Solar Radiation Management Kick-Off Meeting

Gabriel Vecchi
Princeton University

Weakening the CO2 Greenhouse Effect via Stratospheric Aerosol Injection

Stratospheric aerosol injection (SAI) represents one of the primary potential options for intentionally modifying the climate to offset the warming from increasing greenhouse gases. The hypothesized strategy typically involves the injection of scattering aerosols in the lower stratosphere to increase the amount of sunlight reflected to space, thereby reducing the amount of sunlight absorbed by Earth. We demonstrate a new and potentially more efficient approach to SAI, using it to induce a weakening of the Earth’s greenhouse effect. We show that the injection of absorptive aerosols in the upper stratosphere (~10 hPa) increases the emission of top-of-atmosphere infrared radiation. Warming the emission level of CO2 weakens the greenhouse effect by altering the thermal structure of the upper stratosphere rather than the concentration of greenhouse gases. Climate model simulations indicate that the reduction in global temperatures induced through this process is an order of magnitude larger (per unit aerosol mass) than the injection of more traditional reflective aerosols. These results argue for further research into the possible impacts, particularly unintended deleterious side effects, of injecting absorptive aerosols in the upper stratosphere as a potential alternative strategy for solar radiation management.
 

Rajan Chakrabarty
Washington University in St. Louis

First-Principles Characterization of Optical Properties of Aluminum Oxide Aerosols

Stratospheric aerosol injection (SAI) is a proposed geoengineering technique to counteract the rising global temperature by injecting sunlight-reflecting particles into the stratosphere. Ideal SAI particles should not absorb ultraviolet-visible-infrared radiation and efficiently backscatter incoming solar radiation. Recent studies have highlighted the advantages of solid aerosol particles (over sulfate aerosol particles) for SAI because of their theoretically high backscatter ratios and negligible absorption. However, irregularities in the crystal structure of these materials may lead to non-trivial absorption. This study presents our preliminary findings on the optical properties of aluminum oxide (Al2O3) aerosols, an SAI candidate.

We procured commercially available alpha- and delta-Al2O3 nanoparticles and aerosolized them using a bench-scale powder dispersion system. The aerosolized nanoparticles were analyzed for their spectral absorption and scattering properties using photoacoustic spectroscopy and nephelometry, respectively. They were also collected on lacey carbon substrates to estimate particle-scale complex refractive indices using electron energy loss spectroscopy. Preliminary analysis reveals amorphous crystal structures with higher absorbing refractive indexes than those reported in literature. Density functional theory calculations are being performed to explain the discrepancies in absorption spectra between the measured and ideal alumina particles. These experimentally measured and inferred optical will provide crucial input of optical properties to climate modeling simulations to assess their feasibility as potential stratospheric aerosol injection particles. The framework developed to determine the optical properties of these particles will also be useful to evaluate the viability of other candidate particles. Moreover, since alumina particles are emitted into the stratosphere during spacecraft reentry, with emissions likely to increase in the coming years, it is therefore essential to understand the optical properties of these particles to accurately estimate their climate impacts.
 

Zamin Kanji
ETH Zürich Foundation

Atmospheric Aging Impacts on Aerosol for Cirrus Cloud Seeding

Cirrus cloud thinning (CCT) is a potential framework to mitigate climate warming in the near term, while we adopt energy choices with low carbon footprints. Composed only of ice crystals, cirrus produce a net warming effect on the Earth’s climate because they are optically thin and their ability to shield incoming solar radiation is outweighed by their trapping of outgoing longwave radiation at low temperatures (T < 235 K). Cirrus clouds predominantly form by homogeneous freezing of solution aerosol particles at T < 235 K and supersaturation with respect to ice, Si > 1.4. Particles effective for cirrus seeding should possess surface properties that catalyze ice formation to compete with homogeneous freezing to perturb natural cirrus formation. In this work, we will test synthetic and naturally occurring silica and mineral dust particles respectively. These are expected to be highly effective ice nucleators, which can form ice at Si lower than for natural cirrus, grow into large ice crystals, sediment and suppress natural cirrus formation. If these particles are used in cirrus seeding, they will undergo atmospheric aging in the upper troposphere. We aim to investigate the influence that particle aging processes such as in/organic acid condensation, OH oxidation and cloud cycling have on their ability to retain their ice nucleation potential. These results will aid in evaluating if seeding with silica or dust particles produce ice crystals at the desired temperature and supersaturation conditions, thus depleting water vapor that would be otherwise available for natural cirrus formation in the upper troposphere.
 

