  Incremental particle growth in turbulent protoplanetary nebulae is limited by
a combination of barriers that can slow or stall growth. Moreover, particles
that grow massive enough to decouple from the gas are subject to inward radial
drift which could lead to the depletion of most disk solids before
planetesimals can form. Compact particle growth is probably not realistic.
Rather, it is more likely that grains grow as fractal aggregates which may
overcome this so-called radial drift barrier because they remain more coupled
to the gas than compact particles of equal mass. We model fractal aggregate
growth and compaction in a viscously evolving solar-like nebula for a range of
turbulent intensities $\alpha_{\rm{t}} = 10^{-5}-10^{-2}$. We do find that
radial drift is less influential for porous aggregates over much of their
growth phase; however, outside the water snowline fractal aggregates can grow
to much larger masses with larger Stokes numbers more quickly than compact
particles, leading to rapid inward radial drift. As a result, disk solids
outside the snowline out to $\sim 10-20$ AU are depleted earlier than in
compact growth models, but outside $\sim 20$ AU material is retained much
longer because aggregate Stokes numbers there remain lower initially.
Nevertheless, we conclude even fractal models will lose most disk solids
without the intervention of some leap-frog planetesimal forming mechanism such
as the Streaming Instability (SI), though conditions for the SI are generally
never satisfied, except for a brief period %for a brief stage around $\sim 0.2$
Myr at the snowline for $\alpha_{\rm{t}}=10^{-5}$.
