α‑Pinene Autoxidation Products May Not Have Extremely Low Saturation Vapor Pressures Despite High O:C Ratios

COSMO-RS (conductor-like screening
model for real solvents) and
three different group-contribution methods were used to compute saturation
(subcooled) liquid vapor pressures for 16 possible products of ozone-initiated
α-pinene autoxidation, with elemental compositions C10H16O4–10 and C20H30O10–12. The saturation vapor pressures predicted
by the different methods varied widely. COSMO-RS predicted relatively
high saturation vapor pressures values in the range of 10–6 to 10–10 bar for the C10H16O4–10 “monomers”, and 10–11 to 10–16 bar for the C20H30O10–12 “dimers”. The group-contribution
methods predicted significantly (up to 8 order of magnitude) lower
saturation vapor pressures for most of the more highly oxidized monomers.
For the dimers, the COSMO-RS predictions were within the (wide) range
spanned by the three group-contribution methods. The main reason for
the discrepancies between the methods is likely that the group-contribution
methods do not contain the necessary parameters to accurately treat
autoxidation products containing multiple hydroperoxide, peroxy acid
or peroxide functional groups, which form intramolecular hydrogen
bonds with each other. While the COSMO-RS saturation vapor pressures
for these systems may be overestimated, the results strongly indicate
that despite their high O:C ratios, the volatilities of the autoxidation
products of α-pinene (and possibly other atmospherically relevant
alkenes) are not necessarily extremely low. In other words, while
autoxidation products are able to adsorb onto aerosol particles, their
evaporation back into the gas phase cannot be assumed to be negligible,
especially from the smallest nanometer-scale particles. Their observed
effective contribution to aerosol particle growth may therefore involve
rapid heterogeneous reactions (reactive uptake) rather than effectively
irreversible physical absorption.