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