  Due to its Earth-like minimum mass of 1.27 M$_{\text{E}}$ and its close
proximity to our Solar system, Proxima Centauri b is one of the most
interesting exoplanets for habitability studies. Its host star, Proxima
Centauri, is however a strongly flaring star, which is expected to provide a
very hostile environment for potentially habitable planets. We perform a
habitability study of Proxima Centauri b assuming an Earth-like atmosphere
under high stellar particle bombardment, with a focus on spectral transmission
features. We employ our extensive model suite calculating energy spectra of
stellar particles, their journey through the planetary magnetosphere,
ionosphere, and atmosphere, ultimately providing planetary climate and spectral
characteristics, as outlined in Herbst et al. (2019). Our results suggest that
together with the incident stellar energy flux, high particle influxes can lead
to efficient heating of the planet well into temperate climates, by limiting
CH$_4$ amounts, which would otherwise run into anti-greenhouse for such planets
around M-stars. We identify some key spectral features relevant for future
spectral observations: First, NO$_2$ becomes the major absorber in the visible,
which greatly impacts the Rayleigh slope. Second, H$_2$O features can be masked
by CH$_4$ (near infra-red) and CO$_2$ (mid to far infra-red), making them
non-detectable in transmission. Third, O$_3$ is destroyed and instead HNO$_3$
features become clearly visible in the mid to far infra-red. Lastly, assuming a
few percent of CO$_2$ in the atmosphere, CO$_2$ absorption at 5.3 $\mu$m
becomes significant (for flare and non-flare cases), strongly overlapping with
a flare related NO feature in Earth's atmosphere.
