2025 Solar Radiation Management Annual Meeting

Jonathan P. D. Abbatt
University of Toronto

Ice nucleation properties of aerosol particles for potential cirrus cloud thinning

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 catalyse 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. We report on development, validation, and verification of the required cloud chambers needed to perform experiments with the cirrus cloud seeding particles in addition to the synthesis of silica particles with desired porosity. The results will aid in evaluating if seeding with silica or dust particles produces ice crystals at the desired temperature and supersaturation conditions, thus depleting water vapour that would be otherwise available for greenhouse gas warming and natural cirrus formation in the upper troposphere.
 

Rajan Chakrabarty
Washington University in St. Louis

Visible light absorption by alumina aerosols

A desired property of solid particles for stratospheric aerosol injection (SAI) is their minimal absorption cross-sections across the near-ultraviolet-visible-infrared wavelength range. Alumina (Al2O3) is a proposed candidate for SAI; however, the absorption properties of these particles are not well established. Alumina particles are also emitted into the stratosphere during spacecraft reentry, with emissions likely to increase in the coming years. It is therefore essential to understand the spectral absorption properties of these particles to accurately estimate their climate impacts.

The research community relies on optical property measurements of alumina crystals reported in a handbook [Tropf and Thomas, 1998] that do not include direct particle-scale measurements in the visible wavelengths. This talk will present progress made during the first year of this project that includes the development of state-of-the-art photoacoustic spectrometers to measure in-situ aerosol light absorption, investigation of the particle absorption cross-sections and complex refractive indices using photoacoustic spectrometers and electron energy-loss spectroscopy (EELS), and findings from molecular-scale density functional theory (DFT) calculations of the complex refractive indices of different polymorphs of alumina.
 

Frank Keutsch
Harvard University

Evolution of alternative stratospheric aerosol injection materials under simulated stratospheric ageing

We investigate the chemical and physical properties of alternative solid materials for stratospheric aerosol injection (SAI), moving beyond sulfate-based options. Since real-world analogues for these materials are lacking, precise laboratory and modeling studies are essential to assess their climate and broader environmental impact. Our research focuses on the physical interactions of these materials with radiation and their chemical stability and reactivity under stratospheric conditions. Using advanced spectroscopic techniques, including scanning near-field optical microscopy (s-SNOM), we analyze individual aerosol particle interaction with radiation and determine their refractive index across stratospheric temperatures and after simulated ageing. FTIR spectroscopy and UV-visible spectrophotometry track the evolution of optical properties, while X-ray photoelectron spectroscopy (XPS) reveals chemical changes influencing potential refractive index changes. These findings provide a foundation for evaluating solid aerosols for SAI that adds to our previous findings on this and the ice nucleating properties of alternative SAI materials.
 

Jasper Kok
University of California, Los Angeles

Progress in understanding the potential for cooling the poles 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 with mineral dust or other ice nucleating particles, thereby causing these clouds to thin and rain out. Because these clouds warm in the polar night, this would generate cooling at the poles, exactly where global warming is most rapid.

Our initial results find that, for fixed meteorological conditions, Antarctic low-level (1-3 km) clouds are ~10% icier than Arctic clouds. This hemispheric contrast between the (low dust) Antarctic and the (dustier) Arctic supports the basic hypothesis underlying MCT. Assuming cloud glaciation differences between the poles are due to enhanced dust ice nucleating particles, we estimate that the wintertime Southern Ocean could hypothetically be an ~8% icier with similar dust levels, which would have important implications for cooling the surface. We also report on progress in accurately representing the aerosol concentrations in the Arctic and in understanding the climate impact of applying wintertime polar cooling, which we contrast with the mostly summertime cooling that would be generated with a polar-focused stratospheric aerosol injection.
 

