  We investigated the off-limb spicules observed in the Mg II h and k lines by
IRIS in a solar polar coronal hole. We analyzed the large dataset of obtained
spectra to extract quantitative information about the line intensities, shifts,
and widths. The observed Mg II line profiles are broad and double-peaked at
lower altitudes, broad but flat-topped at middle altitudes, and narrow and
single-peaked with the largest Doppler shifts at higher altitudes. We use 1D
non-LTE vertical slab models (i.e. models which consider departures from Local
Thermodynamic Equilibrium) in single-slab and multi-slab configurations to
interpret the observations and to investigate how a superposition of spicules
along the line of sight (LOS) affects the synthetic Mg II line profiles. The
used multi-slab models are either static, i.e. without any LOS velocities, or
assume randomly assigned LOS velocities of individual slabs, representing the
spicule dynamics. We conducted such single-slab and multi-slab modeling for a
broad set of model input parameters and showed the dependence of the Mg II line
profiles on these parameters. We demonstrated that the observed line widths of
the h and k line profiles are strongly affected by the presence of multiple
spicules along the LOS. We later showed that the profiles obtained at higher
altitudes can be reproduced by single-slab models representing individual
spicules. We found that the multi-slab model with a random distribution of the
LOS velocities ranging from -25 to 25 km s$^{-1}$ can well reproduce the width
and the shape of Mg II profiles observed at middle altitudes.
