Introduction
During Arctic spring, photochemical reactions on snow, ice, and aerosol
particle surfaces convert, and may also recycle, halide anions that
originate from sea salts to potent halogen oxidizers that cause ozone depletion
events (Barrie et al., 1988; Simpson et al., 2007b),
oxidize hydrocarbons (Jobson
et al., 1994; Gilman et al., 2010; Hornbrook et al., 2016), and oxidize
mercury (Schroeder et al., 1998; Steffen et al.,
2008), leading to enhanced deposition of pollutants to the Arctic. This
chemistry depends upon the presence of sea ice, which is rapidly changing
(Nghiem et al., 2007, 2013;
Stroeve et al., 2012), but understanding of environmental controls
(Abbatt et al., 2012; Simpson et al., 2015) on this
chemical process is very limited.
Satellite remote sensing has detected enhanced bromine monoxide (BrO) during
Arctic spring (Richter et al., 1998; Wagner and
Platt, 1998), but satellite-detected hot spots were sometimes not observed by
in situ aircraft studies (Jacob et al., 2010;
Salawitch et al., 2010). Satellite-observed BrO was correlated with high
winds (Jones et al., 2009; Choi et al., 2012) and
potential frost flowers (Kaleschke et al., 2004), while
ground-based studies found little relationship between BrO and these proxy
measurements (Simpson
et al., 2007a; Halfacre et al., 2014; Peterson et al., 2016). To investigate
these aspects of springtime reactive bromine chemistry, we carried out the
BRomine, Ozone, and Mercury EXperiment (BROMEX)
(Nghiem et al., 2013) near Barrow
(Utqiaġvik), Alaska, in spring 2012. This study provided an unprecedented
opportunity to investigate the relationship between sea ice and atmospheric
chemical processes.
Past work has demonstrated that BrO is related to ozone abundance (Simpson et al.,
2007a; Helmig et al., 2012; Peterson et al., 2015), snowpack composition and
pH (Simpson et al.,
2005; Grannas et al., 2007; Krnavek et al., 2012; Pratt et al., 2013),
aerosol particles (Frieß et al., 2011), and atmospheric
stability (Peterson et al., 2015). Also during BROMEX, Moore
et al. (2014) demonstrated that sea ice leads caused vertical
mixing that brought ozone and mercury down from aloft. Peterson et al. (2015) expanded the idea that vertical mixing was important by
showing that BrO vertical profiles were affected by atmospheric stability,
finding that temperature inversions were correlated to shallow BrO layers.
Pratt et al. (2013) showed that halogen activation was efficient
in snowpack if that snow was acidic and had enriched
Br- / Cl- ratios. Frieß et al. (2011) showed that BrO was more likely to be elevated in cases
where aerosol particles were present (as indicated by aerosol extinction
measurements) and suggested that aerosol production from blowing snow
(Yang et al., 2010) may have been responsible.
A particular aspect needing consideration for BrO observations is BrOx
(BrOx = BrO + Br) repartitioning that occurs at low ozone mixing
ratios (MRs). The two fastest reactions for the BrOx family are BrO
photolysis (Reaction R1) and reaction of Br with ozone
(Reaction R2):
BrO+hv→Br+OBr+O3→BrO+O2.
These two reactions interconvert BrOx family members but
do not change BrOx abundance. When ozone is depleted to levels
∼ 1–2 nmol mol-1, and for near-noon photolysis conditions,
these reactions are of roughly equal rates. Therefore, when ozone goes below
a few nanomoles per mole (nmol mol-1), reactive bromine (BrOx) may
be present, but not all of that BrOx is spectroscopically visible as
BrO. BrOx is instead present as Br atoms, which rapidly oxidize mercury
(Holmes et al., 2006, 2009; Stephens et al., 2012; Moore et al., 2014),
affecting the fate of this pollutant. This low-ozone-induced BrOx
repartitioning has been observed as decreased surface BrO during very low
surface ozone periods in multiple field studies (Simpson et al., 2007a;
Helmig et al., 2012; Peterson et al., 2015) and is a common feature of
chemical modeling studies (Sander et al., 1997; Evans, 2003; Thomas et al.,
2011; Toyota et al., 2014).
In the BROMEX study, we used multiple-axis differential optical absorption
spectroscopy (MAX-DOAS) instruments deployed via helicopter in an
upwind–downwind array to determine typical horizontal length scale and
vertical structure of BrO events. These gradient observations complement
Peterson et al. (2017), who used airborne
DOAS to study an episode of reactive halogen transport. Here, we use these
observations to study BrO spatial structures and the effects of
sea-ice-lead-induced vertical mixing on reactive bromine.
Methods
MAX-DOAS spectroscopy and analysis
BrO and aerosol optical properties inferred from oxygen
collisional dimer (O2-O2 or O4) absorption were measured by
MAX-DOAS spectroscopy, as described in Peterson et al. (2015),
who adapted the methods of Frieß et al. (2011); see the
Supplement for details. Three MAX-DOAS instruments of design
similar to that described in prior references
(Carlson et al., 2010; Peterson et al., 2016) were
used. Two of the MAX-DOAS instruments were housed on mobile solar-powered
instrument packages called “IceLanders” that were deployed by helicopter
onto sea ice near the Barrow Arctic Research Center (BARC) building, where
the third MAX-DOAS instrument was located. Section 3 describes locations of
the sites.
All MAX-DOAS instruments followed a scan pattern that included below-horizon
viewing elevations through the zenith (nominally -2, -1, 0, 1, 2, 3, 5,
10, 20, and 90∘ elevation angles). Spectra were analyzed relative to
the zenith spectrum in an elevation sequence to result in differential slant
column densities, dSCDs, of O4, BrO, NO2, and O3 as
described in Peterson et al. (2015) with modifications described in the
Supplement. At the negative elevation angles, the physical pathlength is
shortened due to viewing the ground, causing a cutoff in the dSCD(O4)
as compared to above-horizon spectra. For elevation scans when the
near-horizon positive-elevation dSCD(O4) was greater than 2×1043 molecule2 cm-5, automated software determined the
below-horizon half-cut elevation angle. Radiative transfer modeling with
SCIATRAN (version 3.2.5) (Rozanov et al., 2005) indicated that this half-cut
elevation angle was a function of instrument optical inlet height, occurring
at -0.1∘ elevation for 3 m above ground level (a.g.l.) telescope
height, which was appropriate to the IceLander platform, and -0.3∘
elevation for 14 m a.g.l. telescope height, which was appropriate for BARC
building deployment. This optical measurement of the horizon elevation was
then used to adjust the view elevations and resulted in shifts to the
observation elevation of < ±0.3∘ from the nominal
elevation. These corrected elevation angles were used in the subsequent
analysis. All data are available from the NASA Earth Exchange (NEX) platform
(https://nex.nasa.gov/nex/projects/1388/).
