CLOUD collaboration challenges current understanding of aerosol particle formation in polar and marine regions

Atmospheric aerosol particles exert a strong net cooling effect on the climate by making clouds brighter and more extensive, reflecting more sunlight back into space. However, how aerosol particles form in the atmosphere remains poorly understood, particularly in polar and marine regions.

Globally, the main formation of vapor particles is believed to be sulfuric acid, stabilized by ammonia. However, because ammonia is frequently lacking in polar and marine regions, models generally underestimate aerosol particles in these regions.

A new study from the CLOUD collaboration now challenges this view by showing that iodized oxoacids work synergistically with sulfuric acid to significantly improve particle formation rates.

The new findings, described in an article published in the journal Science, build on previous CLOUD studies that showed that iodine oxoacids rapidly form particles even in the complete absence of sulfuric acid. The results imply that climate models significantly underestimate aerosol particle formation rates in marine and polar regions.

“Our results show that climate models need to include iodine oxoacids as well as sulfuric acid and other vapors,” said CLOUD spokesperson Jasper Kirkby. “This is particularly important in polar regions, which are very sensitive to small changes in aerosol particles and clouds. Here, aerosol particles actually produce a warming effect by absorbing infrared radiation otherwise lost in the ‘space and redirecting it towards the surface.’

The CLOUD experiment studies how aerosol particles form and grow from vapor mixtures under atmospheric conditions in a large chamber. It differs from previous experiments both in maintaining an ultra-low level of contaminants and in its precise control of all experimental parameters under conditions found in the real atmosphere. This includes using a CERN particle beam to simulate ions formed by galactic cosmic rays at any altitude in the troposphere.

The new CLOUD results show that iodized oxoacids significantly increase the rate of sulfuric acid particle formation. At concentrations of iodine oxoacid typical of marine and polar regions – between 0.1 and 5 compared to those of sulfuric acid – CLOUD measurements show that the rate of formation of sulfuric acid particles increases 10-fold to 10,000 compared to previous estimates. .

The CLOUD team found that this increase is due to two effects: first, iodized acid replaces ammonia to stabilize newly formed sulfuric acid particles against evaporation and, second, iodic acid facilitates the formation of charged sulfuric acid clusters. Using quantum chemistry, the collaboration confirmed the synergy between iodine oxoacids and sulfuric acid, and calculated particle formation rates that closely agreed with CLOUD measurements.

“Global marine iodine emissions have tripled over the past 70 years due to thinning sea ice and increasing ozone concentrations, and this trend is likely to continue,” Kirkby says.

“The resulting increase in marine aerosol particles and clouds, suggested by our results, will have created a positive feedback that accelerates sea ice loss in polar regions, while simultaneously introducing a cooling effect at lower latitudes. The next generation of climate models must take iodine vapor into account.”