  Short-lived (100s or less), sub-arcsec to a couple of arcsec size features of
enhanced brightenings in the narrowband images at the $\mathrm{H_{2V}}$ and
$\mathrm{K_{2V}}$ positions of the Ca II H and K lines in the quiet Sun are
known as bright grains.
  With simultaneous observations of a quiet Sun internetwork region in the Fe I
6173 {\AA}, Ca II 8542 {AA}, and Ca II K lines acquired by the CRisp Imaging
Spectro-Polarimeter and the CHROMospheric Imaging Spectrometer instruments on
the Swedish 1-m Solar Telescope, we performed multi-line non-local
thermodynamic equilibrium inversions using the STockholm inversion Code to
infer the time-varying stratified atmosphere's physical properties such as the
temperature, line-of-sight (LOS) velocity, and microturbulence.
  The Ca II K profiles of bright grains show enhancement in the
$\mathrm{K_{2V}}$ peak intensities with absence of the $\mathrm{K_{2R}}$
features. At the time of maximum enhancement in the $\mathrm{K_{2V}}$ peak
intensities, we found average enhancements in temperature at lower
chromospheric layers (at $\log\tau_{500}$ $\simeq$ $-$4.2) of about 1.1 kK with
maximum enhancement of about 4.5 kK. These temperature enhancements are
colocated with upflows, as strong as $-$6 $\mathrm{km\;s^{-1}}$, in the
direction of the LOS. The LOS velocities at upper chromospheric layers at
$\log\tau_{500}$ < $-$4.2 show consistent downflows greater than $+$8
$\mathrm{km\;s^{-1}}$. The retrieved value of microturbulence in the atmosphere
of bright grains is negligible at chromospheric layers.
  The study provides observational evidence to support the interpretation that
the bright grains observed in narrowband images at the $\mathrm{H_{2V}}$ and
$\mathrm{K_{2V}}$ positions of the Ca II H and K lines are manifestations of
upward propagating acoustic shocks against a background of downflowing
atmospheres.
