  The innermost regions in active galactic nuclei (AGNs) were not being
spatially resolved so far but spectropolarimetry can provide us insight about
their hidden physics and the geometry. From spectropolarimetric observations in
broad emission lines and assuming equatorial scattering as a dominant
polarization mechanism, it is possible to estimate the mass of supermassive
black holes (SMBHs). We explore the possibilities and limits and to put
constraints on the usage of the method for determining SMBH masses using
polarization in broad emission lines by providing more in-depth theoretical
modeling. Methods. We use the Monte Carlo radiative transfer code STOKES for
exploring polarization of Type 1 AGNs. We model equatorial scattering using
flared-disk geometry for a set of different SMBH masses assuming Thomson
scattering. In addition to the Keplerian motion in the BLR, we also consider
cases of additional radial inflows and vertical outflows. We model the profiles
of polarization plane position angle, degree of polarization and total
unpolarized line for different BLR geometries and different SMBH masses. Our
modeling confirms that the method can be widely used for Type-1 AGNs when
viewing inclinations are between 25 and 45 degrees. We show that the distance
between the BLR and scattering region (SR) has a significant impact on the mass
estimates and the best mass estimates are when the SR is situated at the
distance 1.5-2.5 times larger than the outer BLR radius. Our models show that
if Keplerian motion can be traced through the polarized line profile, then the
direct estimation of the mass of the SMBH can be performed. When radial inflows
or vertical outflows are present in the BLR, this method can be applied if
velocities of the inflow/outflow are less than 500 km/s. We find that models
for NGC4051, NGC4151, 3C273 and PG0844+349 are in good agreements with
observations.
