As part of the CLOUD international project at CERN, researchers at PSI have discovered that so-called sesquiterpenes - gaseous hydrocarbons released by plants - are a major factor in cloud formation. The discovery could reduce uncertainty in climate models and help make more accurate predictions. The research has now been published in the journal Science Advances.
According to the latest projections from the Intergovernmental Panel on Climate Change (IPCC), by 2100, the global climate will be 1.5 to 4.4 degrees Celsius warmer than pre-industrial levels. This figure is based on various hypothetical scenarios describing how anthropogenic greenhouse gas emissions will develop in the future. So, in the best-case scenario, if we can control emissions quickly and completely, we can still achieve the 1.5 degrees Celsius target set out in the Paris Agreement. In the worst-case scenario, we will far exceed this target.
However, there is also a certain degree of uncertainty in this prediction. For example, in the worst-case scenario, if emissions continue to rise sharply, temperature rise could be as low as 3.3 degrees Celsius or as high as 5.7 degrees Celsius instead of 4.4 degrees Celsius.
These uncertainties in predicting how specific developments in greenhouse gas emissions will cause temperatures to change are largely due to the fact that scientists do not yet fully understand all the processes that occur in the atmosphere - the interactions between various gases and aerosols in the atmosphere. Atmospheric researchers at the CERN nuclear research center in Geneva have launched an international collaborative project, the CLOUD (Cosmic Droplets Leaving Outdoors) project, which aims to establish these processes. PSI helped build the CLOUD test chamber and is a member of the project's steering committee.
The mystery of cloud formation
How clouds will form in the future, in particular, remains largely a mystery. However, this is a key factor in predicting climate because more clouds reflect more solar radiation, cooling the Earth's surface.
To form the water droplets that make up clouds, water vapor needs condensation nuclei, which are solid or liquid particles on which it can condense. These particles are provided by a variety of aerosols, which are tiny solid or liquid particles between 0.1 and 10 microns in diameter that are produced and released into the air by nature and human activities. These particles include salt from the ocean, sand from deserts, pollutants from industry and traffic, or smoke particles from fires.
However, about half of all condensation nuclei are actually formed when different gaseous molecules in the air combine to form a solid, a phenomenon experts call "nucleation" or "new particle formation" (NPF). At first, these particles are very tiny, only a few nanometers in size, but over time they grow through the condensation of gaseous molecules and then become condensation nuclei.
Greenhouse Gases You Can Smell
The main anthropogenic gas that causes particle formation is sulfur dioxide in the form of sulfuric acid, primarily from the burning of coal and oil. The most important of these natural gases are the so-called isoolefins, monoterpenes and sesquiterpenes. These are hydrocarbons released primarily by vegetation. They are the main components of essential oils, which we smell when we mow the grass or take a walk in the woods, for example. When these substances oxidize, that is, react with ozone, they form aerosols in the air.
Lubna Dada, atmospheric scientist at PSI, said: "It should be noted that the concentration of sulfur dioxide in the air has fallen significantly in recent years and will continue to fall due to more stringent environmental legislation. On the other hand, the concentration of terpenes is increasing due to plant growth "
So a big question for improving climate predictions is which factors will dominate, causing more or less cloud formation. To answer this question, we need to know how each of these substances contributes to the formation of new particles. Much is already known about sulfuric acid, and the role of monoterpenes and isoprene is now better understood, thanks to field measurements and laboratory experiments such as CLOUD in which PSI participates.
Sesquiterpenes are rare but potent
"So far, sesquiterpenes have not been the focus of research." Dada explains: "This is because they are difficult to measure. First First, because they react very quickly with ozone, and secondly, because they appear much less frequently than other substances. "
About 465 million tons of isoprene and 91 million tons of monoterpenes are released on the earth every year, while sesquiterpenes account for only 24 million tons. Nonetheless, these compounds play an important role in cloud formation. According to measurements, they form ten times more particles than the other two organic substances at the same concentration.
To determine this, Dada and her collaborators used the unique CLOUD laboratory at the European Center for Nuclear Research (CERN). The test chamber is a sealed room that simulates different atmospheric conditions. This climate chamber has nearly 30 cubic meters and is the purest of its kind in the world. The purity is so high that sesquiterpenes can be studied even at the very low concentrations recorded in the atmosphere.
That's exactly what this study was about. This study aimed to simulate the formation of biological particles in the atmosphere. More specifically, the researchers were interested in studying the pre-industrial era when there were no anthropogenic sulfur dioxide emissions. This will allow the impacts of human activities to be more clearly determined and predicted for the future. However, man-made sulfur dioxide emissions have long been ubiquitous in nature. This is another reason why only cloud chambers are feasible. It can also produce pre-industrial mixtures under controlled conditions.
Persistent particles bring more clouds
The experiments found that the oxidation of natural mixtures of isoprene, monoterpenes and sesquiterpenes in pure air produces a large number of organic compounds, so-called ULVOCs (ultra-low volatile organic compounds). As the name suggests, these organic compounds are not very volatile and therefore very efficient at forming particles that grow in size over time and become condensation nuclei. The dramatic effect of sesquiterpenes became apparent when the researchers added them to a suspension containing only isopentenes and monoterpenes. Even adding just 2% doubled the rate of new particle formation. This can be explained by the fact that sesquiterpene molecules are composed of 15 carbon atoms, while monoterpenes have only 10 carbon atoms and isopentenes have only 5 carbon atoms.
On the one hand, this study reveals another way that vegetation affects weather and climate. But most importantly, the findings suggest that sesquiterpenes should be included as a separate factor in future climate models, alongside isopentenes and monoterpenes, to make their predictions more accurate. Especially given the reduction in sulfur dioxide concentrations in the atmosphere and the simultaneous increase in biological emissions due to climate stress, this means that the latter is likely to have an increasing impact on future climate. However, additional research is needed to further improve cloud formation predictions. The Atmospheric Chemistry Laboratory is already planning these studies.
Imad El-Haddad, head of the Atmospheric Molecular Processes research group, said: "Next, we and our CLOUD partners want to investigate what exactly happened during industrialization, when the natural atmosphere was increasingly mixed with anthropogenic gases such as sulfur dioxide, ammonia and other anthropogenic organic compounds."