Vapor–Wall
Deposition in Chambers: Theoretical
Considerations
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Abstract
In
order to constrain the effects of vapor–wall deposition
on measured secondary organic aerosol (SOA) yields in laboratory chambers,
researchers recently varied the seed aerosol surface area in toluene
oxidation and observed a clear increase in the SOA yield with increasing
seed surface area (Zhang, X.; et al. <i>Proc. Natl. Acad. Sci.
U.S.A.</i> <b>2014</b>, <i>111</i>, 5802). Using
a coupled vapor–particle dynamics model, we examine the extent
to which this increase is the result of vapor–wall deposition
versus kinetic limitations arising from imperfect accommodation of
organic species into the particle phase. We show that a seed surface
area dependence of the SOA yield is present only when condensation
of vapors onto particles is kinetically limited. The existence of
kinetic limitation can be predicted by comparing the characteristic
time scales of gas-phase reaction, vapor–wall deposition, and
gas–particle equilibration. The gas–particle equilibration
time scale depends on the gas–particle accommodation coefficient
α<sub>p</sub>. Regardless of the extent of kinetic limitation,
vapor–wall deposition depresses the SOA yield from that in
its absence since vapor molecules that might otherwise condense on
particles deposit on the walls. To accurately extrapolate chamber-derived
yields to atmospheric conditions, both vapor–wall deposition
and kinetic limitations must be taken into account