11 research outputs found
Organic Constituents on the Surfaces of Aerosol Particles from Southern Finland, Amazonia, and California Studied by Vibrational Sum Frequency Generation
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
Stereochemical transfer to atmospheric aerosol particles accompanying the oxidation of biogenic volatile organic compounds
Asymmetric emission profiles of the stereoisomers of plant-derived volatile organic compounds vary with season, geography, plant type, and stress factors. After oxidation of these compounds in the atmosphere, the low-vapor pressure products ultimately contribute strongly to the particle-phase material of the atmosphere. In order to explore the possibility of stereochemical transfer to atmospheric aerosol particles during the oxidation of biogenic volatile organic compounds, second-order coherent vibrational spectra were recorded of the particle-phase organic material produced by the oxidation of different stereoisomeric mixes of alpha-pinene. The spectra show that the stereochemical configurations are not scrambled but instead are transferred from the gas-phase molecular precursors to the particle-phase molecules. The spectra also show that oligomers formed in the particle phase have a handed superstructure that depends strongly and nonlinearly on the initial stereochemical composition of the precursors. Because the stereochemical mix of the precursors for a material can influence the physical and chemical properties of that material, our findings suggest that chirality is also important for such properties of plant-derived aerosol particles. Citation: Ebben, C. J., S. R. Zorn, S.-B. Lee, P. Artaxo, S. T. Martin, and F. M. Geiger (2011), Stereochemical transfer to atmospheric aerosol particles accompanying the oxidation of biogenic volatile organic compounds, Geophys. Res. Lett., 38, L16807, doi: 10.1029/2011GL048599.National Science Foundation Atmospheric Chemistry divisionNational Science Foundation Atmospheric Chemistry division[NSF ATM-0533436]U.S. Department of Energy (DOE)Office of Science (BES), U.S. Department of Energy (DOE)[DE-FG02-08ER64529
On molecular chirality within naturally occurring secondary organic aerosol particles from the central Amazon Basin
In this perspectives article, we reflect upon the existence of chirality in atmospheric aerosol particles. We then show that organic particles collected at a field site in the central Amazon Basin under pristine background conditions during the wet and dry seasons consist of chiral secondary organic material. We show how the chiral response from the aerosol particles can be imaged directly without the need for sample dissolution, solvent extraction, or sample preconcentration. By comparing the chiral-response images with optical images, we show that chiral responses always originate from particles on the filter, but not all aerosol particles produce chiral signals. The intensity of the chiral signal produced by the size resolved particles strongly indicates the presence of chiral secondary organic material in the particle. Finally, we discuss the implications of our findings on chiral atmospheric aerosol particles in terms of climate-related properties and source apportionment.Northwestern Initiative for Sustainability and Energy at Northwestern (ISEN)National Science Foundation NSFAchievement Rewards for College Scientists (ARCS)Schlumberger Oilfield Chemical Products, LLCFAPESPCNPqOffice of Science (BES), U.S. Department of Energy (DOE)[DE-FG02-08ER64529]National Science Foundation Atmospheric Chemistry division[NSF ATM-0533436]Division of Chemistry[CHE-0937460]Irving
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
Size-Resolved Sea Spray Aerosol Particles Studied by Vibrational Sum Frequency Generation
We present vibrational sum frequency
generation (SFG) spectra of
the external surfaces and the internal interfaces of size-selected
sea spray aerosol (SSA) particles generated at the wave flume of the
Scripps Hydraulics Laboratory. Our findings support SSA particle models
that invoke the presence of surfactants in the topmost particle layer
and indicate that the alkyl chains of surfactant-rich SSA particles
are likely to be disordered. Specifically, the SFG spectra suggest
that across the range of sizes studied, surfactant-rich SSA particles
contain CH oscillators that are subject to molecular orientation distributions
that are broader than the narrow molecular distribution functions
associated with well-ordered and well-aligned alkyl chains. This result
is consistent with the interpretation that the permeability of organic
layers at SSA particle surfaces to small reactive and nonreactive
molecules may be substantial, allowing for much more exchange between
reactive and nonreactive species in the gas or the condensed phase
than previously thought. The SFG data also suggest that a one-component
model is likely to be insufficient for describing the SFG responses
of the SSA particles. Finally, the similarity of the SFG spectra obtained
from the wave flume microlayer and 150 nm-sized SSA particles suggests
that the SFG active CH oscillators in the topmost layer of the wave
flume and the particle accumulation mode may be in similar chemical
environments. Needs for additional research activities are discussed
in the context of the results presented
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
Stereochemical transfer to atmospheric aerosol particles accompanying the oxidation of biogenic volatile organic compounds
ClNO2 yields from aircraft measurements during the 2015 WINTER campaign and critical evaluation of the current parameterization
Nitryl chloride (ClNO2) plays an important role in the budget and distribution of tropospheric oxidants, halogens, and reactive nitrogen species. ClNO2 is formed from the heterogeneous uptake and reaction of dinitrogen pentoxide (N2O5) on chloride-containing aerosol, with a production yield, ϕ(ClNO2), defined as the moles of ClNO2 produced relative to N2O5 lost. The ϕ(ClNO2) has been increasingly incorporated into 3-D chemical models where it is parameterized based on laboratory-derived kinetics and currently accepted aqueous-phase formation mechanism. This parameterization models ϕ(ClNO2) as a function of the aerosol chloride to water molar ratio. Box model simulations of night flights during the 2015 Wintertime INvestigation of Transport, Emissions, and Reactivity (WINTER) aircraft campaign derived 3,425 individual ϕ(ClNO2) values with a median of 0.138 and range of 0.003 to 1. Comparison of the box model median to those predicted by two other field-based ϕ(ClNO2) derivation methods agreed within a factor of 1.3, within the uncertainties of each method. In contrast, the box model median was 75–84% lower than predictions from the laboratory-based parameterization (i.e., [parameterization − box model]/parameterization). An evaluation of factors influencing this difference reveals a positive dependence of ϕ(ClNO2) on aerosol water, opposite to the currently parameterized trend. Additional factors may include aqueous-phase competition reactions for the nitronium ion intermediate and/or direct ClNO2 loss mechanisms. Further laboratory studies of ClNO2 formation and the impacts of aerosol water, sulfate, organics, and ClNO2 aqueous-phase reactions are required to elucidate and quantify these processes on ambient aerosol, critical for the development of a robust ϕ(ClNO2) parameterization