  Many of the exoplanets for which we can obtain the highest SNR transit
spectra are tidally locked. The atmospheres on tidally-locked planets likely
exhibit large differences between the day and night side of the planet, the
poles, and the morning versus evening terminators. In this paper, we illustrate
how the combined effects of aerosols and day-night temperature gradients shape
transit spectra of tidally-locked exoplanets when full 3D structures are taken
into account and evaluate the implications for retrievals of atmospheric
properties. To do this, we have developed a new code, METIS, which can compute
transit spectra for an arbitrary longitude-latitude-altitude grid of
temperatures and pressures. Using METIS, we pair flexible treatments of clouds
and hazes with simple parameterized day-night temperature gradients to compute
transit spectra and perform retrieval experiments across a wide array of
possible exoplanet atmospheric properties. Our key findings are that: (1) the
presence of aerosols typically increases the effects of day-night temperature
gradients on transit spectra; (2) ignoring day-night temperature gradients when
attempting to perform Bayesian parameter estimation will still return biased
results when aerosols are present, as has already been shown for clear
atmospheres in the literature; (3) when a day-night temperature gradient is
present and accounted for in the retrieval model, some transit spectra can
provide sufficient information to constrain temperatures and the width of the
transition from day to night. The presence of clouds and hazes can actually
tighten such constraints, but also weaken constraints on metallicity. This
paper represents a step towards the larger goal of developing models and theory
of adequate complexity to match the superior quality data that will soon be
available.
