  With the recent discoveries of terrestrial planets around active M-dwarfs,
destruction processes masking the possible presence of life are receiving
increased attention in the exoplanet community. We investigate potential
biosignatures of planets having Earth-like (N$_2$-O$_2$) atmospheres orbiting
in the habitable zone of the M-dwarf star AD Leo. These are bombarded by high
energetic particles which can create showers of secondary particles at the
surface. We apply our cloud-free 1D climate-chemistry model to study the
influence of key particle shower parameters and chemical efficiencies of NOx
and HOx production from cosmic rays. We determine the effect of stellar
radiation and cosmic rays upon atmospheric composition, temperature, and
spectral appearance. Despite strong stratospheric O$_3$ destruction by cosmic
rays, smog O$_3$ can significantly build up in the lower atmosphere of our
modeled planet around AD Leo related to low stellar UVB. N$_2$O abundances
decrease with increasing flaring energies but a sink reaction for N$_2$O with
excited oxygen becomes weaker, stabilizing its abundance. CH$_4$ is removed
mainly by Cl in the upper atmosphere for strong flaring cases and not via
hydroxyl as is otherwise usually the case. Cosmic rays weaken the role of
CH$_4$ in heating the middle atmosphere so that H$_2$O absorption becomes more
important. We additionally underline the importance of HNO$_3$ as a possible
marker for strong stellar particle showers. In a nutshell, uncertainty in NOx
and HOx production from cosmic rays significantly influences biosignature
abundances and spectral appearance.
