Iodine sea-air emissions and atmospheric impacts in a changing world (I-SEA)

The chemistry of the troposphere (lowest ~12 km of the atmosphere) plays a critical role in climate change, air quality degradation and biogeochemical cycling. Our understanding of the complexity of tropospheric chemistry has developed immensely over the last decades. One of the more recent developments is halogen (Cl, Br, I) chemistry. Halogen atom processes can fundamentally challenge current perspectives of tropospheric (and stratospheric) chemistry, and the uncertainty in the science generates impacts on air pollution and climate predictions. Restricted observational constraints, coupled to a lack of suitable modelling tools, translate into large uncertainties in (the few) calculations of the impact of halogens on regional or global scales, and their role in modifying the response of the Earth system to anthropogenic perturbations.

Together with collaborators, we have shown that reactive halogens play a significant and pervasive role in determining the composition of the troposphere. Of the halogens, iodine has the most profound impact on tropospheric ozone (O3) cycling, and significantly modifies the atmospheric response to anthropogenic perturbations. We identified that the reaction between O3 and iodide (I-) at the ocean surface drives the majority of atmospheric iodine emissions and showed that this process has resulted in a tripling of atmospheric iodine in some regions over the latter half of the 20th century due to increased anthropogenic O3, meaning that iodine-driven O3 loss is more active now than in the past. However, simulations of the impacts of halogens through the 21st century have so far made no account of any potential changes in surface ocean I-, due to a lack of mechanistic understanding.

Our team have constructed the first model of marine iodine cycling and find that the surface iodide distribution is impacted primarily by biological productivity, nitrification rates, mixed layer depth and advection. Indeed, under the scenario where nitrification rates are reduced by up to 44% in the next 20 - 30 years due to ocean acidification, the model predicts a doubling of surface [I-] in some regions (due to decreased bacterial I- oxidation).This result indicates a new coupling between climate-induced oceanographic changes and atmospheric air quality and climate, and suggests the need for an integrated approach to fully understand the impacts of iodine.

Translating knowledge of [I-] into predictions of sea-air iodine emissions and their resulting impacts on the atmosphere is also highly uncertain due to a lack of measurements at environmentally representative concentrations and complex additional dependencies of iodine fluxes, over and above on [O3] and [I-], on water-side turbulent mixing and on surfactants/organic material.

I-SEA is a multidisciplinary collaboration between atmospheric and marine scientists and geochemists from leading Earth System science institutes. We propose to bring new technology and ideas to address major uncertainties in the biogeochemical cycling of iodine in order to address our key hypothesis, that global change will drive significant changes in atmospheric iodine emissions over the coming century which will impact on air quality and climate. Ultimately the project will provide transformative new knowledge of the feedbacks between environmental change and the impact of reactive halogens on air quality, ecosystems and climate change.