On Surface
Order and Disorder of α‑Pinene-Derived
Secondary Organic Material
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Abstract
The surfaces of secondary organic
aerosol particles are notoriously
difficult to access experimentally, even though they are the key location
where exchange between the aerosol particle phase and its gas phase
occurs. Here, we overcome this difficulty by applying standard and
sub- 1 cm<sup>–1</sup> resolution vibrational sum frequency
generation (SFG) spectroscopy to detect C–H oscillators at
the surfaces of secondary organic material (SOM) prepared from the
ozonolysis of α-pinene at Harvard University and at the University
of California, Irvine, that were subsequently collected on Teflon
filters as well as CaF<sub>2</sub> windows using electrostatic deposition.
We find both samples yield comparable SFG spectra featuring an intense
peak at 2940 cm<sup>–1</sup> that are independent of spectral
resolution and location or method of preparation. We hypothesize that
the SFG spectra are due to surface-active C–H oscillators associated
with the four-membered ring motif of α-pinene, which produces
an unresolvable spectral continuum of approximately 50 cm<sup>–1</sup> width reminiscent of the similar, albeit much broader, O–H
stretching continuum observed in the SFG spectra of aqueous surfaces.
Upon subjecting the SOM samples to cycles in relative humidity (RH)
between <2% RH and ∼95% RH, we observe reversible changes
in the SFG signal intensity across the entire spectral range surveyed
for a polarization combination probing components of the vibrational
transition dipole moments that are oriented parallel to the plane
of incidence, but no signal intensity changes for any other polarization
combination investigated. These results support the notion that the
C–H oscillators at the surfaces of α-pinene-derived SOM
deposited on CaF<sub>2</sub> windows shift back and forth between
two different molecular orientation distributions as the RH is lowered
(more ordered) or raised (less ordered). The findings thus point toward
the presence of a reversible surface switch for hindering (more ordered,
<2%RH) and promoting (less ordered, ∼95%RH) exchange between
the aerosol particle phase and its gas phase