Particulate matter is a health problem in many metropolitan areas because the tiny particles can penetrate deep into the lungs.
An experiment at the CERN research center is now proving that part of the fine dust is generated by a previously unrecognized mechanism: at low temperatures, it is secondary to ammonia and the nitric acid produced from nitrogen oxides. Both are air pollutants that are released in large cities primarily by road traffic. This educational mechanism explains why winter smog often measures more fine dust than there should be.
Particulate matter is a global problem. The tiny particles, less than a few micrometers in size, are harmful to health and could be responsible for millions of deaths worldwide. In Europe alone, around a third of all asthma cases in children could be attributed to particulate matter. At the same time, the ultrafine fine dust particles act as condensation nuclei for clouds. Fine dust can also arise through natural processes, but a large part today comes from anthropogenic sources. Particulate matter that is smaller than 2.5 micrometers is primarily released directly through combustion processes, for example in motor vehicles or heating systems. One speaks of primary particulate matter. However, the tiny particles can also be created secondarily by the addition of air pollutants to nanoparticles that are already floating in the air. Especially with winter smog in conurbations, this secondary fine dust makes up a significant part of the pollution.
Rapid particle growth in the cloud chamber
However, the known mechanisms of origin can only explain a part of the fine dust pollution measured in large cities. Especially in the Asian megacities, Wintersmog often measures higher levels of ultra-fine dust than there should be. Because, according to the common view, the smallest proportions of fine dust do not last long: Nanoparticles of less than ten nanometers in size quickly attach themselves to larger suspended matter, which should reduce the particle density, explain Mingyi Wang from Carnegie Mellon University in Pittsburgh and her colleagues. You therefore speak of a “valley of death” for these fine dust sizes. In an experiment at the CERN research center near Geneva, the researchers have now investigated where the additional ultrafine dust could come from. In the cloud chamber of the CLOUD experiment, they simulated the conditions that prevail in the streets of a big city during winter smog. In these weather conditions, a warm layer of air lies like a blanket over a colder, deeper air mass, preventing it from rising and mixing the air. As a result, the exhaust fumes from traffic, households and other emitters remain trapped in the street canyons and concentrate there.
The experiments showed that, contrary to expectations, the airborne nanoparticles are not “swallowed” by larger ones, but remain under certain conditions – and then become germs for ultra-fine dust. This always happens when the two air pollutants ammonia and nitric acid formed from car exhaust gases temporarily accumulate strongly in the street air. So far, it was thought that these two pollutants only play a passive role in particle formation, ”says Jasper Kirkby, head of the CLOUD experiment at CERN. However, as the team found, these pollutants attach to the nanoparticles, form ammonium nitrate and lead to the particles growing into ultra-fine dust. “We have observed that these nanoparticles grow very quickly within a few minutes,” says Joachim Curtius from Goethe University Frankfurt. The growth rates are in some cases 100 times what is known so far. The result of this process is the dense smog of fine dust and ultra-fine dust, which creates “thick air” in winter, especially in large Asian cities.
Winter smog and cloud formation
“In urban metropolitan areas, the process we have observed thus makes an important contribution to the formation of fine dust in winter smog,” says Curtius. Even if the local concentrations of the two air pollutants often only last for a few minutes, because of the rapid growth of the particles, this is sufficient to increase the smog. The conditions for this newly discovered mechanism are particularly favorable in winter. “Because the process only runs at temperatures of less than about plus five degrees Celsius,” explains Curtius. If the air is warmer, the particles are too volatile and do not remain stable long enough to grow into fine dust. In order to effectively combat winter smog, especially in the megacities of Asia that are still badly affected, emissions of nitrogen oxides – the precursors to nitric acid – and ammonia should therefore be reduced more. “The nitrogen oxide emissions are already controlled, but this is not the case with ammonia,” says Kirkby. “The emission of this pollutant could even increase with the latest catalytic converters in petrol and diesel vehicles.”
However, this formation of aerosol particles from ammonia and nitric acid probably not only occurs in cities and metropolitan areas, but can also take place in higher atmospheric layers, as the researchers explain. The main driving force behind this is ammonia, which is produced in agriculture. It reaches the upper troposphere through rising air currents from ground-level air and mixes there with nitric acid, which was created by lightning from the nitrogen in the air. “At the low temperatures prevailing there, new ammonium nitrate particles form that play a role as condensation nuclei in the cloud formation,” explains Armin Hansel from the University of Innsbruck. The newly identified mechanism is therefore also relevant for climate development.