How do trees affect cloud formation?

As part of the international CLOUD project at CERN, researchers at PSI have identified so-called sesquiterpenes — gaseous hydrocarbons released by plants — as a key factor in cloud formation. This result could reduce uncertainties in climate models and help provide more accurate forecasts. The study has now been published in the journal Science advances.

According to the latest forecasts of the Intergovernmental Panel on Climate Change (IPCC), the global climate will be 1.5 to 4.4°C warmer than pre-industrial levels by 2100. This figure is based on different scenarios that describe how anthropogenic emissions of greenhouse gases might evolve. In the future. So, at best, if we can reduce emissions quickly and radically, we can still meet the 1.5°C target in the Paris agreement.

In the worst case, we will end much higher than that. However, these forecasts are also subject to some uncertainty. In a worst-case scenario, for example, as emissions continue to rise sharply, the rise in temperatures could reach 3.3°C or 5.7°C, instead of 4.4°C.

These uncertainties in predicting how temperatures will change as a result of measurable developments in greenhouse gas emissions are mainly due to the fact that scientists do not yet fully understand all of the processes that occur in the atmosphere—the interactions between the various gases and the aerosols in it. Their creation is the goal of the CLOUD (Cosmics Leaving Outdoor Droplets) project, an international collaboration between atmospheric researchers at CERN in Geneva. PSI helped build the CLOUD Room and is a member of the project’s steering committee.

The secret of cloud formation

In particular, how cloud cover will evolve in the future remains largely a mystery at present. However, this is a key factor in predicting climate because more clouds reflect more solar radiation, thus cooling the Earth’s surface.

To form the droplets that form clouds, water vapor needs condensation nuclei, which are solid or liquid particles on which to condense. It is provided by a wide range of aerosols, which are small solid or liquid particles between 0.1 and 10 micrometers in diameter, which are produced and released into the air by nature and by human activity. These particles can include salt from the sea, sand from the desert, pollutants from industry and traffic, or soot particles from fires, for example.

However, about half of condensation nuclei actually form in the air when different gaseous particles combine and turn into solids, a phenomenon experts call “nucleation” or “new particle formation” (NPF). At first, these particles are very small, barely more than a few nanometers in size, but over time they can grow through condensation of gaseous particles and then act as condensation nuclei.

Greenhouse gases you can smell

The main anthropogenic gas that contributes to the formation of particles is sulfur dioxide in the form of sulfuric acid, which is mainly produced from the burning of coal and oil. The most important natural gases concerned are the so-called isoprenes, monoterpenes, and sesquiterpenes. These are hydrocarbons that are mainly released by plants. They are essential components of the essential oils that we smell, for example, when the grass is cut or when we go for a walk in the woods. When these substances oxidize, that is, react with ozone, they form aerosols in the air.

“It is worth noting that the concentration of sulfur dioxide in the air has decreased significantly in recent years due to stricter environmental legislation, and will continue to decline,” says Lubna Dada, an atmospheric scientist at PSI.

“On the other hand, the concentration of terpenes increases because plants release more of them when they are stressed – for example when there is an increase in temperatures and extreme weather conditions and the vegetation is more susceptible to drought.”

Therefore, the big question for improving climate predictions is which factors will prevail, leading to an increase or decrease in cloud formation. To answer this question, we must know how each of these substances contributes to the formation of new molecules. Much is already known about sulfuric acid, and the role of monoterpenes and isoprene is now better understood thanks to measurements made in field trials and experiments such as CLOUD, in which PSI participated.

Sesquiterpenes are rare but effective

To date, sesquiterpenes have not been the focus of research. “This is because it is difficult to measure,” Dada explains. “Firstly because it reacts very quickly with ozone, and secondly because it occurs much less frequently than other substances.”

About 465 million metric tons of isoprene and 91 million metric tons of monoterpenes are released each year, while sesquiterpenes account for only 24 million metric tons. However, the new study, of which Dada is lead author, shows that these compounds play an important role in cloud formation. According to measurements, it forms ten times more molecules than the other two organic substances at the same concentration.

To determine this, Dada and co-authors used the unique CLOUD room at CERN, the European Organization for Nuclear Research. The chamber is an enclosed room in which different weather conditions can be simulated. “This climate chamber is about 30 cubic metres, the purest of its kind in the world,” says Dada. “So pure that it allows us to study sesquiterpenes even at the lowest concentrations recorded in the atmosphere.”

This was exactly what the study set out to do. It is designed to simulate the composition of biological particles in the atmosphere. More specifically, the researchers were interested in studying pre-industrial times, when there were no human-caused sulfur dioxide emissions. This allows the impact of human activities to be more clearly identified and projected into the future. However, human-generated sulfur dioxide has long been ubiquitous in nature. This is another reason why a CLOUD-only room is viable. It also allows the production of pre-industrial mixtures under controlled conditions.

Fixed particles lead to more drag

Experiments revealed that oxidation of a natural mixture of isoprene, monoterpenes, and sesquiterpenes in fresh air produces a large variety of organic compounds – so-called ULVOCs (ultra-low volatility organic compounds). As the name suggests, it is not very volatile and therefore forms particles very efficiently, which over time can grow into condensation nuclei.

The dramatic effect of sesquiterpenes was revealed when the researchers added sesquiterpenes to the chamber with a suspension of only isoprenes and monoterpenes. Even adding just 2% doubled the rate of new particle formation. “This can be explained by the fact that the sesquiterpene molecule has 15 carbon atoms, while the monoterpene has only 10 and isoprene has only five,” Dada says.

On the other hand, the study reveals another way in which vegetation can affect weather and climate. However, above all, the research results suggest that sesquiterpenes should be included as a separate factor in future climate models, alongside isoprenes and monoterpenes, to make their predictions more accurate.

This is especially true given the declining concentrations of sulfur dioxide in the atmosphere and the simultaneous increase in biotic emissions as a result of climate stress, which means that the latter is likely to become increasingly important to our climate in the future. However, other studies are also needed to further improve predictions of cloud formation. This is already being planned in the Atmospheric Chemistry Laboratory.

“The next step is that we and our partners at CLOUD want to investigate what exactly happened during industrialization, when the natural atmosphere became increasingly mixed with anthropogenic gases such as sulfur dioxide, ammonia and other anthropogenic gases,” says Emad El Haddad, Group Lead for Molecular Processes in the Atmosphere. . Organic compounds.”