Beiping Luo
ETH Zürich Foundation

Stratospheric aerosol injection across scales (SAIaS): From near-field agglomerates and size distributions to global and regional impacts

This project aims to investigate how sub-grid processes in earth system models (ESMs) influence the evolution of particle size distribution and morphology, especially of solid particles, over time and space scales, with a focus on the 1 millisecond to 1 hour range. The results will generate physics-based input parameters for ESMs that quantify the radiative impact of stratospheric aerosol injection (SAI) from emission into the stratosphere to re-entry into the troposphere.

The major findings in the first period are:
– A theoretical study on collision mechanisms highlights the importance of turbulent coagulation in the near field immediately after emission.
– Extensive numerical simulations demonstrate that considering the shape of solid aggregates has considerable influence on calculated radiative efficiency through effects on backscattering efficiency and sedimentation velocity. Experiments to validate these simulation results currently are in progress.
– The chemical side effects of solid particle injection are highly uncertain. Diamond dust has very high cooling efficiency and the least stratospheric heating, and is therefore a promising candidate for solid particle SAI that has been underestimated until recently.
– Most of the thin cirrus clouds (as observed by the spaceborne lidar CALIPSO) can be only explained in terms of heterogeneous nucleation. The reentry of solid particles could modify the properties of ice nucleating particles at cirrus cloud level.
– We have observed the coagulation of injected aerosol particles over a period of > 1 day under turbulent conditions in our aerosol coagulation chamber. The formation of aggregates will change the injected particle scattering and settling properties.
 

Vivian Faye McNeill
Columbia University

Practical considerations for risk assessment of stratospheric aerosol injection strategies

High-altitude aerosols have the potential to lower global temperatures by backscattering a small fraction of incident shortwave radiation, a phenomenon best understood through the formation of stratospheric sulfate aerosol in the wake of volcanic eruptions. Solid mineral candidates have been proposed as an alternative to sulfate SAI (stratospheric aerosol injection), potentially limiting associated risks of ozone depletion and longwave stratospheric heating. But practical limitations of SAI have largely gone unexplored. These limitations span disciplines, with macroscopic factors like geography, governance and supply chains influencing resultant sAODs and feasible candidate selection, while microscopic factors, predominantly dispersal and injection technology, influencing resultant size distributions and radiative properties of the aerosol. Here, we show that these practical considerations fundamentally dictate the magnitude of and distribution of risk associated with an SAI strategy. Technical, geopolitical, and economic constraints will ultimately narrow the design (and associated risk) space for feasible SAI strategies.
 

Manas Mohanty
University of California, Los Angeles

Sulfur chemistry and aerosol-cloud interactions following SO2 emissions from a degassing volcano

Volcanic degassing is a significant source of sulfur dioxide (SO2) emissions, contributing to the formation of sulfate aerosols. Interactions of SO2 and resulting aerosols with clouds can lead to acidification of cloud water and affect aerosol-cloud radiation feedback. This study uses high-resolution (4 km) simulations of WRF-Chem to model the degassing plume from the Taal volcano in the Philippines, located 60 kms south of Metro Manila that has been active since 2020. Daily SO2 emissions (5000-12000 Tons/day) and injection heights (300-1200m) estimated by the PHIVOLCS were added to WRF-Chem and simulated during the period of the NASA ASIA-AQ campaign (7th–14th February 2024) which provided a suite of in-situ aircraft measurements. On adding the SO2 emissions, the model captures the pattern and magnitude of the SO2 and the formation of the aerosol plume as observed by satellite retrievals. Locations of the SO2 plumes and sulfate aerosol in the in-situ measurements agreed well with the model simulations. High concentrations of sulfate aerosols (30 µg/m³) were observed at altitudes of 900–1100 m above sea level and about 40 km downwind from the volcano, indicating significant and rapid SO2 to sulfate conversion. The presence of sulfate aerosols was associated with a reduction in net downward shortwave radiation by approximately 5–20 W/m² over regions with elevated sulfate concentrations, indicating potential radiative impacts. The sulfate aerosols exhibited a pronounced diurnal variability that was correlated with shortwave radiation flux. Interactions between the volcanic plume and low- to mid-level clouds led to localized enhancements in cloud droplet number concentrations. The in-situ comparisons, which had a very short interaction with the plume showed that more than 50% of particles were in the 3–10 nm range and 40% in the 10–100 nm range, indicating that new particle formation and growth attributed to SO₂-to-SO₄ conversion in the volcanic plume, whereas the model primarily represents condensation on pre-existing accumulation-mode particles (>100 nm). Over the majority of the domain, the aerosol number distribution was characterized by a dominance of fine-mode aerosols in the model, with accumulation-mode particles contributing more significantly than nucleation-mode particles within the fine-mode fraction. Minor enhancements in cloud liquid water path were observed in regions influenced by sulfate aerosols formed from volcanic emissions, suggesting potential aerosol-cloud interactions. Further analysis is on quantifying the role of aerosol hygroscopicity in cloud microphysical processes and its implications for cloud radiative effects may assert the impact of the sulfate aerosols formed by the volcanic plume. These observations provide some important chemical and physical insights into how SO2 plumes used for SAI would be oxidized and transformed into aerosol.
 

