Isoprene was discovered to drive intensive formation of aerosol particles in the upper troposphere of the tropics (altitude of 8 to 14 km). A recent article published in Nature demonstrates that after their descent to lower altitudes, these particles may provide a globally-important source of cloud condensation nuclei, enhancing cloudiness that strongly influences Earth’s radiative balance.
Aerosol particles cool the climate by directly reflecting sunlight and by modifying clouds, increasing both their reflectivity and extent. More than half of aerosol particles originate from the spontaneous condensation of trace vapours – known as nucleation or new particle formation.
Globally, the most important vapour is thought to be sulfuric acid, which is formed in the atmosphere by oxidation of sulfur dioxide. Emissions from fossil fuels have increased aerosols and clouds since preindustrial times, but they remain persistently uncertain and underrepresented in global climate models.
The net cooling effect is estimated to be around one half of the warming from increased carbon dioxide, but with large uncertainties that make it difficult to predict the additional warming expected later this century as anthropogenic emissions and aerosol fall. It is especially important to understand how aerosol particles arise from biogenic sources since they will become more important as sulfur dioxide levels are reduced with emissions controls.
CLOUD is studying in the laboratory how aerosol particles form and grow from reactive gases under atmospheric conditions. Using CERN know-how, the CLOUD chamber has achieved much lower contaminants than previous experiments, allowing us to measure particle nucleation and growth rates from precisely-controlled mixtures of vapours at ultra-low concentrations found in the atmosphere. With a particle beam from the CERN Proton Synchrotron, CLOUD is also investigating how these processes are affected by ions from galactic cosmic rays.
“During experimental campaigns, our team assembles the world’s most comprehensive array of mass spectrometers and other instruments to analyse the particles and vapours in the CLOUD chamber,” says Jasper Kirkby, the leader of the CLOUD collaboration. “As with other experiments at CERN, CLOUD combines fundamental experiments and modelling - in our case, aerosol and cloud processes, and regional and global climate - within a single team of international researchers.”
Aircraft observations over the last 20 years have found widespread new particle formation in the tropical upper troposphere over the Amazon, and the Atlantic and Pacific Oceans, but the source of these particles has so far remained a mystery. However, recent satellite observations have revealed surprisingly high levels of isoprene in the tropical upper troposphere. Isoprene – which contains 5 carbon and 8 hydrogen atoms – is released by trees and is efficiently lofted in deep convective clouds. It is the most abundant non-methane hydrocarbon emitted into the atmosphere (0.5 billion tonnes per year) but is currently thought to be incapable of forming new particles. ”In these new experiments with CLOUD, we have studied new particle formation from the oxidation of isoprene at upper tropospheric temperatures of −30 oC and −50 oC,” says Jasper Kirkby.
“We find that isoprene oxygenated organic molecules nucleate at concentrations found in the upper troposphere, without requiring any additional vapours. Moreover, the nucleation rates are enhanced 100-fold by extremely low concentrations of sulfuric acid that are ubiquitous in the upper troposphere, reaching rates that can account for the high particle number concentrations observed. We find that Isoprene oxygenated organic molecules also drive rapid growth of particles to sizes where they can seed clouds and influence climate. These rapid nucleation and growth rates persist in the presence of nitrogen oxides (NOx) at upper tropospheric concentrations from lightning.”
An accompanying paper in the same issue of Nature, reports aircraft observations made over the Amazon at the same time as the CLOUD experiments. The two papers are complementary, mutually consistent, and together provide a compelling picture of the importance of isoprene-driven new particle formation and its relevance for the atmosphere.
Isoprene is the most abundant non-methane hydrocarbon emitted into the atmosphere but its ability to nucleate particles in the boundary layer is considered negligible. “Our findings show, however, that isoprene is driving copious new particle formation over vast regions of the tropical upper troposphere. After continued growth and descent to lower altitudes, these particles may provide a globally-important source of cloud condensation nuclei for shallow continental and marine clouds, which strongly influence Earth’s radiative balance. Isoprene from forests therefore represents a major source of biogenic particles that is currently missing in climate models. It implies that the biosphere produced cloud condensation nuclei in the pristine pre-industrial atmosphere more extensively than previously thought.“
“This will reduce the contrast with today’s polluted atmosphere and so tend to reduce estimates of Earth’s climate sensitivity and, in turn, projections of warming later this century. It also implies that the biosphere may more effectively be able to partially buffer the reduction of aerosol and the associated warming later this century as sulfur dioxide levels fall with emissions controls,” says Jasper Kirkby.
“Our Aerosol Physics group at UEF has been a member of the CLOUD collaboration since its inception in 2009, which currently includes more than 15 institutes and industry partners from across Europe and the United States,” adds Professor Siegfried Schobesberger, University of Eastern Finland. “Our intensive measurement campaigns at CERN usually take place yearly during autumn. UEF’s contributions have been instruments for investigating the composition of molecular ion clusters and the ability of larger aerosol particles to absorb water vapour or to nucleate ice crystals. Those measurements allow us to understand which vapours are involved in the formation of new particles, and to assess how the larger particles affect clouds.“
For more information, please contact:
Professor Siegfried Schobesberger, University of Eastern Finland, siegfried.schobesberger@uef.fi, tel. +358 50 339 0647
Doctoral Researcher Lejish Vettikkat Parameswaran, University of Eastern Finland, lejish.vettikkat.parameswaran@uef.fi, tel. +358 50 478 0742
Research article:
Jiali Shen, Douglas M. Russell, Jenna DeVivo, Felix Kunkler, Rima Baalbaki, Bernhard Mentler, Wiebke Scholz, Wenjuan Yu, Lucía Caudillo-Plath, Eva Sommer, Emelda Ahongshangbam, Dina Alfaouri, João Almeida, Antonio Amorim, Lisa J. Beck, Hannah Beckmann, Moritz Berntheusel, Nirvan Bhattacharyya, Manjula R. Canagaratna, Anouck Chassaing, Romulo Cruz-Simbron, Lubna Dada, Jonathan Duplissy, Hamish Gordon, Manuel Granzin, Lena Große Schute, Martin Heinritzi, Siddharth Iyer, Hannah Klebach, Timm Krüger, Andreas Kürten, Markus Lampimäki, Lu Liu, Brandon Lopez, Monica Martinez, Aleksandra Morawiec, Antti Onnela, Maija Peltola, Pedro Rato, Mago Reza, Sarah Richter, Birte Rörup, Milin Kaniyodical Sebastian, Mario Simon, Mihnea Surdu, Kalju Tamme, Roseline C. Thakur, António Tomé, Yandong Tong, Jens Top, Nsikanabasi Silas Umo, Gabriela Unfer, Lejish Vettikkat, Jakob Weissbacher, Christos Xenofontos, Boxing Yang, Marcel Zauner-Wieczorek, Jiangyi Zhang, Zhensen Zheng, Urs Baltensperger, Theodoros Christoudias, Richard C. Flagan, Imad El Haddad, Heikki Junninen, Ottmar Möhler, Ilona Riipinen, Urs Rohner, Siegfried Schobesberger, Rainer Volkamer, Paul M. Winkler, Armin Hansel, Katrianne Lehtipalo, Neil M. Donahue, Jos Lelieveld, Hartwig Harder, Markku Kulmala, Doug R. Worsnop, Jasper Kirkby, Joachim Curtius & Xu-Cheng He. New particle formation from isoprene under upper tropospheric conditions. Nature 636, 115–123 (2024). https://www.nature.com/articles/s41586-024-08196-0