Romaric Odoulami
University of Cape Town

Global and Regional Climate Response to Stratospheric Aerosol Injection Materials

Previous studies assessing the potential cooling of stratospheric aerosol geoengineering (SAG) mostly use sulphate aerosols despite their likely negative influence on the ozone layer and human health. Here, we propose to characterize the global and regional climate response to non-sulphate materials for SAG using climate model simulations that incorporate more advanced stratospheric chemistry representations currently being developed at the National Centre for Atmospheric Research (NCAR). This project will also explore the implications of the simulated climate response for agriculture, biodiversity, energy and water resources across Africa. Preliminary analysis of SAG simulations with the SOCOLv4 model using sulphur, alumina, calcite and diamond as injection materials suggest the following: (i) all these injection materials have a cooling potential, as they could keep global annual and seasonal means temperatures in the range of SSP2-4.5; (ii) a partial SAG-induced cooling, as widespread regions across the globe could become warmer; (iii) all injection materials are more effective in offsetting the maximum than minimum temperature; and (iv) the impact on precipitation is less linear and spatially heterogeneous. In conclusion, lower levels of warming and drying could still be achieved with a similar conventional mitigation scenario (e.g., SSP2-4.5).
 

Paul Wennberg
California Institute of Technology

Constraining Aerosol-Cloud-Radiation Interactions Using Cloud & Sulfur Observations Downwind of a Low Altitude Volcano

Global emissions of sulfate precursors by shipping have decreased by more than fivefold since January 2020. This change in emissions has been discussed as potentially playing a role in the extraordinary increase in the rate of warming over the past three years — effectively marine cloud SRM in the reverse direction. Yet the impact of sulfur emissions on altering the albedo over the oceans remains poorly understood. In this project, we will explore these interactions using extant observations of cloud-aerosol-sulfur interactions downwind of Taal, a volcano near Manila approximately 300 meters above sea level. We further this investigation studying the change in lightening — a measure of convective — over shipping lanes in South East Asia following the vast reduction in sulfur from ships in 2020.
 

Simone Tilmes
National Center for Atmospheric Research

Stratospheric Aerosol Injection with Alternative Materials: Modeling of the Feasibility and Impacts using the NCAR Community Earth System Model (CESM)

A holistic study of the effects of alternative materials for SAI requires a synergy of three components: (1) laboratory studies that characterize particle optical properties, heterogeneous chemistry and microphysics, (2) comprehensive earth system modeling (ESM) capabilities whichintegrate the different SAI materials and identify knowledge gaps that inform on required laboratory studies and (3) modeling feasibility studies that quantify the global and regional impacts of introducing SAI methods and their associated uncertainties. We are working on performing comprehensive model studies to assess the feasibility and impacts of SAI using alternative materials, calcite (CaCO3), alumina (Al2O3) and diamond. In contrast to using sulfate aerosols for SAI modeling studies, these materials are expected to induce less stratospheric heating and associated side effects. However, so far, the effects of using these materials on atmospheric composition, ozone, ice clouds and feedbacks of the climate system remain largely unknown. In this presentation, we outline the main work packages of the funded project and an overview of some initial results. This work integrates new solid materials into the NCAR Community Earth System Model (CESM) based on collaborative input from new laboratory results to assess the direct effects of the material on chemistry and dynamics, polar stratospheric ice clouds and cirrus clouds. We will quantify the influence of model resolution on solid material injections on aerosol and cloud microphysics and transport by utilizing horizontally refined resolution over injection regions. We further enable capacity building in developing countries while collaboratively developing downscaling capacities to identify regional and local impacts of different SAI materials. Finally, we plan to perform future global CESM simulations with and without solid aerosol injections to quantify the full impacts of these materials on the Earth System.
 

Timofei Sukhodolov
Physical Meteorological Observatory Davos / World Radiation Center

Exploring the Chemical & Climatic Impacts of Solid Particles for Stratospheric Solar Geoengineering

Our project aims at investigating the chemical and climatic impacts of alternative materials for potential solar radiation management schemes through stratospheric aerosol injections (SAI). The project team will mainly focus on the alumina, calcite and diamond solid particles, which are hypothesized to produce fewer side effects compared to the traditionally considered SAI approach with sulfate aerosols. Such side effects include stratospheric ozone depletion and lower-stratospheric heating, which has important implications for large-scale stratospheric and tropospheric circulation and regional climates. The project will involve six institutes from three countries (Switzerland, Spain and India), three global models (SOCOL, WACCM and IITM-ESM) and three chemical lab facilities (Ambient Pressure X-ray Photoelectron Spectroscopy, AP-XPS; Heavy Ion Elastic Recoil Detection Analysis, HI-ERDA; and Rutherford Backscattering Spectrometry, RBS). The lab measurements will be performed to get information about the surface and bulk chemistry processes associated with the solid particles, without which their effects on the ozone layer cannot be properly assessed. The project will thus establish a combination of laboratory and numerical multi-model results and experts in chemistry, aerosol microphysics, stratospheric dynamics and tropospheric circulation, which will provide a strong basis for an improved assessment of risks and benefits of SAI via alternative materials. Besides the project overview, this talk will present some results of incorporating and testing the solid particles in a global model SOCOL, which will be the key tool of the project, as it will supply the other two models with the aerosol forcing. With SOCOL, we performed transient experiments to intercompare the impacts of the differences in microphysical and radiative properties between the various SAI materials, as well as investigated the uncertainty ranges in their potential ozone impacts based on the available chemical kinetics data.
 

