Aqueous Photochemistry of Secondary Organic Aerosol of α‑Pinene and α‑Humulene Oxidized with Ozone, Hydroxyl Radical, and Nitrate Radical

Formation
of secondary organic aerosols (SOA) from biogenic volatile
organic compounds (BVOC) occurs via O3- and OH-initiated
reactions during the day and reactions with NO3 during
the night. We explored the effect of these three oxidation conditions
on the molecular composition and aqueous photochemistry of model SOA
prepared from two common BVOC. A common monoterpene, α-pinene,
and sesquiterpene, α-humulene, were used to form SOA in a smog
chamber via BVOC + O3, BVOC + NO3, and BVOC
+ OH + NOx oxidation. Samples of SOA were
collected on filters, water-soluble compounds from SOA were extracted
in water, and the resulting aqueous solutions were photolyzed to simulate
the photochemical aqueous processing of SOA. The extent of change
in the molecular level composition of SOA over 4 h of photolysis (approximately
equivalent to 64 h of photolysis under ambient conditions) was assessed
with high-resolution electrospray ionization mass spectrometry. The
analysis revealed significant differences in the molecular composition
between SOA formed by the different oxidation pathways. The composition
further evolved during photolysis with the most notable change corresponding
to the nearly complete removal of nitrogen-containing organic compounds.
Hydrolysis of SOA compounds also occurred in parallel with photolysis.
The preferential loss of larger SOA compounds during photolysis and
hydrolysis made the SOA compounds more volatile on average. This study
suggests that aqueous processes may under certain conditions lead
to a reduction in the SOA loading as opposed to an increase in SOA
loading commonly assumed in the literature.