The dSCD measurements were inverted to vertical profiles of BrO mixing ratio
and aerosol particle light extinction using the University of Heideleberg
Profile (HeiPro) optimal-estimation (OE) modeling software. HeiPro version 1.4
was used for retrieving aerosol optical properties, and HeiPro 1.3 was
used for BrO trace gas analysis. All data were processed as hourly averages,
resulting in vertical profiles from the surface to 4 km. Only positive-elevation-angle
(above-horizon) dSCD measurements were used in the OE modeling
by selecting observations at elevations greater than the instrumental field
of view (0.7∘ FWHM). The aerosol grid consisted of 20 layers, each
0.2 km thick, and the BrO grid consisted of 40 layers, each 0.1 km thick.
As discussed in Peterson et al. (2015), and following the work of Payne et
al. (2009), the full grid over-represents the true information content in the
retrieved vertical profiles, which was typically ∼ 2–3
degrees of freedom for signal (DOFS) for both BrO and
aerosol extinction (see Fig. S1 in the Supplement). Therefore, the BrO
amounts and vertical distributions were represented by two quantities: the
lower-tropospheric vertical column density (LT-VCD) and the fraction of that
LT-VCD in the lowest 200 m (f200). We refer to this representation of
the data as “information-content-based retrievals”. The BrO LT-VCD is
nominally the integral of the vertical profile from the surface to the top of
the model but loses sensitivity above about 2000 m. The f200 is the
partial VCD from the surface to 200 m divided by the LT-VCD. For BrO
retrieval, if either the DOFS in the lowest 200 m is < 0.7 or the
DOFS from 200 to 2000 m is < 0.5, we do not report the data,
ensuring that only retrievals that are well constrained by the observations
are used. For some figures, we calculated the average BrO MR in the 0–200 m
layer, represented in picomoles per mole (pmol mol-1). Aerosol
extinction vertical profiles were retrieved by modeling dSCD(O4). The
aerosol extinction profiles were then integrated from the surface to 4 km to
result in an aerosol optical depth (AOD, unitless), which was typically small
for much of the campaign due to the predominance of clear skies. Larger AOD
values were likely to be caused by clouds. Figure S1 shows reduction in the
lofted information (DOFS for BrO in the 200–2000 m a.g.l. layer) at high
AOD, which is consistent with Peterson et al. (2015).
Ozone measurements
Surface ozone mixing ratios were measured on each IceLander platform with a
modified 2B Technologies Model 205 photometric ozone monitor (Halfacre et al., 2014). These
instruments had a manufacturer-specified detection limit of ozone of 1.0 nmol mol-1. The surface ozone mixing ratio was measured in Barrow
(Utqiaġvik) (McClure-Begley et al., 2014) at the NOAA
Global Monitoring Division (GMD) site (71.3230∘ N,
156.6114∘ W), 2 km east of the BARC site.
Panel (a) shows the locations of IL1, BARC, and IL2
instruments overlaid on the 23 March 2012 sea ice map from daily RGB
composite 250m resolution MODIS images of ice conditions using bands 7, 2, and 1
(2105–2155, 841–876 and 620–670 nm wavelengths, respectively) from the NASA Aqua
satellite. In this composite, sea ice is light blue, open water is black, and
clouds are white. The red arrows show the MAX-DOAS viewing direction, over
which BrO was averaged. The wind was from the northeast (can be seen by thin
“cloud streets”, which are wind-parallel horizontal convective rolls) and
was pushing the lead open and causing IL2 to break away from shore-fast ice
and drift westward. IL1 was the most upwind instrument, BARC in the middle,
and IL2 downwind. Panel (b) shows the location of the two mobile
platforms (IL1 and IL2) relative to BARC building during the drift phase. See
Fig. 2 for drift distance versus time.
Meteorological measurements
Barrow meteorological data were measured at the NOAA Barrow (Utqiaġvik)
airport (PABR) Automated Surface Observing System (ASOS) site
(71.286∘ N, 156.766∘ W), which is 5.5 km SW from the BARC
building. Winds were measured at 10 m a.g.l. using an ultrasonic anemometer,
and temperature was measured at 2 m a.g.l. with an aspirated thermometer. Each
IceLander platform carried cup anemometers for wind speed measurements that
were recorded by data loggers (Campbell Scientific). The IceLander winds
were recorded at approximately 2.5 m a.g.l..
Meteorological data from the BROMEX campaign and drift information
of IL1 and IL2. Panel (a) shows Barrow (Utqiaġvik) temperature
(2 m a.g.l.) measured at the NWS-AWOS site. Panel (b) shows wind
speed at all three sites, but the Barrow winds were measured at
10 m a.g.l., while IL1 and IL2 were measured at 2.5 m a.g.l.
Panel (c) shows wind direction at Barrow (Utqiaġvik, green
circles) and in blue and red lines the direction of the IL platforms from the
BARC building. The wind direction was bimodal (Fig. S2); west winds are
plotted with a shaded background, and east winds without shading.
Panel (d) shows the distance of the IL platforms from BARC. Both IL
platforms started at BARC and were deployed on 8 and 9 March. On 23 March,
IL2's sea ice broke away from the land, starting its drift phase; on
27 March, IL1 returned to BARC.
BROMEX field campaign and meteorological situation
The BARC instrument was located at 71.325∘ N, 156.668∘ W
and was used as a point of reference for the measurements at the IceLander
sites, called IL1 and IL2. Initially, both IceLanders were co-located with
BARC for intercomparison purposes. On the afternoon of March 8, IL2 was
deployed 27 km west of BARC (to 71.2745∘ N, 157.295∘ W),
and on 9 March, IL1 was deployed 36 km east of BARC (to 71.355∘ N,
155.668∘ W). At about 13:00 AKST on 23 March, the sea ice on which
IL2 was located broke away from the landfast ice, and IL2 entered a drift
phase for the remainder of the campaign. Figure 1 shows the locations of
instrument packages overlaid on a map of the sea ice near local solar noon
(∼ 13:30 AKST) on 23 March, and the animation in the Supplement
shows the temporal evolution of sea ice and motion of the packages. IL1 was
recovered from the sea ice via snowmobile near 12:00 AKST on 27 March and
was operated co-located with the BARC instrument until 31 March. Following
opening of the lead, IL2 drifted, reporting MAX-DOAS data until it tipped
over on 10 April. The locations of IL2 and IL1 compared to the BARC
instrument location are shown in the lower panel of Fig. 1.
