Abstract
Iodine affects the radiative budget and the oxidative capacity of the atmosphere and is consequently involved in important climate feedbacks. A fraction of the iodine emitted by oceans ends up in aerosols, where complex halogen chemistry regulates the recycling of iodine to the gas-phase where it effectively destroys ozone. The iodine speciation and major ion composition of aerosol samples collected during four cruises in the East and West Pacific and Indian Oceans was studied to understand the influences on iodine’s gas-aerosol phase recycling. A significant inverse relationship exists between iodide (I–) and iodate (IO3–) proportions in both fine and coarse mode aerosols, with a relatively constant soluble organic iodine (SOI) fraction of 19.8% (median) for fine and coarse mode samples of all cruises combined. Consistent with previous work on the Atlantic Ocean, this work further provides observational support that IO3– reduction is attributed to aerosol acidity, which is associated to smaller aerosol particles and air masses that have been influenced by anthropogenic emissions. Significant correlations are found between SOI and I–, which supports hypotheses that SOI may be a source for I–. This data contributes to a growing observational dataset on aerosol iodine speciation and provides evidence for relatively constant proportions of iodine species in unpolluted marine aerosols. Future development in our understanding of iodine speciation depends on aerosol pH measurements and unravelling the complex composition of SOI in aerosols.
Highlights
Iodine is ubiquitous in the troposphere where it plays an active role in atmospheric chemistry and important climate feedbacks (Prados-Roman et al, 2015a)
Coarse mode aerosols (>1 μm) in the datasets presented here are dominated by sea salt aerosols, formed through primary aerosol production mechanisms, such as sea spray and bubble bursting (Smith et al, 1993)
Mineral dust appears to be a minor component of the TransBrom, SHIVA, OASIS, and M138 aerosols
Summary
Iodine is ubiquitous in the troposphere where it plays an active role in atmospheric chemistry and important climate feedbacks (Prados-Roman et al, 2015a). Its volatile compounds contribute to aerosol particle formation mechanisms that affect the atmospheric radiative budget (O’Dowd et al, 2002; Read et al, 2008). It has been recognised to alter the oxidative capacity of the atmosphere by significantly destroying ozone (a pollutant) in the lower troposphere (Chameides and Davis, 1980; Chatfield and Crutzen, 1990; Davis et al, 1996; Mahajan et al, 2010). The dominant source of iodine in the atmosphere is the surface ocean through biotic and abiotic processes (Mahajan et al, 2012). Rapid photolysis in the atmosphere releases the iodine atoms from these compounds, which subsequently participate in efficient ozone destruction cycles (Saiz-Lopez et al, 2012a,b)
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