Abstract
Abstract. Ground- and satellite-based measurements have reported high concentrations of iodine monoxide (IO) in coastal Antarctica. The sources of such a large iodine burden in the coastal Antarctic atmosphere remain unknown. We propose a mechanism for iodine release from sea ice based on the premise that micro-algae are the primary source of iodine emissions in this environment. The emissions are triggered by the biological production of iodide (I−) and hypoiodous acid (HOI) from micro-algae (contained within and underneath sea ice) and their diffusion through sea-ice brine channels, ultimately accumulating in a thin brine layer (BL) on the surface of sea ice. Prior to reaching the BL, the diffusion timescale of iodine within sea ice is depth-dependent. The BL is also a vital component of the proposed mechanism as it enhances the chemical kinetics of iodine-related reactions, which allows for the efficient release of iodine to the polar boundary layer. We suggest that iodine is released to the atmosphere via three possible pathways: (1) emitted from the BL and then transported throughout snow atop sea ice, from where it is released to the atmosphere; (2) released directly from the BL to the atmosphere in regions of sea ice that are not covered with snowpack; or (3) emitted to the atmosphere directly through fractures in the sea-ice pack. To investigate the proposed biology–ice–atmosphere coupling at coastal Antarctica we use a multiphase model that incorporates the transport of iodine species, via diffusion, at variable depths, within brine channels of sea ice. Model simulations were conducted to interpret observations of elevated springtime IO in the coastal Antarctic, around the Weddell Sea. While a lack of experimental and observational data adds uncertainty to the model predictions, the results nevertheless show that the levels of inorganic iodine (i.e. I2, IBr, ICl) released from sea ice through this mechanism could account for the observed IO concentrations during this timeframe. The model results also indicate that iodine may trigger the catalytic release of bromine from sea ice through phase equilibration of IBr. Considering the extent of sea ice around the Antarctic continent, we suggest that the resulting high levels of iodine may have widespread impacts on catalytic ozone destruction and aerosol formation in the Antarctic lower troposphere.
Highlights
Over the past two decades, evidence has accumulated for the role of atmospheric iodine in the catalytic destruction of tropospheric ozone (e.g. Chameides and Davis, 1980; Solomon et al, 1994; Vogt et al, 1999; McFiggans et al, 2000; Calvert and Lindberg, 2004a; Saiz-Lopez et al, 2007a, 2012a, 2014; Read et al, 2008; Sommariva and von Glasow, 2012; Carpenter et al, 2013)
Considering the extent of sea ice around the Antarctic continent, we suggest that the resulting high levels of iodine may have widespread impacts on catalytic ozone destruction and aerosol formation in the Antarctic lower troposphere
In order to study the link between polar marine micro-algae iodine emissions and the potential for iodine release from sea ice, we developed the multiphase chemical model (Condensed Phase to Air Transfer Model, CON-AIR)
Summary
Over the past two decades, evidence has accumulated for the role of atmospheric iodine in the catalytic destruction of tropospheric ozone (e.g. Chameides and Davis, 1980; Solomon et al, 1994; Vogt et al, 1999; McFiggans et al, 2000; Calvert and Lindberg, 2004a; Saiz-Lopez et al, 2007a, 2012a, 2014; Read et al, 2008; Sommariva and von Glasow, 2012; Carpenter et al, 2013). In the polar regions the source of reactive inorganic bromine and chlorine includes heterogeneous reactions involving sea-salt bromide on sea ice, snowpack, or marine aerosol surfaces (e.g. Saiz-Lopez and von Glasow, 2012, and references therein). These heterogeneous reactions take part in an autocatalytic cycle that destroys ozone while preserving atomic halogen radicals. The suggested coupling between biology, sea ice and overlying atmosphere is investigated using a multiphase chemical model
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