Vibrational Sum Frequency Generation Spectroscopy
of Secondary Organic Material Produced by Condensational Growth from
α‑Pinene Ozonolysis
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Abstract
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