Figure 2 summarizes the meteorological conditions during the campaign.
Temperatures were cold (-15 to -35 ∘C), but many days showed diurnal
heating due to the returning sun of March/April. Surface wind speeds varied
from calm to 12 m s-1 and were approximately 10 m s-1 at the time
of ice break-off on 23 March. When wind speeds were low, shear was evident
with higher winds at 10 m a.g.l. (measured at the Barrow airport site) but less
wind speed closer to the surface at the 2.5 m a.g.l. IL sites. Two periods
(28 March–2 April and 6–8 April) of zero reported wind speed at IL2
were likely caused by icing of the cup anemometer at that site and are
probably artifacts.
The wind direction in this period was bimodal with a predominant wind from
the north-through-east sector and a secondary wind peak from the west (see
wind direction histograms in Fig. S2). We divided the campaign
into periods of “east” wind and “west” wind by taking the sector from
160 to 340∘ as “west” and from 340 through
0 to 160∘ as “east”. The predominant “east” wind
was from the northeast (average direction: 48∘; standard deviation:
29∘), and the less-frequent “west” wind was from the west
(average direction 262∘; standard deviation: 34∘). The
design of the experiment was to have one IL platform upwind and one downwind
of BARC, and this design often worked, as evident by the wind direction
being relatively parallel to the ENE–WSW direction of the IL1–BARC–IL2 line
of sites. For the majority of the deployment phase, IL1 was upwind of
BARC, and IL2 was downwind. The lowest panel of Fig. 2 shows the distance of the
IL platforms from BARC. IL1 never moved significantly, but IL2's drift
brought it ∼ 250 km west, typically downwind, from BARC.
IceLander 2 MAX-DOAS observations stopped on 10 April at 17:30 AKST, when
IL2 tipped 33∘, preventing its optical scanner from observing the
horizon.
Intercomparison of BrO measurements between BARC, IL1, and IL2 when
all instruments were co-located at BARC. Error bars (1σ) are shown on
each data point and were typically around 5×1012 molecule cm-2 for BrO LT-VCD and 3 pmol mol-1 for the
BrO 0–200 m mixing ratio (MR).
For this analysis, we divided the campaign into four periods:
intercomparison: 2–8 March, in which all instruments were compared at
BARC;
period B1: 8–18 March, which had no open leads and during which the array was
deployed;
period B2: 18–28 March, which covered the lead opening event
when IL2 drifted west;
period B3: 28 March–10 April, during which time IL1 was
returned to BARC and shut down and IL2 continued observing until tipped.
Hourly BrO, aerosol optical depth (AOD), and ozone (O3) data
from each of the three sites (IL1, BARC, IL2) during period B1.
Panel (a) shows the lower-tropospheric VCD of BrO (integrated
concentration from surface to top of model). Panel (b) shows the
fraction of BrO LT-VCD in the lowest 200 m; f200 = VCD
(0–200 m) / LT-VCD. Panel (c) shows the vertical integral of
the aerosol extinction from 0 to 4000 m (the AOD). Panel (d) shows
the in situ surface ozone mixing ratio measured on the IL platforms and as
measured by NOAA-GMD ∼ 2 km northeast of the BARC building.
Results
Intercomparison of MAX-DOAS observations when co-located
To assure intercomparability of the MAX-DOAS measurements, all hours when
the IceLander instruments were at BARC were correlated. This selection led
to 24 h of IL2–IL1 comparison and ∼ 50 h of BARC–IL1
comparison. Because IL1 was recovered, pre- and post-deployment
intercomparison data were determined, but IL2 was lost, so only
pre-deployment intercomparison was possible. Figure 3 shows the results of
these comparisons. For the BrO 0–200 m mixing ratio, we found high R2
correlations between instruments of 0.92 and 0.95, intercepts < 0.5 pmol mol-1, and slopes of 1.08 and 1.29. Typical errors (1σ) for
the BrO surface mixing ratio were 2–3 pmol mol-1. For the BrO LT-VCD,
R2 correlations were 0.76 and 0.87 with intercept statistics within
1σ of zero and slope within 2σ of unity. Typical BrO LT-VCD
errors (1σ) were ∼ 5 × 1012 molecule cm-2.
These results demonstrate good agreement between three independent
instruments and allow us to use the instruments to determine horizontal
gradients.
The same as in Fig. 4 but for period B2. IL2 began drifting away
from BARC on 23 March.
The same as in Fig. 4 but for period B3. IL1 was recovered to BARC
on 27 March, was co-located with the BARC instrument during this period,
and was shut down at the end of March.
Gradient observations during phases B1–B3
Figures 4–6 show atmospheric chemical observations during these three
phases. During this campaign, there was a great deal of variability of BrO
by all measures. The LT-VCD varied ∼ 0 to ∼ 8 × 1013 molecule cm-2, and the fraction in the lowest 200 m
(f200) varied the full possible range: 0 to 1. Generally, period B3 had
lower f200 values, and period B2 lacked very high column events.
During period B1, BrO at the three sites followed the same behavior. Large
changes such as the precipitous drop in BrO LT-VCD from > 6 × 1013 molecule cm-2 on 15 March at sunrise down to near-zero values
in the late morning happened at all three sites. This change appeared to be
at least partially due to low-ozone-induced BrOx repartitioning at the
surface, as discussed in Sect. 5.3. Decreasing AOD was also observed at
this time, which was probably the result of an air mass change (e.g., frontal
passage). The vertical structure (f200) also agreed very well between
sites. There were some time shifts of up to ∼ 2 h between
sites, which was consistent with the corresponding transport time, but the
sites generally followed the same pattern even if they were shifted in time
by an hour or two.
Period B2 started with consistent meteorology from period B1, although the
wind increased from 18 March up until the ice break-off event on 23 March.
