A Comprehensive Sulfur Chemistry Network with Excited S(1D) and SO(1Δ) for XODIAC Photochemical Model: Validation on Venus and Implications for Exo-Venus Atmospheres

TL;DR

This work addresses the incomplete sulfur chemistry in Venus and Venus-like exoplanet atmospheres by computing kinetic parameters for S($^3$P) and S($^1$D) reacting with CO$_2$, and by deriving NASA $7$-term polynomial coefficients for ground- and excited-state S and SO. Using high-level electronic-structure calculations, master-equation kinetics (MESS), and Arkane-fitted thermochemistry, the authors construct an extended XODIAC-2025.v2 network that includes S($^1$D) and SO($^1\Delta$) chemistry and updated photochemical channels, then apply a 1D photochemical model to Venus and three exo-Venus analogs. The Venus results largely agree with observations for major sulfur species, but S$_3$ and S$_4$ remain underpredicted unless a near-surface atomic sulfur source is assumed, highlighting a missing deep-atmosphere sulfur flux or unmodeled chemistry. In exo-Venus scenarios, strong irradiation and isothermal layers markedly reshape vertical sulfur profiles above ~30 km, underscoring the importance of excited-state kinetics in predicting sulfur signatures for Venus-like exoplanets. Overall, the study demonstrates the necessity of incorporating excited-state sulfur processes and updated kinetic data in planetary atmosphere models to accurately interpret sulfur-bearing species in Venus and exo-Venus atmospheres.

Abstract

Sulfur chemistry is fundamental to understanding the structure, cloud formation, and atmospheric composition of Venus and Venus-like exoplanets. However, many key reactions involving ground- and excited-state sulfur species remain poorly characterized, and current photochemical models rely on networks that lack accurate kinetic data under high-temperature, high-CO2 conditions. We compute kinetic parameters for reactions of ground-state S(3P) and excited-state S(1D) with CO2 under Venus-like conditions. These reactions form SO(3Sigma), SO(1Delta), and CO. The potential energy surfaces reveal intermediates, and temperature-dependent rate coefficients are obtained using a master-equation approach based on the chemically significant eigenvalue method. NASA 7-term polynomial coefficients are also derived for ground- and excited-state S and SO for consistent use in photochemical models. Incorporating these data into the one-dimensional photochemical model XODIAC shows that these reactions exert only a minor influence above 60 km in the Venus atmosphere due to competing pathways. The model agrees with observations for most sulfur-bearing species except S3 and S4. Introducing a 1 ppm near-surface atomic sulfur mixing ratio, representing a possible deep-atmosphere or surface source or accounting for missing sulfur processes, increases S3 and S4 by 1-2 orders of magnitude and improves agreement with measurements. For exo-Venus analogs with stratospheric isotherms and strong stellar irradiation, the new reactions significantly modify the vertical profiles of major sulfur species above 30 km and enhance S(1D) and SO(1Delta) more strongly than in scenarios with only isotherms or only irradiation. These results underscore the importance of accurate excited-state sulfur kinetics and the need for updated reaction networks when modeling Venus and exo-Venus atmospheres.

Figures (13)

• Figure 1: Reaction pathways for the S + CO2 system. The relative potential energy profile (in kJ mol$^{-1}$) was computed at the CCSD(T)/aug-cc-pVTZ level of theory. Path 1 corresponds to the ground-state triplet surface, while Paths 2(a) and 2(b) correspond to the excited-state singlet surface. Key stationary points are labeled with their relative energies in parentheses. Optimized geometries at the M06-2X/aug-cc-pVTZ level are shown with selected bond lengths (Å) and bond angles (°).
• Figure 2: Calculated temperature dependence of the rate coefficients over 150 K-2000 K. denotes the forward reaction, while depicts the corresponding backward reaction.
• Figure 3: Comparison of Gibbs free energy (kcal/mol) as a function of temperature using NASA polynomial coefficients. Solid lines represent values from the Burcat thermochemical database (ground state), while dotted lines show ground-state values computed with Gaussian and Arkane, as listed in Table \ref{['tab:nasa7_table']}. The Gibbs free energies for the corresponding excited states, S($^1$D) and $\mathrm{SO}(^1\Delta)$, computed in this work, are shown with star-shaped markers.
• Figure 4: Predicted volume mixing ratios of S$_n$ ($\mathrm{n=1\text{--}8}$), SO, SO$_2$, H$_2$S, H$_2$SO$_4$, OCS, and CO (including both gaseous and condensed S$_2$ and H$_2$SO$_4$) as a function of altitude (km) in the Venusian atmosphere, using the ARGO-STAND2020 network (solid black, similar to the cloud‐chemistry model of Rimmer_2021) and the XODIAC-2025.v1 network (dashed orange). Data from various Venus missions, listed in Table 4 of Rimmer_2021, are also shown in gray. Although observational data for polysulfurs S$_2$, S$_5$, S$_6$, S$_7$, and S$_8$ are not available, we include them to compare polysulfur chemistry between the ARGO-STAND2020 and XODIAC-2025.v1 networks, since S$_3$ and S$_4$ deviate from observed values in both cases.
• Figure 5: Predicted volume mixing ratios of S$_n$ ($\mathrm{n=1\text{--}8}$), SO, SO$_2$, H$_2$S, H$_2$SO$_4$, OCS, and CO (including both gaseous and condensed S$_2$ and H$_2$SO$_4$) as a function of altitude (km) in the Venusian atmosphere, using the XODIAC-2025.v1 network (solid black; M0), the XODIAC-2025.v1 network with a 1 ppm surface mixing ratio of atomic sulfur (dashed green; M1), the XODIAC-2025.v2 network (dashed-dotted orange; M2), and the XODIAC-2025.v2 network with a 1 ppm surface mixing ratio of atomic sulfur (dotted purple; M3). Data from various Venus missions, listed in Table 4 of Rimmer_2021, are also shown in gray. Although observational data for polysulfurs S$_2$, S$_5$, S$_6$, S$_7$, and S$_8$ are not available, we include them to compare polysulfur chemistry across the networks. We do not show $\mathrm{S(^1D)}$ and $\mathrm{SO(^1\Delta)}$, as their mixing ratios reach a maximum of only $\sim 10^{-15}$, which lies far below observational detectability.