Stratospheric solar geoengineering without ozone loss David W. Keitha,b,1, Debra K. Weisensteina, John A. Dykemaa, and Frank N. Keutscha,c aJohn A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138; bJohn F. Kennedy School of Government, Harvard University, Cambridge, MA 02138; and cDepartment of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138 Edited by John H. Seinfeld, California Institute of Technology, Pasadena, CA, and approved November 3, 2016 (received for review September 20, 2016) Injecting sulfate aerosol into the stratosphere, the most frequently HNO3, shifting halogens from reservoir species to reactive com- analyzed proposal for solar geoengineering, may reduce some pounds and altering the NOx budget via hydrolysis of N2O5 to climate risks, but it would also entail new risks, including ozone HNO3. Previous studies found that injection of sufficient SO2 or − loss and heating of the lower tropical stratosphere, which, in turn, particulate sulfate to produce −2W·m 2 of radiative forcing—a would increase water vapor concentration causing additional useful benchmark for SRM—reduced average column ozone by 1 ozone loss and surface warming. We propose a method for strato- to 13% (2, 6–8). spheric aerosol climate modification that uses a solid aerosol com- The use of solid, high refractive index, aerosol particles for posed of alkaline metal salts that will convert hydrogen halides SRM was first suggested (9) in the 1990s. That work and most and nitric and sulfuric acids into stable salts to enable stratospheric subsequent analyses focused on the mass-specific scattering ef- geoengineering while reducing or reversing ozone depletion. Rather ficiency, with the implication that solid aerosol might be able to than minimizing reactive effects by reducing surface area using high reduce the total mass required for SRM (9–12). In prior work refractive index materials, this method tailors the chemical reactivity. (2), we explored the possibility that solid aerosol might reduce Specifically, we calculate that injection of calcite (CaCO3)aerosol important environmental risks of SRM including (i) heating of particles might reduce net radiative forcing while simultaneously the lower stratosphere, (ii) diffuse scattering of incident sun- increasing column ozone toward its preanthropogenic baseline. light, and (iii) ozone loss. Using a model that included the − A radiative forcing of −1W·m 2, for example, might be achieved microphysical interactions of solid aerosol with natural back- − with a simultaneous 3.8% increase in column ozone using 2.1 Tg·y 1 ground sulfate aerosol, we found that, although some ozone of 275-nm radius calcite aerosol. Moreover, the radiative heating of impacts came from reactions such as HCl + ClONO2 on hy- the lower stratosphere would be roughly 10-fold less than if that drophilic oxide surfaces, the dominant impact was from an same radiative forcing had been produced using sulfate aerosol. increase in sulfuric acid surface as the preexisting background Although solar geoengineering cannot substitute for emissions cuts, sulfuric acid was distributed over a large surface area of solid it may supplement them by reducing some of the risks of climate particles (2). change. Further research on this and similar methods could lead to Here we consider the possibility of using alkaline (basic) salts reductions in risks and improved efficacy of solar geoengineering of group 1 and 2 metals, such as Na and Ca, to neutralize the methods. acids involved in the catalytic destruction of ozone. These metals might be introduced to the stratosphere in metallic form, as climate change | geoengineering | stratospheric ozone | oxides, or as salts formed with weak acids such as carbonic acid. climate engineering | atmospheric chemistry Gas-phase acids will then react to form neutral, solid salts such as NaCl, Ca(NO3)2,andNa2SO4 that are stable in the stratosphere. eliberate introduction of aerosol into the stratosphere, a Dform of solar geoengineering or Solar Radiation Manage- Significance ment (SRM), may reduce impacts of climate change, including regional changes in precipitation and surface temperature (1, 2), The combination of emissions cuts and solar geoengineering by partially and temporarily (3) offsetting radiative forcing from could reduce climate risks in ways that cannot be achieved by greenhouse gases. The most salient direct risk of sulfate aerosol, emissions cuts alone: It could keep Earth under the 1.5-degree the material most commonly proposed for SRM, is stratospheric mark agreed at Paris, and it might stop sea level rise this century. ozone loss (4, 5). In a 2006 paper (5) that triggered recent in- However, this promise comes with many risks. Injection of sul- terest in SRM, Crutzen suggested that soot particles could warm furic acid into the stratosphere, for example, would damage the the polar stratosphere and “thereby hinder the formation of ozone layer. Injection of calcite (or