Understanding Iodine Chemistry Over the Northern and Equatorial Indian Ocean

Anoop S. Mahajan*, Liselotte Tinel, Amit Sarkar, Rosie Chance, Lucy J. Carpenter, Shrivardhan Hulswar, Prithviraj Mali, Satya Prakash, P. N. Vinayachandran

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review


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 as a part of the Second International Indian Ocean Expedition (IIOE-2). The expedition took place on board the oceanographic research vessel Sagar Nidhi during 4–22 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 [I2]) was computed using the predicted iodide concentrations, measured atmospheric ozone, and wind speed. Iodine oxide (IO) mixing ratios peaked at 0.47 ± 0.29 pptv (parts per trillion by volume) in the remote open ocean environment. The estimated iodide concentrations and HOI and I2 fluxes peaked at 200/500 nM, 410/680 nmol·m−2·day−1, and 20/80 nmol·m−2·day−1, respectively, depending on the parameterization used. The calculated fluxes for HOI and I2 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 NO2 observations to show that titration of IO by NO2 is the main reason for this result. These observations show that inorganic iodine fluxes and atmospheric IO show similar trends in the Indian Ocean marine boundary layer, but the impact of inorganic iodine emissions on iodine chemistry is buffered in elevated NOx environments, even though the estimated oceanic iodine fluxes are higher.

Original languageEnglish
Pages (from-to)8104-8118
Number of pages15
JournalJournal of Geophysical Research: Biogeosciences
Issue number14
Early online date18 Jul 2019
Publication statusPublished - 15 Aug 2019

Bibliographical note

©2019. American Geophysical Union. All Rights Reserved. Uploaded in accordance with the publisher’s self-archiving policy. Further copying may not be permitted; contact the publisher for details


  • halogens
  • iodine flux
  • marine boundary layer
  • ozone

Cite this