  Methods. We algorithmically tracked thousands of individual particles through
four OSIRIS/NAC image sequences of 67P's near-nucleus coma. We then traced
concentrated particle groups back to the nucleus surface, and estimated their
potential source regions, size distributions, and projected dynamical
parameters. Finally, we compared the observed activity to dust coma
simulations. Results. We traced back 409 decimeter-sized particles to four
suspected source regions. The regions strongly overlap and are mostly confined
to the Khonsu-Atum-Anubis area. The activity may be linked to rugged terrain,
and the erosion of fine dust and the ejection of large boulders may be mutually
exclusive. Power-law indices fitted to the particle size--frequency
distributions range from $3.4 \pm 0.3$ to $3.8 \pm 0.4$. Gas drag fits to the
radial particle accelerations provide an estimate for the local gas production
rates ($Q_\text{g} = 3.6 \cdot 10^{-5}$ kg s$^{-1}$ m$^{-2}$), which is several
times higher than our model predictions based on purely insolation-driven water
ice sublimation. Our observational results and our modeling results both reveal
that our particles were likely ejected with substantial nonzero initial
velocities of around 0.5$-$0.6 m s$^{-1}$. Conclusions. Our findings strongly
suggest that the observed ejection of decimeter-sized particles cannot be
explained by water ice sublimation and favorable illumination conditions alone.
Instead, the local structures and compositions of the source regions likely
play a major role. In line with current ejection models of decimeter-sized
particles, we deem an overabundance of CO$_2$ ice and its sublimation to be the
most probable driver. In addition, because of the significant initial
velocities, we suspect the ejection events to be considerably more energetic
than gradual liftoffs.
