  To leverage the angular resolution of interferometry at high contrast, one
must employ specialized beam-combiners called interferometric nullers. Nullers
discard part of the astrophysical information to optimize the recording of
light present in the dark fringe of the central source. Asgard/NOTT will deploy
a beam-combination scheme offering good instrumental noise rejection when
phased appropriately, but for which information is degenerate on the outputs,
prompting a dedicated tuning strategy using the science detector. The
dispersive effect of water vapor can be corrected with prisms forming a
variable thickness of glass. But observations in the L band suffer from an
additional and important chromatic effect due to longitudinal atmospheric
dispersion coming from a resonance of CO2 at 4.3 micron. To compensate for this
effect efficiently, a novel type of compensation device will be deployed
leveraging a gas cell of variable length at ambient pressure. After reviewing
the impact of water vapor and CO2, we present the design of this atmospheric
dispersion compensation device and describe a strategy to maintain this tuning
on-sky.
