2
1
Geophysical Institute and Department of Chemistry, University of Alaska Fa irbanks,
Fairbanks, AK 99775-6160, USA
2
Institute for Environmental Physics, University of Heidelberg, Im Neuenheimer Feld 229,
69120 Heidelberg, Germany
3
U.S. Army Cold Regions Research and Engineering Laboratory, P.O. Box 35170, Fort
Wainwright, AK 99703-0170, USA
†
deceased
Received: 23 October 2006 – Accepted: 6 November 2006 – Published: 7 November 2006
Correspondence to: W. R. Simpson (ffwrs@uaf.edu)
11051
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Abstract
Reactive halogens are responsible for boundary-layer ozone depletion and mercur y
deposition in Polar Regions during springtime. To investigate the source of reactive
halogens in the air arriving at Barrow, Alaska, we measured BrO, a marker of reactive
halogen chemistry, and correlated its abundance with airmass histories derived from5
meteorological back trajectories and remotely sensed sea ice properties. The BrO is
found to be positively correlated to first-year sea-ice contact (R
2
=0.55), and weakly
negatively correlated to potential frost flower (PFF) contact (R
2
=0.04). These data
indicate that snow contaminated with sea salts on first-year sea ice is a more probable
bromine source than are frost flowers. Recent climate-driven changes in Arctic sea ice10
are likely to alter frost flower and first year sea ice prevalence, suggesting a significant
change in reactive halogen abundance, which will alter the chemistry of the overlying
Arctic atmosphere.
1 Introduction
During late winter and spring, oxidation chemistry in the Arctic troposphere shifts from15
being dominated by ozone photochemistry to being dominated by halogen chemistry,
especially reactive bromine chemistry. This shift has profound influences that range
from depletion of tropospheric ozone to deposition of mercury to alteration of the fate
and lifetime of organic pollutants (Barrie et al., 1988; Schroeder et al., 1998). The
majority of the bromine atoms responsible for ozone depletion and mercury deposi-20
tion come from salts containing bromide (Br-) that originate from the ocean and are
oxidized to bromine atomic radicals through an autocatalytic reaction pathway known
as the bromine explosion (Fan and Jacob, 1992; McConnell et al., 1992). The trans-
fer mechanism by which the salts become atmospherically accessible is not currently
understood, yet this mechanism plays a fundamental role in activating halogens that25
deposit airborne contaminants (e.g. Hg) to the terrestrial cryosphere.
11052
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Two hypotheses have been presented for the mechanism of bromide salt transport
from the ocean to reactive bromine in the atmosphere. Frost flowers, which are highly
saline, have been proposed to provide the surface from which bromine is released
(Rankin et al., 2002). Kaleschke et al. (2004) and co-workers used remotely sensed
sea ice data to predict where frost flowers may be forming on sea ice. They introduced5
a proxy they called potential frost flowers (PFF), which is calculated by a thermody-
namic model of sea ice and frost flower growth and found that PFF is correlated with
BrO detected by satellite (Kaleschke et al., 2004). Jones et al. (2006) found that ozone
depletion events detected at Halley Bay, Antarctica were correlated with airmass mo-
tions that brought air in contact with a large coastal polyna that often produces frost10
flowers. Snow contaminated with salts, which are prevalent on first-year sea ice, has
also been proposed to be the surface from which bromine is released. Direct mass-
spectroscopic evidence of release of Br
2
and BrCl (precursors of BrO) from snow (Fos-
ter et al., 2001), and depletions of Br- in coastal snow (Simpson et al. , 2005) provide
evidence for the salty snow hypothesis. Satellite observations (Wagner et al., 2001)15
and ground-based observations of BrO in Antarctica (Frieß et al., 2004) also point to
first-year ice as an important region for bromine activation.
Freezing sea water separates ice from brine, which is a concentrated salt solution.
Some of this brine is forced to the new ice surface, causing high initial surface salini-
ties of 50–100 parts per thousand by weight (‰, where Arctic Ocean sea-water salin-20
ity ≈30‰). As the ice ages, the surface salinity decreases to approximately 5–10‰.
