Clouds: bright and with a fresh pine scent


A new study has shed light on the first step of cloud formation, revealing that organic oxides - such as the molecule responsible for giving pine forests their smell - are a vital ingredient.

The CLOUD experiment (Cosmic Leaving OUtdoor Droplets), conducted at the CERN laboratory in Switzerland by an international team, including academics from the University of Leeds, will help scientists to better understand the effect that clouds have on climate change. The study was published online today in the journal Science.

“Clouds have a major cooling effect on climate, but we don’t have a good understanding yet of how pollution has affected natural clouds, and what this means for climate change,” said study co-author Professor Ken Carslaw from the
School of Earth and Environment at the University of Leeds.

Indeed, the
latest report of the Intergovernmental Panel on Climate Change (IPCC) has once more pointed at clouds as the largest source of uncertainty in current climate models.

A large share of the uncertainty stems from the complexity of cloud formation. Cloud droplets form when water vapour in the atmosphere condenses, but the process requires the helping hand of solid or liquid particles to which water vapour needs to attach before a cloud droplet is born.

While such particles can be emitted directly from sources like sea spray, dust or vehicle emissions, most of them form in the atmosphere from gases emitted by natural sources or human activity.

This transformation of gases into solid or liquid particles – called ‘nucleation’ – is the very first step to forming clouds. However, scientists still don’t know which molecules are needed to make the particles stable so that they can grow large enough to serve as ‘cloud seeds’.

To study the mechanism behind this stability, the CLOUD experiment research team used a lab-based cloud chamber, equipped with a wide range of external instrumentation to monitor and analyse its contents.

“With CLOUD we can change the concentrations of chemicals involved in nucleation and then measure the rate at which new particles are created with extreme precision,” said study co-author Professor Urs Baltensperger, head of the Laboratory of Atmospheric Chemistry at the Paul Scherrer Institue. “Also parameters like temperature, pressure and humidity can be easily changed at will. Such control on ambient conditions is virtually impossible to attain in field measurements.”

Individual molecules can only nucleate into particles if they first create clusters of molecules large enough to prevent them from falling apart.

The new study shows that certain oxidised organic vapours of biological origin can act as this ‘glue’ to hold the clusters together. The scientists tried a mixture containing oxidised products of alpha-pinene – a molecule that gives pine forests their characteristic smell – and found a dramatic rise in nucleation rates.

“When we tested these new experimental data in our climate model, we were able to show that gases emitted from vegetation are having a large effect on particles in the atmosphere,” said Professor Carslaw. 

“These results are a major step forwards in understanding how natural processes affect Earth’s climate, and they will help us to build much more realistic climate models.”

Image information

The cloud chamber used in the Cosmic Leaving OUtdoor Droplets (CLOUD) experiment at the CERN laboratory in Switzerland.

Image credit: CERN

Further information

Professor Ken Carslaw is available for interview. Please contact the press office on on 0113 343 4031 or email

The research paper, ‘Oxidation products of biogenic emissions contribute to nucleation of atmospheric particles’, is published in the journal Science.