Carnegie Mellon University
February 05, 2021

Carnegie Mellon Chemists Help Shed Light on How Iodine-Containing Molecules Contribute to the Formation of Atmospheric Aerosols

By Ben Panko

As part of a worldwide collaboration, Carnegie Mellon University chemists have helped discover that iodic acids can rapidly form aerosol particles in the atmosphere, giving scientists more knowledge of how iodine emissions can contribute to cloud formation and climate change.

"Essentially all uncertainty around climate change and the atmosphere has something to do with particles and cloud droplets," said Neil Donahue, Thomas Lord University Professor of Chemistry and a professor in the departments of Chemical Engineering, and Engineering and Public Policy. The Donahue lab has been a longtime member of the CERN CLOUD experiment, an international collaboration of scientists that use a special chamber at CERN in Switzerland to remedy that uncertainty by studying how cosmic rays affect the formation of particles and clouds in the atmosphere. The chamber allows researchers to precisely mix vaporous compounds and observe how particles form and grow from them.

In a study published today in the journal Science, the CLOUD collaboration looked in particular at how vapors containing iodine affect this nucleation process. For reasons not yet fully understood, Donahue said, concentrations of iodine-containing vapor compounds have been increasing in recent years in the atmosphere. 

"This represents a new pathway for particle formation, which in turn governs the properties of clouds in the marine atmosphere," Donahue said.

Building on previous research his lab conducted on discovering a new rapid mechanism for atmospheric particle formation from nitric acid and ammonia vapors, Donahue and his team have now helped the CLOUD collaboration discover that the nucleation rates of iodic acid particles are very fast. This means that increasing concentrations of iodine-containing vapors in the atmosphere can lead to large increases in the number of particles that form clouds.

Specifically, Donahue and his collaborators, including current Ph.D. candidates Mingyi Wang and Victoria Hofbauer, alumna Qing Ye and former postdoctoral scholar Dexian Chen, contributed their use of a state-of-the-art chemical ionization mass spectrometer that can measure the amount and composition of extremely small particles less than 10 nanometers in size just following their formation. 

"The CMU measurements showed that the newly formed particles are composed largely of iodic acid, confirming that this critical molecule not only is present as a vapor while particles are forming but definitively drives their growth," Donahue said.

While clouds forming may sound like a relatively benign outcome, clouds play an important role in regulating Earth's temperature because they are highly reflective. Much of the sun's energy is reflected by clouds back into space, keeping Earth from becoming too hot. However, that reflectivity can work both ways, which is a particular problem at Earth's poles. Typically, the white snow and ice surfaces reflect a lot of sunlight back into space, thus keeping the surface there cool. However, increased cloud formation in those regions can mean that the light reflected off the surface can be reflected back onto the ice and snow by the cloud cover.

"The Arctic is an especially vulnerable region, with twice the rate of warming and the huge consequences of both sea ice and ice sheet melting," Donahue said. He and his lab are already planning future research into the complex feedbacks between iodic acid and sulfur compounds and how these affect the polar atmosphere and climate change.

"We have a great deal more to learn in this area, especially regarding the interactions of the iodine compounds and particles, and dimethyl sulfide oxidation and its particle formation," Donahue said.

Additional study authors include Xu-Cheng He, Yee Jun Tham, Lubna Dada, Jiali Shen, Birte Rörup, Rima Baalbaki, Tuija Jokinen, Nina Sarnela, Lisa J. Beck, Federico Bianchi, Biwu Chu, Jonathan Duplissy, Juha Kangasluoma, Deniz Kemppainen, Totti Laitinen, Katrianne Lehtipalo, Tuukka Petäjä, Roseline C. Thakur, Yonghong Wang, Yusheng Wu, Chao Yan, Qiaozhi Zha, Putian Zhou, Markku Kulmala, Veli-Matti Kerminen, Theo Kurtén, Douglas R. Worsnop and Mikko Sipilä with the University of Helsinki; Henning Finkenzeller, Theodore K. Koenig, Randall Chiu, Andrea C. Wagner, Roy L. Mauldin and Rainer Volkamer with the University of Colorado; Dominik Stolzenburg, Sophia Brilke, Loïc Gonzalez Carracedo, Christian Tauber, Miguel Vazquez-Pufleau and Paul M. Winkler with the University of Vienna; Siddharth Iyer and Matti Rissanen with Tampere University; Mario Simon, Andreas Kürten, Lucía Caudillo, Manuel Granzin, Martin Heinritzi, Guillaume Marie, Tatjana Müller, Marcel Zauner-Wieczorek and Joachim Curtius with Goethe University; Siegfried Schobesberger, Zijun Li and Arttu Ylisirniö with the University of Eastern Finland; Dongyu S. Wang, Andrea Baccarini, Josef Dommen, Imad El Haddad, Chuan Ping Lee, Ruby Marten, Mao Xiao and Urs Baltensperger with the Paul Scherrer Institute; João Almeida, Hanna E. Manninen, Serge Mathot, Antti Onnela, Joschka Pfeifer, Stefan K. Weber and Jasper Kirkby with CERN's European Organization for Nuclear Research; Stavros Amanatidis, Changhyuk Kim, Weimeng Kong, Benjamin Schulze, Richard C. Flagan with the California Institute of Technology; António Amorim and António Dias with the University of Lisbon; Farnoush Ataei with the Leibniz Institute for Tropospheric Research; Barbara Bertozzi and Ottmar Möhler with the Karlsruhe Institute of Technology; Aijun Ding and Wei Nie with Nanjing University; Armin Hansel, Markus Leiminger, Bernhard Mentler, Wiebke Scholz and Gerhard Steiner with the University of Innsbruck; Heikki Junninen with the University of Tartu; Jordan E. Krechmer with Aerodyne Research; Aleksander Kvashin, Vladimir Makhmutov, Maxim Philippov and Yuri Stozhkov with the Russian Academy of Sciences; Ananth Ranjithkumar with the University of Leeds; Alfonso Saiz-Lopez with the Institute of Physical Chemistry Rocasolano; Imre Salma with Eötvös University; Simone Schuchmann with Johannes Gutenberg University Mainz and António Tomé with the University of Beira Interior.

Funding for this research was provided by Academy of Finland (projects 316114, 307331, 310682, 266388, 3282290, 306853, 296628, 229574, 326948, and 1325656); the European Research Council (projects 692891, 616075, 764991, 316662, 742206, and 714621); CSC – Finnish IT center; the EC Seventh Framework Programme and the EU H2020 programme Marie Skłodowska Curie ITN “CLOUD-TRAIN” (316662) and “CLOUD-MOTION” (764991); Austrian Science Fund (FWF) (J3951-N36 and P27295-N20); the Swiss National Science Foundation (20FI20_159851, 200021_169090, 200020_172602, and 20FI20_172622); the U.S. National Science Foundation (grants AGS1447056, AGS1439551, AGS1801574, AGS1620530, AGS1801897, AGS153128, AGS1649147, AGS1801280, AGS1602086, and AGS1801329); MSCA H2020 COFUND-FP-CERN-2014 fellowship (665779); German Federal Ministry of Education and Research: CLOUD-16 (01LK1601A); Portuguese Foundation for Science and Technology (CERN/FIS-COM/0014/2017); Academy of Finland Centre of Excellence in Atmospheric Sciences (grant 272041); European Regional Development Fund (project MOBTT42); Estonian Research Council (project PRG714); Hungarian National Research, Development and Innovation Office (K116788 and K132254); NASA Graduate Fellowship (NASANNX16AP36H); and ACTRIS 2TNA H2020 OCTAVE (654109).