  For type-1 active galactic nuclei (AGNs) for which the equatorial scattering
is the dominant broad line polarization mechanism, it is possible to measure
the supermassive black hole mass by tracing the Keplerian motion across the
polarization plane positionangle. So far this method has been used for 30
objects but only for H${\alpha}$ emission line. We explore the possibilities
this method for determining SMBH masses using polarization in broad emission
lines by applying it for the first time to Mg II${\lambda}$2798AA spectral
line. We use 3D Monte Carlo radiative transfer code stokes for simultaneous
modeling of equatorial scattering of H${\alpha}$, H${\beta}$ and Mg II lines.
We included vertical inflows and outflows in the MgII broad line region (BLR).
We find that polarization states of H${\alpha}$ and H${\beta}$ lines are almost
identical and SMBH mass estimates differ by 7%. For Mg II line, we find that
polarization angle ${\varphi}$ exhibits an additional "plateau" with a constant
${\varphi}$, which deviates than the profiles expected for pure Keplerian
motion. SMBH mass estimates using Mg II line are higher by up to 35% than those
obtained from H${\alpha}$ and H${\beta}$ lines. Our model shows that for
vertical inflows and outflows in the BLR that is higher or comparable to
Keplerian velocity, this method can be applied as a first approximation for
obtaining SMBH mass.
