  Lightning has been suggested to play a role in triggering the occurrence of
bio-ready chemical species. Future missions (PLATO, ARIEL, HWO, LIFE) and
ground-based ELTs will investigate the atmospheres of potentially habitable
exoplanets. We aim to study the effect of lightning on the atmospheric
chemistry, how it affects false-positive and false-negative biosignatures, and
if its effect would be observable on an exo-Earth and on TRAPPIST-1 planets. We
use a combination of laboratory experiments, photochemical and radiative
transfer modelling. With spark discharge experiments in N2-CO2-H2 gas mixtures,
representing a range of possible rocky-planet atmospheres, we investigate the
production of potential lightning signatures (CO, NO), possible biosignature
gases (N2O, NH3, CH4), and important prebiotic precursors (HCN, Urea).
Photochemical simulations are conducted for oxygen-rich and anoxic atmospheres
for rocky planets in the habitable zones of the Sun and TRAPPIST-1 for a range
of lightning flash rates. Synthetic spectra are calculated using SMART to study
the atmosphere's reflectance, emission, and transmission spectra. Lightning
enhances the spectral features of NO, NO2, and, in some cases, CO; CH4 and C2H6
may be enhanced indirectly. Lightning at a flash rate slightly higher than on
modern Earth can mask the ozone features of an oxygen-rich, biotic atmosphere,
making it harder to detect the biosphere. Lightning flash rates at least ten
times higher than on modern Earth can mask the presence of ozone in the anoxic,
abiotic atmosphere of a planet orbiting a late M dwarf, reducing the potential
for a false-positive life-detection. The threshold lightning rates to eliminate
oxygen and ozone false positive biosignatures on planets orbiting ultra-cool
dwarfs is up to ten times higher than the modern flash rate, suggesting that
lightning cannot always prevent these false-positive scenarios.
