2026 Solar Radiation Management Annual Meeting

Allan Betram
University of British Columbia

SOLACE: Aims, Initial Results, and Ongoing Work on Engineered Aerosols for Solar Radiation Management

The SOLACE project (Sustainable Organic and Inorganic Laboratory Aerosols for Climate Engineering) aims to evaluate the effectiveness and unintended consequences of engineered aerosols proposed for solar radiation management. Led by an interdisciplinary team at the University of British Columbia, the project focuses on two central questions. First, do solid inorganic particles proposed for stratospheric aerosol injection unintentionally promote ozone-depleting polar stratospheric clouds? Second, can engineered organic particles efficiently thin cirrus and Arctic mixed-phase clouds?

SOLACE integrates laboratory experiments, atmospheric simulations, and theoretical studies to investigate aerosol–cloud interactions under both stratospheric and tropospheric conditions. This contribution outlines the project aims and highlights initial results and ongoing work.

Initial results show that titanium dioxide particles are extremely efficient at nucleating nitric acid trihydrate–containing polar stratospheric clouds. Silicon carbide and α-alumina particles also nucleate nitric acid trihydrate at concentrations above background stratospheric levels, whereas nucleation on γ-alumina is weak. These findings indicate that several solid inorganic particles under consideration for stratospheric aerosol injection can promote crystalline polar stratospheric cloud formation, with implications for stratospheric ozone depletion. Ongoing work is examining the efficiency of solid inorganic aerosols for nucleating ice-containing polar stratospheric clouds.

We also report initial results on engineered organic particles for cirrus and Arctic mixed-phase cloud thinning. We have generated and purified the ice-nucleating protein InaZ, derived from Pseudomonas syringae, and an engineered variant inspired by InaZ, and show that both nucleate ice efficiently at extremely low concentrations. These results suggest that only small material loadings may be required for cloud modification. Ongoing work includes attaching InaZ and the engineered variant to carrier particles, such as mineral dust, to create highly efficient ice nuclei while minimizing injected mass.
 

Luc Deike, Michael E. Mueller, Marissa L. Weichman
Princeton University

New Cloud Chamber and Single-Particle Platforms for Solar Radiation Management Science

We will provide an update from our team at Princeton on the implementation of two new experimental setups aimed at interrogating the droplet and ice nucleating propensities, light-matter interactions, and chemistry of aerosol particles relevant to proposed schemes for stratospheric aerosol injection (SAI).

First, we will discuss the newly-commissioned rapid expansion aerosol chamber (REACh) facility. REACh is a meter-scale chamber that can be prepared with specific particle seeding conditions, pressure, temperature, and humidity. The primary chamber volume can be vented on-demand into an evacuated expansion chamber. This expansion causes a sudden pressure and temperature drop that supersaturates the air in the primary chamber, leading to droplet and/or ice nucleation on the seeding aerosol particles. We are using this chamber to study the nucleation and growth dynamics, aggregation, and aging of SAI-relevant inorganic aerosols, leveraging a suite of diagnostics. We have recently upgraded the chamber to operate at lower temperatures relevant to the stratosphere and upper troposphere. This development allows us to study the ice nucleating properties of proposed SAI candidate particles, and therefore the possible feedback of these particles on cirrus cloud fields as they sediment into the upper troposphere or migrate to the poles.

We will also introduce a single-particle electrodynamic balance and spectrometer that we are using to examine isolated aerosols of well-characterized size and composition. Active feedback on the balancing voltage permits trapping and monitoring the same particle over prolonged timescales of hours to days. The balance is enclosed within an environmentally-controlled chamber that can be held at appropriate pressure, temperature, and humidity to simulate atmospheric processing of SAI-relevant particles. We use Mie imaging and broadband spectroscopy to optically track particle size, refractive index, and extinction as particles age.
 

Zamin Kanji, Diego Villanueva and Lauren Zamora
ETH Zurich and University of Maryland, College Park

Cloud Thinning: Observation and Inference from the Micro to the Macro Scale

Cloud thinning is proposed as a technique where the optical depth of clouds with a warming climate effect is reduced, or their formation is suppressed by cloud seeding interventions. Seeding can be applicable to high altitude cirrus clouds or polar wintertime mixed-phase clouds. In both cloud cases, effective aerosol seeds are necessary to form ice at conditions relevant for the respective clouds. In the case of cirrus cloud thinning (CCT,) aerosol seeds that form ice at saturation (Si) conditions much lower than natural cirrus clouds are desirable so that clouds form at lower altitude, with large ice crystal size and shorter lifetime, therefore dehydrating the upper troposphere to curb natural cirrus cloud formation which requires significantly higher Si than required for seeding. In this work, we present ice nucleation threshold conditions for feldspar and silica particles (hollow and porous), demonstrating how the chosen morphology and composition can drive ice crystal formation conditions at temperatures below -40 º C, close to Si = 1 (the minimum possible Si for bulk ice crystals to form and grow). A newly developed cold cell is used to represent longer nucleation exposure times more similar to those in the atmosphere. Furthermore, we present observations on how the ice crystal seeding efficiency can deteriorate depending on the type of dispersion techniques (water versus air) and from atmospheric aging resulting in organic coatings. Future experiments are to explore inorganic coatings, and the fate of the seeding particles that remain in the troposphere with respect to oxidation processes and second cloud formation cycles.

