David Keith and Debra Weisenstein Harvard University Climate Engineering Conference

1. Stratospheric Geoengineeringby Injection of Solid Particles: Risks and BenefitsPreliminary Assessment of Al2O3Geoengineering:Microphysics, Simplified RF, Ozone impacts No climate impacts or feedbacks David Keith and Debra Weisenstein Harvard University Climate Engineering Conference 18-21 August 2014 Berlin, Germany

2. Why investigate solid aerosols? Long history of suggestions for use of solid particles for SRM • Teller et al. [1997], Novim (Blackstock et al., 2009), SPICE (e.g. Pope 2012), • Several papers with radiative calculations with exotic particles. None of the existing literature has a quantitative treatment of: • Coalescence of solid particles, or • Interaction of solids with background sulfate aerosol droplets. The fundamental goal of this study was to develop a model for the dynamics of the interacting solid and liquid aerosols which could be used to understand the impact of such aerosols on stratospheric ozone chemistry.

3. Why Al2O3? Al2O3was chosen as a first exercise for the model, though model could be readily adapted to other solids. One pragmatic reason to look at Al2O3 is that there is pre-existing literature about it’s impact on atmospheric chemistry due to its atmospheric deposition from solid rocket motors such as the Space Shuttle. Benefits: • Efficient RF per MT emission • Less diffuse radiation than sulfate geoengineering • Less tropopause heating than sulfate geoengineering Risks: • Toxicity: Al2O3 common on Earth’s crust, but nanoparticles may present particular risks that needs to be assessed • Ozone loss: Similar to sulfate geoengineering, but more unknowns • Impact on upper tropospheric cloudiness • Unexpected climate impacts: No natural analogs

4. Modeling Solid Metal Oxide Aerosols • Al2O3 particles form sparse fractal structures on coagulation • AER 2-D Sulfate Aerosol Model modified to include Al2O3 monomer and fractal particles, and their interaction with background atmospheric sulfate. • 18 size bins tracked for pure Al2O3 particles,sulfate-coated Al2O3 particles, and H2SO4 on coated Al2O3particle, 40 bins for sulfate particles. Size bins are sectional by volume doubling. • Details of fractal modeling presented in Poster Session • Geoengineering emissions between 30°S-30°N, 20-25 km continuous in time until steady-state achieved • Inject monodispersedspherical Al2O3 solid particles with R=80 nm, 160 nm, 240 nm, 320 nm • Emission rates of 1, 2, 4, 8 MT/yr • Parametric calculations to bracket two uncertainties: • Fractals remain sparse structures -OR- become more compact on wetting/aging  affects particle lifetime, burden • Fractals are easily coated with sulfate (hydrophilic) –OR- remain dry (hydrophobic)  affects ozone loss Electron microscope image of Al2O3 fractal aggregate. [From Karasev et al., 2001]

5. Sedimentation Velocities of Alumina Monomers and Fractals Coated and Compacted Fractals Sparse Fractals • Al2O3 monomers fall faster than sulfate particles of the same diameter due to 2x greater density • Sparse fractals fall slower than the monomers they are composed of • Compact fractals fall faster than the monomers they are composed of • Compared to average tropical upwelling rate, Al2O3 particles ≥ 160 nm have significant sedimentation in lower stratosphere.

6. Alumina Mixing Ratio (ppbm) R0=240 nm R0=80 nm Alumina Particle Concentration (#/cm3) R0=80 nm R0=240 nm

7. Fractal Distribution of Al2O3ParticlesGlobally and Seasonally Averaged 1 MT/yr R0=160 nm 1 MT/yr R0=240 nm 1 MT/yr R0=80 nm 4 MT/yr R0=80 nm 4 MT/yr R0=240 nm 4 MT/yr R0=160 nm

8. Stratospheric Burden of Alumina vsEmission Rate and Monomer Radius • Al2O3 burdens quite linear with emission. Sulfate burden not linear. • Compaction of coated fractals on aging has large impact on lifetime and burden.

9. Scattering physics: Comparing sulfate and high refractive index solids • Al2O3 (blue) • H2SO4+H2O (red) • Model: • Mie code plus integration over spectra, scattering angle, and latitude using an efficient impulse function method. • Uncertainties and omissions: Index of refraction data uncertain, monomer and fractal shapes uncertain, no multiple scattering. • Summary: • Al2O3 particles about 3 × more scattering per unit volume, but density is about 3 × higher, so scattering per unit mass about the same. • Scattering from Al2O3 particles is more sharply dependent on particle size. • Alumina has about twice the upscatter cross section of sulfate. • Alumina has about half the forward scattering as does sulfate

10. Calculated Change in Radiative Forcing vs Emission Rate and Monomer Radius • Simplified radiative model: • Use an impulse function to calculate annual, global, spectrally-averaged upscatter fraction per MT per aerosol size bin from Mie theory. • Assume thin sulfate coating doesn’t affect alumina optical properties, monomers perfect spheres. • Fractals scatter inefficiently: 80 nm emission has low RF. • No advantage to R0=320 nm: lower burden, same RF per MT as R0=240nm. • Compaction of coated particles has little impact on RF. • Al2O3 emission at R0=240 nm better RF/(MT/yr) than SO2 emission, comparable RF to H2SO4 emission.

11. Stratospheric Burden of Sulfate on Coated Alumina • Most of stratospheric sulfate on Al2O3 particles for R0 ≤ 160 nm. • Greater emissions leads to thinner coating on more Al2O3 particles. • Total sulfate increases for R0=80 nm, decreases for R0 ≥ 240 nm.

12. Alumina and Sulfate Surface Area Density: Mediate ozone loss Heterogeneous reactions on sulfate surfaces: N2O5 + H2O  2 HNO3 ClONO2 + HCl Cl2 + HNO3 HOCl+ HCl Cl2 + H2O etc Heterogeneous reaction on Al2O3 surfaces: ClONO2 + HCl Cl2 + HNO3

13. Global Column Ozone Change • Large ozone depletion for R0 = 80 nm. • Ozone depletion comparable to SO2, H2SO4 emission for R0 ≥ 160 nm. • Hydrophobic Al2O3 gives less depletion  large impact of enhanced sulfate surface area density when hydrophilic, especially for R0=80 nm.

14. Conclusions of this Study • Atmospheric burden sensitivity to injected monomer size andlinear with emission rate • Radiative Forcing sensitivity to particle size and burden in fractals vs monomers • Radiative forcing optimal for injected monomers of240 nm radius • Ozone impact from both alumina surface and enhanced sulfate surface Further Key Work Needed • Toxicity of submicron alumina particles • Long wave absorption and tropopause heating, RF refinement • Heterogeneous chemistry on Al2O3 surfaces – Catalytic or surface poisoning? Unknown reactions? • Emission method, near-field microphysics, particles engineered to avoid sulfate coating or neutralize stratospheric sulfate • Effects of solid particles on upper tropospheric cloud properties