  We present a study of interstellar comet 2I/2019 Q4 (Borisov) using both
preperihelion and postperihelion observations spanning late September 2019
through late January 2020. The intrinsic brightness of the comet was observed
to continuously decline throughout the timespan, likely due to the decreasing
effective scattering cross-section as a result of volatile sublimation with a
slope of $-0.43 \pm 0.02$ km$^{2}$ d$^{-1}$. We witnessed no significant change
in the slightly reddish colour of the comet, with mean values of $\left \langle
g - r \right \rangle = 0.68 \pm 0.04$, $\left \langle r - i \right \rangle =
0.23 \pm 0.03$, and the normalised reflectivity gradient across the $g$ and $i$
bands $\overline{S'} \left(g,i\right) = \left(10.6 \pm 1.4\right)$ % per $10^3$
\AA, all unremarkable in the context of solar system comets. Using the
available astrometric observations, we confidently detect the nongravitational
acceleration of the comet following a shallow heliocentric distance dependency
of $r_{\rm H}^{-1 \pm 1}$. Accordingly, we estimate that the nucleus is most
likely $\lesssim$0.4 km in radius, and that a fraction of $\gtrsim$0.2% of the
total nucleus in mass has been eroded due to the sublimation activity since the
earliest observation of the comet in 2018 December by the time of perihelion.
Our morphology simulation suggests that the dust ejection speed increased from
$\sim$4 m s$^{-1}$ in September 2019 to $\sim$7 m s$^{-1}$ around perihelion
for the optically dominant dust grains of $\beta \sim 0.01$, and that the
observable dust grains are no smaller than micron size.