The BrO LT-VCD during this period was lower than its peak earlier in the
campaign, and very shallow (e.g., f200 > 0.5) events were
observed. The association of shallow BrO layers with small column density
was noted by Peterson et al. (2015) and was interpreted as a
result of poor vertical mixing preventing propagation of surface-based BrO
aloft. During the static phase (prior to ice break-off and IL2 motion) of
the study (8 to 23 March), correlations between BrO measurements at BARC and
IL2 compared to IL1 were still high despite horizontal separation (see
Fig. S3). The BARC–IL1 LT-VCD correlation R2 value was 0.84,
and the IL2–IL1 correlation was R2=0.79. The surface BrO mixing
ratios were similarly correlated, with R2 values of 0.85 and 0.81,
respectively. These correlation coefficients were similar to the co-located
period, despite separation between sites of 36 km (BARC–IL1) and 63 km
(IL2–IL1). Figure S4 shows histograms of LT-VCD
differences between sites. This analysis shows that the probability of
having a difference with an absolute value less than 1013 molecule cm-2 (2σ of the BrO LT-VCD measurement error) is 87 % for
IL1–IL2 and 83 % for IL1–BARC, again indicating that, at most times
without open sea ice leads, strong spatial gradients in BrO are not
observed.
Figures 4 and 5 also demonstrate that ozone at the three locations was
highly correlated before the lead opening event on 23 March. Generally,
before lead opening, surface ozone mixing ratios were low (< ∼ 15 nmol mol-1), but when the wind speed increased
(10 and 11 March, and surrounding the lead opening on 23 March), ozone mixing
ratios increased, consistent with ozone downward transport associated with
wind-induced mixing (Jacobi et al., 2010). We also observed
reduced wind shear between the Barrow measurements (at 10 m a.g.l.) and the IL
platforms (2.5 m a.g.l.) during the higher-wind periods, consistent with reduced
stratification of air near the surface.
Upon lead opening on the afternoon of 23 March, changes to the vertical
structure of BrO appeared. The most downwind site, IL2, was within a
surface-based cloud formed by the open lead, and the AOD increased
significantly at that location as compared to the other sites. This cloud
precluded BrO observations at IL2 until the next day (24 March), on which a
gradient in f200 developed between IL1 (upwind), BARC (middle), and IL2
(downwind), with a shallower BrO distribution (higher f200 values) at
the upwind site and a more vertically mixed behavior (lower f200 values)
at IL2. Note that the surface ozone mixing ratio was high enough (more than a
few nanomoles per mole) that repartitioning of BrOx at the surface was not
responsible for these lower f200 values at IL2. This gradient in BrO
vertical structure persisted until the morning on 26 March, when the three
sites appeared to be in different air masses (as indicated by different ozone
mixing ratios at the three sites, in contrast to their prior highly
correlated behavior). This change in ozone at Barrow (BARC) on 26 March was
noted by Moore et al. (2014) as a reduction in vertical mixing due to re-freezing of
previously open leads upwind of BARC.
Altitude–time profiles of aerosol particle extinction (a)
and BrO mixing ratio (b) on selected days during BROMEX in 2012.
Ticks on the time axis occur every 3 h. Black pixels indicate extinction
above 1 km-1, which may be too optically thick to be reliably
calculated via the optimal-estimation analysis and probably make
higher-altitude measurements at that time unreliable as well. BrO mixing ratios above
40 pmol mol-1 are shown as black and are most likely artifacts of the
limited vertical resolution of the optimal-estimation analysis. White periods
indicate missing data by lack of sun, instrumental problems, or low
visibility (e.g., afternoon of 23 March at IL2 due to the lead opening
event).
On the afternoon of 27 March, IL1 was recovered and resumed operation at
BARC. During this post-deployment co-location period, IL1 and BARC agreed
well (Fig. 6). On 28 March (Fig. 6), there was a different vertical profile
at IL2 than at BARC, with a surface-based event on the sea ice at IL2
that had higher f200 values than BARC. During the downwind drift period
(B3), the highly correlated behavior observed prior to lead opening on 23 March
was replaced with notable discrepancy between IL2 and BARC. However, the
daily-timescale values of all quantities vary similarly between these sites
despite the large distance (∼ 130 to 260 km) between sites.
Selected cases
Figure 7 shows altitude–time profiles of aerosol extinction and BrO mixing
ratio on the selected days of 9, 15, 16, and 22–24 March. These cases were
selected to have similar meteorological conditions, with winds from the
northeast, moderate to low AOD (except for around the lead opening event,
particularly at IL2, on the afternoon of 23 March), and cold temperatures
(-25 to -35 ∘C). There was generally good agreement
between the three sites, in agreement with the information-content-based
analysis shown in Figs. 4–6. These altitude–time profile presentations are
informative despite the fact that they over-represent the vertical
information, particularly aloft, where the averaging kernels show that the
vertical resolution is broadened significantly (Frieß et al., 2011; Peterson et al., 2015). It is
likely that some subtle differences aloft are simply due to lack of vertical
resolution, but the consistent features between sites are likely well
represented in these profile plots.
9 March
This case shows a vertically thick surface-based aerosol layer, with
log10(extinction) > -1 up to ∼ 1 km. During
this day, surface ozone was sufficiently low (< 1 nmol mol-1)
after noon to cause BrOx repartitioning at the surface, which is
evident by reduced values of f200 < 0.1 (Fig. 4). The time
profile plots show that BrO was not present at the surface, but the peak
mixing ratio moved aloft. There was a moderate decay in the LT-VCD on this
day, but the occurrence of surface BrOx repartitioning did not
eliminate BrO aloft.
15 March
This day had dramatic BrO changes with nearly temporally coordinated
behavior at all three sites. Figure 4 shows that in the morning there was
high BrO LT-VCD (> 7×1013 molecule cm-2), which
declined to near-zero values (< 1×1013 molecule cm-2) at
noon and then recovered moderately (∼ 2×1013 molecule cm-2).
The morning vertical distribution of BrO showed f200=0.3, which decreased to lower values, consistent with surface-based
BrOx repartitioning at low ozone levels, and then f200 increased
again to > 0.5 late in the afternoon, indicating a surface-based
BrO layer. The BrO profiles (Fig. 7) show a relatively thick BrO layer (to
1 km) in the morning that decayed to zero at noon and then built a shallow
event in the afternoon. Aerosol extinction on this day was at relatively
high values in the morning but decreased to low levels (log(extinction)
< -1.2), particularly above the first 200 m a.g.l., in the afternoon.
