Vibrational sum frequency generation spectroscopy of secondary organic material produced by condensational growth from α-pinene ozonolysis

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) × 107 cm-3 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) × 1014 nm3 cm-3. 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-3 and (1.2 ± 0.1) × 103 μg m-3 for an assumed density of 1200 kg m-3. 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. © 2013 American Chemical Society.