  We utilize state-of-the-art 3D hydrodynamical and classical 1D stellar model
atmospheres to study the influence of convection on the formation properties of
various atomic and molecular spectral lines in the atmospheres of four red
giant stars, located close to the base of the red giant branch, RGB
($T_{\mathrm eff}\approx5000$ K, $\log g=2.5$), and characterized by four
different metallicities, [M/H] = 0.0, -1.0, -2.0, -3.0. The role of convection
in the spectral line formation is assessed with the aid of abundance
corrections, i.e., the differences in abundances predicted for a given
equivalent width of a particular spectral line with the 3D and 1D model
atmospheres. We find that for lines of certain neutral atoms the abundance
corrections strongly depend both on metallicity of a given model atmosphere and
the line excitation potential. While abundance corrections for all lines of
both neutral and ionized elements tend to be small at solar metallicity, for
lines of neutral elements with low ionization potential and low-to-intermediate
$\chi$ they quickly increase with decreasing metallicity, reaching in their
extremes to -0.6...-0.8 dex. In all such cases the large abundance corrections
are due to horizontal temperature fluctuations in the 3D hydrodynamical models.
Abundance corrections of molecular lines are very sensitive to metallicity of
the underlying model atmosphere and may be larger (in absolute value) than -0.5
dex at [M/H] = -3.0 (-1.5 dex in the case of CO). We also find that an
approximate treatment of scattering in the 3D model calculations leads to the
abundance corrections that are altered by less than ~0.1 dex, both for atomic
and molecular (CO) lines, with respect to the model where scattering is treated
as true absorption throughout the entire atmosphere, with the largest
differences for the resonance and low-excitation lines.
