  Infrared solar occultation measurements are well established for remote
sensing of Earth's atmosphere, and the corresponding primary transit
spectroscopy has turned out to be valuable for characterization of extrasolar
planets. Our objective is an assessment of the detectability of molecular
signatures in Earth's transit spectra.
  To this end, we take a limb sequence of representative cloud-free
transmission spectra recorded by the space-borne ACE-FTS Earth observation
mission (Hughes et al., ACE infrared spectral atlases of the Earth's
atmosphere, JQSRT 2014) and combine these spectra to the effective height of
the atmosphere. These data are compared to spectra modeled with an atmospheric
radiative transfer line-by-line infrared code to study the impact of individual
molecules, spectral resolution, the choice of auxiliary data, and numerical
approximations. Moreover, the study serves as a validation of our infrared
radiative transfer code.
  The largest impact is due to water, carbon dioxide, ozone, methane, nitrous
oxide, nitrogen, nitric acid, oxygen, and some chlorofluorocarbons (CFC11 and
CFC12). The effect of further molecules considered in the modeling is either
marginal or absent. The best matching model has a mean residuum of 0.4 km and a
maximum difference of 2 km to the measured effective height. For a quantitative
estimate of visibility and detectability we consider the maximum change of the
residual spectrum, the relative change of the residual norm, the additional
transit depth, and signal-to-noise ratios for a JWST setup. In conclusion, our
study provides a list of molecules that are relevant for modeling transmission
spectra of Earth-like exoplanets and discusses the feasibility of retrieval.
