Recent advancements in high-performance observational instruments have enabled
significant progress in the study of exoplanetary atmospheres. The atmospheres of hot
giant exoplanets in close proximity to their host stars have been the most intensively
investigated, owing to the large signal provided by their extended
atmospheres. Observations of these exoplanets, combined with simulations using general
circulation models, have revealed atmospheric inhomogeneities, including day–night
temperature gradients, strong eastward zonal winds, localized clouds, and spatial
variations in chemical composition. However, analytical techniques for probing these
inhomogeneities remain under development. In particular, fully leveraging the
high-precision data provided by instruments such as JWST requires further methodological
advances.

To address this challenge, we have developed a new method for investigating atmospheric
inhomogeneities in exoplanets using transmission spectroscopy. This approach links
chromatic variations in conventional transit model parameters --central transit time,
total and full durations, and transit depth-- to atmospheric asymmetries. It enables us
to probe differences between planetary limbs slightly offset from the terminator on the
dayside and the nightside. We applied this method to JWST's NIRSpec/G395H observations
of the hot Saturn exoplanet WASP-39 b. Our analysis suggests a higher abundance of CO2
on the evening limb compared to the morning limb and indicates a greater probability of
SO2 on the limb slightly offset from the terminator on the dayside relative to the
nightside. These findings should be considered in the context of photochemical processes
and atmospheric circulation.