  Recent progress in high-resolution transmission spectroscopy has offered new
avenues in which to characterise the atmospheres of transiting exoplanets.
High-resolution cross-correlation spectroscopy allows for the unambiguous
detection of molecules/atoms. It has also been used to map both atmospheric
dynamics and longitudinal variations in the abundance of species across the
morning and evening limbs. We present multiple VLT/ESPRESSO observations of the
ultra-hot Jupiter WASP-121b, from which we constrain relative abundances of
various neutral metals consistently across all observations, whilst accounting
for the distortion of the exoplanet's signal caused by traditional data
processing techniques. We also constrain planetary orbital velocities and
$T$-$P$ profiles. We compare our abundance constraints with previous
constraints using VLT/UVES transmission spectroscopy of WASP-121b, and find our
results to be consistent between observations, and also in agreement with
stellar values for species previously detected in the atmosphere of WASP-121b.
Our retrieval framework can also be used to identify potential exospheric
species, resulting in extended absorption features beyond the transit
equivalent Roche limit of WASP-121b ($R_{\rm eqRL}$ $\sim$ 1.3 $R_{\rm p}$).
H$\alpha$, Fe II, and Ca II were found to extend to high altitudes
($1.54\pm0.04$ $R_{\rm p}$, $1.17\pm0.01$ $R_{\rm p}$, and $2.52\pm0.34$
$R_{\rm p}$, respectively), which are broadly consistent with literature
values. The consistency of our constraints across multiple high-resolution
observations is a strong validation of our model filtering and retrieval
framework, as well as the stability of the atmosphere over the timescales of
months/years, and could allow for planet formation processes to be inferred
from future ground-based observations of exoplanetary atmospheres.
