Estimation of reactive inorganic iodine fluxes in the Indian and Southern Ocean marine boundary layer

Inamdar, Swaleha; Tinel, Liselotte; Chance, Rosie; Carpenter, Lucy J.; Sabu, Prabhakaran; Chacko, Racheal; Tripathy, Sarat C.; Kerkar, Anvita U.; Sinha, Alok K.; Bhaskar, Parli Venkateswaran; Sarkar, Amit; Roy, Rajdeep; Sherwen, Tomás; Cuevas, Carlos; Saiz-Lopez, Alfonso; Ram, Kirpa; Mahajan, Anoop S.

Iodine chemistry has noteworthy impacts on the oxidising capacity of the marine boundary layer (MBL) through the depletion of ozone (inline-formulaO3) and changes to inline-formulaHOx (inline-formulaOH∕HO2) and inline-formulaNOx (inline-formulaNO∕NO2) ratios. Hitherto, studies have shown that the reaction of atmospheric inline-formulaO3 with surface seawater iodide (Iinline-formula) contributes to the flux of iodine species into the MBL mainly as hypoiodous acid (HOI) and molecular iodine (inline-formulaI2). Here, we present the first concomitant observations of iodine oxide (IO), inline-formulaO3 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 40inline-formula 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 inline-formulaI2, 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 inline-formulaa showed a significant correlation with atmospheric IO (inline-formulaR=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.

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Inamdar, Swaleha / Tinel, Liselotte / Chance, Rosie / et al: Estimation of reactive inorganic iodine fluxes in the Indian and Southern Ocean marine boundary layer. 2020. Copernicus Publications.

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