Two new chemical mechanisms behind cloud formation have been discovered in experiments at Cern. The mechanisms, which involve the interaction of iodine oxoacid and sulfuric acid vapours to nucleate water droplets, provide new insights into cloud formation above oceans and in the polar regions and will help to improve the accuracy of important climate change models.

Cloud cover affects temperature by both reflecting incoming heat to space and preventing outgoing heat from escaping and so is a significant factor in climate models. Before water vapour can condense into an aerosolised droplet, it requires a seed particle. But the complexity of the various coupled chemical reactions involved in this process forces climatologists to run drastically over-simplified simulations, explains atmospheric scientist Xu-Cheng He, currently a visiting scholar at the University of Cambridge in the UK. ‘They may just use a few reactions to describe the aerosol precursors,’ he notes. In particular, models usually focus on sulfuric acid.

To explain observed formation rates, sulfuric acid requires a stabiliser. In urban areas, this is usually ammonia. But over pristine environments, such as the ocean, ammonia is much scarcer, so modelling cloud formation becomes difficult. This is especially problematic as marine clouds are the most important to Earth’s radiation balance. ‘From space, water is literally dark, so it can absorb more solar radiation, whereas if you have white coverage of clouds, they reflect a lot of incoming radiation back to space,’ says He.

In experiments at Cern’s Cloud (Cosmics Leaving Outdoor Droplets) chamber in 2021, He and his colleagues discovered that iodic acid and iodous acid could nucleate aerosol particles at rates comparable with sulfuric acid in pristine marine conditions. In the new research, they show that the two nucleation mechanisms are not completely independent. Under ionised conditions in the atmosphere, iodic acid can enhance the ion-induced nucleation of sulfuric acid. More strangely, iodous acid can substitute for ammonia and behave as a base, accepting a proton from sulfuric acid to form a neutral dimer. In fact, it is a much more effective substitute: less than one part per trillion by volume of iodous acid gives the same rate as 500 parts per trillion of ammonia.

The overall implications of the research are highly uncertain, says He. Levels of sulfuric acid in the atmosphere have been falling thanks to increased pollution controls, whereas iodine emissions have tripled since the 1950s and continue to rise for many reasons including increased ozone concentrations and arctic ice thinning. Further investigations will therefore require field research and global Earth system modelling, although He speculates that ‘recently it’s become a hot topic that air pollution control in cities is going to warm the climate because it will lead to less sulfuric acid concentration … I think that should be partly revised.’

Hamish Gordon of Carnegie Mellon University in Pittsburgh, US, believes the research is ‘potentially a very important finding’. He says the role of iodine species in aerosol chemistry is not well understood, with the atmospheric mechanism(s) by which iodous acid is produced being uncertain. ‘This is an impressive result because usually the lowest concentration species in these kinds of nucleating systems is sulfuric acid, and measuring sulfuric acid concentrations is difficult enough, so measuring concentrations of species that matter in even lower concentrations is kind of neat,’ he says. ‘There’s going to be a lot of interesting work to figure out where iodous acid could be important and where it’s unlikely to be produced even in the really low concentrations needed for making particles.’