  Magnetic reconnection is often considered as the primary particle
acceleration mechanism in a magnetized blazar zone environment. The majority of
radiation in the reconnection layer comes from plasmoids and their mergers. In
particular, plasmoid mergers can produce strong multi-wavelength flares and
major variations in synchrotron polarization signatures. However, radiative
properties of plasmoid mergers have not been well explored due to difficulties
in tracking the merging processes. Here we use an image processing method that
combines the magnetic vector potential and density to identify isolated and
merging plasmoids. We find that this method can clearly distinguish radiation
contributions from isolated plasmoids, merging plasmoids, and the primary
current sheet of reconnection. This new method enables us to study the
radiative properties of plasmoids and mergers statistically. Our results show
that isolated plasmoids have similar emissivity regardless of their sizes, and
they generally have nonzero polarization degree (PD) due to their
quasi-circular shape. Flares due to plasmoid mergers have relative amplitudes
that are anti-proportional to the size ratio of the plasmoids participating in
the mergers. Finally, only mergers between plasmoids of comparable sizes (width
ratio $\lesssim 5$) can lead to significant spectral hardening and polarization
angle (PA) variations; the amplitude of the PA variations is between 0 and
$180^{\circ}$ and has a mean value of $90^{\circ}$. Our analyses on 2D
simulations can pave the way for future analyses and machine learning
techniques on radiative properties of 3D magnetic reconnection simulations.
