Iodine emissions from the sea ice of the Weddell Sea

H M Atkinson,B Davison,T Hoffmann,R.-J Huang,A S Mahajan,P S Liss,H K Roscoe,A Schönhardt,A Saiz-Lopez,C Hughes,R Chance

doi:10.5194/acp-12-11229-2012

Abstract

Abstract. Iodine compounds were measured above, below and within the sea ice of the Weddell Sea during a cruise in 2009, to make progress in elucidating the mechanism of local enhancement and volatilisation of iodine. I2 mixing ratios of up to 12.4 pptv were measured 10 m above the sea ice, and up to 31 pptv was observed above surface snow on the nearby Brunt Ice Shelf – large amounts. Atmospheric IO of up to 7 pptv was measured from the ship, and the average sum of HOI and ICl was 1.9 pptv. These measurements confirm the Weddell Sea as an iodine hotspot. Average atmospheric concentrations of CH3I, C2H5I, CH2ICl, 2-C3H7I, CH2IBr and 1-C3H7I were each 0.2 pptv or less. On the Brunt Ice Shelf, enhanced concentrations of CH3I and C2H5I (up to 0.5 and 1 pptv respectively) were observed in firn air, with a diurnal profile that suggests the snow may be a source. In the sea ice brine, iodocarbons concentrations were over 10 times those of the sea water below. The sum of iodide + iodate was depleted in sea ice samples, suggesting some missing iodine chemistry. Flux calculations suggest I2 dominates the iodine atom flux to the atmosphere, but models cannot reconcile the observations and suggest either a missing iodine source or other deficiencies in our understanding of iodine chemistry. The observation of new particle formation, consistent with the model predictions, strongly suggests an iodine source. This combined study of iodine compounds is the first of its kind in this unique region of sea ice rich in biology and rich in iodine chemistry.

Highlights

The presence of iodine oxides, formed via reactions of iodine compounds with ozone that may contribute to ozone depletion events (Chameides and Davis, 1980), may lead to new particle formation (O’Dowd et al, 2002)
Our measurements confirmed that the Weddell Sea is an iodine hotspot, with large amounts of IO being observed in line with earlier studies (Saiz-Lopez et al, 2007; Schonhardt et al, 2008)
I2 and AIC were observed above sea ice of the Weddell Sea and the nearby ice shelf, and the large concentrations of I2 were comparable to midlatitude measurements near macroalgae beds exposed at low tide (Saiz-Lopez and Plane, 2004; Huang et al, 2010a)

Summary

Introduction

The presence of iodine oxides, formed via reactions of iodine compounds with ozone that may contribute to ozone depletion events (Chameides and Davis, 1980), may lead to new particle formation (O’Dowd et al, 2002). Given the large amounts of IO observed by a ground-based spectrometer near the Weddell Sea (Saiz-Lopez et al, 2007), understanding the local enhancement of atmospheric iodine is an essential part of understanding control of regional climate and chemistry. Satellite measurements (Schonhardt et al, 2008) showed that such large amounts of IO exist over much of the sea ice of the Weddell Sea. an iodine-selective mechanism must exist that concentrates and volatilises iodine from the ocean, but does not concentrate bromine – bromide is over 1000 times more abundant than iodide in seawater, whereas atmospheric concentrations of IO near the Weddell Sea are similar to those of BrO (Saiz-Lopez et al, 2007).

Methods

Discussion

Conclusion