The way in which cloud cover will develop is a key factor in predicting the climate because more clouds reflect more solar radiation, thus cooling the earth’s surface. Researchers at the University of Helsinki in collaboration with colleagues from Paul Scherrer Institute (PSI) have identified so-called sesquiterpenes – gaseous hydrocarbons that are released by plants – as being a major factor in cloud formation. The study has now been published in the journal Science Advances. The research was carried out as part of the international CLOUD project at the nuclear research centre CERN.
The main anthropogenic gas that contributes to the formation of particles is sulphur dioxide (SO2) in the form of sulphuric acid, mainly emitted from burning coal and oil. The most important natural gases involved are hydrocarbons that are mainly released by trees and shrubs, so-called isoprenes, monoterpenes and sesquiterpenes. They are key components of the essential oils that we smell when, for example, grass is cut or we go for a walk in the woods. When these substances oxidise, i.e. react with ozone, in the air they form aerosols.
“It should be noted that the concentration of sulphur dioxide in the air has decreased significantly in recent years due to stricter environmental legislation, such as regulations on installing filters in factories, and it will continue to decrease,” says Lubna Dada, an atmospheric scientist currently at PSI and formerly at INAR when these experiments took place.
“The concentration of terpenes, on the other hand, is increasing because plants release more of them when they experience stress – for example when there is an increase in temperatures and extreme weather conditions and vegetation is more frequently exposed to droughts or flooding.”
The big question for improving climate predictions is therefore which of the factors will predominate, leading to an increase or a decrease in cloud formation. To answer this, one would need to know how each of these substances contributes to the formation of new particles.
Persistent particles lead to more clouds
Until now, sesquiterpenes have not been a focus of research. “This is because they are quite difficult to measure,” explains Dada.
“Firstly because they react very quickly with ozone, and secondly because they occur much less frequently than the other substances.”
Around 465 million tonnes of isoprene and 91 million tonnes of monoterpenes are released into the air every year, whereas sesquiterpenes account for just 24 million tonnes. Nevertheless, the new study, of which Dada is the lead author, has shown that they play an important role in cloud formation. The measurements indicate that they form ten times more particles than the other two organic substances at the same concentration.
To determine this, Dada and her partners used the unique, almost CLOUD chamber at the European Organisation for Nuclear Research, CERN. The chamber is a large sealed, 30 cubic metre room in which different atmospheric conditions can be simulated.
The study reveals another mean by which plants – especially trees and shrubs – influence the weather and climate. Above all, the research results suggest that sesquiterpenes should be included as a separate factor in future climate models to make their predictions more accurate.
Lubna Dada et al., Role of sesquiterpenes in biogenic new particle formation.Sci. Adv.9,eadi5297(2023).DOI:10.1126/sciadv.adi5297