Halogen Transport in the Sea Ice Zone

Polar tropospheric ozone depletion events (ODE) are an early springtime phenomena strongly correlated with increased concentrations of reactive bromine gases (BrO and Br), whereby Br serves as a catalyst in the breakdown of ozone into oxygen through a series of photochemical and heterogeneous reactions. This process involves the autocatalytic production of reactive bromine from bromide ions originating in the ocean, in what is termed the "bromine explosion." During an ODE, atmospheric oxidation potentials can be altered, with unique halogen oxidation pathways dominating atmospheric chemistry, resulting in consequences such as the depletion of gaseous mercury and subsequent mercury deposition in polar regions. However, the mechanism by which Br enters the troposphere is not well understood. Sea ice is known to play a critical role in mediating the exchange of heat, gases, and chemical species across the ocean-atmosphere interface. This research focuses on the transport of Br, which originates in sea water and is hypothesized to enter the atmosphere via blowing snow over first year sea ice. Using ion chromatography, x-ray micro-computed tomography, synchrotron x-ray micro-fluorescence, and scanning electron microscopy, we aim to identify the microstructural and stratigraphic location of Br and other salts in the snow and ice. Knowing whether these salts exist at grain boundaries or deeper within the crystal lattice helps assess the potential that blowing snow can loft Br into the atmosphere. With the ratio of first-year to multi-year sea ice increasing with climate change, understanding this mechanism is critical for assessing the impact of ODEs on future atmospheric chemistry.


Fieldwork

Ross Sea, Antarctica (October - November 2012)

Two field sites (Butter Point and Iceberg) were chosen to represent different first-year sea ice environments within the Ross Sea. Blowing snow collection towers were erected at each site and collected snow samples during a period of four weeks. Weather permitting, each site was visited via helicopter every other day to collect surface snow and any blowing snow captured in the baskets. Sea ice cores of nearly 2 m length were also extracted from each site.

In addition to the main field sites, a 10-point transect sampled surface snow perpendicular to the ice edge. This transect also included the extraction of three sea ice cores. Sea ice cores were collected at several other points around Ross Island in conjunction with colleagues measuring tropospheric halogen gas concentrations.

BROMEX: Barrow, Alaska (March 2012)


Characterization Techniques

X-Ray Micro Computed Tomography (X-ray microCT)
X-ray microCT provides a non-destructive three-dimensional analysis of the microstructure of sea ice with 15 micron resolution. This enables us to examine the brine channel networks within our samples.

Ion Chromatography (IC)
This technique allows us to measure the concentration of different salts, such as chloride, bromide, nitrate, phosphate, sulfate, sodium, potassium, and calcium. Knowledge of the salt concentrations in both the snow and ice will aid us in tracking the transport of bromide from the ocean into the atmosphere.

Synchrotron X-Ray MicroFluorescence
We use Argonne National Laboratory's Advanced Photon Source to image our samples using synchrotron x-ray micro-fluorescence. This enables us to determine the microstructural location of salts within our samples. Click here for a link to the summary video describing our work at Argonne.


Collaborators

University of Colorado: Lars Kalnajs

University of Heidelberg, Germany: Udo Friess, Denis Poehler, Johannes Zielcke,

NIWA: Karin Kreher

US Naval Academy: John Woods


This research was supported by US National Science Foundation (NSF) grant NSF-1043145. The views and conclusions contained herein are those of the authors and should not be interpreted as representing official policies, either expressed or implied, of the NSF or the United States Government.