On Surface Order and Disorder of α-Pinene-Derived Secondary Organic Material

Abstract

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(-1) 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 CaF2 windows using electrostatic deposition. We find both samples yield comparable SFG spectra featuring an intense peak at 2940 cm(-1) 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(-1) 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 CaF2 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.

Kinetically controlled glass transition measurement of organic aerosol thin films using broadband dielectric spectroscopy

“…Recent studies have confirmed that organic aerosols, which comprise approximately half of the total submicron aerosol mass in the atmosphere, can change from liquid to glassy state at ambient humidity levels and temperatures (Zobrist et al, 2008;Virtanen et al, 2010;Shrestha et al, 2014;Zhang et al, 2015). The effect of temperature may be especially important when aerosol particles are lifted into the free troposphere, where the temperature change can rapidly alter their phase from liquid to glass .…”

Changing shapes and implied viscosities of suspended submicron particles

“…An underestimated viscosity and hence overestimated diffusion coefficient can result in an overprediction of particle mass concentrations as well as an underprediction of gasphase concentrations (Shiraiwa and Seinfeld, 2012). Particle reactivity (Shiraiwa et al, 2011;Kuwata and Martin, 2012a;Zelenyuk et al, 2012;Zhou et al, 2013;Slade and Knopf, 2014), number concentrations (Riipinen et al, 2011), and diameters (Shiraiwa et al, 2013b) can also be influenced by viscosities and diffusion rates, in turn influencing model predictions related to air quality and climate.…”

Atmospheric β-Caryophyllene-Derived Ozonolysis Products at Interfaces

“…A comprehensive description of the Harvard flow tube reactor used in this work can be found in our previously published work. 25,91 Briefly, β-caryophyllene was introduced into the flow tube as a solution of β-caryophyllene (≥98.5%, Sigma-Aldrich Inc.) diluted in 1-butanol 25,[92][93] at selected injection rates that altered the gas-phase concentration in the range of 300-500 ppb, which subsequently changed the organic particle mass loading in the flow tube.…”

“…Spectra were referenced to the ppp-polarized nonresonant SFG response of gold deposited on fused silica to account for the incident IR energy distribution, and frequencies were calibrated using a polystyrene film. 23,91,[100][101] For the high resolution SFG system, vapor phase compound spectra were acquired by placing 1-2 drops of liquid sample at the edge of a shallow Teflon beaker that was then capped with a fused silica optical window. The vapor was allowed to equilibrate for ~10-45 minutes before spectral data acquisition.…”