Aerosols: When scents influence our climate

Dominik Stolzenburg of the Institute of Materials Chemistry at TU Wien is working to better understand these processes and now summarizes the current state of research in a review article in the  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 take a deep breath and it smells really nice like a forest," says Dominik Stolzenburg. "This is due to volatile organic substances emitted from the resin of trees, but also from leaves and tree needles."

These are not greenhouse gases -- but it is precisely these organic substances that have an important influence on our climate: "They oxidize in the air, and this produces reaction products that adhere to each other very easily," explains Dominik Stolzenburg.

More and more molecules cluster together until finally a tiny cluster has formed that can grow to a diameter of about 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 remain suspended in the air for long periods of time.

Condensation nuclei for water

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

"It can be shown that high concentrations of these aerosols make clouds denser and whiter than they would otherwise be, and that they take longer to dissipate," Dominik Stolzenburg reports. "This means that a greater proportion of sunlight is reflected by the cloud layer and the Earth becomes cooler." If it turns out that this effect is stronger than has been 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 greater cloud density resulting from human-made increases in the production of tiny clusters in the atmosphere.

Room for improvement for climate models

The now published review article focuses on the growth of the newly formed aerosol particles. These small clusters are very mobile and quickly collide with larger particles such as pollen or soot -- thus they disappear and no longer play a role as condensation nuclei. Only the fastest growing particles are relevant for the climate. Current research shows: Across continents, organic molecules are the key ingredient that ensures the survival of these clusters, allowing them to reach the size necessary to serve as condensation nuclei for water vapor. Over the oceans iodine related substances or sulfate compounds are crucial but they are often not even included in global models.

There has been great progress in this area in recent years -- especially in the field of measurement technology. But truly reliable models that represent the complexity of aerosol growth with the necessary accuracy are still lacking. Consequently, conclusions on the human-made masking of climate change cannot be drawn reliably at present, explains Dominik Stolzenburg: "This is precisely what we need to work on in the future: We want to understand exactly which substances interact with each other and in what way, and what processes need to be included into our models to make them better."