Photochemistry is a fundamental process of planetary atmospheres that regulates the atmospheric composition and stability1. However, no unambiguous photochemical products have been detected in exoplanet atmospheres so far. Recent observations from the JWST Transiting Exoplanet Community Early Release Science Program2,3 found a spectral absorption feature at 4.05 μm arising from sulfur dioxide (SO2) in the atmosphere of WASP-39b. WASP-39b is a 1.27-Jupiter-radii, Saturn-mass (0.28 MJ) gas giant exoplanet orbiting a Sun-like star with an equilibrium temperature of around 1,100 K (ref. 4). The most plausible way of generating SO2 in such an atmosphere is through photochemical processes5,6. Here we show that the SO2 distribution computed by a suite of photochemical models robustly explains the 4.05-μm spectral feature identified by JWST transmission observations7 with NIRSpec PRISM (2.7σ)8 and G395H (4.5σ)9. SO2 is produced by successive oxidation of sulfur radicals freed when hydrogen sulfide (H2S) is destroyed. The sensitivity of the SO2 feature to the enrichment of the atmosphere by heavy elements (metallicity) suggests that it can be used as a tracer of atmospheric properties, with WASP-39b exhibiting an inferred metallicity of about 10× solar. We further point out that SO2 also shows observable features at ultraviolet and thermal infrared wavelengths not available from the existing observations.

The hydrogen-abstraction-C 2 H 2-addition (HACA) chemistry of naphthalenyl radicals has been studied extensively, but there is a significant discrepancy in product distributions reported or predicted in literature regarding appearance of C 14 H 8 and C 14 H 10 species. Starting from ab initio calculations, a comprehensive theoretical model describing the HACA chemistry of both 1-and 2naphthalenyl radicals is generated. Pressure-dependent kinetics are considered in the C 12 H 9 , C 14 H 9 , and C 14 H 11 potential energy surfaces including formally direct well-skipping pathways. On the C 12 H 9 PES, reaction pathways were found connecting two entry points: 1-naphthalenyl (1-C 10 H 7) + acetylene (C 2 H 2) and 2-C 10 H 7 + C 2 H 2. A significant amount of acenaphthylene is predicted to be formed from 2-C 10 H 7 + C 2 H 2 , and the appearance of C 14 H 8 isomers is predicted in the model simulation, consistent with high-temperature experimental results from Parker et al. At 1500 K, 1-C 10 H 7 + C 2 H 2 mostly generates acenaphthylene through a formally direct pathway, which predicted selectivity of 66% at 30 Torr and 56% at 300 Torr. The reaction of 2-C 10 H 7 with C 2 H 2 at 1500 K yields 2-ethynylnaphthalene as the most dominant product, followed by acenaphthylene mainly generated via isomerization of 2-C 10 H 7 to 1-C 10 H 7. Both the 1-C 10 H 7 and 2-C 10 H 7 reactions with C 2 H 2 form some C 14 H 8 products, but negligible phenanthrene and anthracene formation is predicted at 1500 K. A rate-of-production analysis reveals that C 14 H 8 formation is strongly affected by the rates of H-abstraction from acenaphthylene, 1-ethynylnaphthalene, and 2ethynylnaphthalene, so the kinetics of these reactions are accurately calculated at the high level G3(MP2,CC)//B3LYP/6-311G ** level of theory. At intermediate temperatures like 800 K, acenaphthylene + H are the leading bimolecular products of 1-C 10 H 7 + C 2 H 2 , and 1-acenaphthenyl radical is the most abundant C 12 H 9 isomer due to its stability. The predicted product distribution of 2-C 10 H 7 + C 2 H 2 at 800 K, in contrast to the results of Parker et al is predicted to consist primarily of species containing three fused benzene rings-for example, 752

The formation of polycyclic aromatic hydrocarbons (PAHs) during combustion has a substantial impact on environmental pollution and public health. The hydrogen-abstraction-acetylene-addition (HACA) mechanism is expected to be a significant source...