Reproducing springtime Arctic tropospheric ozone depletion events in an outdoor mesocosm sea-ice facility

Mapping Intimacies ◽

10.5194/acp-2021-157 ◽

2021 ◽

Author(s):

Zhiyuan Gao ◽

Nicolas-Xavier Geilfus ◽

Alfonso Saiz-Lopez ◽

Feiyue Wang

Keyword(s):

Boundary Layer ◽

Sea Ice ◽

Gas Phase ◽

Ozone Depletion ◽

Surface Ozone ◽

Environmental Research ◽

The Arctic ◽

Ozone Loss ◽

Ozone Concentrations ◽

Air Temperatures

Abstract. The episodic build-up of gas-phase reactive bromine species over sea ice and snowpack in the springtime Arctic plays an important role in the boundary layer, causing annual concurrent depletion of ozone and gaseous elemental mercury during polar sunrise. Extensive studies have shown that these phenomena, known as bromine explosion events (BEEs), ozone depletion events (ODEs) and mercury depletion events (MDEs), respectively, are all triggered by gas-phase reactive bromine species that are photochemically activated from bromide via multi-phase reactions under freezing air temperatures. However, major knowledge gaps exist in both fundamental cryo-photochemical processes causing these events and meteorological conditions that may affect their timing and magnitude. Here, we report an outdoor mesocosm-scale study in which we successfully reproduced ODEs at the Sea-ice Environmental Research Facility (SERF) in Winnipeg, Canada. By monitoring ozone concentrations inside large, acrylic tubes over bromide-enriched artificial seawater during entire sea ice freeze-and-melt cycles, we observed mid-day photochemical ozone loss in winter in the boundary layer air immediately above the sea ice surface in a pattern that is characteristic of BEE-induced ODEs in the Arctic. The importance of UV radiation and the presence of a condensed phase (experimental sea ice or snow) in causing such surface ozone loss was demonstrated by comparing ozone concentrations between UV-transmitting and UV-blocking acrylic tubes under different air temperatures. The ability of reproducing BEE-induced ODEs at a mesocosm scale in a non-polar region provides a new approach to systematically studying the cryo-photochemical and meteorological processes leading to BEEs, ODEs, and MDEs in the Arctic, their role in biogeochemical cycles across the ocean-sea ice-atmosphere interfaces, and their sensitivities to climate change.

Reproducing polar springtime bromine explosion events, ozone depletion events and atmospheric mercury depletion events in an outdoor mesocosm sea-ice facility

10.5194/egusphere-egu21-6296 ◽

2021 ◽

Author(s):

Zhiyuan Gao ◽

Feiyue Wang ◽

Nicolas-Xavier Geilfus

Keyword(s):

Sea Ice ◽

Ozone Depletion ◽

Polar Region ◽

Surface Ozone ◽

Elemental Mercury ◽

Atmospheric Mercury ◽

Environmental Research ◽

The Arctic ◽

Gaseous Elemental Mercury ◽

Air Temperatures

Every year during polar sunrise, a series of photochemical events are observed episodically in the troposphere over the Arctic and Antarctic, including bromine explosion events (BEEs), ozone depletion events (ODEs), and mercury depletion events (MDEs). Extensive studies show that all these events are triggered by gas-phase reactive bromine species that are photochemically activated from sea-salt bromide via multi-phase reactions under freezing air temperatures. However, major knowledge gaps exist in both fundamental cryo-photochemical processes and local meteorological conditions that may affect the timing and magnitude of those events. Here, we present an outdoor mesocosm-scale experiment in which we studied the depletion of surface ozone and gaseous elemental mercury at the Sea-ice Environmental Research Facility (SERF) in Winnipeg, Canada, in an urban and non-polar region. Temporal changes in ozone and gaseous elemental mercury concentrations inside acrylic tubes were monitored over bromide-enriched artificial seawater during entire sea ice freeze-and-melt cycles and open water periods. Mid-day photochemical loss of both gas species was observed in the boundary layer air immediately above the sea ice surface, in a pattern that is characteristic of BEE-induced ODEs and MDEs in the Arctic. The importance of UV radiation and sea ice presence in causing such observations was demonstrated by sampling from UV-transmitting and UV-blocking acrylic tubes under different air temperatures. The ability of reproducing mesocosm-scale BEE-induced ODEs and MDEs in a non-polar region provides a new platform with opportunities to systematically study the cryo-photochemical mechanisms leading to BEEs, ODEs, and MDEs in the Arctic, their role in biogeochemical cycles across the ocean-sea ice-atmosphere interfaces, and their sensitivities to a changing climate.&#160;

