An overview of iodine chemistry over the Indian and Southern Ocean waters using ship-based observations and modelling

This study presents an overview of observations and modelling of reactive iodine chemistry in the marine boundary layer (MBL) of the Indian and Southern Ocean. Ship observations of iodine oxide (IO) from 2015 to 2017 show its ubiquitous presence with values up to 1 pptv (parts per trillion) in this region. To identify the source of iodine in this region, we computed inorganic fluxes of iodine using tropospheric ozone (O3), sea surface iodide concentration, and wind speed. The estimated fluxes of hypoiodous acid (HOI) and elemental iodine (I2) did not adequately explain the observed IO levels in the Indian and Southern Ocean region. However, a significant correlation of IO with chlorophyll-a indicates a possible biogenic control on iodine chemistry in the Indian Ocean MBL. To understand the role of organic and inorganic precursors in MBL iodine chemistry, we used the Weather Research and Forecast model coupled with Chemistry (WRF-Chem version 3.7.1) incorporating halogen (Br, Cl, and I) chemistry. The modelling study shows that including only organic sources of iodine underestimate the detected IO in the northern Indian Ocean MBL. This highlights the importance of inorganic emissions as a source of iodine over the ocean. However, the inorganic flux emissions in the model had to be reduced by 40% to match the detected IO levels in this region. The reduced emission produces an overall good match between the observed and modelled IO levels. This discrepancy with flux emissions and IO levels in both the modelled IO simulation and observation highlights that there may be uncertainties in estimating the fluxes or that the flux parameterisation does not perform well for the Indian and Southern Ocean region. The model results show that inclusion of iodine chemistry causes significant regional changes to O3 (up to 25%), nitrogen oxides (up to 50%), and hydroxyl radicals (up to 15%) affecting the chemical composition of open ocean MBL and coastal regions of the Indian sub-continent. Accurate estimation of iodine precursors in the MBL calls for an urgent need to improve the existing parameterisation of inorganic fluxes. Direct measurements of the HOI and I2 may prove useful in the accurate quantification of iodine precursors in the marine atmosphere.