How do they survive?

When new clusters of a few molecules are born in the atmosphere, they need to survive long enough to gain more and more mass by the addition of other molecules to become relevant for climate and health. This is what NPF-PANDA is about.

However, do we use the right tools to estimate the survival probability of those tiny particles? Typically, we measure the particle number size distribution, which means we deploy instruments which provide us with two types of information: what is the size of the particles and how many of each size do we have present? Improving our abilities to derive this particle number size distribution was described already here. However, how can we determine a probability for each individual particle to survive to larger sizes by just looking into the evolution of the ensemble of all particles?

In a recently published preprint we explore the most common methods to estimate the survival probability of particles during new particle formation. Surprisingly, we found that different methods are needed for different types of aerosol size distributions and misapplication of the conventional methods to typical new particle formation events may underestimate the survival probability. Using the correct approaches case-by-case in urban Beijing, we can demonstrate that on average the survival probability of particles in this highly polluted environment can be described by a competition of their growth and loss rates, as predicted by theory. This contrasts with the original hypothesis in NPF-PANDA that the survival of particles in highly polluted megacities might not be fully explained yet. What a major success for NPF-PANDA!

A unique atmospheric experiment

Besides the many tragedies caused by the global Covid-19 pandemic since 2020, the various lockdowns have also introduced a unique atmospheric experiment, which was unthinkable before: due to the restrictions and many people staying home, traffic originated air pollution was essentially turned off. Researchers from the Institute of Atmospheric and Earth System Research of the University of Helsinki have investigated how the Covid-19 lockdown related dramatic reductions in traffic emissions have influenced the air quality in Beijing, one of the cities, which experienced very strong lockdown measures in February 2020. The international group of researchers including many Chinese collaborators compared the observations at the Beijing University of Chemical Technology Aerosol and Haze Laboratory from the pre-lockdown and lockdown periods in 2020 with previous years. The atmospheric scientists focused on the drivers of new particle formation. During that process gas molecules cluster together and form new solid or liquid particles in the air.  New particle formation can be a large source of aerosol particles in the urban atmosphere and contribute to air pollution. Surprisingly, the strong reductions in traffic emissions, especially NOx and directly emitted aerosol particles, did not result in a decrease of particles produced via new particle formation. In contrast, the researchers observed an even enhanced importance of these newly formed particles as they were growing faster to sizes where they contributed to the formation of strong pollution episodes, which were still occurring during the lockdown period. The recently published pre-print therefore shows that the strict lockdowns enabled unique atmospheric research, which could finally investigate the role of traffic in the formation of air pollution. It demonstrates that reductions in traffic emissions alone cannot improve air quality in megacities directly, but that the atmospheric chemical cocktail is much more complex and depends on the interplay of various pollution sources.

The particle size-distribution: How much and how big?

Air pollution and climate change patterns are linked to the abundance of aerosols. However, it is one property of the aerosol particles which is really important with respect to its role in the atmosphere. And that is the particle size. Aerosol particles can have many sizes, starting from small molecular clusters, which dimensions of about one nanometer (a billionth part of a meter) up to the sizes of visible dust particles and raindrops (millimeter). As you can imagine, the physics governing the motion and properties of such differently sized particles can be quite different. Therefore, we describe aerosols often with their so-called particle size-distribution, which expresses how many particles of a certain size (typically expressed by their diameter) are found in a sample. For example, if we find 100 particles smaller than 100 nm and 5 particles larger than 100 nm in 1 cubic centimeter of air, we would already know something about the abundance of large and small particles.

Typical aerosol size-distribution measuring instruments do resolve the question of how many particles of which sizes do we find in a sample on a finer grained grid. The important challenge here is that such instruments need to measure the size of a particle and then count how many of them are found in a sample. This can be especially challenging when we want to assess the size-distribution of very small particles (below 10 nanometer for example). Here both processes are challenging: Measuring the size and counting them. Therefore a variety of different instruments with different approaches exist. However, a recent paper showed that there are still huge discrepancies between these instruments when they measure the same aerosol.

We therefore investigated if it is possible to combine the information of several such instruments in order to better estimate the particle size-distribution in the sub-10 nm range. In this paper published in the Journal of Aerosol Science, we show that mathematical methods can indeed improve the quality of the measured size-distributions when several instruments and their information are combined. With that approach it is therefore possible to better understand the processes related to such small particles, for example how fast they grow and become important for the climate and air quality.

The hunger of newborns can cause smog

Air pollution kills. Some studies estimate that bad air quality is responsible for 10,000 pre-mature deaths every day. Aerosol particles with diameters smaller than 2.5 micron (a millionth meter), so called PM2.5, is especially threatening Human Health. If more mass is suspended in the air in form of such particles, the bigger the health thread. It is well known that industry, traffic and other human activities, such as cooking or heating, emit such particles. However, how do large numbers of them accumulate? Moreover, why is severe haze still happening in urban China and other highly industrialized regions? Stricter and stricter controls and better filtration techniques should prevent the massive direct emissions of large particles during the last decades.

The answer are so-called secondary processes, already explained here. Many aerosol particles are not directly emitted into the atmosphere, but form in the air from gases. In 2014, a study showed that even if the particles are born at a size of 1 nanometer, they can grow up to 1-2.5 micron during the following days, causing severe haze in Beijing. This means they grow by a factor of 1000 in size, having 1 billion times the mass compared to their weight at birth: Imagine a human baby born with 3 kg, reaching 3 million tons when reaching adulthood.

However, for a long time, it was unclear, if this new particle formation mechanism is causing the haze. Or would severe pollution also occur, when there are no newborns? Could all the mass also be consumed by the adult aerosols, that they become so heavy that the air becomes difficult to breath? Our new study, published in Faraday Discussions, makes a strong argument that the newborns are much more effective in building haze. Well, most of you might know the hunger of newborns, so this seems no surprise to you?

Scientifically it is. Without the formation of new particles, severe air pollution caused by aged aerosols would occur later and hence less frequently throughout the year. Our study therefore suggests that if we fix the formation of small clusters in the atmosphere, we could fix the problem of bad air quality in modern megacities!

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?