Thomas Preston
McGill University

Single Particle Measurements of Aerosols for Informed Solar Radiation Management

Our research utilizes single-particle trapping and spectroscopy techniques to obtain high-precision, wavelength-dependent optical properties of aerosols, which are directly relevant to solar radiation management (SRM) strategies, such as stratospheric aerosol injection. By using surface-free platforms like aerosol optical tweezers and electrodynamic traps, we can measure aerosols in metastable states across a wide range of temperatures and relative humidities, simulating both tropospheric and stratospheric conditions. This talk will provide an overview of our analytical instrumentation and ongoing research, with a primary focus on how we measure and model the complex refractive index of aerosols, which is essential for calculating their impact on light scattering and radiative forcing. Additionally, our team plans to explore the role of chemical and photochemical transformations, including reactions with common atmospheric oxidants and acids, on the optical properties and longevity of aerosols in the atmosphere, which will be briefly discussed. Overall, our project aims to investigate the behavior of stratospheric aerosol injection materials post-injection, with a central focus on their optical properties under atmospherically relevant conditions, and our methodology and approach will be covered in this presentation.
 

Robert Wood
University of Washington

Modeling Atmospheric Turbulence and its Impacts on Plume Dispersion for Stratospheric Aerosol Injection

Stratospheric aerosol injection (SAI) would inject aerosols or their precursors into the stratosphere, likely using aircraft, to increase the reflection of incoming sunlight back to space, thereby cooling the Earth’s climate and mitigating some of the impacts of global warming. Previous studies have shown that injected aircraft or rocket plumes (containing aerosols) can keep their quasi-linear structure for several days or weeks with plumes as long as thousands of kilometers and cross-sections as narrow as a few kilometers. The evolution (including physical, chemical and aerosol processes) of these line-shape plumes highly depends on near-field (less than 100 km) atmospheric turbulence, which is poorly represented in global climate models (GCMs) due to the controlling processes occurring at the subgrid-scale for these models, and due to limited observations of both plumes and turbulence in the stratosphere with which to constrain the models. Better understanding of stratospheric near-field turbulence and its impacts on the evolution of injected plumes is needed to improve representations of stratospheric turbulence and plume evolution for SAI in global-scale modeling (e.g., GCMs), which will affect the evaluation of both the cooling magnitude and side effects (e.g., stratospheric warming, ozone depletion, changes in precipitation) of SAI.

Two main goals of this study include: (1) using large eddy simulations (LES, with a sufficiently high vertical and horizontal resolution to explicitly simulate much of the energy-containing atmospheric turbulence) to accurately assess the near-field (less than 100 km) stratospheric turbulence and its impacts on the dispersion of SAI’s aerosol plumes, and (2) atmospheric turbulence results from LES will be used to optimize the turbulence parameterizations (e.g., diffusion coefficient) of the Lagrangian plume model that we have developed to better represent the evolution of line-shape plumes (e.g., aircraft plumes containing injected aerosols for SAI). We can couple the optimized Lagrangian plume model (with improved representations of aerosols and turbulence) into a GCM to build a multiscale plume-in-grid model. This study will be carried out as a collaboration between the University of Washington and Sandia National Laboratories. This presentation will focus on our development of tools and a theoretical framework to simulate and represent plume spreading in lower dimensional models that can be used as physical parameterizations with large scale models. Examples will be presented from our work on plume spreading from point source shipping emissions in the marine boundary layer, and the presentation will highlight how these methods will be adapted to study stratospheric plume spreading.
 

V. Faye McNeill
Columbia University

Laboratory Characterization and Modeling of Stratospheric Aerosol Heterogeneous Chemistry for SAI

We will provide an update on laboratory measurements of stratospheric heterogeneous chemistry of aerosols proposed for use in stratospheric aerosol injection (SAI). Understanding and quantifying these processes is essential for predicting the impact of SAI on stratospheric ozone and other atmospheric chemical systems (Huynh and McNeill, 2024). We will also discuss the results of a recent analysis of technical, logistical and economic feasibility issues surrounding SAI with solid particles.
 

