Chemical Impact of Stratospheric Alumina Particle Injection for Solar Radiation Modification and Related Uncertainties.

Compared to stratospheric SO2 injection for climate intervention, alumina particle injection could reduce stratospheric warming and associated adverse impacts. However, heterogeneous chemistry on alumina particles, especially chlorine activation via ClONO2+HCl→surfCl2+HNO3 ${\text{ClONO}}_{2}+\text{HCl}\stackrel{\text{surf}}{\to }{\text{Cl}}_{2}+{\text{HNO}}_{3}$, is poorly constrained under stratospheric conditions, such as low temperature and humidity. This study quantifies the uncertainty in modeling the ozone response to alumina injection. We show that extrapolating the limited experimental data for ClONO2 + HCl to stratospheric conditions leads to uncertainties in heterogeneous reaction rates of almost two orders of magnitude. Implementation of injection of 5 Mt/yr of particles with 240 nm radius in an aerosol‐chemistry‐climate model shows that resulting global total ozone depletions range between negligible and as large as 9%, that is more than twice the loss caused by chlorofluorocarbons, depending on assumptions on the degree of dissociation and interaction of the acids HCl, HNO3, and H2SO4 on the alumina surface. Plain Language Summary: Global warming caused by increasing greenhouse gases could be temporarily reduced by introducing aerosol particles into the stratosphere. The most frequently studied approach to climate intervention uses H2SO4‐H2O aerosols, which, however, could result in undesirably strong warming of the stratosphere and significant ozone depletion. This might be improved by injecting solid particles, for example, made of aluminum oxide. However, here we show that the extremely limited availability of experimental studies on heterogeneous chemistry on alumina under the influence of stratospheric concentrations of HCl, HNO3, H2SO4, and H2O leads to large uncertainties in the impact of alumina injection on stratospheric ozone. In order to quantify these uncertainties, we integrated the currently available knowledge about the most important heterogeneous reaction ClONO2+HCl→surfCl2+HNO3 ${\text{ClONO}}_{2}+\text{HCl}\stackrel{\text{surf}}{\to }{\text{Cl}}_{2}+{\text{HNO}}_{3}$ into an aerosol‐chemistry‐climate model. We conclude that the uncertainty in the resulting heterogeneous reaction rate is more than two orders of magnitude depending on the partitioning of HCl, H2SO4, and HNO3 on the alumina surface. This could lead to global ozone column depletion ranging between almost negligible and up to 9%, which would be more than twice as much as the ozone loss caused by chlorofluorocarbons in the late 1990s. Key Points: Heterogeneous chemistry on solid alumina particles is highly uncertain and depends strongly on the partitioning of acids onto the surfaceThe reaction rate of ClONO2 with HCl on alumina particles is uncertain by up to two orders of magnitude under stratospheric conditionsInjection of 5 Mt/yr of alumina particles could double global ozone reductions compared to chlorofluorocarbons in the late 1990s [ABSTRACT FROM AUTHOR]