Snow on the sea ice is contaminated by salts when brine wicks up the snow or the
wind scours the snow to re-expose saline surfaces or salt aerosols deposit to the snow
(Domine et al., 2004). Sea ice that sur vives the summer has a much lower surface
salinity because the brine drains away during summer melting. Therefore, multi-year25
sea ice (MYI) and land surfaces contain less atmospherically accessible salt and are
less likely to be reactive halogen sources than first-year sea ice (FYI). Frost flowers
form when open leads or polynyas freeze over and small nodules in the forming ice act
as condensation nuclei for vapor deposition of angular ice crystals that look like deli-
11053
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cate flowers. The flowers wick brine from the sea-ice surface producing highly saline
(≈100‰) crystals with specific surface areas similar to that of surface hoar (Domine
et al., 2005; Perovich and Richter-Menge, 1994).
2 Methods
The contact time of an air parcel with potential frost flowers (PFF) or first-year ice5
(FYI) is calculated by combining back-trajectory analysis, which provides air motion
and temperature with remotely sensed sea ice data, which provides information on
where open water and ice are present. In the PFF calculation, open water is assumed
to be freezing over and producing new frost flowers with increasing efficiency as the
temperature decreases (Kaleschke et al. , 2004). Figure 1 shows an example of this10
calculation where a trajectory passes near a coastal polynya in the vicinity of Banks
Island, Canada.
2.1 PFF calculation
For each hour during the winter/spring of 2005, we calculate a 72 h back trajector y
at 50 m arrival height using the HYSPLIT trajectory model (Draxler and Rolph, 2003).15
In the time period shown (DOY 80–120), there are 960 total trajectories considered.
Other arrival heights (of 100 and 300 m) were explored and shown to have similar re-
sults. The open water fraction, assumed to be 1 – sea ice concentration, comes from
ASI algorithm (Kaleschke et al., 2001) sea-ice maps (based upon AMSR-E data) at the
6.25 km resolution downloaded from http://www.seaice.de/. Airmasses are propagated20
back along the prescribed trajectory and minutes of contact with open water are calcu-
lated on a pixel-by-pixel basis using the appropriate daily sea-ice map. For example, if
the trajectory spends 30 min in a particular pixel, and the sea-ice fraction is 66% in that
pixel, then there are (1−0.66) × 30 min =10 min of open water contact for this pixel. We
then use the PFF parameterization (Kaleschke et al., 2004) based upon the parcel’s25
11054
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temperature to calculate the percent of open water that could maximally be covered
by frost flowers. Continuing the example, say the PFF parameterization indicates 50%
PFF coverage. We then multiply the 10 min in this pixel by 0.5 to get 5 min of PFF
contact. The PFF contact is then summed over the entire 72 h trajectory to get the total
PFF contact for the airmass arriving at Barrow at the arrival time. If the trajectory goes5
over 1 km height, it is assumed to no longer have sea-ice contact. This 1 km height
was chosen as a typical height of the convective cloud that would be in the presence
of a lead.
Airmass dispersion occurs in the atmosphere but is not accounted for in the trajec-
tory calculation. Additionally, inaccurate meteorological fields cause back trajectory10
errors. Therefore, we added a feature to the calculation where the average sea ice
concentration in a circle centered at the trajectory’s calculated position is used instead
of simply the immediate pixel of the calculated position. This circle increases in radius
as time is propagated backwards at 2 km/hr. This “diffusion” rate is about 1/10th of the
wind speed and results in a radius of ≈140 km three days before arrival. Substituting15
this algorithm for the simple pixel-by-pixel calculation smoothed some high-frequency
noise in the PFF time series, and was chosen as more realistic. In the example shown
in Fig. 1, the PFF contact is with the polynya areas near Banks Island, Canada and
arises from the radial averaging algorithm causing overlap of the trajectory with the
open water shown in that vicinity.20
We performed many tests where the recently past sea ice conditions (up to 5 days
earlier) were considered instead of the current sea-ice conditions. The idea of these
tests was to see if frost flowers that may have formed a few days before the trajec-
tory passed over a pixel were responsible for halogen activation. None of these tests
showed significantly better correlation with BrO, and thus we present the results using25
the formulation used by Kaleschke et al. (2004) because it has been accepted in the
literature.
