  The determination of supermassive black hole (SMBH) masses is the key to
understanding the host galaxy build-up and the SMBH mass assembly histories.
The SMBH masses of non-local quasars are frequently estimated via the
single-epoch virial black-hole mass estimators, which may suffer from
significant biases. Here we demonstrate a new approach to infer the mass
distribution of SMBHs in quasars by modelling quasar UV/optical variability.
Our inferred black hole masses are systematically smaller than the virial ones
by $0.3\sim 0.6$ dex; the $\sim 0.3$ dex offsets are roughly consistent with
the expected biases of the virial black-hole mass estimators. In the upcoming
time-domain astronomy era, our methodology can be used to constrain the cosmic
evolution of quasar mass distributions.