Analysis of reactive bromine production and ozone depletion in the Arctic boundary layer using 3-D simulations with GEM-AQ: inference from synoptic-scale patterns

Atmospheric Chemistry and Physics ◽

10.5194/acp-11-3949-2011 ◽

2011 ◽

Vol 11 (8) ◽

pp. 3949-3979 ◽

Cited By ~ 53

Author(s):

K. Toyota ◽

J. C. McConnell ◽

A. Lupu ◽

L. Neary ◽

C. A. McLinden ◽

...

Keyword(s):

Boundary Layer ◽

Sea Ice ◽

Ozone Depletion ◽

Surface Ozone ◽

High Arctic ◽

Heterogeneous Reactions ◽

The Arctic ◽

Synoptic Scale ◽

Arctic Boundary Layer ◽

Surface Snow

Abstract. Episodes of high bromine levels and surface ozone depletion in the springtime Arctic are simulated by an online air-quality model, GEM-AQ, with gas-phase and heterogeneous reactions of inorganic bromine species and a simple scheme of air-snowpack chemical interactions implemented for this study. Snowpack on sea ice is assumed to be the only source of bromine to the atmosphere and to be capable of converting relatively stable bromine species to photolabile Br2 via air-snowpack interactions. A set of sensitivity model runs are performed for April 2001 at a horizontal resolution of approximately 100 km×100 km in the Arctic, to provide insights into the effects of temperature and the age (first-year, FY, versus multi-year, MY) of sea ice on the release of reactive bromine to the atmosphere. The model simulations capture much of the temporal variations in surface ozone mixing ratios as observed at stations in the high Arctic and the synoptic-scale evolution of areas with enhanced BrO column amount ("BrO clouds") as estimated from satellite observations. The simulated "BrO clouds" are in modestly better agreement with the satellite measurements when the FY sea ice is assumed to be more efficient at releasing reactive bromine to the atmosphere than on the MY sea ice. Surface ozone data from coastal stations used in this study are not sufficient to evaluate unambiguously the difference between the FY sea ice and the MY sea ice as a source of bromine. The results strongly suggest that reactive bromine is released ubiquitously from the snow on the sea ice during the Arctic spring while the timing and location of the bromine release are largely controlled by meteorological factors. It appears that a rapid advection and an enhanced turbulent diffusion associated with strong boundary-layer winds drive transport and dispersion of ozone to the near-surface air over the sea ice, increasing the oxidation rate of bromide (Br−) in the surface snow. Also, if indeed the surface snowpack does supply most of the reactive bromine in the Arctic boundary layer, it appears to be capable of releasing reactive bromine at temperatures as high as −10 °C, particularly on the sea ice in the central and eastern Arctic Ocean. Dynamically-induced BrO column variability in the lowermost stratosphere appears to interfere with the use of satellite BrO column measurements for interpreting BrO variability in the lower troposphere but probably not to the extent of totally obscuring "BrO clouds" that originate from the surface snow/ice source of bromine in the high Arctic. A budget analysis of the simulated air-surface exchange of bromine compounds suggests that a "bromine explosion" occurs in the interstitial air of the snowpack and/or is accelerated by heterogeneous reactions on the surface of wind-blown snow in ambient air, both of which are not represented explicitly in our simple model but could have been approximated by a parameter adjustment for the yield of Br2 from the trigger.