16 March
This day had a shallow surface-based BrO event with f200 values between
0.4 and 0.8. Sufficient ozone was present at the surface to prevent
repartitioning of BrOx. The aerosol profiles show that there was very
little aerosol extinction aloft and that there were only small amounts in the lowest few
hundred meters. Figure 7 shows some evidence of a lofted aerosol particle
layer, but that layer was decoupled from the surface aerosol layer and was
not associated with BrO enhancements.
22 March
This case, which was the day before the lead opening event, shows an
interesting contrast to 16 March. There was again a surface-based BrO event,
with f200 > 0.6, which was slightly shallower than on 16 March.
However, the aerosol extinction was both higher in magnitude and
distributed much more aloft on this date than on 16 March. Figure 7
demonstrates that the aerosol layer descended throughout the day and seemed
to be overlapping the surface layer. However, despite the presence of
aerosol particles aloft, Fig. 7 shows that BrO does not appear aloft (as it
had on 9 and 15 March in the morning), as discussed in Sect. 5.4.
23 March
This was the day of the lead opening event. All three sites began with a
shallow BrO event in the morning. There was moderate aerosol extinction,
mostly based at the surface but extending aloft. At the time of the lead
opening event, the aerosol extinction at IL2 (downwind) went high,
> 1 km-1, in the lowest 400–600 m a.g.l., consistent with
that instrument being within the convective lead cloud. Unfortunately, the
lead cloud prevented BrO LT-VCD or f200 from being observed at IL2, but
observations become valid at all three sites on the next day.
24 March
This case shows the next-day response of BrO to this lead opening event.
Downwind of the open and re-freezing lead, IL2 observed a decrease in BrO
mixing ratio at the surface (Fig. 7) and a broadening of the BrO vertical
profile to greater heights. Unlike most times earlier in the campaign, BrO
and aerosol extinction (Fig. 7) show spatial gradients between the sites, as
was discussed using BrO LT-VCD and f200 earlier in this section. The
downwind IL2 site had high aerosol extinction in a thick
(> 400 m) surface based layer, which decayed in the afternoon.
Ozone was high (∼ 30 nmol mol-1) at all sites, eliminating
BrOx repartitioning as a cause for this difference.
Discussion
BrO measurements were highly correlated on ∼ 30 km length
scales in the absence of leads
During the pre-lead-opening period (before 23 March), Figs. 4 and 5 show
that measurements at the three sites correlate despite physical separation
between sites of ∼ 30 km, and even ∼ 60 km from
IL2 to IL1. The correlation (Fig. S3) and R2 values of these separated
measurements in this period are quite similar to the times when the
instruments were co-located at BARC (Fig. 3). It is evident from examination
of the time series data (Figs. 4 and 5) that some changes in BrO occurred at
one site before another, with temporal shifts of a couple hours. This type
of temporal shift would have decreased the hourly correlation coefficient
and was likely responsible for some of the reduction of R2 between BrO
column densities for the deployed site locations and the co-located sites.
The high correlation of measurements separated by length scales similar to
satellite pixel dimensions (Richter et al., 1998;
Wagner and Platt, 1998; Begoin et al., 2010; Choi et al., 2012; Sihler et
al., 2012) is an important finding that generally indicates that
satellite-based BrO observations are likely to represent horizontal spatial
features effectively. Variability of BrO in the stratosphere (Theys et al., 2009; Salawitch et al., 2010)
or free troposphere (Theys et al., 2011; Choi et al.,
2012) could affect this conclusion, but one would expect less horizontal
inhomogeneity aloft because of a lack of small-scale features such as leads or
topography, which are only present at ground level. Although we observe that
the general behavior of BrO is high correlation despite spatial separation,
transport events that have gradients significantly sharper than satellite
length scales are clearly evident in the data. For example, on 13 March,
Fig. 4 shows time-staggered changes in BrO LT-VCD and f200. Peterson et
al. (2017) used airborne DOAS to study
the 13 March case and observed a very steep BrO gradient that transports
with the wind, clearly indicating that features on the edges of air masses
are < 30 km. Therefore, we interpret the BrO distribution as being
large regions of relatively consistent BrO on > 30 km length
scales with sharp contrasts at their edges that are much smaller than
satellite length scales.
Snowpack-based BrO events were common during BROMEX
Many of the BrO events that occurred during BROMEX were ground-based with a
high fraction of the BrO LT-VCD in the lowest 200 m (large f200).
Peterson et al. (2015) showed that shallow events are associated
with stable meteorological conditions, which predominated during much of
this campaign, particularly before the lead opening event. These shallow
events are consistent with a snowpack source of reactive bromine (Simpson et al., 2007a;
Pratt et al., 2013). Reactive bromine is relatively short lived due to
termination reactions, which often lead to HBr or HOBr bromide reservoirs
(Platt and Hönninger, 2003). However, these reservoir species
can recycle to reactive halogens through heterogeneous chemical reactions.
Fan and Jacob (1992) proposed that heterogeneous Reaction (R3) on
aerosol surfaces was a critical step for recycling reactive bromine and
activating particle-bound bromide (Br-) to reactive bromine after
photolysis of Br2.
HOBr+H++Br-→Br2+H2O
Subsequent laboratory studies have demonstrated that Reaction (R3) is
efficient on saline liquid, ice, and aerosol particle surfaces (Fickert
et al., 1999; Huff and Abbatt, 2000, 2002; Wachsmuth et al., 2002; Abbatt et
al., 2012; Wren et al., 2013; Roberts et al., 2014). Other mechanisms of
halogen activation also exist, such as the reaction of ozone with aerosol or
ice-bound bromide (Oum et al., 1998;
Hunt et al., 2004), but these processes are typically slower than
photochemical bromine release. Because heterogeneous chemistry is required
for reactive bromine recycling, we interpret these surface-based events as
recycling reactive bromine on snowpack surfaces.
As evident from the case studies on 16 and 22 March, shallow surface-based
BrO events can have different relationships to aerosol vertical structures.
On 16 March, there was little aerosol extinction aloft, implying low aerosol
surface area density, which would have slowed heterogeneous recycling on
lofted aerosol particles. One might interpret the lack of aerosol particles
aloft as causing the event to be surface based. However, on 22 March, there
was significantly more aerosol extinction detected aloft, but the event
remained based at the surface. Potential reasons for BrO not being observed
aloft on 22 March despite the presence of aerosol particles could be a lack of
vertical mixing due to meteorological inversions (Peterson et
al., 2015), which were common during the campaign. Specifically, on 22 March,
the meteorological sounding balloon launched from Barrow (Utqiaġvik)
at 15:00 AKST showed an inversion with dT / dz of +15 K km-1 in the
lowest 200 m a.g.l.. An alternative hypothesis for the lack of reactivity on the
lofted aerosol on this date could be that the particles had a chemical
composition that was not conducive to halogen release. For example, if the
particles had not contained bromide (Br-), Reaction (R3) would not have occurred.
