Researchers at CERN’s CLOUD experiment have uncovered a new mechanism that could accelerate the loss of Arctic sea ice. In a paper published in Science on 5 February, the team showed that aerosol particles made of iodic acid can form extremely rapidly in the marine boundary layer – the portion of the atmosphere that is in direct contact with the ocean. Aerosol particles are important for the climate because they provide the seeds on which cloud droplets form. Marine new-particle formation is especially important since particle concentrations are low and the ocean is vast. However, how new aerosol particles form and influence clouds and climate remain relatively poorly understood.
In polar regions, aerosols and clouds have a warming effect because they absorb infrared radiation otherwise lost to space and then radiate it back down to the surfaceJasper Kirkby
“Our measurements are the first to show that the part-per-trillion-by-volume iodine levels found in marine regions will lead to rapid formation and growth of iodic acid particles,” says CLOUD spokesperson Jasper Kirkby of CERN, adding that the particle formation rate is also strongly enhanced by ions from galactic cosmic rays. “Although most atmospheric particles form from sulphuric acid, our study shows that iodic acid – which is produced by the action of sunlight and ozone on molecular iodine emitted by the sea surface, sea ice and exposed seaweed – may be the main driver in pristine marine regions.”
CLOUD is a one-of-a kind experiment that uses an ultraclean cloud chamber to measure the formation and growth of aerosol particles from a mixture of vapours under precisely controlled atmospheric conditions, including the use of a high-energy beam from the Proton Synchrotron to simulate cosmic rays up to the top of the troposphere. Last year, the team found that small inhomogeneities in the concentrations of ammonia and nitric acid can have a major role in driving winter smog episodes in cities. The latest result is similarly important but in a completely different area, says Kirkby.
“In polar regions, aerosols and clouds have a warming effect because they absorb infrared radiation otherwise lost to space and then radiate it back down to the surface, whereas they reflect no more incoming sunlight than the snow-covered surface. As more sea surface is exposed by melting ice, the increased iodic acid aerosol and cloud-seed formation could provide a previously unaccounted positive feedback that accelerates the loss of sea ice. However, the effect has not yet been modelled so we can’t quantify it yet.”
X-C He et al. 2021 Science 371 589.