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Mysteries in the sky: How trees impact cloud formation

Recent breakthroughs at the international CLOUD project, based at the nuclear research center CERN, have revealed fascinating insights into the mysteries of cloud formation. 

This natural process is of paramount importance in comprehending the planet’s shifting climate patterns, as it plays a significant role in reflecting solar radiation, consequently impacting the Earth’s temperature.

Cloud formation

Cloud formation requires water vapor to coalesce around condensation nuclei, which are solid or liquid particles that provide a foundation for water vapor to condense upon. 

Various aerosols serve this purpose, ranging from sea salt and desert sand to pollutants from industrial and vehicular activities. 

Nonetheless, an intriguing aspect of this process is the concept of “new particle formation” (NPF), where gaseous molecules merge in the atmosphere and transition into solids.


The NPF process involves hydrocarbons, including isoprenes, monoterpenes, and the lesser-studied sesquiterpenes, which are emitted by vegetation. 

These hydrocarbons are not only fundamental to the scents we encounter during woodland strolls or after cutting grass, but they also undergo an oxidation process with atmospheric ozone to form aerosols.

Factors influencing cloud formation 

“With tighter environmental regulations, there’s been a noteworthy decline in the concentration of sulphur dioxide in the atmosphere,” said Dr. Lubna Dada, an atmospheric scientist from PSI.

“Contrastingly, there’s a surge in terpene concentrations, particularly when plants undergo stress due to temperature spikes, extreme weather shifts, and increased drought exposure.”

This dynamic presents a crucial question for climatologists: which among these factors will dominate in influencing cloud formation? 


Sesquiterpenes are gaseous hydrocarbons that are released by trees and other plants. While prior knowledge is extensive on sulphuric acid and both isoprene and monoterpene roles have been demystified, sesquiterpenes remained a rather untouched frontier in atmospheric studies. 

“Sesquiterpenes have remained under the radar primarily due to their rapid reaction with ozone and their comparative rarity in the environment,” said Dada.

Yet, Dada’s latest study, emphasizes the integral role sesquiterpenes play in cloud formation. Remarkably, at equivalent concentrations, they form particles at a rate ten times higher than their counterparts.

The CLOUD chamber at CERN, globally recognized as the purest of its kind, allowed for this discovery. This sealed environment simulates varied atmospheric conditions, granting researchers the capacity to study even the low sesquiterpene concentrations found in nature.

What the researchers discovered 

Dada’s team delved into mimicking biogenic particle formation from pre-industrial times, a period without anthropogenic sulphur dioxide emissions, providing a clearer picture of human-induced impacts on climate. 

The results were revealing. The oxidation of a natural blend of isoprene, monoterpenes, and sesquiterpenes led to the creation of Ultra-Low-Volatility Organic Compounds (ULVOCs). These compounds are adept at particle formation, which can eventually mature into condensation nuclei.

“A single sesquiterpene molecule comprises 15 carbon atoms, while monoterpenes have ten and isoprenes merely five,” said Dada.

Significance of the study 

This research does not only reveal another dimension by which flora influences meteorological patterns. It makes a compelling argument for the inclusion of sesquiterpenes in future climate modeling.

Given the reduction in atmospheric sulphur dioxide and the rise in biogenic emissions from climate stress, the research is increasingly significant.

“Next, we and our CLOUD partners want to investigate what exactly happened during industrialisation, when the natural atmosphere became increasingly mixed with anthropogenic gases such as sulphur dioxide, ammonia and other anthropogenic organic compounds,” said Imad El Haddad, group leader at the Laboratory for Atmospheric Chemistry

The research is published in the journal Science Advances.

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