CLOUD news
"The CLOUD experiment at CERN recently revealed a new mechanism behind smog. The results have been published in Nature"
Winter smog episodes in cities occur when new aerosol particles form in polluted air trapped below a temperature inversion. The warmer air above the inversion inhibits convection, causing the polluted layer to stay and grow near the ground. However, how additional aerosol particles manage to form has been a mystery until now, because they should be rapidly destroyed by colliding with pre-existing aerosols. In the new study, CLOUD scientists investigated the role of ammonia and nitric acid at atmospheric concentrations. The results show that small inhomogeneities in the concentrations of ammonia and nitric acid can lead to particle growth rates 100 times faster than previously seen, but only in short jets. These ultrafast growth rates are sufficient to make the newly formed particles less likely to be destroyed by pre-existing particles. The end result is a dense smog episode with a high number of particles.
While global emissions of ammonia are dominated by farming, in cities the presence of both ammonia and nitric acid is largely due to vehicles. “Although the emission of nitrogen oxides is regulated, ammonia emissions are not and may even be increasing with the latest catalytic converters used in gasoline and diesel vehicles. Our study shows that regulating ammonia emissions from vehicles could contribute to reducing urban smog,” concludes Jasper Kirkby, head of the CLOUD experiment. The results now published in Nature could help inform policies for reducing air pollution.
It is worth noting that the experiment has Portuguese participation: António Amorim (CENTRA and FCUL) and António Tomé (CLOUD uses a special cloud chamber to recreate the diverse aspects of Earth’s atmosphere, with precise control of the conditions. Cosmic rays can also influence aerosol formation, and their contributions are studied by varying the intensity of a pion beam from CERN’s Proton Synchrotron, which passes through the chamber.