limestone) particles rather ozone holes.” Crutzen’s approach rests on manipulating radia- than sulfuric acid could counter ozone loss by neutralizing acids tive properties to indirectly influence chemistry, whereas here resulting from anthropogenic emissions, acids that contribute to we propose that use of solid aerosol composed of alkaline the chemical cycles that destroy stratospheric ozone. Calcite aero- metal salts that react with hydrogen halides and nitric and sol geoengineering may cool the planet while simultaneously re- sulfuric acid to form stable salts may enable stratospheric pairing the ozone layer. SRM while allowing partially independent manipulation of the catalytic reactions that control stratospheric ozone. In Author contributions: D.W.K. conceived the idea; D.W.K., D.K.W., J.A.D., and F.N.K. de- contrast to Crutzen’s approach, this provides a direct chem- signed research; F.N.K. developed our treatment of kinetics and interaction between acids; D.W.K., D.K.W., J.A.D., and F.N.K. performed research; D.K.W. performed the chem- ical method that may restore ozone concentrations that are ical-transport modeling experiments; J.A.D. performed the radiative transfer calculations reduced by anthropogenic NOx and halogens while simulta- and contributed analysis tools; D.K.W., J.A.D., and F.N.K. analyzed data; and D.W.K., D.K.W., neously providing SRM radiative forcing with minimal J.A.D., and F.N.K. wrote the paper. stratospheric heating. The authors declare no conflict of interest. Three acids, HNO3, HCl, and HBr, serve as reservoirs for the This article is a PNAS Direct Submission. nitrogen, NOx, chlorine, ClOx, and bromine, BrOx, radical families Freely available online through the PNAS open access option. that participate in coupled catalytic cycles that destroy ozone. 1To whom correspondence should be addressed. Email: [email protected]. Sulfuric acid, H2SO4, can accelerate ozone loss by forming aqueous This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. aerosol that catalyzes reactions such as HCl + ClONO2 → Cl2 + 1073/pnas.1615572113/-/DCSupplemental. 14910–14914 | PNAS | December 27, 2016 | vol. 113 | no. 52 www.pnas.org/cgi/doi/10.1073/pnas.1615572113 Downloaded by guest on September 28, 2021 − Fig. 1. Particle aggregation, spatial distribution and chemistry. All plots represent annual average conditions resulting from a 5.6-Tg·y 1 steady-state in- jection of calcite. (Left) The fraction of solid particle mass per sectional bin vs. number of monomers in the fractal aggregate. (Middle) Particle number density −3 (cm ) as a function of latitude and altitude. (Right) Composition of solid particles resulting from reaction with acids showing total (black line) and CaCO3, CaCl2, Ca(NO3)2, and CaSO4 mixing ratios (parts per billion by volume) averaged from 60°S to 60°N. The surfaces of these salts have low rates for acid-catalyzed reac- formation of Ca(NO3)2 being favored over CaCl2 due to the high tions, as they are neutral, and do not contribute to bulk hydrolysis vapor pressure of HCl. At low calcite loadings, the coagulation reactions, as they are solid. process with sulfuric acid aerosol will therefore reduce the ef- Particles composed of, or coated with, alkaline compounds fectiveness of removing gas-phase HNO3 and HCl. However, the might reduce ozone loss through reaction with the background resulting solid, nonacidic CaSO4 surfaces have much lower cat- sulfate aerosol or reaction with gas-phase acids. The rate of the alytic activity than liquid sulfuric acid aerosol for acid-catalyzed first mechanism depends on the flux of H2SO4 onto particles by and liquid-phase reactions. coalescence or condensation. The liquid−solid reaction will Fig. 1 shows the extent of particle aggregation along with the rapidly neutralize the acid to form a dry salt if there is sufficient spatial distribution and composition of solid particles for a cal- − base. The rate of the second mechanism will depend on the ki- cite injection rate of 5.6 Tg·y 1, the maximum injection rate we − netics of the gas−solid reactions. studied, which was chosen to produce approximately −2W·m 2 As a specific example, we model the use of calcite (CaCO3) of global average radiative forcing. The concentration of solid − aerosol for SRM using an extension of the model we developed particles in the lower stratosphere ranges from about 4 cm 3 to − for solid aerosols such as alumina and diamond (2). We simulate 8cm 3, and only about a third of the solid aerosol coalesces into a monodisperse 275-nm-radius calcite aerosol injected uniformly aggregates. The dominant salt formed is Ca(NO3)2, consistent between 20 km and 25 km altitude within 30 degrees of the with the greater stratospheric abundance of HNO3 relative to equator. We use a 2D chemical transport and aerosol micro- HCl, HBr, and H2SO4. Fig. 2 shows the resulting changes in the physics model that includes a prognostic size distribution for individual catalytic cycles and the ozone distribution.