Mixed-phase cloud thinning (MCT) has been proposed as a potential climate intervention approach to reduce polar warming by depleting long-lived mixed-phase clouds. The GLANCE project has recently moved beyond global correlation studies toward identifying the physical limits, regional sensitivities, and potential risks associated with MCT. Using a combination of long-term satellite observations, laboratory constraints, and climate-model simulations, we show that mineral dust explains much of the variability in cloud-top ice over the Northern Hemisphere, making it a crucial proxy for MCT. High-resolution modeling further reveals that cloud-thinning efficacy can be reduced when strong updrafts replenish liquid water, buffering clouds against glaciation. We also identify critical “over-seeding” thresholds, where excessive aerosol injection reverses the intended cooling effect and leads to regional warming in the Community Earth System Model (CESM) climate model. Finally, new satellite evidence provides insights into the temporal evolution of cloud glaciation—an important constraint that is currently missing in most models. Together, these findings highlight fundamental limitations of MCT and underscore the need for better observationally constrained MCT modelling.

Complementing GLANCE, the UCLA-led team assessed the viability of MCT by combining satellite-constrained cloud contrasts, improved representation of high-latitude dust sources, and modeling of the climate response to MCT forcing. Using satellite and reanalysis data, we compared wintertime low-level clouds in matched meteorological regimes over the Southern and Arctic Oceans to infer how Southern Ocean clouds would respond if seeded to Arctic-like aerosol conditions. Southern Ocean stratus clouds are less glaciated and exert stronger longwave surface warming, especially over sea ice, which is consistent with MCT-type cooling. To reduce a key uncertainty in modeling aerosol impacts on mixed-phase clouds, we implemented locally generated Arctic dust in the GEOS modeling system using high-resolution dust emissions. This local source contributes ~20–40% of wintertime low-level dust north of 60° N and is validated against extensive in-situ and satellite observations. In parallel, idealized CESM2 simulations show that a wintertime Arctic infrared forcing of -10 W m⁻² produces ~ -2.3 °C regional cooling and increases annual-mean sea-ice area by ~1.3 million km², with approximately linear additivity relative to marine cloud brightening. We also developed and deployed high-school curricula on climate change and climate intervention.
 

Odran Sourdeval
University of Lille

Assessing Cirrus Cloud Thinning Strategies by Learning From Aerosol-Cirrus Interactions in Natural and Perturbed Conditions

Cirrus clouds exert a strong yet highly uncertain influence on the Earth’s radiation budget due to their competing longwave warming and shortwave cooling effects. Their net radiative impact critically depends on poorly constrained microphysical processes governing ice crystal formation and growth. This uncertainty limits our ability to quantify aerosol-cirrus interactions and hampers a robust assessment of cirrus cloud thinning (CCT) as a potential solar radiation management strategy. While aerosol-cloud interactions in liquid clouds have been extensively investigated, their counterpart in ice clouds remains insufficiently constrained by both observations and models.

Within the framework of the ACCTS project, we investigate the sensitivity of cirrus microphysical properties to aerosol perturbations by combining satellite remote sensing, aerosol reanalyses, Lagrangian transport diagnostics, and high-resolution cloud-resolving simulations. The approach targets three complementary questions: (i) how cirrus microphysical properties respond to aerosol variability across dynamical and thermodynamical regimes, (ii) to what extent high-pollution events can be used as natural experiments to isolate aerosol-driven signals, and (iii) how these observational constraints can inform and improve the representation of aerosol-cirrus interactions in regional and global models, with implications for CCT assessments.

The results presented here focus on the first of these questions and provide initial observational constraints on the sensitivity of ice crystal number concentration (Nice) to both natural and anthropogenic aerosols. Estimates of Nice from combined lidar-radar retrievals (DARDAR) are analysed together with aerosol fields from the Copernicus Atmospheric Monitoring Service (CAMS), enabling a global assessment of how ice cloud properties respond to aerosol loading. Regime-based analyses, stratified by season and region, are used to disentangle meteorological variability from aerosol signatures. Joint-histogram diagnostics reveal an overall positive relationship between Nice and aerosol mass, particularly near cloud top, although negative sensitivities are found under specific meteorological conditions.

