4 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
Vibrational Mode Assignment of α‑Pinene by Isotope Editing: One Down, Seventy-One To Go
This study aims to reliably assign
the vibrational sum frequency
generation (SFG) spectrum of α-pinene at the vapor/solid interface
using a method involving deuteration of various methyl groups. The
synthesis of five deuterated isotopologues of α-pinene is presented
to determine the impact that removing contributions from methyl group
C–H oscillators has on its SFG response. 0.6 cm<sup>–1</sup> resolution SFG spectra of these isotopologues show varying degrees
of differences in the C–H stretching region when compared to
the SFG response of unlabeled α-pinene. The largest spectral
changes were observed for the isotopologue containing a fully deuterated
vinyl methyl group. Noticeable losses in signal intensities allow
us to reliably assign the 2860 cm<sup>–1</sup> peak to the
vinyl methyl symmetric stretch. Furthermore, upon removing the vinyl
methyl group entirely by synthesizing apopinene, the steric influence
of the unlabeled C<sub>9</sub>H<sub>14</sub> fragment on the SFG response
of α-pinene SFG can be readily observed. The work presented
here brings us one step closer to understanding the vibrational spectroscopy
of α-pinene
Accurate Line Shapes from Sub‑1 cm<sup>–1</sup> Resolution Sum Frequency Generation Vibrational Spectroscopy of α‑Pinene at Room Temperature
Despite the importance of terpenes
in biology, the environment,
and catalysis, their vibrational spectra remain unassigned. Here,
we present subwavenumber high-resolution broad-band sum frequency
generation (HR-BB-SFG) spectra of the common terpene (+)-α-pinene
that reveal 10 peaks in the C–H stretching region at room temperature.
The high spectral resolution resulted in spectra with more and better
resolved spectral features than those of the Fourier transform infrared,
femtosecond stimulated Raman spectra in the bulk condensed phase and
those of the conventional BB-SFG and scanning SFG spectroscopy of
the same molecule on a surface. Experiment and simulation show the
spectral line shapes with HR-BB-SFG to be accurate. Homogeneous vibrational
decoherence lifetimes of up to 1.7 ps are assigned to specific oscillators
and compare favorably to lifetimes computed from density functional
tight binding molecular dynamics calculations. Phase-resolved spectra
provided their orientational information. We propose the new spectroscopy
as an attractive alternative to time domain vibrational spectroscopy
or heterodyne detection schemes for studying vibrational energy relaxation
and vibrational coherences in molecules at molecular surfaces or interfaces
Size-Dependent Changes in Sea Spray Aerosol Composition and Properties with Different Seawater Conditions
A great deal of uncertainty exists
regarding the chemical diversity
of particles in sea spray aerosol (SSA), as well as the degree of
mixing between inorganic and organic species in individual SSA particles.
Therefore, in this study, single particle analysis was performed on
SSA particles, integrating transmission electron microscopy with energy
dispersive X-ray analysis and scanning transmission X-ray microscopy
with near edge X-ray absorption fine structure spectroscopy, with
a focus on quantifying the relative fractions of different particle
types from 30 nm to 1 μm. SSA particles were produced from seawater
in a unique ocean-atmosphere facility equipped with breaking waves.
Changes to the SSA composition and properties after the addition of
biological (bacteria and phytoplankton) and organic material (ZoBell
growth media) were probed. Submicrometer SSA particles could be separated
into two distinct populations: one with a characteristic sea salt
core composed primarily of NaCl and an organic carbon and Mg<sup>2+</sup> coating (SS-OC), and a second type consisting of organic carbon
(OC) species which are more homogeneously mixed with cations and anions,
but not chloride. SS-OC particles exhibit a wide range of sizes, compositions,
morphologies, and distributions of elements within each particle.
After addition of biological and organic material to the seawater,
a change occurs in particle morphology and crystallization behavior
associated with increasing organic content for SS-OC particles. The
fraction of OC-type particles, which are mainly present below 180
nm, becomes dramatically enhanced with increased biological activity.
These changes with size and seawater composition have important implications
for atmospheric processes such as cloud droplet activation and heterogeneous
reactivity