The heterogeneous reaction of HNO3 and HCl with CaCO3 in the context of stratospheric aerosol injection and its impact on stratospheric ozone

Lack of action in climate change mitigation is driving research on solar radiation modification via stratospheric aerosol injection (SAI), i.e., the injection of aerosols or their precursors into the stratosphere to increase Earth’s albedo, inducing global cooling. The idea evolved from observations of the cooling effect of large volcanic eruptions, which emitted SO₂ into the stratosphere. Therefore, SAI research mainly focused on sulfur dioxide (SO₂) injection, the main precursor of H₂SO₄ aerosols. However, SO₂ injection could lead to adverse side effects such as stratospheric ozone depletion, stratospheric heating, and sizable effects on the large-scale atmospheric circulation. Recent studies suggested that injection of solid particles such as calcite (CaCO₃), alumina (Al₂O₃) and diamond (C) instead of SO₂ could reduce some of these adverse side effects. However, the expected improvements are subject to large uncertainties. Heterogeneous chemistry on solid aerosols in the stratosphere can increase ozone depletion by moving passive chlorine reservoir species such as HCl or ClONO₂ into their active, ozone depleting form (e.g., ClO). Furthermore, alkaline materials such as CaCO₃ are subject to acid-base reactions resulting in an uptake of acidic gases which could impact stratospheric ozone. We constrain some of these uncertainties by experimental work on heterogeneous chemistry of CaCO₃ in presence of gaseous HCl, HNO₃, and H₂SO₄ under near-stratospheric conditions. Single crystalline CaCO₃ {001} and {104} faces were exposed to controlled gas mixtures closely above either a binary HNO₃/H₂SO₄ or HCl/H₂SO₄ solution, or a ternary HNO₃/HCl/H₂SO₄ solution for several days. Fixed temperature ranging between -20°C and -60°C were investigated, reaching lower temperature conditions than in previous experiments. Various Relative Humidities (RH) were as well probed. Elastic Recoil Detection Analysis (ERDA), an ion beam analysis technique to obtain elemental concentration depth profile of up to 300 nm, was used to observe surface reaction and diffusion in the material. Uptake coefficients were calculated from these observations. This work presents a path forward for climate intervention research and more specifically for more reliably assessing the impact of SAI of solid particles on stratospheric ozone.