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Cracking the chemical code on how iodine helps form clouds

Published on 22.11.2022
Tampere University
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Researchers tested their findings on Réunion Island on the southern Indian Ocean. Photo: Sergey Zhesterev / Unsplash
A new experiment by an international research team demonstrates the mechanism for how the most stable gas-phase form of iodine known as iodic acid is formed. The researchers suggest it has a catalytic role in atmospheric particle formation. This significant result for air quality and climate research was made possible by quantum chemical modelling and field experiments by researchers at the Universities of Tampere and Helsinki in Finland, and University of Colorado Boulder in the United States.

The international team of experts in theoretical molecular modelling and experimental chemical reaction research combined results from field experiments and detailed laboratory chamber simulations to resolve the first molecular steps of particle formation from iodine emissions. Their article was published in the prestigious Nature Chemistry journal on 14 November 2022.

The new particle formation is an immensely important phenomena in the atmosphere. It affects our well-being from the immediate influence on local air quality to the global climate change. Atmospheric secondary particles form as a result of rapid chemical reactions transforming volatile gas molecules into condensable aerosol pre-stages. They are mainly formed by oxidation of three elements: sulfur, carbon and iodine.

The current finding is a key piece in the iodine puzzle and an important leap toward describing the changing atmosphere. During the last decades the sulfur related particle formation has decreased due to cleansing of combustion processes, whereas the amount of atmospheric iodine has steadily increased. 

The work conducted was highly collaborative and included field data from Réunion island on Indian Ocean, chamber simulations from the CERN CLOUD project, and state-of-the-science quantum chemical computations.

DisCERNing chemical processes

The research team studied this missing link in CERN, where they could study it in the pristine conditions required to observe and collect data on the particles. Here, a unique experiment known as CLOUD (Cosmics Leaving Outdoor Droplets) has become the world's leading laboratory experiment to study the remaining poorly understood aspects of aerosol and cloud formation.

In the CLOUD chamber at CERN, the researchers had access to a laboratory environment with perfect control over conditions like temperature, pressure, humidity, ozone concentration, and iodine concentration, as well as access to different light sources resembling different aspects of the solar spectrum.

By setting up this artificial indoor atmosphere where certain reactions may or may not happen, the scientists could accurately gather data on iodine chemical reactions that form and grow particles.

To determine whether what they observed in the laboratory translated to the real world, they also tested their findings in the air surrounding the Maïdo observatory on Réunion Island on the southern Indian Ocean. In this remote location free of the influence of human activity they were able to corroborate their laboratory results.

“This has been a long path from the first field observations to refining the chemistry and physics behind the process in detailed experimental and theoretical simulations. Elucidating and confirming this key finding has truly required multifaceted approach and it would not been possible without all the effort devoted to it,” ponders Associate Professor Matti Rissanen from Tampere University.


Matti Rissanen
+358 45 873 0170
matti.rissanen [at]