A laboratory characterisation of inorganic iodine emissions from the sea surface: dependence on oceanic variables and parameterisation for global modelling

S M Macdonald,L J Carpenter,A Saiz-Lopez,R Chance,J M C Plane,J C Gómez Martín,S Warriner

doi:10.5194/acp-14-5841-2014

Abstract

Abstract. Reactive iodine compounds play a significant role in the atmospheric chemistry of the oceanic boundary layer by influencing the oxidising capacity through catalytically removing O3 and altering the HOx and NOx balance. The sea-to-air flux of iodine over the open ocean is therefore an important quantity in assessing these impacts on a global scale. This paper examines the effect of a number of relevant environmental parameters, including water temperature, salinity and organic compounds, on the magnitude of the HOI and I2 fluxes produced from the uptake of O3 and its reaction with iodide ions in aqueous solution. The results of these laboratory experiments and those reported previously (Carpenter et al., 2013), along with sea surface iodide concentrations measured or inferred from measurements of dissolved total iodine and iodate reported in the literature, were then used to produce parameterised expressions for the HOI and I2 fluxes as a function of wind speed, sea-surface temperature and O3. These expressions were used in the Tropospheric HAlogen chemistry MOdel (THAMO) to compare with MAX-DOAS measurements of iodine monoxide (IO) performed during the HaloCAST-P cruise in the eastern Pacific ocean (Mahajan et al., 2012). The modelled IO agrees reasonably with the field observations, although significant discrepancies are found during a period of low wind speeds (< 3 m s−1), when the model overpredicts IO by up to a factor of 3. The inorganic iodine flux contributions to IO are found to be comparable to, or even greater than, the contribution of organo-iodine compounds and therefore its inclusion in atmospheric models is important to improve predictions of the influence of halogen chemistry in the marine boundary layer.

Highlights

Reactive iodine compounds play an important role in the chemistry of the marine boundary layer (MBL) through their influence on ozone depletion and the oxidising capacity via repartitioning of HOx and NOx (Saiz-Lopez et al, 2012)
The experimental results reported show that over the large range of conditions considered in the laboratory, parameters such as salinity can have a large effect on the resulting total inorganic iodine flux, these effects are likely to be minimal under real seawater conditions where the variations in salinity tend to be relatively small
None of the organics used in the reported experiments had any significant effect on the resulting inorganic iodine fluxes, but we cannot rule out the possibility that surface organic films may play a role in limiting the transfer of HOI and I2 to the gas phase in real seawater

Summary

Introduction

Reactive iodine compounds play an important role in the chemistry of the marine boundary layer (MBL) through their influence on ozone depletion and the oxidising capacity via repartitioning of HOx and NOx (Saiz-Lopez et al, 2012). Iodine oxides form new particles spontaneously in coastal (O’Dowd et al, 2002) and polar regions (Atkinson et al, 2012), potentially leading to the production of cloud condensation nuclei. Emissions of molecular iodine and to a lesser extent of halocarbons from macroalgae are found to be the dominant source of reactive iodine to the MBL (O’Dowd et al, 2002). The source of reactive iodine and particles observed over the Antarctic sea ice remains to be determined (Atkinson et al, 2012). Modelling studies have shown that these levels of IO cannot be sustained by the measured iodocarbon fluxes and that an additional source of reactive iodine from the open ocean, equivalent to > 50 % of Published by Copernicus Publications on behalf of the European Geosciences Union

Results

Discussion

Conclusion