Uptake of Stratospheric Species on Minerals Proposed for Stratospheric Aerosol Injection

Solid mineral-based particles have been proposed as alternatives to sulfates for climate intervention by stratospheric aerosol injection, as a possible means for improving optical or chemical characteristics to thereby minimize risks and uncertainties. However, the heterogeneous reactivity of solid particles toward stratospheric trace gases, and possible implications to the ozone layer, is currently not fully constrained, particularly at stratospheric concentrations. Here we present a systematic comparative study of the uptake of nitric acid (HNO₃), hydrogen chloride (HCl), and nitrogen dioxide (NO₂) on four mineral surrogates, calcite, alumina, crystalline silica (quartz), and amorphous silica, using complementary Knudsen cell and flow-through reactor techniques. We find that NO₂ uptake coefficients are relatively small on all surfaces, with estimated heterogeneous removal time scales indicating negligible direct impact on stratospheric nitrogen chemistry. Conversely, measuring HCl uptake over a concentration range spanning five orders of magnitude, we find substantial steady-state uptake rates with a pronounced concentration dependence. Extracting adsorption isotherms, we find that the steady-state surface coverage of HCl at low concentrations differs by four orders of magnitude between the surfaces, with calcite exhibiting the highest HCl uptake capacity and amorphous silica the least, suggesting a dominant role of surface acid–base character. Using HCl surface coverage as a proxy for the reactive uptake coefficient of ClONO₂, we estimate that amorphous silica could produce substantially lower ozone depletion due to chlorine activation than calcite or alumina under equivalent injection scenarios. We also find a marked difference in uptake between the crystalline and amorphous forms of silica, underscoring the sensitivity of heterogeneous chemistry to surface microstructure and the importance of selecting particles with low-reactivity surfaces, in addition to the consideration of bulk characteristics. Our findings motivate the development of particles with surfaces tailored for minimizing SAI risks and uncertainties, including minimal reactivity with stratospheric gases and background sulfate aerosols.