Polar boundary layer bromine explosion and ozone depletion events in the chemistry-climate model EMAC v2.52: Implementation and evaluation of AirSnow algorithm

10.5194/gmd-2017-126 ◽

2017 ◽

Author(s):

Stefanie Falk ◽

Björn-Martin Sinnhuber

Keyword(s):

Boundary Layer ◽

Sea Ice ◽

Atmospheric Chemistry ◽

Dry Deposition ◽

Ozone Depletion ◽

Climate Model ◽

Surface Ozone ◽

The Arctic ◽

Vertical Column ◽

Model Studies

Abstract. Ozone depletion events (ODE) in the polar boundary layer have been observed frequently during spring-time. Most likely, they are related to events of boundary layer enhancement of bromine. Consequently, increased vertical column densities (VCD) of BrO have been observed from satellites. These so called bromine explosion events have been discussed serving as source of tropospheric BrO at high latitudes. We have implemented a treatment of bromine release and recycling on sea ice and snow covered surfaces in the global chemistry-climate model EMAC (ECHAM/MESSy Atmospheric Chemistry) based on the scheme of Toyota et al. (2011). In this scheme, dry deposition fluxes of HBr, HOBr, and BrNO3 over ice and snow covered surfaces are recycled into Br2 fluxes. In addition, dry deposition of O3, dependent on temperature and sunlight, triggers a Br2 release from surfaces associated with first-year sea ice. Many aspects of observed bromine enhancements and associated episodes of near-complete depletion of boundary layer ozone, both in the Arctic and in the Antarctic, are reproduced by this relatively simple approach. We present first results from our global model studies extending over a full annual cycle, including comparisons with GOME satellite BrO VCD and surface ozone observations.

Polar boundary layer bromine explosion and ozone depletion events in the chemistry–climate model EMAC v2.52: implementation and evaluation of AirSnow algorithm

Geoscientific Model Development ◽

10.5194/gmd-11-1115-2018 ◽

2018 ◽

Vol 11 (3) ◽

pp. 1115-1131 ◽

Cited By ~ 9

Author(s):

Stefanie Falk ◽

Björn-Martin Sinnhuber

Keyword(s):

Boundary Layer ◽

Sea Ice ◽

Atmospheric Chemistry ◽

Dry Deposition ◽

Ozone Depletion ◽

Climate Model ◽

Surface Ozone ◽

The Arctic ◽

In Situ Observation ◽

Vertical Column

Abstract. Ozone depletion events (ODEs) in the polar boundary layer have been observed frequently during springtime. They are related to events of boundary layer enhancement of bromine. Consequently, increased amounts of boundary layer volume mixing ratio (VMR) and vertical column densities (VCDs) of BrO have been observed by in situ observation, ground-based as well as airborne remote sensing, and from satellites. These so-called bromine explosion (BE) events have been discussed serving as a source of tropospheric BrO at high latitudes, which has been underestimated in global models so far. We have implemented a treatment of bromine release and recycling on sea-ice- and snow-covered surfaces in the global chemistry–climate model EMAC (ECHAM/MESSy Atmospheric Chemistry) based on the scheme of Toyota et al. (2011). In this scheme, dry deposition fluxes of HBr, HOBr, and BrNO3 over ice- and snow-covered surfaces are recycled into Br2 fluxes. In addition, dry deposition of O3, dependent on temperature and sunlight, triggers a Br2 release from surfaces associated with first-year sea ice. Many aspects of observed bromine enhancements and associated episodes of near-complete depletion of boundary layer ozone, both in the Arctic and in the Antarctic, are reproduced by this relatively simple approach. We present first results from our global model studies extending over a full annual cycle, including comparisons with Global Ozone Monitoring Experiment (GOME) satellite BrO VCDs and surface ozone observations.

Synoptic-scale meteorological control on reactive bromine production and ozone depletion in the Arctic boundary layer: 3-D simulation with the GEM-AQ model

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-10-26207-2010 ◽

2010 ◽

Vol 10 (11) ◽

pp. 26207-26278 ◽

Cited By ~ 1

Author(s):

K. Toyota ◽

J. C. McConnell ◽

A. Lupu ◽

L. Neary ◽

C. A. McLinden ◽

...

Keyword(s):

