  Nitrogen is a biosignature gas that cannot be maintained in its Earth-like
ratio with CO$_2$ under abiotic conditions. It has also proven to be
notoriously hard to detect at optical and infrared wavelengths. Fortunately,
the ultraviolet region, which has only recently started being explored for
terrestrial exoplanets, may provide new opportunities to characterise
exoplanetary atmospheric nitrogen. In this work, the future prospects for
detecting atomic nitrogen absorption lines in the transmission spectrum of an
Earth-like planet orbiting in the habitable zone of a Sun-like star with LUVOIR
are explored. Using the non-local thermodynamic equilibrium spectral synthesis
code Cloudy, we produce a far-ultraviolet atomic transmission spectrum for an
Earth-Sun-like system, and identify several nitrogen features, including both N
I and N II lines. We calculate the number of transits required for 1$\sigma$
and 3$\sigma$ detections of the planetary N{\sc i} $\lambda1200$ triplet signal
with the G120M grating of the LUMOS spectrograph designed for LUVOIR, as a
function of distance to the system and stellar ultraviolet emission. The
minimum number of transit observations necessary for 1$\sigma$ and 3$\sigma$
detections of atomic N are 188 and 1685, respectively, for a system located at
a distance of one pc with 100 times the Solar ultraviolet flux. Given that the
orbital period of an Earth-Sun system is one year, it is not feasible to detect
atomic N in the transmission spectrum for these systems. Future studies in this
direction should therefore focus on Earth-like planets orbiting in the
habitable zone of M dwarfs.

Earth and Planetary Astrophysics