11055
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2.2 First-year sea-ice contact calculation
First year ice (FYI) contact is calculated in a similar manner along the same trajectories,
following the method of Frieß et al. (2004). Regions of older ice (second year or older)
were identified through examination of QuikSCAT sea ice backscatter coefficient, which
is a proxy for sea ice age (Voss et al., 2003). The QuikSCAT data were downloaded5
from http://www.ifremer.fr/. We found that the region of older ice is relatively stationary
during the 40-day period of analysis. Therefore, one mask of old sea ice, shown in
yellow in Fig. 1, was generated from the observation on DOY 100 (10 April 2005). With
this mask, we identify regions of first-year sea ice and sum the ice fraction times the
contact time for each pixel along the trajectory. If the trajector y indicates that the parcel10
is at an altitude over 100 m (twice the 50 m arrival height), we consider the contact with
the surface is lost and do not accumulate any first-year sea-ice contact.
2.3 Chemical measurements
Bromine monoxide (BrO) and ozone were measured continually at Barrow, Alaska
(71.3 N, 156.7 W) over the winter and spring of 2005. Bromine monoxide was mea-15
sured by multiple-axis differential optical absorption spectroscopy (MAX-DOAS) (Hoen-
ninger and Platt, 2002). Measurements were made at multiple viewing elevations to
separate stratospheric BrO from tropospheric BrO and to derive vertical profiles of
the BrO concentration. Strict inversion of the data to concentration profiles is compli-
cated by radiation transfer issues. Therefore, we chose the difference in slant column20
density (DSCD) of BrO between a 2-degree elevation angle measurement and a near-
coincident zenith measurement to quantify the tropospheric BrO abundance. The high
degree of correlation shown in this work argues for the accuracy of this simple quan-
tification of BrO. For clear-sky conditions, 10
14
molecule cm
−2
DSCD corresponds to
about a 5 parts per trillion by volume (pptv) mixing ratio in a 1 km thick mixed layer. The25
peak values shown here are ≈30 pptv, which is similar to values seen at other Arctic lo-
cations where halogen activation is prevalent (Hausmann and Platt, 1994; Hoenninger
11056
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and Platt, 2002; Tuckermann et al., 1997). Hourly in-situ ozone mixing ratios were mea-
sured by the National Oceanic and Atmospheric Administration (NOAA) Earth Systems
Research Laboratory/Global Monitoring Division (ESRL/GMD).
3 Results
Figure 2 shows time series of ozone, BrO, FYI contact, and PFF contact for Barrow,5
Alaska. Ozone and BrO are generally anticorrelated, as expected because BrO de-
stroys ozone. Because it requires time for BrO to destroy ozone, the anticorrelation is
not precise, and BrO is considered a better marker of halogen activation. The BrO data
follow the FYI contact remarkably well, particularly the rapid variations during the pe-
riod DOY 110 to 119. In contrast, the BrO shows little relationship to PFF. The largest10
BrO events (DOY 110 to 112) occur after very little PFF contact, while the largest PFF
contact event, DOY 115, is correlated with essentially no BrO. Therefore, PFF appears
unrelated to BrO.
There are two days where FYI contact is high but BrO is near zero, DOY 88 and 97.
Both of these days have extremely low ozone, nearly always <1 ppbv, as indicated by15
the red coloring of all BrO data when O
3
<1 ppbv. At these low ozone levels, reactive
bromine (the sum of the concentrations of Br and BrO) partitions to Br atoms because
the ozone necessary to form BrO is absent. Therefore, the absence of BrO on DOY 88
and 97 is expected, and we exclude these data from later analysis.
Figure 3 shows the correlations of BrO with FYI and BrO with PFF contact. BrO20
is positively correlated with FYI contact while BrO is weakly anticorrelated with PFF.
