Secondary organic aerosol (SOA) particles have been proposed to undergo phase transitions as a function of relative humidity (RH). While indirect measurements of this process are available in the literature, the surfaces of SOA particles have not yet been probed at high or low RH. Here, we report results from surface-selective vibrational spectroscopic measurements of SOA particles derived from a-pinene in a flow tube and subsequently collected on Teflon filters that were subsequently pressed onto optical windows to transfer SOA particles on to them. SOA particles were also collected directly onto optical windows by electrostatic deposition. The windows obtained from both methods were then placed into an RH-controlled flow cell. Vibrational sum frequency generation (SFG) spectra obtained in the CH stretching region from the surfaces of SOA particles show one dominant spectral feature at 2940 cm-1 that is attributed to a combination of methyl asymmetric and methylene symmetric stretching modes. RH-cycling experiments reveal that the SFG intensity at 2940 cm-1 is up to 32�6% higher at ~0% RH than at ~95% RH. Given that SFG signal intensity reductions are generally associated with increased surface disorder and concomitant loss in net alignment of oscillators, we attribute our observations to substantial increases in surface disorder at high RH. The RH-dependent SFG signal intensity variations decrease in magnitude of observed change in the SFG signal intensity as the particle material density increases.

Vibrational sum frequency generation spectroscopy of secondary organic aerosol particle surfaces as a function of relative humidity

248th National Meeting (2014)

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