How do trees influence cloud formation?

This finding could reduce uncertainties in climate models and help make more accurate predictions.

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The formation of new particles in the atmosphere by biogenic vapors impacts the climate. Numerous studies have been done on monoterpenes and isoprene’s roles in new particle formation (NPF).

As part of the global CLOUD project at the CERN nuclear research facility, scientists from PSI have determined that so-called sesquiterpenes, gaseous hydrocarbons emitted by plants, play a significant role in cloud formation. This discovery may help climate models predict the future more accurately by reducing uncertainty.

The Intergovernmental Panel on Climate Change (IPCC) has updated its forecasts, which state that by 2100, the world’s climate will be 1.5 to 4.4 degrees Celsius warmer than pre-industrial levels. This graph is based on different projections of how future anthropogenic greenhouse gas emissions might change. In the best-case scenario, we can still achieve the 1.5-degree target of the Paris Agreement if we are able to reduce emissions drastically.

However, there needs to be more clarity around these projections. The temperature increase could be lower or higher than 4.4 degrees Celsius in the worst-case scenario, for instance, if emissions continue to climb quickly.

These ambiguities in predicting how temperatures will change in response to specific changes in greenhouse gas emissions are primarily caused by scientists still learning about all the processes that take place in the atmosphere, including the interactions between its different gases and aerosols.

The CLOUD project (Cosmics Leaving Outdoor Droplets) aims to establish them, an international collaboration between atmospheric researchers at the CERN nuclear research center in Geneva. PSI helped to build the CLOUD chamber and is a member of the project’s steering committee.

The future development of cloud cover is still entirely unknown at this point. However, because denser clouds reflect more solar radiation, cooling the earth’s surface is an essential component in predicting the climate.

Water vapor needs condensation nuclei, solid or liquid particles, to condense to create the droplets that makeup clouds. These are given by a wide range of aerosols, which are minute particles of solid or liquid between 0.1 and 10 micrometers in diameter created and released into the atmosphere by natural and artificial processes.

These particles can include, for example, soot from fires, dust from the desert, salt from the sea, and pollution from industry and transportation. However, when various gaseous molecules mix and solidify in the air, a phenomenon known as “nucleation” or “new particle formation” (NPF), around half of the condensation nuclei are created. Such particles are initially very small, no larger than a few nanometers. Still, as time passes, they can develop through the condensation of gaseous molecules and eventually act as condensation nuclei.

Sulphur dioxide in the form of sulphuric acid, primarily from burning coal and oil, is the principal anthropogenic gas that creates particles in the atmosphere. Monoterpenes, sesquiterpenes, and so-called isoprenes are the most significant natural gases. These are hydrocarbons that the vegetation mainly releases. When grass is cut or we walk in the woods, for example, we can smell essential oils that are important parts of such scents. Aerosols are created when these compounds oxidize or react with ozone in the atmosphere.

Lubna Dada, an atmospheric scientist at PSI, said, “It should be noted that the concentration of sulphur dioxide in the air has decreased significantly in recent years due to stricter environmental legislation, and it will continue to decrease.”

“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.”

Which of the variables will prevail, causing an increase or decrease in cloud formation is the key question for improving climate predictions. To respond to this, one would need to be aware of how each of these substances affects the creation of new particles. A lot is already known about sulphuric acid, and measurements made in the field and chamber experiments like CLOUD, in which PSI was involved, have helped to improve our understanding of the roles played by monoterpenes and isoprene.

Dada said, “Until now, sesquiterpenes have not been a research focus. “This is because they are quite difficult to measure. Firstly, they react very quickly with ozone, and secondly, they occur much less frequently than other substances.”

Sesquiterpenes comprise only 24 million tonnes of annual emissions, compared to isoprene’s 465 million tonnes and monoterpenes’ 91 million tonnes. However, a recent study, of which Dada is the principal author, has demonstrated that these substances are crucial for cloud formation. The measurements show they produce ten times more particles at the same concentration as the other two organic compounds.

Dada and her co-authors used the distinctive CLOUD chamber at the European Organization for Nuclear Research, or CERN, to come to this conclusion. To simulate various atmospheric conditions, scientists used a chamber.

Dada said, “At almost 30 cubic meters, this climate chamber is the purest of its kind worldwide. So pure that it allows us to study sesquiterpenes even at the low concentrations recorded in the atmosphere.”

This was the exact goal the study had in mind. It was created to mimic the generation of biogenic particles in the atmosphere. Scientists were particularly interested in researching pre-industrial eras because there were no human sulphur dioxide emissions. This enables a clearer determination of the impact of human actions and their future projection. But human sulphur dioxide is now pervasive throughout nature. This is another factor that made the CLOUD chamber the sole possible option. It also enables the regulated production of a pre-industrial mixture.

The research showed that a wide range of organic molecules, known as ULVOCs (Ultra-Low-Volatility Organic molecules), are produced when an organic combination of isoprene, monoterpenes, and sesquiterpenes is oxidized in pure air. Since they are not very volatile, as their name implies, they form particles quite effectively. Over time, these particles can develop into condensation nuclei. When scientists added sesquiterpenes to the chamber containing a suspension of only isoprenes and monoterpenes, the significant impact of sesquiterpenes became apparent. The rate of new particle creation was increased by a factor of two percent, even.

Dada said, “This can be explained by the fact that a sesquiterpene molecule consists of 15 carbon atoms, while monoterpenes consist of only ten and isoprenes only five.”

The study, on the one hand, offers yet another mechanism by which vegetation might affect the temperature and weather. To improve the accuracy of future climate models, the research findings strongly support the inclusion of sesquiterpenes as a separate element alongside isoprene and monoterpenes. This is especially true in view of the rise in biogenic emissions brought on by climatic stress and the concomitant fall in atmospheric sulphur dioxide concentrations, suggesting that the latter is likely to play a bigger role in our future climate.

However, other studies are also needed to improve cloud formation predictions further. These are already being planned at the Laboratory for Atmospheric Chemistry.

Imad El Haddad, Group Leader for Atmospheric Molecular Processes, said“Next, we and our CLOUD partners want to investigate what exactly happened during industrialization when the natural atmosphere became increasingly mixed with anthropogenic gases such as sulphur dioxide, ammonia, and other anthropogenic organic compounds.”

Journal Reference:

  1. Lubna Dada, Dominik Stolezenburg et al. Role of sesquiterpenes in biogenic new particle formation. Science Advances. DOI: 10.1126/sciadv.adi5297

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