Abstract
Abstract. Iodine chemistry has noteworthy impacts on the oxidising capacity of the marine boundary layer (MBL) through the depletion of ozone (O3) and changes to HOx (OH∕HO2) and NOx (NO∕NO2) ratios. Hitherto, studies have shown that the reaction of atmospheric O3 with surface seawater iodide (I−) contributes to the flux of iodine species into the MBL mainly as hypoiodous acid (HOI) and molecular iodine (I2). Here, we present the first concomitant observations of iodine oxide (IO), O3 in the gas phase, and sea surface iodide concentrations. The results from three field campaigns in the Indian Ocean and the Southern Ocean during 2015–2017 are used to compute reactive iodine fluxes in the MBL. Observations of atmospheric IO by multi-axis differential optical absorption spectroscopy (MAX-DOAS) show active iodine chemistry in this environment, with IO values up to 1 pptv (parts per trillion by volume) below latitudes of 40∘ S. In order to compute the sea-to-air iodine flux supporting this chemistry, we compare previously established global sea surface iodide parameterisations with new region-specific parameterisations based on the new iodide observations. This study shows that regional changes in salinity and sea surface temperature play a role in surface seawater iodide estimation. Sea–air fluxes of HOI and I2, calculated from the atmospheric ozone and seawater iodide concentrations (observed and predicted), failed to adequately explain the detected IO in this region. This discrepancy highlights the need to measure direct fluxes of inorganic and organic iodine species in the marine environment. Amongst other potential drivers of reactive iodine chemistry investigated, chlorophyll a showed a significant correlation with atmospheric IO (R=0.7 above the 99 % significance level) to the north of the polar front. This correlation might be indicative of a biogenic control on iodine sources in this region.
Highlights
Iodine chemistry in the troposphere has gained interest over the last 4 decades after it was first discovered to cause depletion of tropospheric ozone (O3) (Chameides and Davis, 1980; Jenkin et al, 1985) and cause changes to the atmospheric oxidation capacity (Davis et al, 1996; Read et al, 2008)
In the open-ocean environment, ozone mixing ratios did not show this diurnal variation, and values of ozone dropped during daytime, indicating photochemical destruction during both ISOE-8 and ISOE-9 (Fig. 5b)
Region-specific parameterisation tools were devised for sea surface iodide (SSI) estimation following previous SSI estimation methods from Chance et al (2014) and MacDonald et al (2014)
Summary
Iodine chemistry in the troposphere has gained interest over the last 4 decades after it was first discovered to cause depletion of tropospheric ozone (O3) (Chameides and Davis, 1980; Jenkin et al, 1985) and cause changes to the atmospheric oxidation capacity (Davis et al, 1996; Read et al, 2008). Iodine studies in the remote open ocean are important considering its role in tropospheric ozone destruction Iodine chemistry in the remote open ocean is still not completely understood, with uncertainties remaining around the sources and impacts of atmospheric iodine (Saiz-Lopez et al, 2012; Simpson et al, 2015). Fluxes of volatile organic iodine (e.g. CH3I, CH2ICl, CH2I2) compounds including those originating from marine algae (Saiz-Lopez and Plane, 2004) were considered to be the primary source of iodine in the marine atmosphere (Carpenter, 2003; Vogt et al, 1999). Inorganic iodine emissions are considered to be the dominant sources contributing to the open-ocean boundary layer iodine (Carpenter et al, 2013). Laboratory investigations revealed that at the ocean surface, iodide (I−) dissolved in the seawater reacts with the deposited gas-phase ozone to release hypoiodous acid (HOI) and molecular iodine (I2) via the following reactions (Carpenter et al, 2013; Gálvez et al, 2016; MacDonald et al, 2014): I− + O3 → IOOO−, IOOO− → IO− + O2, IO− + H+ HOI,
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