Romaric C. Odoulami
University of Cape Town

Africa’s climate response to stratospheric aerosol injection materials

This study assessed Africa’s climate response to different injection materials for stratospheric aerosol geoengineering (SAG) using simulations from the SOCOLv4 model, which provides a set of SAG simulations following the G6sulfur experiment in the Geoengineering Multi-model Intercomparison Project (GeoMIP). We analysed four SAG experiments, which used four injection materials (sulphur, alumina, calcite, and diamond) referred to as G6sulfur, G6alumina, G6calcite, and G6diamond, respectively. All SAG experiments used a high-emission pathway (SSP5-8.5) as baseline in which each material was injected into the equatorial stratosphere to keep global warming levels similar to an intermediate emission pathway (SSP2-4.5). We assessed Africa’s climate response to these SAG materials by quantifying the end-of-century (2080–2099) mean changes in minimum and maximum temperatures and precipitation relative to SSP2-4.5. Our findings suggest that all injection materials show a cooling potential by keeping annual and seasonal minimum and maximum temperatures below SSP2-4.5 across most parts of the continent. Maximum and minimum temperatures could decrease the most between 10°S and 10°N, along the Guinean coast of west Africa and parts of Central Africa, by up to -2°C and -4°C, respectively. This SAG-induced cooling remains partial over north Africa where a residual warming of about 1°C could persist at the end of the century relative to the SSP2-4.5, irrespective of the injection material. On the other hand, the impact on precipitation is less linear and spatially heterogeneous. However, SAG could reverse the SSP5-8.5 projected mean continental and regional (especially over Central Africa) increases in annual and seasonal precipitation, inducing a dryer future under SAG across the continent, independent of the injection material. Our results further suggest that, relative to SSP2-4.5, G6alumina could cause the largest precipitation decrease and G6diamond the slightest decrease at the end of the century. In summary, our results show that regardless of the injection material, SAG could significantly decrease temperatures across Africa.
 

Thomas Preston
McGill University

Using single particle measurements of aerosols to evaluate candidate materials for solar radiation management

In this presentation, we provide an update on our research using single-particle trapping and spectroscopy to obtain wavelength-dependent optical properties of aerosols relevant to solar radiation management (SRM). We will discuss recent measurements of the complex refractive indices of candidate materials for stratospheric aerosol injection, including sulfate, diamond, and metal-oxide aerosols, conducted under varying humidity and temperature conditions. Within the context of these measurements, we will explain how particle morphology (e.g., core-shell structure) can be used to enhance Bond albedo and reduce the settling velocity of particles. We will also briefly cover a proposal into whether ultra-fine metal oxide nanoparticles produced by rocket fuels are effective for SRM.
 

