Aerosols: When fragrances influence our climate

Various molecules can form mini-particles, which then have an influence on the c
Various molecules can form mini-particles, which then have an influence on the climate.

One of the great unknowns in climate models: The behavior of certain gases, which often have a strong smell and cause water to condense. The TU Vienna provides new insights into this.

It has long been clear that man-made greenhouse gases are changing the climate - but there are still important details of climate change that are not well understood. These include the behavior of tiny particles that form all by themselves from molecules in the air and can lead to the formation of clouds.

Dominik Stolzenburg from the Institute of Materials Chemistry at TU Wien is working to better understand these processes and has now summarized the current state of research in a review article in the renowned journal "Reviews of Modern Physics". This research should make climate models even more accurate in the future.

Fragrant gases become mini-particles

"You know this from a walk in the woods: you breathe in deeply and it smells really nice like the forest," says Dominik Stolzenburg. "This is due to volatile organic substances that are released from the resin of the trees, but also from leaves and tree needles."

Of course, these are not greenhouse gases - but these organic substances nevertheless have an important influence on our climate: "They oxidize in the air, creating reaction products that stick together very easily," explains Dominik Stolzenburg.

More and more molecules clump together until finally a tiny cluster is formed, which can grow to a diameter of around 100 to 200 nanometers. These particles are still far too small to be visible to the human eye. They do not simply fall to the ground, but can float in the air for long periods of time.

Condensation nuclei for water

These particles now have a decisive influence on the water vapor in the air: If there are such tiny aerosols in the air, water molecules can attach themselves to these particles. The particles thus become a condensation nucleus on which a water droplet forms - this is the only way to create fog or a cloud.

However, small clusters are very mobile and quickly collide with larger particles such as pollen or soot - so they disappear and no longer play a role as condensation nuclei. Only the fastest growing particles are relevant for the climate. Current research shows that over the continents, organic molecules are the key ingredient that ensures the survival of these clusters so that they can reach the necessary size to serve as condensation nuclei for water vapor.

"It can be shown that a large number of such aerosols make clouds denser and whiter than usual, and that it takes longer for them to dissipate again," reports Dominik Stolzenburg. "This means that a greater proportion of the sunlight is reflected by the cloud layer and the Earth becomes cooler." If it turns out that this effect is stronger than taken into account in current climate models, this would mean that global warming due to CO2 is even stronger than previously assumed. Part of its effect would then be offset by increased cloud density, which would result from the increased production of tiny clusters in the atmosphere by humans.

There has been great progress in this area in recent years - not least in the field of measurement technology. However, truly reliable models that depict the complexity of organic aerosol growth with the necessary accuracy are still lacking. As a result, such conclusions cannot yet be drawn reliably, explains Dominik Stolzenburg: ,,This is exactly what we are working on: We want to understand exactly which substances interact with each other and how."

What will the city of the future smell like?

Of course, this does not only apply to the smell of the forest, but also to many other organic compounds - tens to hundreds of thousands of them are likely to play a role in our atmosphere. As part of a "Vienna Research Group" funded by the Vienna Science and Technology Fund, Stolzenburg and his research group will investigate the question of how the city of the future will smell and how these odors contribute to aerosol formation and the climate footprint of urban regions.

Increasing e-mobility will also change the composition of urban air. Other gases are coming into focus: what do asphalt, solvents, window cleaners or your deodorant spray emit? We can already clearly see the imprint of these substances in our data. The question is whether the city of the future will still suffer from increased aerosol pollution and what we could perhaps do better in terms of material design," explains Stolzenburg. This is why he came from the University of Helsinki to the Institute of Materials Chemistry at TU Wien to supplement the existing expertise here, for example in the field of asphalt testing.

Geoengineering? Better not!

Because prevention is better than fighting symptoms, says Stolzenburg. It is precisely the lack of reliability of the models, which he pointed out in the review article, that he also cites as the reason why he takes a critical view of so-called geoengineering projects: There are ideas to counteract climate heating by artificially increasing cloud density, for example by deliberately releasing additional aerosols or their precursor gases. Most of the colleagues I discuss this with agree that the interrelationships are far too complex to venture such experiments," he says.

Original publication:

D. Stolzenburg et al, Atmospheric nanoparticle growth, Rev. Mod. Phys. 95, 045002 (2023).

Vienna Research Group: Aerosol Formation