Boundary Layer ◽

Sea Ice ◽

Dry Deposition ◽

Ozone Depletion ◽

Surface Ozone ◽

High Arctic ◽

Heterogeneous Reactions ◽

The Arctic ◽

Synoptic Scale ◽

Arctic Boundary Layer

Abstract. Episodes of high bromine levels and surface ozone depletion in the springtime Arctic are simulated by an online air-quality model, GEM-AQ, with gas-phase and heterogeneous reactions of inorganic bromine species and a simple scheme of air-snowpack chemical interactions implemented for this study. Snowpack on sea ice is assumed to be the only source of bromine to the atmosphere and capable of converting relatively stable bromine species to photolabile Br2 via air-snowpack interactions. A "bromine explosion", by which Br− retained in the snowpack is autocatalytically released to the atmosphere as a result of dry deposition of HOBr and BrONO2, is assumed to occur on young, first-year (FY) sea ice (or its overlying snowpack), whereas the snowpack on old, multi-year (MY) sea ice and over land is assumed only to recycle a part (but up to 100%) of bromine reservoirs lost via dry deposition back to Br2. Model runs are performed for April 2001 at a horizontal resolution of approximately 100 km × 100 km in the Arctic. The model simulates temporal variations in surface ozone mixing ratios as observed at stations in the high Arctic and the synoptic-scale evolution of enhanced BrO column amounts ("BrO clouds") as seen from satellite reasonably well. The results strongly suggest: (1) a ubiquitous source of reactive bromine exists on the FY sea ice during the Arctic springtime; and (2) the timing of bromine release to the atmosphere is largely controlled by meteorological forcing on the transport of ozone to the near-surface air. Also, if the surface snowpack supplies most of the reactive bromine in the Arctic boundary layer, it should be capable of releasing reactive bromine at temperatures as high as −10 °C, particularly on the FY sea ice in the central and eastern Arctic Ocean. Dynamically-induced BrO column variability in the lowermost stratosphere appears to interfere with the use of satellite BrO column measurements for interpreting BrO variability in the lower troposphere but probably not to the extent of totally obscuring "BrO clouds" associated with the surface source of bromine in the high Arctic. Contrary to our original intention, the present air-snowpack interaction scheme yields a majority of atmospheric bromine input via Br2 release associated empirically with a dry deposition of ozone on the snow/ice surface under sunlight to represent a trigger of bromine explosion. This implies that the bromine explosion actually occurs in the interstitial air of snowpack and/or is accelerated by heterogeneous reactions on the surface of wind-blown snow in ambient air, both of which are missing in our model but could have been approximated by a parameter adjustment for the yield of Br2 from the trigger.

The mechanism of halogen liberation in the polar troposphere

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-4-3607-2004 ◽

2004 ◽

Vol 4 (3) ◽

pp. 3607-3652 ◽

Cited By ~ 2

Author(s):

E. Lehrer ◽

G. Hönninger ◽

U. Platt

Keyword(s):

Boundary Layer ◽

Sea Ice ◽

Gas Phase ◽

Ozone Depletion ◽

Primary Source ◽

Temperature Inversion ◽

The Arctic ◽

Ice Surface ◽

Reactive Halogen Species ◽

Halogen Species

Abstract. Sudden depletions of tropospheric ozone during spring were reported from the Arctic and also from Antarctic coastal sites. Field studies showed that those depletion events are caused by reactive halogen species, especially bromine compounds. However the source and seasonal variation of reactive halogen species is still not completely understood. There are several indications that the halogen mobilisation from the sea ice surface of the polar oceans may be the most important source for the necessary halogens. Here we present a 1-D model study aimed at determining the primary source of reactive halogens. The model includes gas phase and heterogeneous bromine and chlorine chemistry as well as vertical transport between the surface and the top of the boundary layer. The autocatalytic Br release by photochemical processes (bromine explosion) and subsequent rapid bromine catalysed ozone depletion is well reproduced in the model and the major source of reactive bromine appears to be the sea ice surface. The sea salt aerosol alone is not sufficient to yield the high levels of reactive bromine in the gas phase necessary for fast ozone depletion. However, the aerosol efficiently 'recycles' less reactive bromine species (e.g. HBr) and feeds them back into the ozone destruction cycle. Isolation of the boundary layer air from the free troposphere by a strong temperature inversion was found to be critical for boundary layer ozone depletion to happen. The combination of strong surface inversions and presence of sunlight occurs only during polar spring.

