Frank Keutsch — Heterogeneous Chemistry and Ageing of Designer Aerosol Particles to Assess the Risk of Solar Geoengineering

This project proposes laboratory investigations of alternate materials with properties that may make them more suitable candidates for stratospheric SRM. Specifically, the project proposes studies that address direct physical risks associated with SRM, i.e., effects on stratospheric ozone and temperature, and consequences when particles settle through the lower stratosphere into the troposphere, i.e. impacts on tropospheric chemistry, composition and radiation. This only represents one vignette of the risks associated with stratospheric SRM, but it is critical to quantify these risks as soon as possible to help evaluate the efficacy and risks of different stratospheric SRM strategies. These studies can rule out materials found to have clearly unsuitable properties, but field tests are required for materials that “pass” the laboratory stage, to quantify the risks in the actual environment.
Frank Keutsch

Frank Keutsch

Stonington Professor of Engineering and Atmospheric Science
Professor of Chemistry and Chemical Biology


Dai, Zhen, Debra K. Weisenstein, Frank N. Keutsch, and David W. Keith. “Experimental reaction rates constrain estimates of ozone response to calcium carbonate geoengineering.” Communications Earth & Environment 1, no. 63 (2020). Publisher's VersionAbstract
Stratospheric solar geoengineering (SG) would impact ozone by heterogeneous chemistry. Evaluating these risks and methods to reduce them will require both laboratory and modeling work. Prior model-only work showed that CaCO3 particles would reduce, or even reverse ozone depletion. We reduce uncertainties in ozone response to CaCO3 via experimental determination of uptake coefficients and model evaluation. Specifically, we measure uptake coefficients of HCl and HNO3 on CaCO3 as well as HNO3 and ClONO2 on CaCl2 at stratospheric temperatures using a flow tube setup and a flask experiment that determines cumulative long-term uptake of HCl on CaCO3. We find that particle ageing causes significant decreases in uptake coefficients on CaCO3. We model ozone response incorporating the experimental uptake coefficients in the AER-2D model. With our new empirical reaction model, the global mean ozone column is reduced by up to 3%, whereas the previous work predicted up to 27% increase for the same SG scenario. This result is robust under our experimental uncertainty and many other assumptions. We outline systematic uncertainties that remain and provide three examples of experiments that might further reduce uncertainties of CaCO3 SG. Finally, we highlight the importance of the link between experiments and models in studies of SG.