  Solid particles in protoplanetary disks that are sufficiently super-solar in
metallicity overcome turbulence generated by vertical shear to gravitationally
condense into planetesimals. Super-solar metallicities result if solid
particles pile up as they migrate starward as a result of aerodynamic drag.
Previous analyses of aerodynamic drift rates that account for mean flow
differences between gas and particles yield particle pile-ups. We improve on
these studies not only by accounting for the collective inertia of solids
relative to that of gas, but also by including the transport of angular
momentum by turbulent stresses within the particle layer. These turbulent
stresses are derived in a physically self-consistent manner from the structure
of marginally Kelvin-Helmholtz turbulent flows. They are not calculated using
the usual plate drag formulae, whose use we explain is inappropriate.
Accounting for the relative inertia of solids to gas retards, but does not
prevent, particle pile-ups, and generates more spatially extended regions of
metal enrichment. Turbulent transport hastens pile-ups. We conclude that
particle pile-up is a robust outcome in sufficiently passive protoplanetary
disks. Connections to observations of circumstellar disks, including the Kuiper
Belt, and the architectures of planetary systems are made.