Although the functional form of the relationship between BrO and FYI is unknown, a
simple linear model appears to fit well and can explain 55% of the variance in BrO
(R
2
=0.55). This correlation is quite impressive given inaccuracies in the trajectory
model and inevitable variations in other important meteorological parameters, such as25
wind speed and inversion height. On the other hand, a linear model of the depen-
dence of BrO on PFF can explain <5% of the variance (R
2
=0.04), and the slope of the
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correlation is opposite to the expectation that PFF should produce BrO. The intercept
of this correlation, which indicates significant BrO in the absence of PFF contact also
does not make sense. These findings clearly indicate that contact with first-year sea
ice provides the majority of reactive bromine to airmasses that arrive at Barrow during
the ozone depletion season.5
4 Discussion
The potential frost flower algor ithm was chosen for this work because Kaleschke et al.
(2004) reported that PFF from large polynya areas spatially matched regions of en-
hanced BrO detected by satellite. In the Kaleschke study, PFF came mostly from large
coastal polynyas because those areas are a major source of open water in the Polar10
Regions. In our study, we also find that most airmasses with high PFF contacts can
be traced to coastal polynyas or leads. Although there is commonly a lead a few kilo-
meters from Barrow, this lead opens when winds drive ice away from the shore, and
this lead seldom affects airmasses that blow onshore to the Barrow site. Therefore, the
leads/polynyas that contribute to PFF typically impact the airmass days before arrival at15
Barrow and the overall PFF contact time is quite low. If the influence of polynyas/leads
occurs relatively local to the source, their influence could have diminished before air-
masses impact Barrow, which might explain the lack of correlation seen here. If it is
the case that PFF produces only a local effect, then there must be another source of
reactive bromine, associated with sea ice, to explain the high levels of BrO observed20
at Barrow in this study.
In the Jones et al. (2006) study, ozone depletion was observed when airmasses
passed over a coastal polyna, which they interpreted as being covered by frost flowers
and these frost flowers providing reactive bromine to deplete the ozone. The trajecto-
ries shown in the Jones et al. paper also pass across areas of first-year sea ice before25
traversing the coastal polynya, so it is possible that a sea-ice source of bromine caused
their ozone depletions instead of the interpretation that frost flowers caused the ozone
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depletion. In our analysis of data arriving at Barrow, the different mechanisms of PFF
contact and FYI contact allow the PFF and FYI contact time series to be very different
and allow separation of the sources. It is unclear from the Jones et al. analysis whether
potential frost flower contact and first-year sea-ice contacts are different in their data.
Therefore, we cannot speculate on whether their data uniquely point to frost flowers5
as the source of reactive bromine, but simply note that their data probably could be
alternatively interpreted as due to first-year sea-ice contact.
Our study clearly shows that first-year sea ice is correlated with reactive halogen
production, but we need to consider what specific property of these geographic areas
is responsible for the halogen production. Ice motion, even in the middle of the pack,10
causes cracks to form such that on the order of a percent of the ice area is open water.
Given cold temperatures, these open leads freeze over, forming surface brine. Frost
flower formation additionally requires supersaturated water vapor, which depends on
the presence of nearby open leads to source the water vapor (Andreas et al., 2002).
Because small leads may be difficult to detect by satellite remote sensing, we cannot15
preclude the possibility that some frost flowers have formed in the first-year ice even
though the calculated PFF contact is very small for these trajectories. As brine and
frost-flower covered areas age, on the time scale of a few days, they may be covered by
snow or the frost flowers could be blown away by wind (Perovich and Richter-Menge,
1994). Therefore, frost flower formation is temporally transient in addition to being20
spatially limited. The property of frost flowers thought to make up for these limiting
factors is their specific surface area (the ice surface area per mass), which was initially
estimated to be very large (Rankin et al., 2002). However, the specific surface area of
frost flowers has recently been shown to be much smaller and similar to that of snow
(Domine et al., 2005), making frost flowers appear less likely as the bromine source.25
Overall, frost flowers that may be present in the first-year ice would only provide a
saline surface over small areas and for short periods of time.
