The gecko story: Sulfuric acid grows nanoparticles faster than expected

A gecko is sitting at the ceiling of a room, waiting to catch a fly. It is quite surprising that animals like geckos, which have the weight of a chocolate bar, can just hang at walls. When closer investigated, they do not use any sticking substance or suction cups. Their feet are soft without any claws. Recent experiments performed at the CERN CLOUD chamber found that the gecko force, which keeps geckos sticking to walls, is also important on the molecular level, sticking molecules together. The study concludes that this magic gecko force could be one puzzle piece to explain why air pollution is formed from tiny aerosol particles in metropolises around the globe.

In urban areas, power plants, residential heating and traffic lead to higher concentrations of sulfur dioxide. Once emitted into the atmosphere sulfur dioxide is oxidized and forms sulfuric acid. And sulfuric acid is one of the main trace substance responsible for the formation of aerosols. While extremely sticky, sulfuric acid rapidly forms small clusters, when two molecules meet. However, this takes a long time in the atmosphere. The typical concentrations in most urban areas are nowadays quite low as sulfur dioxide emissions were strongly reduced during the last decades. Once two sulfuric acid molecules met, this pair cannot grow as it takes again some time to find a third or a fourth molecule.

But time is crucial for small clusters in urban areas. The air is full of large soot particles emitted from trucks or chimneys. The small clusters move around very fast and quickly adhere to the large particles before finding more small molecules, growing, becoming slower and hence surviving that scavenging. For small clusters it is all about grow or die, as explained earlier. So how can they quickly find other sulfuric acid molecules? The study published in Atmospheric Chemistry and Physics in July, investigated the growth of nanoparticles from sulfuric acid with high precision aerosol instruments down to the smallest particle sizes of about 1 nanometer. Here, the gecko force becomes important. Typically, we think of the collisions of clusters as if two tiny spheres would collide. And if two solid spheres just miss each other – they miss each other.

Sulfuric acid is different. Its molecular structure is different. If another sulfuric acid molecule just passes, they get attracted to each other and finally collide. This attraction is called the van-der-Waals force, which also causes the adherence of millions of hair like lamellae structures on the gecko’s footpad to the wall. The gecko force helps sulfuric acid molecules to find each other, which lead to a more than two times faster growth of the smallest sulfuric acid clusters. This is one possibility how they can grow and survive in cities, becoming also relevant for air pollution. So next time you see a gecko at the wall, keep in mind what physics make this happen!

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.

What is NPF-PANDA about?

Air pollution is one of the major health risks in the 21st century. 91 % of the global population breath bad air. Around 7 million people die each year from air pollution, according to the WHO. And it is a rising problem: Air pollution is still growing, especially in the megacities around the globe.

How does air pollution form? What major sources influence air quality? Air pollution has many faces. Hazardous gases like ozone and nitrogen oxides can cause respiratory diseases.  Fine particulate matter can penetrate deep into the human lungs and some of these small particles may even get into your bloodstream.

Particulate matter in the atmosphere are tiny liquid or solid droplets suspended in the air, so-called aerosols. Some of them are directly emitted in to the atmosphere like soot from combustion engines or dust particles during sand-storms. However, a major fraction of aerosols are of secondary origin, formed directly in the atmosphere from low volatile gases.

Secondary aerosols are important drivers for heavy smog events. However, these secondary processes are extremely complex. They involve heterogeneous chemistry, physical chemistry and fundamental physics.  Moreover, ambient observations always are masked by meteorology. Thus, scientists around the world are trying to understand secondary aerosol formation and its impact on air quality.

NPF-PANDA, funded by the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie Actions, will look into New Particle Formation in Polluted Atmospheres. New Particle Formation (NPF) is frequently observed in the atmosphere throughout the globe. During this process, certain gas molecules start to stick at each other upon collisions. They form small molecular clusters, which can grow to larger aerosol particles. A phase transition of low volatile gases to liquid or solid particulate matter occurs. Usually, this process is observed during daytime. Particles smaller than 3 nanometer appear in the atmosphere and grow to sizes of 30-100 nanometer during the afternoon.

This process also occurs in heavily polluted environments, where it actually should not occur. Why? When the air is highly polluted a lot of pre-existing particles are already present, for example soot particles from combustion engines. The formed low volatile vapors would just condense onto those particles, not finding other vapor molecules fast enough to form clusters.

We need a better understanding of the nanocluster dynamics in order to find out how they form under polluted condistions. What grows these clusters? Are there any reasons why they won’t stick to the pre-existing particles?

New Particle Formation in Polluted Atmospheres by Nanocluster Dynamics Assessment (NPF-PANDA) will try to answer these questions and hence contribute to the question, what drives air pollution? How can we mitigate it?