Frank Keutsch
Harvard University

Ice Nucleation and Optical Properties of Alternative SAI Materials

Stratospheric aerosol injection (SAI) has gained attention as a potential climate intervention strategy, with sulfuric acid aerosol being the most commonly studied material. However, alternative materials may mitigate some of the risks associated with sulfuric acid, such as stratospheric heating. Recent modeling work has suggested the potential benefits of these alternatives, primarily based on assumptions of optical properties derived from literature and databases. In this study, we present experimental results on the optical properties of various commercially available materials for SAI. Our findings reveal significant deviations in properties not only compared to literature but also among the materials themselves. Additionally, we explore the ice-nucleating properties of commercial diamond, both in its pure form and when coated with sulfuric acid. Our experiments show notable ice nucleation, underscoring the importance of conducting experimental investigations with real materials to validate and refine model predictions. These results highlight the need for further research on alternative materials for SAI to fully understand their environmental impact and feasibility.
 

Ulrike Lohmann
ETH Zürich Foundation

Mixed-Phase Clouds: Can Experiences from Weather Modification be Used for Climate Intervention Purposes?

Clouds at temperatures below 0ºC often consist of supercooled cloud droplets, because of the lack of ice nucleating particles (INPs). This fact has been exploited by the weather modification community, because a deliberate injection of INPs will initiate freezing of cloud droplets. A mix of cloud droplets and ice crystals is thermodynamically unstable because of the lower vapor pressure over ice than over liquid water. Then the ice crystals will grow at the expense of the evaporating cloud droplets until the cloud is glaciated. The ice crystals will sediment, and the cloud will dissolve. Supercooled clouds in polar winter warm the climate. If they are caused to precipitate, more longwave radiation will be emitted to space and Earth will cool, offsetting regionally some of the greenhouse gas warming. In the talk, I will discuss both our field experiments as well as simulations of mixed-phase clouds and discuss an extrapolation to potential climate implications of an artificially induced glaciation process.
 

Beiping Luo
ETH Zürich Foundation

Stratospheric Aerosol Injection Across Scales (SAIaS): From Near-Field Agglomerates & Size Distributions to Global & Regional Impacts

The goal of SAIaS is to investigate the influence of small-scale processes in the plume of aerosol-emitting aircraft on the evolution of the aerosol size distribution and particle morphology in order to provide optical and aerodynamic properties of aerosols for Earth System Models that in turn quantify the radiative effect of SAI from the moment of injection into the stratosphere until re-entry into the troposphere, including its regional impacts.

We focused on the following research topics:
(i) We will measure bouncing, sticking and fragmentation of primary aerosol particles and their clusters in a chamber with different turbulence to quantify processes in the turbulent near-field plume and to inform large-scale-eddy simulation, which will determine size distributions, morphologies, sedimentation speeds and optical properties of different solid particle candidate materials.
(ii) We will use box model simulations for the atmospheric mesoscale processes, which translate size distribution and morphology of the primary aerosol particles and their clusters to scales relevant as input to global Earth System Models.
(iii) We will improve the optical and aerodynamic (sedimentation and coagulation) properties of aerosols and aggregates based on laboratory experiments and T-matrix calculations and estimate the consequences for the overall cooling efficiency.
(iv) Finally, we will estimate the impacts on global surface climate of solid particle schemes in comparison with SO2 emissions and with climate scenarios without any intervention. Specifically, we will determine the impacts on regions that are particularly vulnerable to climate change, such as tropical and subtropical islands.
 

Jasper Kok
University of California Los Angeles

Exploring the Potential for Regionally Cooling the Earth by Seeding Wintertime Mixed-Phase Clouds

We investigate the viability of a recently proposed climate intervention technique termed mixed-phase cloud thinning (MCT). This technique would generate cooling in the wintertime poles by enhancing ice formation in shallow mixed-phase clouds through seeding, thereby causing these clouds to thin and rain out. Because these clouds warm in the polar night, this would generate a cooling at the poles, exactly where global warming is most rapid. Two important advantages of this technique over stratospheric aerosol injection (SAI) and marine cloud brightening (MCB) are that it might interfere less with the hydrological cycle and that the generated cooling would likely be mostly confined to the poles, possibly reducing the governance challenges that arise with SAI and MCB.

Because models struggle to adequately represent aerosol-cloud interactions, which are especially complex for mixed-phase clouds because of the interactions of three phases of water, we assess the viability of MCT by obtaining two separate observationally based estimates of its cooling potential. First, we will use the hemispheric contrast in aerosol concentration between the (relatively pristine) Antarctic and the (more polluted) Arctic. Second, we will use the natural analog of cloud seeding by ice nucleating desert dust aerosols to estimate the cooling that could be generated with MCT. Using modeling, we will then determine the climate system’s response to this observationally estimated cooling and assess the resulting positive and negative impacts of MCT, including on Arctic communities.

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Overview:
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Meals:
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