CLOUD Role of iodine oxoacids in atmospheric aerosol nucleation

• Main Author: Fichet, Jacques Herve
• Format: Article in Journal/Newspaper
• Language: English
• Published: CERN 2021
• Subjects: CLOUD; climate; weather; Arctic; Kirkby; Climate change; Sea ice
• Online Access: https://dx.doi.org/10.17181/cds.2751181; https://videos.cern.ch/record/2751181

Interview of Jasper Kirkby CLOUD spokesperson and some animations and footage. CLOUD Role of iodine oxoacids in atmospheric aerosol nucleation Supporting information to press briefing on Science publication by the CLOUD collaboration: He, X.-Ch., et al. Role of iodine oxoacids in atmospheric aerosol nucleation. Science, doi 10.1126/science.abe0298 (2021). Background to the CERN CLOUD experiment. CLOUD is studying how aerosol particles form and grow from reactive gases under carefully controlled atmospheric conditions in the laboratory. Aerosol particles can modify clouds and climate or contribute to urban smog. Using a particle beam from the CERN Proton Synchrotron, CLOUD is also investigating whether these processes are affected by ions from galactic cosmic rays. Atmospheric aerosol particles generally cool the climate by reflecting sunlight and by forming more numerous but smaller cloud droplets, making clouds brighter and more long-lasting. However, in polar regions increased aerosols and clouds have a warming effect since they reduce longwave radiation lost to space. Accurate projections of climate change are limited by the uncertainty in how much aerosols and clouds have increased since pristine pre-industrial times and how they may continue to change in the future as anthropogenic emissions are reduced. How marine particles form is especially poorly understood yet is important for the climate as the ocean is vast, and marine clouds are highly sensitive to cloud condensation nuclei since their number concentrations are low. What is special about the CLOUD experiment? Using CERN know-how, the CLOUD chamber has achieved much lower contaminants than previous experiments, allowing us to measure particle nucleation and growth from a precisely controlled mixture of vapours. A special feature of CLOUD is its capability to measure nucleation enhanced by ionisation from galactic cosmic rays between ground level and, using a CERN pion beam, the top of the troposphere - or with all the effects of ionisation completely suppressed by an internal electric field. During experimental campaigns, our team assembles the world’s most comprehensive array of mass spectrometers and other instruments to characterise both the physical and chemical state of the particles and vapours in the CLOUD chamber. As with other experiments at CERN, CLOUD combines fundamental experiments and modelling - in our case, aerosol and cloud processes, and regional and global climate - within a single team of international researchers. What has the CLOUD team studied? CLOUD has studied how new particles form from iodine-containing vapours under marine boundary layer conditions. In the present experiments we measured the nucleation and growth rates as well as the composition of freshly formed particles from iodic oxoacids (iodic acid and iodous acid) from +10 °C to -10 °C. These vapours derive from photolysis and oxidation of molecular iodine, for which the ocean surface is a major source. We found that the conversion to iodine oxoacids takes place rapidly even under weak daylight conditions, without requiring any ultraviolet. What has the CLOUD team discovered? We have found that the nucleation rates of iodic acid particles are extremely rapid, even exceeding those of sulfuric acid-ammonia under similar conditions. The particles form especially rapidly on ions from galactic cosmic rays, limited only by the rate of collision with iodic acid molecules (termed the kinetic limit). We have also found that freshly formed particles are composed almost entirely of iodic acid, which drives rapid growth at the kinetic limit. Although we identify iodic acid as the key vapour, a related species – iodous acid – plays an important stabilizing role in the initial steps of neutral (uncharged) particle formation. Our global boundary layer measurements of iodic acid indicate the conditions for abundant iodine new particle formation and rapid growth are frequently reached at coastal mid-latitude and polar sites. Why is it important for our understanding of climate? Our results indicate that iodic acid particle formation can compete with sulfuric acid in pristine regions of the atmosphere. Moreover, it is a very efficient source of cloud condensation nuclei since a single vapour species drives both nucleation and rapid growth. Iodic acid particle formation is likely to be especially important in marine regions where sulfuric acid and ammonia concentrations are extremely low. Indeed, frequent new particle formation over the high Arctic pack ice has recently been reported, driven by iodic acid with little contribution from sulfuric acid. The implications for the future are notable. Global iodine emissions have increased three-fold over the last 70 years and may continue to increase in the future as sea ice becomes thinner. The resultant increase of iodic acid cloud condensation nuclei could increase longwave radiative forcing from clouds and provide a positive feedback mechanism that accelerates the loss of sea ice in the Arctic.