title: 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.
authors.affiliation: †Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States.
authors.forename: Mona
authors.initials: M
authors.lastname: Shrestha
authors.forename: Yue
authors.initials: Y
authors.lastname: Zhang
authors.affiliation: †Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States.
authors.forename: Mary Alice
authors.initials: MA
authors.lastname: Upshur
authors.forename: Pengfei
authors.initials: P
authors.lastname: Liu
authors.affiliation: §Department of Chemistry, University of California, 1102 Natural Sciences 2, Irvine, California 92697, United States.
authors.forename: Sandra L
authors.initials: SL
authors.lastname: Blair
authors.affiliation: ∥Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, 3335 Innovative Boulevard, Richland, Washington 99354, United States.
authors.forename: Hong-Fei
authors.initials: HF
authors.lastname: Wang
authors.affiliation: §Department of Chemistry, University of California, 1102 Natural Sciences 2, Irvine, California 92697, United States.
authors.forename: Sergey A
authors.initials: SA
authors.lastname: Nizkorodov
authors.affiliation: †Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States.
authors.forename: Regan J
authors.initials: RJ
authors.lastname: Thomson
authors.forename: Scot T
authors.initials: ST
authors.lastname: Martin
authors.affiliation: †Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States.
authors.forename: Franz M
authors.initials: FM
authors.lastname: Geiger
mesh_terms: D000336:Aerosols / Q000737:chemistry; D001272:Atmosphere / Q000737:chemistry; D000081008:Bicyclic Monoterpenes; D018625:Microscopy, Atomic Force; D039821:Monoterpenes / Q000737:chemistry*; D013057:Spectrum Analysis; D014867:Water / Q000737:chemistry
chemical_list: D000336:Aerosols; D000081008:Bicyclic Monoterpenes; D039821:Monoterpenes; D014867:Water; C005451:alpha-pinene