Timofei Sukhodolov
Physical Meteorological Observatory Davos / World Radiation Center

Solid particles for stratospheric solar geoengineering: Climatic impacts and chemical uncertainties

Our project investigates the viability of solid particles such as alumina, calcite, and diamond as alternatives to sulfate aerosols for stratospheric aerosol injection (SAI) in solar radiation management (SRM). Through a combination of global climate modeling and experimental chemistry (AP-XPS, HI-ERDA), we aim to assess the potential benefits and uncertainties associated with solid particle injections. In the first year of this project, we focused on refining climate models, improving experimental setups, and conducting initial simulations and laboratory experiments. In our modelling activities, we mostly focused on long-term transient experiments to investigate the SAI impacts on climate. We found that solid particles induce less stratospheric heating and Arctic residual warming compared to sulfate aerosols, with diamond emerging as the most effective material for mitigating global warming. Alumina and calcite injections are also leading to smaller stratospheric heating and reduced radiative side effects relative to sulfur-based SAI, though uncertainties in heterogeneous chemistry persist. To address this uncertainty, our experimental chemistry efforts have focused on analyzing the uptake of stratospheric acids on calcite surfaces via XPS and HI-ERDA. We measured the uptake of HNO3 and HCl on calcite under near-stratospheric conditions, and determined their penetration into deeper layers below the surface and the chemical transformation of calcite into calcium chlorides and nitrates. We found that the uptake coefficient of HCl and HNO3 decreases with stratospheric exposure time. The reason for this is a layer that is increasingly enriched with nitrogen- and chlorine-containing reaction products (Ca(NO3)2 and CaCl2 hydrates), whose depth increases with exposure, as evidenced by the ERDA depth profiles. This, in turn, leads to an increasing protection of the underlying CaCO3 core. Over a typical stratospheric residence time of one year, uptake coefficients decrease, but a 5 Mt burden of CaCO₃ particles could still significantly reduce HNO₃ and HCl concentrations and convert half of the calcite mass to nitrates and chlorides. Consequently, the impact on ozone may be lower than previously estimated. In the second year of the project, we will start laboratory experiments with alumina particles, start combining our new lab data with the modelling activities, as well as launch a first multi-model intercomparison activity focusing on solid particle SAI.
 

Simone Tilmes
University Corporation for Atmospheric Research

Solid material integration into the CARMA box model and CESM2/CARMA

In the first part of our project, we have focused on implementing solid material, particularly aluminum, into the Community Climate Earth System Model (CESM) using the Community Aerosol and Radiation Model for Atmospheres (CARMA) aerosol model. CARMA is a general-purpose sectional microphysics scheme that studies various aerosols in planetary atmospheres. It can be used as a “standalone” box (or column) model and is a helpful tool for testing new implementations of solid material before running them in the full earth system model. We present capabilities of an updated, more user-friendly version of the CARMA box model that can be used for research and educational purposes. In particular, we present examples that help understand sensitivities depending on the assumed aerosol properties (optical properties, monomer size, etc.) of different solid matter. We further present some results of the effects of alumina particles integrated into the CESM configuration (CESM2) coupled with the CARMA. We developed two implementations, one where alumina is assumed not to react with other gases and materials and another allowing the coating with sulfate and interactions with chemistry, including ozone. Comparisons of different monomer sizes of alumina and bin size distributions are performed. Initial comparisons of other solid materials, including diamond and calcite, will be presented. Finally, we will give an overview of additional ongoing work in our team, including progress on dynamical downscaling for assessing the impacts of SAI with a focus on Africa.
 

Gabriel Vecchi
Princeton University

Exploring the impacts of absorbing and scattering stratospheric aerosols with planetary cooling

We build on initial results that showed absorbing aerosols high in the stratosphere as more efficient (per unit mass of aerosol) than scattering aerosols in cooling the planet (He et al. 2025, in review), to explore the ozone and regional climate impacts of these different planetary cooling agents. Initial assessment of the impacts on stratospheric ozone is done using offline calculations from climate model data, in an effort to identify leading mechanisms and set the stage for interpreting more complex interactive models and field data. The response of tropical cyclones to stratospheric aerosols is strongly dependent on the altitude of the aerosols and on whether the aerosols are strongly scattering or absorbing. We also explore the impact on regional temperature extremes and precipitation extremes, and the extent to which the regional and extreme response scales with temperature change.
 

