Understanding Iodine Chemistry over the northern and equatorial Indian Ocean

Abstract

Observations of halogen oxides, ozone, meteorological parameters, and physical and biogeochemical water column measurements were made in the Indian Ocean and its marine boundary layer (MBL) as a part of the 2nd International Indian Ocean Expedition (IIOE-2). The expedition took place on board the oceanographic research vessel Sagar Nidhi during 4th - 22nd December 2015 from Goa, India to Port Louis, Mauritius. Observations of mixed layer depth, averaged temperature, salinity and nitrate concentrations were used to calculate predicted iodide concentrations in the seawater. The inorganic iodine ocean-atmosphere flux (hypoiodous acid, HOI and molecular iodine,(I sub(2)) was computed using the predicted iodide concentrations, measured atmospheric ozone and wind speed. Iodine oxide (IO) mixing ratios peaked at 0.47 plus-minus 0.29 pptv (parts per trillion by volume) in the remote open ocean environment. The estimated iodide concentrations, HOI and I sub(2) fluxes peaked at 200/500 nM, 410/680 nmol m sup(-2) d sup(-1) and 20/80 nmol m sup(-2) d sup(-1), respectively, depending on the parameterization used. The calculated fluxes for HOI and I sub(2) were higher closer to the Indian subcontinent, however atmospheric IO was only observed above the detection limit in the remote open ocean environment. We use NO sub(2) observations to show that titration of IO by NO sub(2) is the main reason for this result. These observations show that inorganic iodine fluxes and atmospheric IO show similar trends in the Indian Ocean MBL, but the impact of inorganic iodine emissions on iodine chemistry is buffered in elevated NO sub(x) environments, even though the estimated oceanic iodine fluxes are higher.