  Soot aggregates form as open, fractal-like structures, but aged atmospheric
particles are often observed to be restructured into more compact shapes. This
compaction has a major effect on the radiative properties of the aggregates,
and may also influence their aerosol-cloud interactions and toxicity. Recent
laboratory studies have presented conflicting arguments on whether this
compaction occurs during condensation or during evaporation. In this three-part
study, we combine theory and experiments to explain these conflicting results.
First, we review the surface-science literature and identify explicit
mechanisms for condensation compaction as well as evaporation compaction. We
also identify a mechanism for avoiding compacting during condensation, which is
predicted from heterogeneous nucleation theory and the kinetic barriers to
capillary formation. Second, we review the soot-restructuring literature and
find evidence for all of these compaction mechanisms, the most common being
condensation-compaction. Some atmospheric studies have reported non-compacted
soot in internal mixtures, which we attribute to coagulation, and which is less
common. Third, we present new experimental results from a study in our
laboratory where the surface tension of anthracene coatings was "switched on"
or "switched off" by using solid or liquid phases during addition and removal.
Consequently, we demonstrated condensation compaction, evaporation compaction,
and no compaction, for the same soot source. Overall, our study indicates that
it is most reasonable to assume that soot will undergo compaction upon coating
condensation in the atmosphere, in combustion systems, and in human lungs.
