First-Principles Physicochemical Characterization of Aerosol Candidates for SAI

Stratospheric aerosol injection (SAI) involves the stratospheric injection of sunlight-reflecting aerosols or aerosol precursor gases that ideally possess the following material properties: (i) they do not absorb radiation in the shortwave solar spectra (0.2–2 μm) in either fresh or aged (atmospherically processed) states, and (ii) they are chemically unreactive and do not destroy stratospheric ozone. Calcite (CaCO3) and Al2O3 are deemed the most promising SAI candidates because they are comparatively more benign, would not cause significant stratospheric heating and may counteract stratospheric ozone loss. However, there is considerable uncertainty in modeling shortwave radiative forcing efficiencies of SAI scenarios involving CaCO3 and Al2O3. This uncertainty is due to a lack of data regarding their aerosol optical properties — i.e., spectrally resolved complex refractive index, mass scattering and absorption cross-sections, single scattering albedo, absorption Ångström exponent and asymmetry parameter — that serve as key input parameters to radiative transfer models. There is also a lack of atomic-scale understanding of the evolution of optical properties with point defects arising during synthesis and atmospheric processing, and interfaces arising from aerosol agglomeration. From a stratospheric ozone chemistry standpoint, there is significant uncertainty regarding the heterogeneous uptake of stratospheric trace gases on the surfaces of CaCO3 and Al2O3.

This proposed three-year study has two objectives. The first objective is to investigate the spectral optical properties of CaCO3 and Al2O3 in non-agglomerated (single crystal) and agglomerated morphologies using first principles probing techniques from the atomic to particle scales. The second objective is to quantify the impacts of CaCO3 and Al2O3 injection on stratospheric ozone by quantifying the heterogeneous uptake of reservoir species (HCl, HNO3, N2O5 and ClONO2). A major deliverable of this project will be a database containing the optical and chemical properties of CaCO3 and Al2O3. For each aerosol type, the extensive and intensive properties in the wavelength range 300–2500 nm will be tabulated as a function of size, composition, wavelength and atmospheric processing. The reactive uptake coefficients of reservoir species will be tabulated as a function of aerosol composition, temperature and aging processes. These parameters can be readily implemented in climate models to accurately assess the environmental risks of SAI scenarios.

Rajan Chakrabarty is currently the Harold D. Jolley Career Development associate professor of energy, environmental and chemical engineering at Washington University in St. Louis. He leads the Aerosol Interdisciplinary Research group, which works at the forefront of addressing grand challenges associated with aerosol radiative forcing, aerosol instrumentation development and engineering techniques, and rapid detection of respiratory pathogens. Chakrabarty received his Ph.D. in chemical physics from the University of Nevada, Reno with dissertation research conducted at the Desert Research Institute. He also holds degrees in atmospheric physics (M.S.) and electronics and instrumentation engineering (B.Eng.). His research contributions have been recognized with several prestigious honors, most notably a 2021 NASA Group Achievement Award for the FIREX-AQ study, the 2019 Kenneth T. Whitby Award from the American Association for Aerosol Research (AAAR), the 2019 Schmauss Award from Gesellschaft für Aerosolforschung e.V. (GAeF), and the 2018 American Geophysical Union Global Environmental Change Early Career Award.

Rohan Mishra is an associate professor of mechanical engineering and materials science, and physics (by courtesy) at Washington University in St. Louis. He obtained his Ph.D. in materials science and engineering from the Ohio State University in 2012. From 2012–2015, he was a postdoctoral researcher in the Scanning Transmission Electron Microscopy group at Oak Ridge National Laboratory with a joint affiliation from the Department of Physics at Vanderbilt University. His research interests are to develop quantitative structure-property correlations in materials starting from the atomic scale using a synergistic combination of electronic structure calculations and aberration-corrected scanning transmission electron microscopy. Mishra has published over 90 journal articles and has given over 30 invited presentations.

Lu Xu is currently an assistant professor in the Department of Energy, Environmental and Chemical Engineering at Washington University in St. Louis. Xu received his Ph.D. in chemical engineering from the Georgia Institute of Technology in 2016. He completed postdoctoral research at the California Institute of Technology and then worked as a research scientist in the Chemical Science Laboratory for the National Oceanic and Atmospheric Administration. Xu’s research interests are in air quality and climate change. He has investigated the atmospheric chemistry of organic compounds from diverse sources (e.g., wildfires, vegetation, power plants) and the subsequent impacts on air quality, human health and climate. He also developed analytical methods based on mass spectrometry to characterize the complex atmospheric composition. Xu’s research involves laboratory studies, field measurements and instrument development.