Ozone in the Boundary Layer air over the Arctic Ocean – measurements during the TARA expedition

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-9-8561-2009 ◽

2009 ◽

Vol 9 (2) ◽

pp. 8561-8586

Author(s):

J. W. Bottenheim ◽

S. Netcheva ◽

S. Morin ◽

S. V. Nghiem

Keyword(s):

Boundary Layer ◽

Arctic Ocean ◽

Ozone Depletion ◽

Surface Ozone ◽

The Arctic ◽

Total Absence ◽

Trajectory Calculations ◽

The Arctic Ocean ◽

Ocean Measurements ◽

Siberian Coast

Abstract. A full year of measurements of surface ozone over the Arctic Ocean far removed from land is presented (81° N – 88° N latitude). The data were obtained during the drift of the French schooner TARA between September 2006 and January 2008, while frozen in the Arctic Ocean. The data confirm that long periods of virtually total absence of ozone occur in the spring (mid March to mid June) after Polar sunrise. At other times of the year ozone concentrations are comparable to other oceanic observations with winter mole fractions of ca. 30–40 nmol mol−1 and summer minima of ca. 20 nmol mol−1. Contrary to earlier observations from ozone sonde data obtained at Arctic coastal observatories, the ambient temperature was well above −20°C during most ODEs (ozone depletion episodes). Backwards trajectory calculations suggest that during these ODEs the air had previously been in contact with the frozen ocean surface for several days and originated largely from the Siberian coast where several large open flaw leads developed in the spring of 2007.

Observation of HClO3 and HClO4 in the Arctic atmosphere

10.5194/egusphere-egu2020-16908 ◽

2020 ◽

Author(s):

Yee Jun Tham ◽

Nina Sarnela ◽

Carlos A. Cuevas ◽

Iyer Siddharth ◽

Lisa Beck ◽

...

Keyword(s):

Gas Phase ◽

Ozone Depletion ◽

Surface Ozone ◽

Negative Ion ◽

The Arctic ◽

Research Station ◽

Visible Absorption ◽

Data Set ◽

Near Surface ◽

Arctic Atmosphere

Atmospheric halogens chemistry like the catalytic reaction of bromine and chlorine radicals with ozone (O3) has been known to cause the springtime surface-ozone destruction in the polar region. Although the initial atmospheric reactions of chlorine with ozone are well understood, the &#64257;nal oxidation steps leading to the formation of chlorate (ClO3-) and perchlorate (ClO4-) remain unclear due to the lack of direct evidence of their presence and fate in the atmosphere. In this study, we present the first high-resolution ambient data set of gas-phase HClO3 (chloric acid) and HClO4 (perchlorate acid) obtained from the field measurement at the Villum Research Station, Station Nord, in high arctic North Greenland (81&#176;36&#8217; N, 16&#176;40&#8217; W) during the spring of 2015. A state-of-the-art chemical ionization atmospheric pressure interface time-of-flight mass spectrometer (CI-APi-TOF) was used in negative ion mode with nitrate ion as the reagent ion to detect the gas-phase HClO3 and HClO4. We measured significant level of HClO3 and HClO4 only during the springtime ozone depletion events in the Greenland, with concentration up to 9x105 molecule cm-3. Air mass trajectory analysis shows that the air during the ozone depletion event was confined to near-surface, indicating that the O3 and surface of sea-ice/snowpack may play important roles in the formation of HClO3 and HClO4. We used high-level quantum-chemical methods to calculate the ultraviolet-visible absorption spectra and cross-section of HClO3 and HClO4 in the gas-phase to assess their fates in the atmosphere. Overall, our results reveal the presence of HClO3 and HClO4 during ozone depletion events, which could affect the chlorine chemistry in the Arctic atmosphere.

The role of open lead interactions in atmospheric ozone variability between Arctic coastal and inland sites

Elem Sci Anth ◽

10.12952/journal.elementa.000109 ◽

2016 ◽

Vol 4 ◽

Cited By ~ 2

Author(s):

Peter K. Peterson ◽

Kerri A. Pratt ◽

William R. Simpson ◽

Son V. Nghiem ◽

Lemuel X. Pérez Pérez ◽

...