These relationships constitute a first step toward an observation-constrained quantification of the radiative forcing associated with aerosol-cirrus interactions, and therefore provide an empirical basis to assess assumptions underlying CCT strategies. Ongoing work aims to link the diagnosed sensitivities to cirrus origin and ice formation pathways using Lagrangian transport modelling, thereby connecting large-scale aerosol environments to the microphysical processes governing cirrus evolution.
 

Simone Tilmes and Rob Wood
National Center for Atmospheric Research and University of Washington

SAI: Climate Impacts from the Near Field to the Global Scale

This presentation synthesizes the currently achieved and future goals of six projects that assess the processes and impacts of stratospheric aerosol injections. These projects focus on modeling SAI using alternative injection materials, integrating relevant input from dedicated physicochemical laboratory experiments, and addressing their impacts at the regional and global scales. The modeling of SAI requires bridging scales from near-field plume injections to long-term global climate modeling, necessitating numerical models with varying spatial and temporal resolution and complexity.

First, we present progress in our understanding of the evolution of plumes from particle injections in the wake of aircraft (scales of 1–1000 m) and how the aerosol plume subsequently disperses and dilutes in the natural background turbulence in the stratosphere (plume dimensions from 10 m to the climate model gridbox scale). A key finding is that particle coagulation is likely strongly dependent on the number density of injected particles, which can significantly affect the effectiveness of SAI. Without significant dispersion, the most radiatively ideal particles may not be achieved. The findings are supported by the development of box models, driven by empirically derived estimates of turbulence, that can assess the microphysical evolution of particle formation.

The next goal is to connect findings from plume studies into different box models and compare them. To bridge near-field SAI injection to the global scale, global model studies are tested by incorporating plume models within global models, considering sub-grid dynamics. Another way to bridge the gap between local injections, the near-field evolution of the aerosol plume, and the large-scale aerosol distribution is to develop high-resolution global models that integrate results from high-resolution turbulence models (including large eddy simulations), which will be the next major collaborative goal. Global model comparisons of solid particle injections without considering plume development show substantial differences in solid aerosol burden, radiative forcing, and size distribution. Reasons for differences are currently explored. In addition to low-absorptivity solid particles, we use three different global models to investigate the impact of highly absorptive materials, here black carbon, to test whether such materials, if injected at high altitudes (40–45 km), can cool the surface climate. We also began incorporating information from laboratory studies into the models, including optical properties to constrain the results. Results for impact-relevant analysis using solid aerosol injections have been produced and are being analysed, including effects of SAI on the large-scale atmospheric circulation and surface climate variability.

The next focus will also be on the effects of cirrus clouds. To produce fully coupled model simulations using different aerosol microphysical models, we developed an approach that prescribes the same aerosol forcing across models. This allows us to more directly compare the dynamical responses and to constrain sources of model uncertainty. For a more comprehensive impact assessment analysis, we are currently devising experiments that employ the same aerosol forcing and injection strategy across different models, making it possible to more accurately quantify underlying uncertainties and assess their sources, In the meantime, the team in South Africa will present an impact analysis using sulfur injection scenarios and available solid-particle injections, and we will present results from the ITM team on Indian precipitation extremes and Hadley cell dynamics based on Geoengineering Model Intercomparison Project (GeoMIP). We also present a preliminary analysis of dynamical downscaling work over Africa. More work on dynamical and statistical downscaling over the Caribbean region and South-East Asia is expected in the next year.
 

Mingyi Wang
University of Chicago

Constraining Aerosol Size Distribution for Quantitative Stratospheric Aerosol Injection

Stratospheric aerosol injection (SAI) proposes to cool the Earth by introducing aerosols or their precursor gases into the stratosphere to scatter incoming solar radiation. To date, much of the SAI research has been driven by climate models that necessarily use simplified aerosol representations, which may not adequately capture aerosol physicochemical properties, and therefore cast doubt on favorable SAI evaluations as potentially overly optimistic or misleading. This limitation cannot be resolved through modeling alone; large uncertainties will persist until we achieve a process-level understanding of aerosol formation and evolution under the near-field conditions, where nonlinear chemistry, microphysics, and wake dynamics collectively determine aerosol composition and size distribution.

Our project addresses this gap by integrating laboratory experiments with multiscale modeling to resolve key chemical and microphysical processes for potentially benign SAI materials, including (a) solid aerosols (e.g., calcite and diamond) and (b) aerosols formed from acid-base and oxidized organic gas precursors. In this presentation, we first demonstrate that organic aerosols may offer a more practical, lower-sulfur alternative to conventional sulfur-based SAI. Then, we investigate how aerosol charge state influences the size distributions and discuss the implications for solid aerosol SAI. Finally, we introduce a multi-scale modeling framework that couples near-field fluid dynamics with aerosol microphysics to more accurately simulate aerosol properties in the line-shape aircraft plume in the stratosphere.
 

