Sulfur dioxide in Venus’ middle atmosphere: determining its chemical pathways.

The depletion of SO₂ in Venus's middle atmosphere, observed from below and above the cloud deck, remains unexplained. SO₂ abundance decreases from 130 ppmv below the clouds [1] to a few hundred ppbv at the cloud top [2,3,4,5]. The VenSpec suite onboard EnVision aims to provide new observations of key gas species (SO₂, H₂O, CO, etc.) in the middle atmosphere to help constrain its chemical landscape.

Modeling efforts have explored different approaches to Venusian sulfur chemistry. [6] proposed that increased SO₂ dissolution, facilitated by salts in cloud droplets, may account for the observed depletion. However, most chemical models instead simplify the issue by reducing the bulk SO₂ reservoir [7, 8, 9, 10, among others]. Despite this simplification, sulfur photochemistry remains intricate, governed by a balance between SO₂ photolysis and the condensation of sulfuric acid against vertical transport. To further investigate the modeled sulfur chemistry in the Venusian middle atmosphere, we employed the chemical pathways analysis tool PaPy [11], which builds on [12]. PaPy has been successfully applied to Early Mars simulations [13] and has now been adapted for use with the Venus Planetary Climate Model (Venus PCM) results.

We will detail the key chemical pathways governing SO₂ behavior in the Venusian cloud deck using 3D outputs from the Venus PCM. We will examine how sulfur chemistry varies with local time and latitude throughout the Venusian day. On the dayside, photolysis dominates SO₂ chemistry and the production of H₂SO₄ is crucial. Nonetheles, we will present how the SO₂ chemistry remains more complicated. On the nightside, the SO₂ chemistry involves significant contributions from NO_x, ClO_x, and S_x species while keeping interesting and unexpected behavior as some altitude ranges still are being dominated by a NET chemical loss of SO₂. At the morning and evening terminators, we will identify the specific chemical pathways responsible for observed variations in SO₂ abundances. These pathways will be analyzed within their local transport context, considering both advective and diffusive dynamics as evaluated by the Venus PCM.

Acknowledgements: Authors FDS, AS, LML and AM acknowledges financial support from the Severo Ochoa grant CEX2021-001131-S funded by MCIN/AEI/ 10.13039/501100011033. Author AS acknowledges financial support from project PID2021-126365NB-C21 funded by MCIN/AEI/10.13039/501100011033/ and FEDER.

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