  Radial velocities (RVs) measured from high-resolution stellar spectra are
routinely used to detect and characterise orbiting exoplanet companions. The
different lines present in stellar spectra are created by several species,
which are non-uniformly affected by stellar variability features such as spots
or faculae. Stellar variability distorts the shape of the spectral absorption
lines from which precise RVs are measured, posing one of the main problems in
the study of exoplanets. In this work we aim to study how the spectral lines
present in M dwarfs are independently impacted by stellar activity. We used
CARMENES optical spectra of six active early- and mid-type M dwarfs to compute
line-by-line RVs and study their correlation with several well-studied proxies
of stellar activity. We are able to classify spectral lines based on their
sensitivity to activity in five M dwarfs displaying high levels of stellar
activity. We further used this line classification to compute RVs with
activity-sensitive lines and less sensitive lines, enhancing or mitigating
stellar activity effects in the RV time series. For specific sets of the least
activity-sensitive lines, the RV scatter decreases by ~ 2 to 5 times the
initial one, depending on the star. Finally, we compare these lines in the
different stars analysed, finding the sensitivity to activity to vary from star
to star. Despite the high density of lines and blends present in M dwarf
stellar spectra, we find that a line-by-line approach is able to deliver
precise RVs. Line-by-line RVs are also sensitive to stellar activity effects,
and they allow for an accurate selection of activity-insensitive lines to
mitigate activity effects in RV. However, we find stellar activity effects to
vary in the same insensitive lines from star to star.
