DOI: 10.25338/b8zk5x
title: Influence of Relative Humidity on the Heterogeneous Oxidation of Secondary Organic Aerosol

description: 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
 (&lt;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.
description: Data were collected at the Advanced Light Source, Berkeley, CA in
 May 2017 at Beamline 9.0.2.  In the experiments here,
 SOA is formed initially under dry conditions and these SOA particles are
 then heterogeneously oxidized by OH at either a low or high RH. Four
 aspects are highlighted: SOA formation, RH control, heterogeneous
 oxidation and SOA measurements. SOA particles were first produced
 chemically in one flow tube under dry conditions (the SOA formation flow
 tube). The airstream was then humidified to the desired level, and the SOA
 particles were reacted heterogeneously with OH radicals in a second flow
 tube (the OH flow tube). The size distribution and chemical composition of
 the particles were then characterized. The size distribution was
 characterized with a scanning mobility particle sizer and the chemical
 composition monitored with the vacuum ultraviolet aerosol mass
 spectrometer.  Model calculations were performed using
 Igor (Wavemetrics) v6.37. 
description: The files here are derived from figures in the manuscript
 "Influence of Relative Humidity on the Heterogeneous Oxidation of
 Secondary Organic Aerosol" published in Atmospheric Chemistry and
 Physics. More details are provided in the two read me files. One describes
 the observational data and one the model code.