  We present the long-term X-ray spectral and temporal analysis of a 'bare-type
AGN' Ark 120. We consider the observations from XMM-Newton, Suzaku, Swift, and
NuSTAR from 2003 to 2018. The spectral properties of this source are studied
using various phenomenological and physical models present in the literature.
We report (a) the variations of several physical parameters, such as the
temperature and optical depth of the electron cloud, the size of the Compton
cloud, and accretion rate for the last fifteen years. The spectral variations
are explained from the change in the accretion dynamics; (b) the X-ray time
delay between 0.2-2 keV and 3-10 keV light-curves exhibited zero-delay in 2003,
positive delay of 4.71 \pm 2.1 ks in 2013, and negative delay of 4.15 \pm 1.5
ks in 2014. The delays are explained considering Comptonization, reflection,
and light-crossing time; (c) the long term intrinsic luminosities obtained
using nthcomp, of the soft-excess and the primary continuum show a correlation
with a Pearson Correlation Coefficient of 0.922. This indicates that the
soft-excess and the primary continuum are originated from the same physical
process. From a physical model fitting, we infer that the soft excess for Ark
120 could be due to a small number of scatterings in the Compton cloud. Using
Monte-Carlo simulations, we show that indeed the spectra corresponding to fewer
scatterings could provide a steeper soft-excess power-law in the 0.2-3 keV
range. Simulated luminosities are found to be in agreement with the observed
values.