Laboratory (Fickert et al., 1999; Huff and Abbatt, 2002; Abbatt et al., 2012; Wren et al., 2013; Roberts
et al., 2014) and field (Pratt et al., 2013) studies indicate
that acidic pH is also required for Reaction (R3), adding another potential
reason. Another alternative could be that the aerosol size distribution
consists of larger particles for which diffusion limits gas–surface reaction
rates or that these particles are long-range-transported Arctic haze
particles (Quinn et al., 2002).
Low-ozone-induced BrOx repartitioning affected BrO vertical
profiles
Past considerations of reactive bromine chemistry has indicated that
BrOx partitioning between Br atoms and BrO can be an important control
on BrO abundance, which has been modeled (Sander et al., 1997; Evans, 2003; Toyota et
al., 2014) and observed (Simpson et al., 2007a;
Helmig et al., 2012). The low ozone mixing ratios observed here (often
< 1–2 nmol mol-1) controlled surface BrOx partitioning
and reduced BrO abundance, and thus affected the vertical distribution of
BrO. Low BrO concentrations would also have reduced the formation of HOBr,
which is necessary for “bromine explosion” events (Wennberg, 1999; Lehrer et al., 2004) that recycle
BrOx via Reaction (R3). Through reduced heterogeneous recycling,
BrOx would have decayed over time as termination reactions (e.g., Br + H2CO) occurred. On 9 March, ozone levels began the day above this
threshold but soon decayed below the threshold, and BrO at the surface
decayed to zero (Fig. 7). This repartitioning effect reduces the f200
value to < 0.1 (Fig. 4), and BrO exists only aloft in the afternoon
(Fig. 7). On 9 March, the reactive bromine aloft was apparently generated at
the surface and moved aloft, where it recycled on aerosol particle surfaces.
On 15 March, low ozone values were observed, and what was a very intense
BrO event in the morning decayed to near-zero LT-VCD at noon. At noon, the
vertical structure of BrO became lofted (f200 < 0.1; Fig. 4),
but aerosol extinction aloft was smaller (Fig. 7; see Sect. 5.4) and BrO
did not propagate aloft after noon (Fig. 7). Interestingly, on 15 March, a
shallow (f200 > 0.5) post-noon BrO column appeared
(Fig. 7), potentially enabled by decreased afternoon photolysis rates and an
increase in O3 in the late afternoon that repartitioned BrOx back
towards BrO.
BrOx repartitioning may also have been responsible for low surface BrO
levels and low f200 values on many of the days during this campaign.
Peterson et al. (2016) found that this period (spring 2012) had particularly
low surface ozone, and Oltmans et al. (2012) showed that March ozone
depletion event (surface O3 < 10 nmol mol-1)
probability has been increasing over the 38-year period from 1972 to 2010.
Therefore, the prevalence of BrOx repartitioning in the BROMEX data set
may not be representative of average behavior and warrants further
climatological investigation through analysis of larger data sets.
Aerosol extinction aloft was necessary but not sufficient for BrO to be
present aloft
The cases presented in Sect. 4.3 and discussed above found that shallow
BrO events sometimes occurred with little aerosol aloft (16 March) and at
other times with significantly more aerosol aloft (22 March). When BrOx
repartitioning affected surface BrO, sometimes the BrO event migrated aloft
in the presence of significant aerosol loading (9 March), but sometimes BrO
decayed both at the surface and aloft (15 March). In all of these cases, BrO
only propagated aloft into layers with log10 (aerosol extinction)
> ∼ -1, meaning aerosol extinction coefficient
(kext) > ∼ 0.1 km-1.
To check if this aerosol extinction coefficient threshold is reasonable, we
can use the observation to estimate heterogeneous chemical rates. We
encourage future photochemical modeling to answer this question more fully.
Aerosol extinction is related to aerosol surface area density by kext=Qext×SA/4, where Qext is the extinction efficiency, which
maximizes for submicron particles at a value close to 4, and SA is the surface
area density. Assuming maximal Qext=4 gives the minimum surface area density
(SA = ∼ 100 µm2 cm-3) that is consistent with the observed threshold
kext=∼ 0.1 km-1, which appears necessary for BrO to propagate aloft.
In the absence of diffusion limitations (e.g.,
typically for submicron particles), the rate of a heterogeneous reaction is
khet=1/4cγSA, where c is the average velocity of the gas, and
γ is the reaction probability. Wachsmuth et al. (2002) indicate that heterogeneous uptake of HOBr on sea
salt aerosol particles is limited by accommodation and has the value
γ=0.6. The actual value of γ may be lower because
ambient particles are likely not solely sea salt. At 100 µm2 cm-3 and thermal velocity of HOBr at 253 K (-20 ∘C), c=255 m s-1, and thus khet=0.0038 s-1, corresponding to an
∼ 4 min HOBr lifetime. Thompson et al. (2015) indicate the
photolysis rate J(HOBr) = 0.0023 s-1 for springtime Barrow
(Utqiaġvik) conditions, so this surface area density results in a
heterogeneous reactivity rate that competes with HOBr photolysis. Photolysis
of HOBr cycles reactive bromine and destroys ozone but does not increase
the reactive bromine pool. On the other hand, Reaction (R3) forms Br2,
and upon Br2 photolysis results in two reactive bromine species from
the one BrO radical that formed HOBr (Wennberg, 1999; Platt and
Hönninger, 2003). Thus, for bromine to “explode”, heterogeneous
reactions must occur fast enough to compensate for reactive bromine losses
(e.g., termination reactions such as Br + H2CO). The observational
threshold found in this study appears to be sufficiently high to allow
heterogeneous recycling of BrOx to compete with BrOx loss.
Therefore, it appears reasonable that current understanding of bromine
chemical kinetics is in agreement with this observed aerosol optical
extinction threshold (aerosol extinction > 0.1 km-1)
required for BrO to exist aloft.
