Influence of Relative Humidity on the Heterogeneous Oxidation of Secondary Organic Aerosol

This dataset contains data from figures and model code used to
generate figures in the paper "Influence of Relative Humidity on the
Heterogeneous Oxidation of Secondary Organic Aerosol" by Li et al.,
published in Atmospheric Chemistry and Physics. A description of the work
follows below. See read me files for details. Secondary organic aerosol
(SOA) is a complex mixture of hundreds of semi‑volatile to extremely
low‑volatility organic compounds that are chemically processed in the
atmosphere, including via heterogeneous oxidation by gas‑phase radicals.
Relative humidity (RH) has a substantial impact on particle phase, which
can affect how SOA evolves in the atmosphere. In this study, SOA from dark
a-pinene ozonolysis is heterogeneously aged by OH radicals in a flowtube
at low and high RH. At high RH (RH = 89%) there is substantial loss of
particle volume (~60%) at an equivalent atmospheric OH exposure of 3
weeks. In contrast, at low RH (RH = 25%) there is little mass loss
(<20%) at the same OH exposure. Mass spectra of the SOA particles
were measured as a function of OH exposure using a vacuum ultraviolet
aerosol mass spectrometer (VUV‑AMS). The mass spectra observed at low RH
overall exhibit minor changes with oxidation and negligible further
changes above an OH exposure = 2 x 1012 molecule cm‑3 s, suggesting
limited impact of oxidation on the particle composition. In contrast, the
mass spectra observed at high RH exhibit substantial and continuous
changes as a function of OH exposure. Further, at high RH clusters of
peaks in the mass spectra exhibit unique decay patterns, suggesting
different responses of various species to oxidation. A model of
heterogeneous oxidation has been developed to understand the origin of the
difference in aging between the low and high RH experiments. Differences
in diffusivity of the SOA between the low and high RH experiments alone
can explain the difference in compositional change but cannot explain the
difference in mass loss. Instead, the difference in mass loss is
attributable to RH‑dependent differences in the OH uptake coefficient
and/or the net probability of fragmentation, with either or both larger at
high RH compared to low RH. These results illustrate the important impact
of relative humidity on the fate of SOA in the atmosphere.