Snow contaminated by salts is ubiquitous in the FYI area. Snow on sea ice has a
wide distribution of depths and large areas are only covered by shallow snow that can
11059
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be influenced by brine wicking from the sea ice surface. Snow on FYI is also shallower
(mean depth 20 cm) and less prevalent than snow on MYI (Sturm et al., 2002). Studies
from Antarctica have shown snow salinities of 5‰ commonly occur at heights up to
20 cm in the snow pack (Massom et al., 2001), and summertime measurements from
the Arctic show highly variable snow salinity with an average of 1‰ that may have been5
reduced by summer brine drainage (Eicken et al., 2002). Wind pumping of air through
snow also increases the depth within the snowpack to which sea salts can become
atmospherically accessible. Frost flowers could produce aerosols that contaminate
the snow surface and this snow could later release bromine, leading to correlation with
first-year sea-ice but not PFF. From these observations, we find that saline snow on the10
first-year sea ice is ubiquitous and contains significant quantities of salts necessary for
halogen activation. Therefore, the common source of salty snow appears more likely
than the transient and spatially limited frost flowers that may have formed but have
been undetected by the PFF method.
5 Conclusions15
Potential frost flower contact time, as calculated by meteorological back trajectories
and remotely sensed sea ice features is not well correlated to BrO arriving at Bar-
row, while first-year sea-ice contact is positively correlated to BrO. These data point to
first-year sea ice areas as the major source region for reactive bromine arriving at Bar-
row during the ozone depletion season with salty snow apparently being the bromine20
source mechanism. The Arctic has recently seen dramatic reductions in sea ice cover-
age and increases in air temperatures, with most models predicting further ice losses
(Johannessen et al., 1999; Over peck et al., 2005; Serreze et al., 2003; Stroeve et al.,
2005). This mar ked modification of sea ice coverage is likely to have different effects
on formation of first-year sea ice and frost flowers. For example, a warmer Arctic with25
less summer sea ice might have less frost flowers in spring due to warmer tempera-
tures but more saline snow on first-year sea ice. A better mechanistic understanding of
11060
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the relative global importance of these two possible reactive bromine sources is critical
for meaningful prediction of future halogen activation, boundary-layer ozone depletion,
and mercury deposition in the context of a rapidly changing Arctic.
Acknowledgements. This work was supported by the National Science Foundation, under grant
OPP-0435922. Logistical support from the Barrow Arctic Science Consortium is gratefully ac-5
knowledged. We also thank U. Frieß for the suggestion to try correlation of BrO with first-year
sea-ice contact. We thank NOAA for providing the ozone measurements and the HYSPLIT
trajectory model.
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Schlosser, P., and V
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and halogen
activation
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3-day back trajectory arriving
at Barrow 0Z 13 Apr 05 (DOY 103.0)
Sea Ice map 10 Apr (DOY100)
Brown = Land Yellow = >1yr. ice
Fig. 1. An example of a 3-day back trajectory arriving at Barrow, Alaska. The arrival time is
00:00 UTC on 13 April 05 (DOY 103.0). The sea-ice map is shown for 10 April because the
main contact of these trajectories with open water occurred on that day. The proper sea-ice
map is used for each day of the trajector y calcul ation. The ice map shows brown for land,
a gradation from black to white via blue tones for ice concentration, from AMSR-E data, and
yellow for the area identified as multi-year ice from QuikSCAT sea ice age estimations.
11064
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6
4
2
0
BrO DSCD / 10
14
molec cm
-2
120110100
9080
DOY 2005
50
40
30
20
10
0
O
3
/ ppbv
4000
3000
2000
1000
0
FYI contact / min
8
6
4
2
0
PFF contact / min
Fig. 2. Ozone, Bromine monoxide (BrO), first-year sea-ice contact (FYI), and potential frost
flowers contact (PFF) timeseries in the period DOY 80–120. BrO data colored red occurred
when ozone <1 ppbv.
11065
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6
5
4
3
2
1
0
BrO DSCD / 10
14
molec cm
-2
40003000200010000
FYI contact / minutes
R
2
= 0.55
6
5
4
3
2
1
0
BrO DSCD / 10
14
molec cm
-2
76543210
PFF contact / minutes
R
2
= 0.04
Fig. 3. Correlation plots of B rO versus first-year sea-ice contact (left) and potential frost flowers
contact (right). Data colored in red occurred when ozone <1 ppbv and were ignored from the
correlation analysis.
11066