Diego Villanueva
ETH Zürich Foundation

The glaciogenic effect of aerosols and its potential for mixed-phase cloud thinning

Clouds with temperatures between –38°C and 0°C can consist of either liquid water or ice. The balance between these phases is expected to affect cloud radiative effect and precipitation and underpins the concept of mixed-phase cloud thinning as a climate intervention strategy. The cloud-top ice-to-total frequency (ITF) quantifies the fraction of clouds with an ice top relative to the total cloud occurrence; however, the factors controlling ITF remain poorly understood. Using 35 years of satellite data, we show for the first time that, in the Northern Hemisphere and between –15°C and –30°C, dust aerosol is strongly correlated with ITF in both time and space. This finding is in agreement with laboratory measurements of droplet freezing and indicates that cloud phase variability can be attributed to the immersion freezing of dust aerosol. Consequently, cloud phase may potentially be influenced by artificial aerosol injections.

A signature of dust-driven cloud glaciation is a higher ice-to-liquid partitioning in northern extratropical clouds, a pattern that is not well captured by current climate models. We find that, in these models, dust immersion freezing may be often masked by spurious sources of ice that are not yet well understood. Furthermore, we demonstrate that simplifying the droplet freezing process in the CESM model produces cloud phase patterns more consistent with observations. This improved configuration increases climate sensitivity to ice-nucleating particles while potentially reducing biases in cloud feedback.

Finally, the contributions of cloud-top temperature, optical thickness, and phase to cloud radiative effect in stratiform clouds are only coarsely constrained, complicating the evaluation of climate models and their sensitivity to cloud glaciation. Based on 20 years of satellite observations, we find that cloud-top temperature and optical thickness explain most of the cloud radiative effect. In contrast, cloud phase plays an indirect role through its correlation with optical thickness and cloud cover, though all three cloud properties contribute to precipitation. In the GLANCE project, we will use these new observational constraints to better represent dust-driven cloud glaciation in two climate models (CESM and ICON). By combining satellite observations, high-resolution models, and more realistic aerosol–cloud parameterizations, we aim to improve our understanding of the potential of ice-nucleating particles for radiation management. This work will help elucidate how aerosols could be used to deliberately thin mixed-phase clouds in the polar winter, mitigate polar warming, and reduce the risk of polar climate tipping points.
 

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.

We have been seeking expertise from observational scientists who specialize in understanding stratospheric mixing and dynamics using balloon and other observational methods. A group at the Leibniz Institute of Atmospheric Physics in Germany has shared balloon data from wintertime flights over Germany and Sweden that show a wide variation in turbulence intensity (as measured by turbulent dissipation) but mostly weak turbulence in the lower stratosphere and intense turbulence in the upper troposphere in one of the flights. We are also gathering data from aircraft campaigns, with an initial focus on NASA’s ACCLIP field campaign that sampled the upper troposphere and lower stratosphere of the Asian Summer monsoon from a base in South Korea. With the intermittent presence of turbulence and its sporadic occurrence there, stochastic modeling of plume spread will likely play a major role in studying the evolution of aerosol plumes over long time scales. We plan to characterize relationships between atmospheric parameters from reanalysis (like stability and wind shear) and turbulence data from balloon and aircraft data. Combining this with information about the autocorrelation of turbulent intensity from balloon measurements will be used to construct a stochastic parameterization for turbulence within a plume-following, stochastic Langevin agrangian particle model that has been used previously to study aerosol plume spreading in the marine boundary layer. We will present an initial configuration of a large eddy simulation of stratospheric conditions to aid in the characterization of how turbulence is related to the atmospheric parameters. While the high resolution required to represent turbulence in the stratosphere may limit the duration of such simulations, they may help further constrain models for turbulence in the stochastic lagrangian particle model.

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