Fangqun Yu
Atmospheric Sciences Research Center, University at Albany

Contrail Cirrus Mitigation and Cirrus Cloud Thinning: Intertwined Climate Strategies

Cirrus clouds, which exert a net warming effect on Earth, form both naturally and anthropogenically as aviation contrails spread into cirrus shields. Cirrus cloud thinning (CCT) and current aviation industry efforts in contrail cirrus mitigation (CCM) are deeply intertwined, as both face uncertainties driven by aerosol and cloud microphysical processes and their representation in global models. Specifically, the strategy investigated in this project—COntrail Mitigation By Ice Nuclei Effective Seeding (COMBINES)—functions essentially as CCT for anthropogenically generated cirrus. Furthermore, a practical CCT approach would likely utilize commercial aircraft to inject ice-nucleating particles (INP) into the upper troposphere. By leveraging extensive field measurements of upper-tropospheric aerosols, contrails, and cirrus clouds, this research aims to bridge key knowledge gaps, explore practical deployment strategies, and assess the risks inherent in both CCM and CCT. Our approach integrates measurement-guided plume-scale, large-eddy, and global simulations. We employ and improve two established global models—each featuring an integrated sectional advanced particle microphysics (APM) model—to simulate global CCM and CCT impacts. The two global aerosol models have previously been used in stratospheric aerosol injection (SAI) study and can be readily employed to investigate potential impacts of SAI on upper tropospheric aerosol concentrations and the associated implications for CCM and CCT. On the other hand, the insights gained during this project, especially the plume scale processes, can benefit SAI research.

During the first year of this project, efforts focused on: (1) analyzing in-situ measurements of particles and contrails within aircraft plumes; (2) constraining a state-of-the-art aerosol and contrail microphysics (ACM) model using recent field data; (3) conducting plume-scale simulations to better understand factors controlling particle and contrail formation; (4) refining the COMBINES strategy and its parametric sensitivities; (5) developing a new global contrail model; and (6) simulating the global efficacy of COMBINES in reducing radiative forcing. Major findings include: (1) Volatile particles dominate contrail ice formation in modern lean-burn engines, where non-volatile soot emissions are reduced by 3–4 orders of magnitude; (2) Volatile particle formation is significantly influenced by fuel sulfur content, lubrication oil emissions, organic compounds, and chemi-ions; (3) Through controlled INP seeding, COMBINES can reduce the number of contrail ice particles by 1–2 orders of magnitude; (4) COMBINES is applicable to both rich-burn and lean-burn engines and all fuel types, and may circumvent some key concerns associated with navigational contrail avoidance; and (5) COMBINES demonstrates high potential for reducing the global radiative forcing of aviation-induced cirrus. This presentation concludes with a discussion of remaining challenges and upcoming research phases.

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Premium Economy Class: When traveling internationally, the maximum allowable class of service is premium economy.

Hotel
Up to 3 nights at the conference hotel, arriving on Wednesday, April 8, 2026, and departing on Saturday, April 11, 2026.6.

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Hotel

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Overview

In-person participants will be reimbursed for meals and local expenses including ground transportation. Expenses should be submitted through the foundation’s online expense reimbursement platform after the meeting’s conclusion.

Expenses accrued because of meetings not directly related to the Simons Foundation-hosted meeting (a satellite meeting or meeting held at another institution, for example) will not be reimbursed by the Simons Foundation and should be paid by other sources.

Below are key reimbursement takeaways; a full policy will be provided with the final logistics email circulated approximately 2 weeks prior to the meeting’s start.

Meals

The daily meal limit is $125; itemized receipts are required for expenses over $24 USD. The foundation DOES NOT provide a meal per diem and only reimburses actual meal expenses up the following amounts.

• Breakfast $20
• Lunch $30
• Dinner $75

Allowable Meal Expenses

• Meals taken on travel days (when you traveled by air or train).
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• Group dinners consisting of fellow meeting participants paid by a single person will be reimbursed up to $75 per person and the amount will count towards the $125 daily meal limit.

Unallowable Meal Expenses

• Meals taken outside those provided by the foundation (breakfast, lunch, breaks and/or dinner).
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• Minibar expenses.
• Meal expenses for a non-foundation guest.

• Ubers, Lyfts, taxis, etc., taken to and from restaurants in Manhattan.

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Ground Transportation

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Transportation to/from satellite meetings are not reimbursable.

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