Constraining the Risks of Alternative SAI Materials via Quantitative Laboratory Experiments

By far the most researched form of stratospheric aerosol injection (SAI) is using sulfuric acid/water aerosol or its gas-phase precursors. This sulfur-based approach has well-documented, significant stratospheric impacts such as ozone depletion and increasing humidity and temperatures. Furthermore, the ability of climate models to predict the changes in global atmospheric composition and circulation due to stratospheric changes caused by SAI is highly uncertain. For example, patterns of stratospheric temperature change from greenhouse gases give consistent increases in the rate of the stratosphere’s large-scale overturning
circulation in models, but observations show the opposite trend. Alternatives to sulfuric acid/water for SAI have been suggested that, in principle, have properties expected to minimize these impacts to the stratosphere. The scientific uncertainty and complexity of SAI decision making, including alternative materials, heightens the urgency of focused research efforts on this topic that are also broad and have substantive engagement of scientists from the Global South.

The true potential of alternative materials is highly uncertain due to lack of fundamental knowledge of aerosol physicochemical properties over their stratosphere lifetime. The stratosphere is a complex environment with low temperatures, UV radiation and exposure to strong acids and highly oxidizing conditions that also introduce challenges for laboratory studies. Materials may age, changing physicochemical properties, which further increases the parameter space. We will use state-of-the-art facilities at Harvard University, the University of São Paulo and the Karlsruhe Institute of Technology to investigate the physicochemical properties of a comprehensive suite of alternative SAI materials. The quantitative experimental results will facilitate reduction of uncertainty in SAI simulations by chemistry-climate models. We will also elucidate the fundamental mechanisms of the evolving optical physics and chemical reactivity underpinning these results, providing an intuition about deviations from these results due to differences between particles in the real world and in the laboratory.

Frank Keutsch is a physical chemist in the School of Engineering and Applied Sciences, Harvard University, whose research spans laboratory experiments, field campaigns and global modeling. His group’s research has focused on understanding anthropogenic influence on atmospheric composition, both gas-phase oxidative chemistry and molecular level studies of organic aerosol formation, fate and properties. His research group has participated in numerous atmospheric chemistry field campaigns on land (in the United States, Europe and the Amazon), in the air (stratosphere on board NASA’s ER-2 and WB-57F aircraft and troposphere) and on the oceans (aboard the South Korean icebreaker Araon). Keutsch is deputy-PI on the NASA Earth Venture Suborbital-3 (EVS-3) Dynamics and Chemistry of the Summer Stratosphere (DCOTSS) campaign, and his group is focusing on aerosol properties and fate in the upper troposphere–lower stratosphere. He has contributed to national and international assessments of stratospheric chemistry and geoengineering, including the 2022 WMO Quadrennial Ozone Assessment and the 2019 National Academies Committee on climate intervention research and governance. He has also had an advisory role regarding atmospheric chemistry measurements and modeling at the National Center for Atmospheric Research.

Paulo Artaxo is a professor of environmental physics at the Institute of Physics, University of São Paulo. He initiated the study of tropical aerosols in 1980s, highlighting the significant climatic role of particles resulting from biomass burning in Amazonia. He has played a leading role in field experiments, including large international campaigns and the Large-Scale Biosphere-Atmosphere Experiment in Amazonia (LBA) investigation. He plays an active international role in fostering scientific development in low- and middle-income countries, including in his role as a member of several scientific steering committees of the International Geosphere-Biosphere Programme. He has been a major contributor to multiple IPCC Assessments, contributing to or leading analyses and reports on a range of topics such as radiative forcing, aerosols and cloud effects, land use, and geoengineering and was the advisor of 60 M.Sc. and Ph.D. students. He has played a major role as a science advisor and advocate, including as vice president of the Brazilian Association for the Advancement of Science (SBPC) and vice president of the São Paulo Academy of Sciences (ACIESP). He has been recognized for his contributions with the 2007 TWAS Earth Science Prize, the 2007 Dorothy Stang Award, the 2010 Fissan-Pui-TSI Award, the 2010 Ordem do Mérito Científico Nacional (National Order of Scientific Merit) at the level of Comendador, and the 2016 Almirante Álvaro Alberto Award, one of the most important science prizes in Brazil.

John Dykema, project scientist in the School of Engineering and Applied Sciences, Harvard University, is an applied physicist focused on research at the intersection of atmospheric chemistry and atmospheric radiation. He brings a background in semiconductor technology to the development of innovative observing systems for radiometry, aerosol measurements and atmospheric profiling. Dykema is a member of the DCOTSS mission, working to extract observational constraints to update the radiative properties of upper troposphere–lower stratosphere aerosols. This investigation revisits the climate impact of stratospheric aerosol, taking account of new discoveries regarding the aerosol’s complex morphology and composition. He has led the design of laser-based calibration technology to link satellite measurements of climate quantities to fundamental physics. To enhance the scientific value of novel instrumentation, he has studied new approaches to data analysis. These methods include linear stochastic modeling, optimal detection/fingerprinting and adaptive signal processing. He has applied these methods to remotely sensed observations and climate model output to infer climate sensitivity, quantify radiative processes and estimate climate impacts. His advisory and advocacy activities include membership in NASA and international measurement teams dedicated to advancing physically rigorous calibration of climate measurements.

Thomas Leisner, professor at the Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology (KIT), is an experimental physicist who works at the interface of the environment and quantum optics. His research elaborates nanoscale aerosol and cloud processes utilizing world-class facilities that include atmospheric chambers, including the Aerosol Interaction and Dynamics in the Atmosphere (AIDA) chamber, mass spectrometers, electrodynamic traps and ultrafast laser systems. These facilities allow the elucidation of fundamental chemistry and thermodynamics that control properties of condensed materials and their interactions with gas-phase species. A recent example highlighted the importance of meteoritic smoke particles for the stability of noctilucent clouds, which can be found at altitudes of up to 85 km and are the highest lying clouds in the atmosphere. In addition to serving as Department Head at KIT, he also performs advocacy and advisory roles for the Japanese Society for the Promotion of Sciences, in environmental physics for the German Physical Society, and as part of the review board of the Atmospheric and Maritime Science of the German Science Foundation. Through his work at the AIDA chamber, he has contributed to the cutting edge of mechanistic understanding of cloud-aerosol climate effects such as ice cloud nucleation, water phase transitions at the molecular level and organic aerosol photochemistry.