Although we found that increased aerosol aloft was necessary for BrO to be
found aloft, there were cases in which BrO remained ground based despite
significant aerosol extinction above. For example, on 22 March, there was
significant aerosol extinction aloft (Fig. 7), but BrO did not show signs of
migrating aloft (Fig. 7). The lack of BrO aloft could be caused by hindered
vertical mixing (Sect. 5.2) or by the particles having incorrect chemical
composition to recycle reactive bromine. Therefore, we find that aerosol
aloft is necessary for BrO to be present aloft, but it is not sufficient to
always cause BrO to propagate vertically when enhanced aerosol extinction is
present. Peterson et al. (2017) used
airborne DOAS to study the case on 13 March and found that a reactive
bromine plume propagated with the wind and was maintained by heterogeneous
chemistry on aerosol particles, complementing the detailed cases explored in
the present study.
Average BrO LT-VCD (a) and BrO fraction in the lowest 200 m,
f200, (b) during the 2 days after the lead opening, when the
lead is open and re-freezing. The bar height is the average, and error bar
length is ±1σ. A t test for significant difference (α=0.05) shows that the LT-VCD at BARC and IL2 was significantly larger than at
IL1 but that BARC and IL2 were not significantly different. For the f200
vertical distribution metric, all three sites were statistically different
from each other with a clear trend from the upwind IL1 site to the downwind
IL2 site.
Sea-ice-lead-associated convection affected the BrO vertical
profile
On 23 March, after the opening of the sea ice lead, and on the following
days (24 and 25 March) at the upwind IL1 site, BrO was present in a shallow
layer (f200 > 0.5) with moderately enhanced (2×1013 molecule cm-2) LT-VCD (Fig. 5). However, the sites near to and downwind
of the lead (BARC and IL2) exhibited decreased f200 values as compared
to the upwind site, as would be expected by vertical entrainment of
reactive-bromine-poor air from above the shallow boundary layer and mixing
of surface air aloft. Consistent with decreased f200, Fig. 7 shows this
vertical mixing decreased the BrO surface mixing ratio downwind of the lead
at IL2. As opposed to the clear surface mixing ratio decrease after lead
opening, the BrO LT-VCD (Fig. 5) does not show strong differences between
sites along the transport direction.
To further explore the effect of the lead opening event, Fig. 8 shows the
average and variability of BrO LT-VCD and vertical distribution
(f200) for 24–25 March, which were the 2 days following the lead
opening event. During these 2 days, IL2 was downwind of a large area of
re-freezing sea water, 71–106 km downwind of BARC. The typical surface
wind speed was ∼ 5 m s-1 coming from 70∘,
nearly parallel to the BARC–IL2 direction. The wind speed increases
aloft; averaged over the 0–600 m a.g.l. region, the Utqiaġvik airport
(PABR) radiosonde wind speeds were ∼ 8.5 m s-1,
indicating a 2–3 h transport time. Figure 8 (left panel) demonstrates
that the BrO column peaks at the middle (BARC) site and not the most
downwind (IL2) site. On average, the BrO LT-VCD at IL1 (upwind) was somewhat
(28 %) smaller than BARC, and IL2 was slightly (5 %) smaller than BARC.
A paired t test shows that the LT-VCD was statistically significantly larger
at both BARC and IL2 than it was at IL1 but that the BARC and IL2 sites
were statistically indistinguishable. Figure 8 (right panel) demonstrates a
clear trend in the f200 metric of BrO vertical distribution, with a
shallower surface layer (larger f200) at the upwind IL1 site trending
towards a more vertically mixed layer (smaller f200) at the downwind IL2 site.
All sites are statistically significantly different from each other for
f200. These observations show that the open lead's primary influence was
to alter the vertical distribution of BrO, increasing its vertical extent,
but the lead only slightly affected the BrO column density on the timescale
of transport between these sites (up to about 3 h).
The presence of the open and re-freezing leads could have had multiple
effects on aerosol particles and BrO. Wind blowing across the lead is likely
to produce aerosol particles (Nilsson
et al., 2001; May et al., 2016), which could be lofted in the convective
environment of the lead cloud. The lead is also re-freezing between BARC and
IL2 during this period, and that new ice is likely covered with frost
flowers, which have been proposed as either a direct source of reactive
halogens (Rankin et al., 2002) or a source of sea salt aerosol
particles (Kaleschke et al., 2004) that could subsequently
produce/recycle reactive bromine via Reaction (3). Figure 7 shows the lead
opening event produced high extinction (> 1 km-1) through
∼ 600 m altitude on 23 March, and this aerosol–cloud layer
persisted into the morning of 24 March. Note that MAX-DOAS measures aerosol
extinction by attenuation of O4 absorption path length, so all
submicron aerosol particles, supermicron particles, and solid/liquid water
droplets in a cloud will increase the aerosol extinction. Interestingly, the
aerosol extinction at IL2 on the afternoon of 24 March is lower than at the
other sites, potentially due to enhanced scavenging by the humid lead cloud
environment. On 25 March, Fig. 5 shows that the AOD at BARC and IL2 went
below 0.2, also potentially caused by scavenging and/or reduced wind speeds
(Fig. 2). Piot and von Glasow (2008) modeled interaction of an
air mass with an open lead and found that the presence of supercooled liquid
water droplets suppresses heterogeneous bromine recycling, which may be
relevant to the lead re-freezing event. Examination of the MODIS sea ice
images (animation in the Supplement) shows that IL2 is not cloudy on both 24 and
25 March, but some clouds are seen between BARC and IL2. Overall, the
response to the lead opening of aerosol extinction, as measured by MAX-DOAS,
was a peak in aerosol extinction that corresponded with the highest winds
especially at the most downwind site (IL2) and then lower aerosol levels as
winds slowed and the lead re-froze. Following the passage of the lead cloud,
which cleared around noon on 24 March, aerosol extinction (Fig. 7) does not
appear to be enhanced at the downwind site (IL2) as compared to the other
sites, potentially indicating that open water and/or frost flowers between
BARC and IL2 are not efficient aerosol particle sources, at least at this
surface wind speed (which dropped to ∼ 4 m s-1).
With regard to BrO observations on the 2 days following lead opening, Fig. 8 showed
that BrO LT-VCD increased 28 % from IL1 to BARC but decreased
5 % from BARC to IL2. The origin of the moderate increase from IL1 to BARC
is not clear, but Peterson et al. (2015) found that shallower
layers (as are observed at IL1 as compared to BARC) are correlated with
lower LT-VCD, so the deepening of the BrO layer, and heterogeneous reactions
on lofted aerosol particles, between IL1 and BARC could be responsible for
that moderate increase in BrO. MODIS images (see animation in the Supplement)
show that the lead between BARC and IL2 is re-freezing in this period;
given the cold temperatures (-23 to -32 ∘C), humidity
from the open water, and presence of re-freezing sea ice, it is highly
likely that the area between BARC and IL2 contained frost flowers, which
have been suggested to be a source of reactive bromine
(Rankin et al., 2002; Kaleschke et al., 2004).
