Molecular dynamics simulations and vibrational sum frequency generation (VSFG) experiments in the methyl-stretching spectral region have been used to study acetonitrile at the silica/liquid, silica/vapor, and liquid/vapor interfaces. Our simulations show that, at the silica/liquid interface, acetonitrile takes on a considerably different structure than in the bulk liquid. The interfacial structure is reminiscent of a lipid bilayer, and this type of ordering persists for tens of Ångstroms into the bulk liquid. This result has important implications for processes involving solid/acetonitrile interfaces, such as heterogeneous catalysis and chromatographic separations. Fitting of VSFG data that have an extremely low nonresonant background contribution provides strong evidence for interfacial populations pointing in opposite directions at these interfaces, in agreement with our simulations. The picture developed from our simulations and experiments reconciles conflicting interpretations of data from previous experimental studies of interfacial acetonitrile.

Previous experiments and simulations have shown that acetonitrile organizes into a lipid-like bilayer at the liquid/silica interface. Recent simulations have further suggested that this bilayer structure persists in mixtures of acetonitrile with water, even at low acetonitrile concentrations. This behavior is indicative of microscopic phase separation of these liquids near silica interfaces and may have important ramifications for the use of acetonitrile in chromatography and heterogeneous catalysis. To explore this phenomenon, we have used vibrational sum-frequency-generation spectroscopy to probe acetonitrile/water mixtures at a silica interface. Our spectra provide evidence that acetonitrile partitions to the hydrated silica interface even when the mole fraction of acetonitrile is as low as 10%. A blue shift is observed in the spectrum of the methyl symmetric stretch upon increasing water mole fraction, in agreement with vibrational spectra of bulk mixtures. Line shape analysis suggests that acetonitrile may exist in the form of bilayer patches at high water mole fractions.

The ability of two techniques, aerosol cavity ring down spectroscopy (A-CRDS) and optical tweezers, to retrieve the refractive index of atmospherically relevant aerosol was compared through analysis of supersaturated sodium nitrate at a range of relative humidities. Accumulation mode particles in the diameter range 300-600 nm were probed using A-CRDS, with optical tweezer measurements performed on coarse mode particles several micrometers in diameter. A correction for doubly charged particles was applied in the A-CRDS measurements. Both techniques were found to retrieve refractive indices in good agreement with previously published results from Tang and Munkelwitz, with a precision of ±0.0012 for the optical tweezers and ±0.02 for the A-CRDS technique. The coarse mode optical tweezer measurements agreed most closely with refractive index predictions made using a mass-weighted linear mixing rule. The uncertainty in the refractive index retrieved by the A-CRDS technique prevented discrimination between predictions using both mass-weighted and volume-weighted linear mixing rules. No efflorescence or kinetic limitations on water transport between the particle and the gas phase were observed at relative humidities down to 14%. The magnitude of the uncertainty in refractive index retrieved using the A-CRDS technique reflects the challenges in determining particle optical properties in the accumulation mode, where the extinction efficiency varies steeply with particle size.

Biomass combustion produces black carbon (BC) and brown carbon (BrC) aerosols that contribute substantially to warming the Earth's atmosphere. Accurate knowledge of their emissions and absorption per unit mass (mass absorption cross-section; MAC) can be used to quantify the radiative impact of these combustion products. We isolated particles generated from laboratory biomass burning fires by morphology and found that some particles from biomass burning do not correspond to either BC or BrC according to common operational definitions. Unlike BrC, these particles strongly absorb red light, with a MAC and spectral dependence of absorption between that of BrC and BC. They also have intermediate volatility: they survive thermodenuding at 250 C but do not heat to incandescence in a single particle soot photometer (SP2) instrument. We also found evidence for intermediate properties in ambient wildfire smoke from the 2013 Rim Fire in California. More work is needed to understand how much this intermediate material contributes to atmospheric light absorption from typical combustion, whether or not it corresponds to "tar balls," and how it may affect previous MAC measurements that were attributed to enhanced absorption by transparent coatings.