Understanding the fundamental interactions within and at the surface of atmospheric aerosol is of the utmost importance as they drive the properties of aerosol that influence global climate and public health. The first work presented herein explores the highly perturbed structure of water within systems inspired by phase separated organic aerosol. An approach is taken that combines polarized Raman spectroscopy and molecular dynamics to reveal the structural changes that occur as water is added incrementally to propylene carbonate (PC), a polar, aprotic solvent that is relevant in the environment and in electrochemical systems. Polarized Raman spectra of PC solutions were collected for water mole fractions 0.003 ≤ Χwater ≤ 0.296, which encompasses the solubility range of water in PC. The novel approach taken to the study of water-in-PC mixtures herein provides additional hydrogen bond and solvation characterization of this system that has not been achieveable in previous studies. Analysis of the polarized carbonyl Raman band in conjunction with simulations demonstrated that the bulk structure of the solvent remained unperturbed upon the addition of water. Experimental spectra in the O-H stretching region were decomposed through Gaussian fitting into sub-bands and studies on dilute HOD in H2O. With the aid of simulations, we identified these different bands as water arrangements having different degrees of hydrogen bonding. The observed water structure within PC indicates that water tends to self-aggregate, forming a hydrogen bond network that is distinctly different from the bulk and dependent on concentration. For example, at moderate concentrations, the most likely aggregate structures are chains of water molecules, each with two hydrogen bonds on average.
The interaction of NO2 with organic interfaces is critical in atmospheric processing of marine and continental aerosol as well as in the development of NO2 sensing and trapping technologies. Recent studies point to the importance of surface oxygen groups in these systems, however the role of specific functional groups on the microscopic level has yet to be fully established. In the second part of this work, the aim is to provide fundamental information on the behavior of NO2 at atmospherically relevant organic interfaces that may also help inform innovation in NO2 sensing and trapping development. An investigation into the structural changes induced by NO2 at the surface of PC, an environmentally relevant carbonate ester, is presented. Surface-sensitive spectra of the PC liquid surface are acquired before, during, and after exposure to NO2 using infrared reflection-absorption spectroscopy (IRRAS). Spectra reveal that NO2 preferentially interacts with the carbonyl of PC at the interface, forming a distribution of binding symmetries. At low ppm levels, NO2 saturates the PC surface within 10 minutes and the perturbations to the surface are constant over time during the flow of NO2. Upon removal of NO2 flow, and under atmospheric pressures, these interactions are reversible, and the liquid surface structure of PC recovers completely within 30 min. Another ester-containing molecule, diethyl sebacate (DES), was also investigated and the same carbonyl-specific interaction with NO2 was intermittently observed.
N2O5 is an important atmospheric oxidant that plays a critical role in the nighttime chemistry of the troposphere, where it is efficiently hydrolyzed at the surface of aerosol particles to form HNO3. The decomposition of N2O5 at the interface proceeds through a unique pathway that requires the separation of the charged species NO2+ and NO3– to be stabilized. This property of N2O5 makes it an ideal probe molecule to inform on the fundamental properties that lead to charge stabilization at soft interfaces (i.e. air/liquid interfaces). The final part of this work describes the design, construction, and troubleshooting of a novel system wherein N2O5 is synthesized and flowed over the surface of low vapor pressure organic liquids (propylene carbonate and diethyl sebacate). Infrared reflection-absorption spectroscopy enables the elucidation of the surface structure before, during, and after N2O5 exposure. IRRAS experiments reveal a side reaction with KBr windows, leading to the buildup of HNO3 over time. Additional analysis of the N2O5 flow demonstrates that there is significant water contamination, leading to high concentrations of HNO3 gas. Possible solutions to remove excess water are discussed. Preliminary results of the N2O5 flow over the surface of PC and DES demonstrate a reversible perturbation to the carbonyl mode for each organic, much like that observed in the second work presented here.