NEWS
The latest news stories in scientific computing
MODELLING AND SIMULATION
CLOUD at CERN reveals a new mechanism behind urban smog
The CLOUD collaboration has revealed a new mechanism that drives winter smog episodes in cities. The results, published in Nature, could help inform policies for reducing urban particle pollution, which ranks fifth in worldwide risk factors for mortality. Winter urban smog episodes occur
when particles form in polluted air trapped below a temperature inversion. The warm air above the inversion inhibits convection, causing pollution to build up near the ground. However, how additional aerosol particles form and grow in this highly polluted air has been a mystery until now, because they should be rapidly lost through scavenging by pre-existing aerosols. A new result from CLOUD could explain the mystery. CLOUD (Cosmics Leaving Outdoor
Droplets) scientists simulated polluted urban conditions and investigated the role of ammonia and nitric acid at atmospheric concentrations. Global emissions of ammonia are dominated by farming. In cities, however, the presence of both ammonia and nitric acid – derived from nitrogen oxides (NOx) – is largely due to vehicles. Jasper Kirkby, head of the experiment,
MODELLING AND SIMULATION Air Flow simulation reduces Covid-19 propagation risk
Airflow simulation software from Dassault Systèmes was recently used for a collaborative project to reduce the risk of virus propagation inside the St Francis Hospital in Marange-Silvange in northeastern France. The project, part of Dassault
Systèmes’ 3DExperience Lab Open Covid-19 online community, is the latest one to leverage collective intelligence on the 3DExperience platform to quickly solve critical issues during the
32 Scientific Computing World Summer 2020
Covid-19 pandemic. The St Francis hospital needed
to convert a floor into an area to accommodate Covid-19 patients and wanted to reduce the risk of virus propagation inside. Based on a floor plan in 2D,
3D modelling made it possible to reassemble the partitions and the walls of the area concerned to quickly provide an idea of the hospital’s current layout. Dassault Systèmes’ team of scientists and analysts then used Simulia
computational fluid dynamics simulation applications to simulate different air flow fields within the building, enabling hospital directors to understand how the corridors were important vectors of propagation, and optimise the interior layout. The team made assumptions
about patient behaviour, the impacts of a cough, and the floor’s ventilation and air conditioning systems, including air leaks in the windows and doors. They studied
different options by simulating reduced ventilation as well as strategic opening of windows, to understand the impact of fresh air on air flow and the concentration of virus particles. Throughout the project engineers provided support and mentoring, and the medical community provided feedback. Hospital technicians are testing
with sensor equipment from local industrial companies to validate the different simulations.
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The CLOUD team at work in CERN
said: ‘Ammonia and nitric acid were previously thought to play a passive role in particle formation, simply exchanging with ammonium nitrate in the particles.’ The CLOUD experiment at CERN involves a special chamber capable of mimicking all the diverse aspects of Earth’s atmosphere, with precise control of the conditions and extremely low contaminants. Data allow a comprehensive understanding of the formation of aerosol particles and their effect on clouds and climate. Ions from 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. However, the new CLOUD study showed that small inhomogeneities in the concentrations of ammonia and nitric
acid - which only persist for a few minutes - can lead to particle growth rates up to more than 100 times faster than previously seen, but only in short spurts that have escaped detection so far. These ultrafast growth rates are sufficient to rapidly transform the newly formed particles to larger size, where they are less prone to being lost through scavenging by pre-existing particles. The end result is a dense smog episode with a high number of particles. ‘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,’ added Kirkby.
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