How plant “smell” causes cloud formation

How plant “smell” causes cloud formation

Trees release volatile substances that can form condensation nuclei for cloud formation. © ae-photos/iStock

On the trail of the “seeds” of clouds: Researchers have gained new insights into how condensation nuclei arise in the atmosphere from plant emissions. They have now been able to demonstrate the important role of a special version of these hydrocarbons in cloud formation. Since the consequences of climate change are expected to increase the release of these plant substances, the results could be important for climate prediction models, say the scientists.

It’s getting warmer – climate researchers largely agree on this. However, it remains difficult to assess exactly how the increasing greenhouse gas levels in the atmosphere and feedback effects in the climate system will affect further development. This is because some factors in the complex climate process are still not sufficiently understood. It is also literally nebulous how the Earth’s cloud cover will develop in the future. However, this is an important factor that influences the climate in a complex way.

Scientists in the CLOUD (Cosmics Leaving Outdoor Droplets) project have been dedicated to researching the processes of cloud formation for several years. As they explain, so-called condensation nuclei in the air are necessary to form the water droplets. These are particles on which water vapor can condense. These so-called aerosols only have a diameter between 0.1 and 10 micrometers and have natural or man-made origins. It could be, for example, disturbed soil particles or substances from air pollution. However, around half of the condensation nuclei arise in the air when certain gaseous molecules combine to form solid particles. Natural and human basic substances can also be responsible for this so-called “nucleation”.

Plant emissions in the sights

Sulfur dioxide comes primarily from the combustion of coal and oil. The most important volatile substances of natural origin include various representatives of terpenes – hydrocarbons that are released by vegetation. We can sometimes detect these substances with our noses: among other things, they are responsible for the typical smell of the forest and are released more intensively by plants when they are stressed. The CLOUD team has already shown in previous research that plant monoterpenes and the terpene isoprene play a role in the formation of condensation nuclei.

Now, for the first time, the focus was on the so-called sesquiterpenes. They are present in the air in significantly lower concentrations than the other two plant terpene substances. But their complex structure suggested that they could have a particularly intense effect. The team now investigated this suspicion through investigations in the special research chamber of the CLOUD project at the CERN nuclear research center in Geneva, in which various atmospheric conditions can be simulated. “In this climate chamber, the study of sesquiterpenes is also possible if the low concentration in the atmosphere is simulated,” says first author Lubna Dada from the Paul Scherrer Institute in Villigen.

Less common – but particularly effective

As the team reports, the results revealed an important role for sesquiterpenes in cloud formation: The substances apparently develop their effect through the combination with the other two terpenes: with only a small addition of sesquiterpene to a mixture of isoprene and monoterpene The analyzes showed that there is a strong formation of complex molecules in the air, from which condensation nuclei are formed very efficiently. Ultimately, the sesquiterpenes have a ten times more intense effect on particle formation than the other two organic substances, say the scientists. Despite the lower concentrations, these plant substances also play a significant role in cloud formation. “This can be explained by the fact that a sesquiterpene molecule contains 15 carbon atoms, while monoterpenes only contain ten and isoprene contains five,” says Dada.

The study now shows another factor through which vegetation can influence weather and climate, say the researchers. This in turn is important in the context of developments associated with climate change: “It should be noted that the concentration of sulfur dioxide in the air, which causes condensation nuclei, has decreased significantly in recent years due to stricter environmental laws and will continue to decrease.” says Dada. “The concentration of terpenes, on the other hand, increases because plants release more of them under stress – for example, when temperatures and weather extremes increase and vegetation is more frequently exposed to drought,” explains the researcher. With regard to climate forecasts, the question arises as to how these developments will affect cloud formation. The results can now contribute to this. However, they also underline the need for further studies to better understand the factors behind cloud formation, the researchers say.

Source: Paul Scherrer Institute, specialist article: Science Advances, doi: 10.1126/sciadv.adi5297

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