2 research outputs found
Vibrational Sum Frequency Generation Spectroscopy of Secondary Organic Material Produced by Condensational Growth from α‑Pinene Ozonolysis
Secondary organic material (SOM)
was produced in a flow tube from
α-pinene ozonolysis, and collected particles were analyzed spectroscopically
via a nonlinear coherent vibrational spectroscopic technique, namely
sum frequency generation (SFG). The SOM precursor α-pinene was
injected into the flow tube reactor at concentrations ranging from
0.125 ± 0.01 ppm to 100 ± 3 ppm. The oxidant ozone was varied
from 0.15 ± 0.02 to 194 ± 2 ppm. The residence time was
38 ± 1 s. The integrated particle number concentrations, studied
using a scanning mobility particle sizer (SMPS), varied from no particles
produced up to (1.26 ± 0.02) × 10<sup>7</sup> cm<sup>–3</sup> for the matrix of reaction conditions. The mode diameters of the
aerosols increased from 7.7 nm (geometric standard deviation (gsd),
1.0) all the way to 333.8 nm (gsd, 1.9). The corresponding volume
concentrations were as high as (3.0 ± 0.1) × 10<sup>14</sup> nm<sup>3</sup> cm<sup>–3</sup>. The size distributions indicated
access to different particle growth stages, namely condensation, coagulation,
or combination of both, depending on reaction conditions. For filter
collection and subsequent spectral analysis, reaction conditions were
selected that gave a mode diameter of 63 ± 3 nm and 93 ±
3 nm, respectively, and an associated mass concentration of 12 ±
2 μg m<sup>–3</sup> and (1.2 ± 0.1) × 10<sup>3</sup> μg m<sup>–3</sup> for an assumed density of
1200 kg m<sup>–3</sup>. Teflon filters loaded with 24 ng to
20 μg of SOM were analyzed by SFG. The SFG spectra obtained
from particles formed under condensational and coagulative growth
conditions were found to be quite similar, indicating that the distribution
of SFG-active C–H oscillators is similar for particles prepared
under both conditions. The spectral features of these flow-tube particles
agreed with those prepared in an earlier study that employed the Harvard
Environmental Chamber. The SFG intensity was found to increase linearly
with the number of particles, consistent with what is expected from
SFG signal production from particles, while it decreased at higher
mass loadings of 10 and 20 μg, consistent with the notion that
SFG probes the top surface of the SOM material following the complete
coverage of the filter. The linear increase in SFG intensity with
particle density also supports the notion that the average number
of SFG active oscillators per particle is constant for a given particle
size, that the particles are present on the collection filters in
a random array, and that the particles are not coalesced. The limit
of detection of SFG intensity was established as 24 ng of mass on
the filter, corresponding to a calculated density of about 100 particles
in the laser spot. As established herein, the technique is applicable
for detecting low particle number or mass concentrations in ambient
air. The related implication is that SFG is useful for short collection
times and would therefore provide increased temporal resolution in
a locally evolving atmospheric environment
On Surface Order and Disorder of α‑Pinene-Derived Secondary Organic Material
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