Iodine volatilisation from the ocean

Reactive iodine species (RIS) in the troposphere affect partitioning in the HOx and NOx chemical families and thereby alter the troposphere’s oxidising capacity. Recent research indicates the main source of RIS globally is the heterogeneous reaction between atmospheric ozone and aqueous iodide at the surface of the oceans. However, uncertainties remain because experimental studies reported in the literature generally use reagent concentrations substantially above ambient conditions.

This project used a broadband cavity enhanced absorption spectroscopy (BBCEAS) instrument with the necessary sensitivity to directly detect molecular iodine produced from ambient concentrations of ozone reacting with iodide at concentrations similar to those in seawater. Laboratory work on synthetic seawater solutions confirmed previous results that iodine emissions (i) increase with increasing concentrations of iodide and ozone, and decreasing pH < 8, and (ii) are suppressed by the addition of humic acid (proxy for dissolved organic matter). This work newly finds that iodine emissions decrease with increasing temperature and chloride concentrations; iodine emissions have a complex dependence on bromide concentrations.

Another BBCEAS instrument made ambient air measurements over the open ocean aboard the RV Sonne during the research vessel’s SO287 transect across the Atlantic Ocean (Dec 2021–Jan 2022). The instrument achieved an impressive iodine detection limit of 0.9 ppt (parts per trillion, 1, 10-minute averaging). However, even in the absence of photolytic iodine losses, night-time I2 concentrations failed to be observed above this detection limit. BBCEAS timeseries of NO2, which absorbs at the same wavelengths as I2, showed low concentration over the open ocean (<200 ppt) punctuated by short-lived spikes of high NO2 (1000–5000 ppt for ~10 minutes) from exhaust plumes of other shipping.

Hypoiodous acid (HOI) is the other main RIS product of the ozone + iodide reaction. Attempts to generate HOI and detect HOI by BBCEAS at near-ultraviolet wavelengths were unsuccessful.