  We revisit the problem of particlization of a QCD fluid into hadrons and
resonances at the end of the fluid dynamical stage in relativistic heavy-ion
collisions in a context of fluctuation measurements. The existing methods
sample an ideal hadron resonance gas, therefore, they do not capture the
non-Poissonian nature of the grand-canonical fluctuations, expected due to QCD
dynamics such as the chiral transition or QCD critical point. We address the
issue by partitioning the particlization hypersurface into locally
grand-canonical fireballs populating the space-time rapidity axis that are
constrained by global conservation laws. The procedure allows to quantify the
effect of global conservation laws, volume fluctuations, thermal smearing and
resonance decays on fluctuation measurements in various rapidity acceptances,
and can be used in fluid dynamical simulations of heavy-ion collisions. As a
first application, we study event-by-event fluctuations in heavy-ion collisions
at the LHC using an excluded volume hadron resonance gas model matched to
lattice QCD susceptibilities, with a focus on (pseudo)rapidity acceptance
dependence of net baryon, net proton, and net charge cumulants. We point out
large differences between net proton and net baryon cumulant ratios that make
direct comparisons between the two unjustified. We observe that the existing
experimental data on net-charge fluctuations at the LHC shows a strong
suppression relative to a hadronic description.
