Iodine accelerates formation of cloud condensation nuclei in the atmosphere

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Iodine cycle between marine biosphere and atmosphere
Iodine cycle between marine biosphere and atmosphere

International research collaboration shows influence of iodine chemistry on formation of new particles in marine atmosphere and recycling of iodine during particle growth

The natural cycling of materials between the biosphere and the atmosphere is of great importance to the Earth’s climate system. The best known example is the cycling of carbon between the atmosphere, the land biosphere and the ocean. It is essentially responsible for life-friendly mean temperatures at the Earth’s surface. But other elemental cycles also play an important role, such as that of sulfur. It is thought that the release of sulfur compounds by phytoplankton into the marine atmosphere creates water vapor condensation nuclei, leading to cloud formation - and thus a natural feedback effect exists that could ultimately stabilize Earth’s surface temperature. Now, an international research team has identified another element that is also linked to marine algae and shows remarkable interlocking cyclic reactions in the marine atmosphere - iodine. The findings were published in the journal Proceedings of the National Academy of Sciences (PNAS).

Iodine belongs to the halogens, a group of elements that occur in large quantities in seawater. Although the iodine concentration is much lower than, for example, the chlorine concentration in the form of sea salt, iodine shows some chemical peculiarities. "Initially, the mechanism is similar to the sulfur cycle," explains Thorsten Hoffmann of Johannes Gutenberg University Mainz (JGU). Phytoplankton convert iodate present in seawater into iodide, probably to use iodide as a simple inorganic antioxidant to protect the cell walls of marine algae. However, iodide also reacts with atmospheric ozone at the sea surface to release molecular iodine. Through a series of rapid atmospheric reactions, this molecular iodine forms iodine oxides, which have an exceptionally high potential to form aerosol particles. "These particles can grow into larger particles and then serve as cloud condensation nuclei, influencing cloud formation," Hoffmann said, describing the process. "However, in the case of iodine, unlike sulfur, the process does not end here."

Iodine as a catalyst for new particle formation?

In the PNAS study, the authors show how molecular iodine is formed again in growing atmospheric particles from previously formed iodine oxides and released into the gas phase. "As far as we know today, iodine is the only element that does not leave the atmosphere again after being released from the Earth’s surface, but can be returned to the gas phase through redox reactions in the particle phase," said Hoffmann, a professor in the Department of Chemistry at JGU. Thus, iodine could play an important catalytic role in the formation of cloud condensation nuclei. A number of unresolved questions remain, such as how human activities, which intervene at various points in this mechanism, influence the unique material cycle of iodine in the Earth system.

In addition to researchers from JGU, scientists from the Chinese Academy of Sciences, the California Institute of Technology in Pasadena (Caltech) and the University of Galway were involved in the work.