Cracking the chemical code on how iodine helps form clouds

A new experiment demonstrates the mechanism for how the most stable gas-phase form of iodine -- known as iodic acid -- forms, and suggests it has a catalytic role in atmospheric particle formation.

Atmospheric new particle formation is an immensely important phenomena influencing our well-being in many scales of life, from the immediate influence on local air quality to the somewhat more distant seeming influence on global climate change. Notwithstanding the importance of this process, and the decades of research poured into it, the molecular details of the formation of new atmospheric particles keep eluding the researchers.  

Atmospheric secondary particles, particles that form as a result of rapid chemical reactions transforming volatile gas molecules into condensable aerosol pre-stages, are mainly formed by oxidation of three elements: Sulfur, carbon and iodine.

The current finding is a key, corner piece in the iodine puzzle and an important leap toward describing the changing atmosphere; Whereas the sulfur related particle formation has decreased during the last decades due to cleansing of combustion processes, the amount of atmospheric iodine has steadily increased over the same time-period. 

In their work published this Monday (14th November) in the prestigious Nature Chemistry journal, an international team of experts in theoretical molecular modelling and experimental chemical reaction research, combining results from field experiments and detailed laboratory chamber simulations, have resolved the first molecular steps of particle formation from iodine emissions.

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

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.

"This is a great example of experiments and computations coming together to answer a question that neither of them could have answered on their own," said Theo Kurten, co-lead author on the study and professor of chemistry at the University of Helsinki.

They key team behind this finding included people from Helsinki, Tampere and Colorado Universities.  

Reference:, The gas-phase formation mechanism of iodic acid as an atmospheric aerosol source. Nature Chemistry 2022

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