However, we see a small decrease in BrO from BARC to IL2, which argues
against frost flowers being a direct source of reactive bromine, consistent
with recent laboratory studies (Roscoe et al., 2011; Yang et al.,
2017).
Relationship of these findings to prior studies
These data show that vertical mixing deepens the atmospheric layer
containing BrO through lead-induced vertical mixing. Peterson et al. (2015) found that more vertically mixed BrO events are correlated
with higher BrO column amounts. McElroy et al. (1999) observed a
large tropospheric BrO column from high-altitude aircraft and associated
this column with sea-ice-lead-induced vertical mixing, consistent with our
observations. Satellite-based spectrometers detect the total (tropospheric + stratospheric) BrO column density, which can be corrected for
stratospheric BrO (Theys et al., 2011; Choi et
al., 2012; Sihler et al., 2012) to give a tropospheric VCD, but satellite
sensors cannot determine vertical profiles of BrO and may not observe
shallow BrO events, which were common during BROMEX. Thus, surface (e.g.,
ground-based CIMS or MAX-DOAS) observations would indicate differing
environmental controls for halogen activation than satellites would have
indicated. These types of differences in environmental controls have been
noted in the literature, depending upon the type of sensor (satellite versus
ground based) that was used to quantify halogen activation (BrO). Jones et
al. (2009) and Yang et al. (2010) found that
satellite-detected BrO is associated with high winds that would decrease
atmospheric stability and thus cause vertical mixing much like the
lead-induced mixing in this example and increase visibility of BrO from
space. However, there may also be differences in wind speed regime between
Antarctic (Jones et al., 2009; Yang et al.,
2010; Theys et al., 2011) and Arctic observations.
The finding that BrO is not increased downwind of frost flowers is in agreement
with measurements of their chemical composition, which has incorrect pH for
reactive bromine production (Kalnajs and Avallone,
2006; Abbatt et al., 2012; Pratt et al., 2013). It is possible that the
“potential frost flowers” (PFF) metric (Kaleschke et al.,
2004), which was devised to diagnose regions of frost flower formation and which involved
a combination of open water and cold temperatures, could have been
diagnosing spatial regions where lead-induced vertical mixing was occurring
(Nghiem et al., 2012), which are correlated
with higher BrO LT-VCD (Peterson et al., 2015). Therefore, the
correlation of PFF with satellite-observed BrO could be expected, not
because frost flowers are directly responsible for halogen activation but
because vertical mixing associated with the PFF proxy enhances the thickness
of the BrO layer.
Airborne observations of BrO were targeted during the NASA ARCTAS field
mission to locations of high satellite-detected BrO column densities, but
little in situ BrO was found (Jacob et al., 2010). This finding is again
consistent with our observations – regions of high column BrO are vertically
mixed, leading to lower in situ mixing ratios of BrO, and thus less detectable by
aircraft in situ techniques. Recent studies (Jones et al., 2009; Begoin et
al., 2010; Toyota et al., 2011; Choi et al., 2012) have found that mesoscale
cyclonic storms that have high winds that destabilize the otherwise stable
Arctic atmosphere are associated with multi-day satellite-remote-sensed BrO
transport events, again in agreement with the finding that vertical mixing
enhances the BrO column density (Peterson et al., 2015).
Conclusions
Analysis of time series of the BrO LT-VCD and
fraction of BrO in the lowest 200 m (f200) at Barrow (Utqiaġvik)
gave the following results. When a large sea ice lead opened and the ocean
re-froze, the vertical distribution of BrO was affected, but no significant
increase in BrO LT-VCD was observed between BARC and IL2, which was downwind
of the re-freezing lead, providing a counterexample to the hypothesis that
frost flowers growing on sea ice are a direct source of BrO. Measurements of
BrO LT-VCD and f200 were highly correlated on ∼ 30 km length
scales when there were no sources of vertical mixing (e.g., open leads) in the
intervening area. During the BROMEX period, which was characterized by clear
skies and cold temperatures that enhance vertical stability, shallow
surface-based BrO events were common. Repartitioning of BrOx due to low
ozone levels caused low surface BrO mixing ratios; depending upon whether
reactive bromine recycling was efficient aloft, BrO either shifted to higher
altitude, becoming a lofted layer, or decreased through the column.
Aerosol extinction aloft was necessary but not sufficient for BrO to be
present aloft. An aerosol extinction larger than 0.1 km-1 appeared
necessary for maintaining BrO aloft. These detailed observations can be
further used in modeling studies that provide insights into how chemistry and
meteorology interact (Holmes et al., 2006, 2010; Thomas et al., 2011, 2012,
Toyota et al., 2011, 2014).
These observations highlight spatial features of BrO events in the Arctic
and their relationship to aerosol extinction. Vertical atmospheric structure
(stability) is a critical control on the nature of reactive bromine events,
with typical “inverted” temperature profiles holding BrO close to the
snowpack (Frieß et al., 2011; Peterson et al.,
2015), where halogen activation reactions occur (Pratt et al.,
2013). Punctuated vertical mixing events, either by sea-ice-lead-induced
convection or by high winds associated with storms, dilute the surface mixing
ratio, but these more vertically mixed events are correlated with enhanced
BrO column density. These detailed observations resolve many past
controversies with respect to halogen activation in the Arctic. The Arctic
sea ice pack is thinning, and multi-year ice is being replaced by seasonal
first-year ice (Maslanik et al., 2011), which has been
predicted to increase the occurrence of leads and has been predicted to have
many further implications (Bhatt et al., 2014).
Moore et al. (2014) showed that ozone and mercury are brought
down from aloft during these lead events. In this work, we showed that
reactive bromine was brought up from near the surface to a thicker layer by
a lead-induced mixing event. These two factors should increase the overlap
of mercury with reactive bromine and thus the oxidation and deposition of
mercury to the Arctic. Predictions of increased sea ice leads would thus be
expected to increase the amount of toxic mercury deposition and have a
greater impact on Arctic free-tropospheric O3.