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JWST has recently detected the first robust photochemical product on anexoplanet: sulfur dioxide (SO$_2$) on WASP-39b (Rustamkulov et al. 2023;Alderson et al. 2023; Tsai et al. 2023b). The data from the NIRISS instrumentalso reveal signs of partial coverage of clouds (Feinstein et al. 2023). Mostof the previous studies have focused on interpreting spectral data with 1Dmodels. To explore how the chemical species and cloud particles are altered byglobal circulation, we applied a 2D photochemical model and a 2D microphysicalcloud model separately to post-process the thermal and dynamical structuressimulated by a 3D general circulation model (GCM) of WASP-39b. We found thatSO$_2$ produced by photochemistry on the dayside can be transported to thenightside owing to the efficient replenishment of horizontal transport. Themorning-evening limb differences in methane (CH$_4$) abundances predicted bythe 1D models disappeared after horizontal transport is included. Similarly,the inclusion of horizontal transport also reduced the limb differences inSO$_2$. Our modeling results suggest that the fast zonal wind results inminimal or negligible limb asymmetry in composition. Based on the syntheticspectra generated by our 2D atmosphere simulations, we propose that observingSO$_2$ absorption in the emission spectra of WASP-39b at different phases mayoffer opportunities to probe the horizontal quenching process of photochemicalproducts. We will focus on the gas-phase chemistry in this paper and leave theresults regarding clouds in the subsequent paper as part of the series.

Day-night transport induced chemistry and clouds on WASP-39b I: Gas-phase composition