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Before We Inject: Assessing the Impact of Silica-Based Aerosols on Stratospheric Chemistry via a Kinetic Model Informed by Molecular Dynamics

Dennis Lima, Saif Al-Kuwari, Ivan Gladich

TL;DR

This work investigates whether silica-based aerosols used in stratospheric aerosol injection could alter stratospheric chemistry, particularly ozone depletion, by examining the HCl + ClONO2 reaction on a dry, hydroxylated α-quartz silica surface using first-principles molecular dynamics. The study reveals a barrierless forward pathway to form Cl2 and HNO3 at the interface, but a MD-informed Langmuir–Hinshelwood kinetic model yields a very small reactive uptake coefficient for ClONO2, γ_{ClONO2} ≈ 1.0 × 10^{-11}, suggesting limited ozone impact. The model additionally shows that adsorption energies strongly control the surface chemistry and that two water molecules can erase barriers in related pathways, underscoring the sensitivity of silica surfaces to environmental conditions. Overall, the findings argue for cautious interpretation and underscore the need for experimental validation and improved frameworks to translate MD results into large-scale atmospheric predictions before considering silica-based SAI deployment.

Abstract

Stratospheric aerosol injection (SAI) has been proposed as a geoengineering strategy to mitigate global warming by increasing Earth's albedo. Silica-based materials, such as diamond-doped silica aerogels, have shown promising optical properties, but their impact on stratospheric chemistry, ozone one in particular, remains largely unknown. Here, we present first-principles molecular dynamics (MD) simulations of the heterogeneous reaction between hydrogen chloride ($\mathrm{HCl}$) and chlorine nitrate ($\mathrm{ClONO_2}$), two main reservoirs of stratospheric chlorine and nitrogen species, on a dry, hydroxylated $α$-quartz silica interface. Our results reveal a barrierless reaction pathway toward the formation of chlorine gas ($\mathrm{Cl}_2$), a major contributor to stratospheric ozone loss. We design a heterogeneous kinetic model informed by our MD simulation and available experimental data: despite the barrierless formation of $\mathrm{Cl_2}$, the higher surface affinities and partial pressures of $\mathrm{HNO_3}$ and $\mathrm{HCl}$ compared to those of $\mathrm{ClONO_2}$ result in a negligible reaction probability, $γ_\mathrm{ClONO_2}$, upon chlorine nitrate collision with the silica surface. Since $γ_\mathrm{ClONO_2}$ enters as a proportionality constant in the definition of the heterogeneous reaction rate, our kinetic model indicates that the injection of silica-based aerosols may have only a limited impact on stratospheric ozone depletion driven by $\mathrm{HCl}$ and $\mathrm{ClONO_2}$ chemistry. At the same time, our findings also underscore the scarcity of experimental data, the need of better theoretical frameworks for the inclusion of MD results into kinetic models, and the urgency for further experimental validations of silica-based SAI technologies before their deployment in climate intervention strategies.

Before We Inject: Assessing the Impact of Silica-Based Aerosols on Stratospheric Chemistry via a Kinetic Model Informed by Molecular Dynamics

TL;DR

This work investigates whether silica-based aerosols used in stratospheric aerosol injection could alter stratospheric chemistry, particularly ozone depletion, by examining the HCl + ClONO2 reaction on a dry, hydroxylated α-quartz silica surface using first-principles molecular dynamics. The study reveals a barrierless forward pathway to form Cl2 and HNO3 at the interface, but a MD-informed Langmuir–Hinshelwood kinetic model yields a very small reactive uptake coefficient for ClONO2, γ_{ClONO2} ≈ 1.0 × 10^{-11}, suggesting limited ozone impact. The model additionally shows that adsorption energies strongly control the surface chemistry and that two water molecules can erase barriers in related pathways, underscoring the sensitivity of silica surfaces to environmental conditions. Overall, the findings argue for cautious interpretation and underscore the need for experimental validation and improved frameworks to translate MD results into large-scale atmospheric predictions before considering silica-based SAI deployment.

Abstract

Stratospheric aerosol injection (SAI) has been proposed as a geoengineering strategy to mitigate global warming by increasing Earth's albedo. Silica-based materials, such as diamond-doped silica aerogels, have shown promising optical properties, but their impact on stratospheric chemistry, ozone one in particular, remains largely unknown. Here, we present first-principles molecular dynamics (MD) simulations of the heterogeneous reaction between hydrogen chloride () and chlorine nitrate (), two main reservoirs of stratospheric chlorine and nitrogen species, on a dry, hydroxylated -quartz silica interface. Our results reveal a barrierless reaction pathway toward the formation of chlorine gas (), a major contributor to stratospheric ozone loss. We design a heterogeneous kinetic model informed by our MD simulation and available experimental data: despite the barrierless formation of , the higher surface affinities and partial pressures of and compared to those of result in a negligible reaction probability, , upon chlorine nitrate collision with the silica surface. Since enters as a proportionality constant in the definition of the heterogeneous reaction rate, our kinetic model indicates that the injection of silica-based aerosols may have only a limited impact on stratospheric ozone depletion driven by and chemistry. At the same time, our findings also underscore the scarcity of experimental data, the need of better theoretical frameworks for the inclusion of MD results into kinetic models, and the urgency for further experimental validations of silica-based SAI technologies before their deployment in climate intervention strategies.
Paper Structure (7 sections, 2 equations, 4 figures, 1 table)

This paper contains 7 sections, 2 equations, 4 figures, 1 table.

Table of Contents

  1. Introduction
  2. Methodology
  3. Results and Discussion
  4. Adsorption
  5. Reactivity
  6. Reactive Uptake Model on Silica
  7. Conclusions

Figures (4)

  • Figure 1: The HCl-ClONO2 (pre)reactive structure at the hydroxylated silica surface, stabilized by hydrogen bonds (dashed red and cyan lines), observed during the FPMD simulation. A plausible mechanism for the production of nitric acid and Cl2 via proton transfer is indicated by black arrows. Atom color code: Si (yellow), Cl (cyan), O (red), H (white), N (blue).
  • Figure 2: Panel (a): snapshot from the FPMD showing HCl adsorbed at the surface forming two hydrogen bonds with interfacial -Si-OH silanols. Panel (b-d), the relative cumulative (over time) hydrogen bond (HB) populations, $D(t)$, for each HB channels, i.e., 0HB (blue line), 1HB (green), and 2HB (orange). Panel (b-d) shows $D(t)$ for HCl, ClONO2, and HNO3, respectivily. Atomic color code: Si (yellow), Cl (cyan), O (red), H(white), N(blue).
  • Figure 3: Panel (a): energy reaction profiles for Reaction \ref{['r1']} from reactants (ClONO2 and HCl, replica 0) to products (HNO3 and Cl2, replica 19) on silica and at different level of density functional theory. Panel (b), energy reaction profiles for the reaction in the gas phase and with 2 water molecules (blue dotted line). Red dashed line is $\Delta E$ for the reaction in the gas phase at MP2/aug-cc-pVTZ+ZPE level.
  • Figure 4: Reaction steps: (a) pre-reactive complex (before the barrier), (b) transition state (at the barrier), (c) post-reactive (after the barrier) from NEB calculations.