Keyword(s):

Boundary Layer ◽

Sea Ice ◽

Ozone Depletion ◽

Surface Ozone ◽

Optical Absorption Spectroscopy ◽

Atmospheric Ozone ◽

Vertical Profiles ◽

Large Spatial Scale ◽

Near Surface ◽

Air Masses

Abstract Boundary layer atmospheric ozone depletion events (ODEs) are commonly observed across polar sea ice regions following polar sunrise. During March-April 2005 in Alaska, the coastal site of Barrow and inland site of Atqasuk experienced ODEs (O3< 10 nmol mol-1) concurrently for 31% of the observations, consistent with large spatial scale ozone depletion. However, 7% of the time ODEs were exclusively observed inland at Atqasuk. This phenomenon also occurred during one of nine flights during the BRomine, Ozone, and Mercury EXperiment (BROMEX), when atmospheric vertical profiles at both sites showed near-surface ozone depletion only at Atqasuk on 28 March 2012. Concurrent in-flight BrO measurements made using nadir scanning differential optical absorption spectroscopy (DOAS) showed the differences in ozone vertical profiles at these two sites could not be attributed to differences in locally occurring halogen chemistry. During both studies, backward air mass trajectories showed that the Barrow air masses observed had interacted with open sea ice leads, causing increased vertical mixing and recovery of ozone at Barrow and not Atqasuk, where the air masses only interacted with tundra and consolidated sea ice. These observations suggest that, while it is typical for coastal and inland sites to have similar ozone conditions, open leads may cause heterogeneity in the chemical composition of the springtime Arctic boundary layer over coastal and inland areas adjacent to sea ice regions.

Pan-Arctic surface ozone seasonality modified by sea-ice-sourced bromine: modelling vs measurements

10.5194/egusphere-egu21-1262 ◽

2021 ◽

Author(s):

Xin Yang ◽

Anne-M Blechschmidt2 ◽

Kristof Bognar ◽

Audra McClure–Begley ◽

Sara Morris ◽

...

Keyword(s):

Boundary Layer ◽

Sea Ice ◽

Surface Ozone ◽

Open Ocean ◽

Arctic Sea Ice ◽

The Arctic ◽

Sea Spray ◽

Research Station ◽

Blowing Snow ◽

Model Control

Within the framework of the International Arctic Systems for Observing the Atmosphere (IASOA), we report a modelling-based study on surface ozone across the Arctic. We use surface ozone from six sites: Summit (Greenland), Pallas (Finland), Barrow (USA), Alert (Canada), Tiksi (Russia), and Villum Research Station (VRS) at Station Nord (North Greenland, Danish Realm), and ozonesonde data from three Canadian sites: Resolute, Eureka, and Alert. Two global chemistry models: a global chemistry transport model (p-TOMCAT) and a global chemistry climate model (UKCA), are used for model-data comparisons. Remotely sensed data of BrO from the GOME-2 satellite instrument at Eureka, Canada are used for model validation.The observed climatology data show that spring surface ozone at coastal Arctic is heavily depleted, making ozone seasonality at Arctic coastal sites distinctly different from that at inland sites. Model simulations show that surface ozone can be greatly reduced by bromine chemistry. In April, bromine chemistry can cause a net ozone loss (monthly mean) of 10-20 ppbv, with almost half attributable to open-ocean-sourced bromine and the rest to sea-ice-sourced bromine. However, the open-ocean-sourced bromine, via sea spray bromide depletion, cannot by itself produce ozone depletion events (ODEs) (defined as ozone volume mixing ratios VMRs < 10 ppbv). In contrast, sea-ice-sourced bromine, via sea salt aerosol (SSA) production from blowing snow, can produce ODEs even without bromine from sea spray, highlighting the importance of sea ice surface in polar boundary layer chemistry.Modelled total inorganic bromine (BrY) over the Arctic sea ice &#160;is sensitive to model configuration, e.g., under the same bromine loading, BrY in the Arctic spring boundary layer in the p-TOMCAT control run (i.e., with all bromine emissions) can be 2 times that in the UKCA control run. Despite the model differences, both model control runs can successfully reproduce large bromine explosion events (BEEs) and ODEs in polar spring. Model-integrated tropospheric column BrO generally matches GOME-2 tropospheric columns within ~50% in UKCA and a factor of 2 in p-TOMCAT. The success of the models in reproducing both ODEs and BEEs in the Arctic indicates that the relevant parameterizations implemented in the models work reasonably well, which supports the proposed mechanism of SSA production and bromide release on sea ice. Given that sea ice is a large source of SSA and halogens, changes in sea ice type and extent in a warming climate will influence Arctic boundary layer chemistry, including the oxidation of atmospheric elemental mercury. Note that this work dose not necessary rule out other possibilities that may act as a source of reactive bromine from sea ice zone.