  Current observational data of exoplanets are providing increasing detail of
their 3D atmospheric structures. As characterisation efforts expand in scope,
the need to develop consistent 3D radiative-transfer methods becomes more
pertinent as the complex atmospheric properties of exoplanets are required to
be modelled together consistently. We aim to compare the transmission and
emission spectra results of a 3D Monte Carlo Radiative Transfer (MCRT) model to
contemporary radiative-transfer suites. We perform several benchmarking tests
of a MCRT code, Cloudy Monte Carlo Radiative Transfer (CMCRT), to transmission
and emission spectra model output. We add flexibility to the model through the
use of k-distribution tables as input opacities. We present a hybrid MCRT and
ray tracing methodology for the calculation of transmission spectra with a
multiple scattering component. CMCRT compares well to the transmission spectra
benchmarks at the 10s of ppm level. Emission spectra benchmarks are consistent
to within 10% of the 1D models. We suggest that differences in the benchmark
results are likely caused by geometric effects between plane-parallel and
spherical models. In a practical application, we post-process a cloudy 3DHD
189733b GCM model and compare to available observational data. Our results
suggest the core methodology and algorithms of CMCRT produce consistent results
to contemporary radiative transfer suites. 3D MCRT methods are highly suitable
for detailed post-processing of cloudy and non-cloudy 1D and 3D exoplanet
atmosphere simulations in instances where atmospheric inhomogeneities,
significant limb effects/geometry or multiple scattering components are
important considerations.
