Nitrogen oxides: From the tailpipe to aerosols

A car starts its engine. It accelerates after a traffic light went green. Or it drives up a hill. We all know the smoke and the smell coming out of the tailpipe of the car in such situations. One major part of those emissions are nitrogen oxides. Nitrogen oxides are gases. Nitric oxide is formed by one nitrogen and one oxygen atom. A colorless sweet-smelling gas. Nitrogen dioxide consists of one nitrogen and two oxygen atoms. A brownish gas with strong, harsh odor. They both can irritate the human throat and lungs, leading to coughing or shortness of breath.

Once emitted out of the tailpipes, nitrogen oxides react with other molecules in the atmosphere, making them a key player in atmospheric chemistry. As part of the urban atmospheric cocktail, they also influence the formation of aerosols. These fine particles suspended in the air are responsible for severe haze and hence millions of air pollution related deaths worldwide. A large part of urban aerosols are formed from gas molecules, which stick together and form small solid or liquid particles. This is different to primary emissions, for example the brownish soot which visibly also comes out of tailpipes. However, in the urban environments, secondary formation from gases is the main driver of bad air.

But how do the tailpipe nitrogen oxides influence this process? An international team of researchers at the European Center for Nuclear Research (CERN) investigated these effects in a huge atmospheric simulation experiment, called CLOUD. In a 26 m3 stainless-steel research chamber, they mixed ultra-pure air with the ingredients typical for urban environments. The chamber allows them to control the experiment very precisely. They can easily adjust the temperature or the illumination of the chamber to simulate the sunlight.  When they injected nitrogen oxides into the chamber they found two competing processes in aerosol formation, which they have recently published in Nature and Science Advances.

What did the tailpipe gases do? It all depended on the mix of gases which was already present in the chamber. In the first set of experiments, they mixed the nitrogen oxides with ammonia. Ammonia is for example emitted by livestock. It is also abundant in polluted mega-cities. When the researches switched on the lights inside the chamber, nitric acid was formed from the nitrogen oxides. Together with ammonia, these two gases combined and could grow the aerosol particles at an unprecedented speed. The process was very sensitive to temperature. The colder it gets, the more important it was. This fast growth of aerosol particles could explain the occurrence of winter haze in metropolitan areas such as Beijing or Moscow.

However, the researchers also identified a competing process. If organic molecules were present in the chamber, the nitrogen oxides had a different effect. Typically, organic molecules are also abundant in cities, responsible for the smell of gasoline or the smell of cleaning detergents. The sunlight initiates a chain of oxidation for those molecules, which then are also very efficient in growing small aerosols. However, in the presence of nitrogen oxides, this oxidation process was altered. The organics became less efficient in growing aerosols.

Altogether, the CLOUD team, with a large contribution from the University of Helsinki, has demonstrated that tailpipe emissions can alter atmospheric chemistry and impact the aerosol formation mechanisms in cities. In order to find out which of the two processes is more important, ambient measurements are necessary. NPF-PANDA will thus investigate the influence of nitrogen oxides on nano-particle growth with direct measurements in Beijing. This could help solving the mystery how tailpipe emissions contribute to bad air.

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