Bulk Contributions Modulate the Sum-Frequency Generation Spectra of Water on Model Sea-Spray Aerosols

Vibrational sum-frequency generation (vSFG) spectroscopy is used to determine the molecular structure of water at a model sea-spray aerosol surface. Both measured and calculated spectra display specific features as a result of third-order contributions to the vSFG response, and these are associated with finite interfacial electric potentials. We demonstrate that theoretical modeling enables separation of the third-order contributions, thus allowing for a systematic analysis of the strictly surface-sensitive, second-order component of the vSFG response. This study provides fundamental insights into the interfacial molecular organization and hydrogen-bonding structure of water, which mediate heterogeneous processes on sea-spray aerosols. Our results emphasize the key role that computer simulations can play in interpreting vSFG spectra and revealing microscopic details at complex aqueous interfaces, which can be difficult to extract from experiments because of the mixing of second-order, surface-sensitive, and third-order bulk-dependent contributions to the vSFG response. Generated by various sources, aerosol particles influence Earth's radiative budget and affect air quality, ecosystems, and public health. By providing a link between the oceans and the atmosphere, sea-spray aerosols play a critical yet poorly understood role in determining Earth's climate. In this study, we combine vibrational spectroscopy with computer simulations to characterize the molecular structure of model sea-spray particles. It is found that electrical potentials present at the particle surfaces generate bulk-dependent effects that modulate the spectral features. Surface-sensitive contributions are then extracted from computer simulations, allowing for an unambiguous characterization of the surface region. This information is key to the molecular-level understanding of fundamental processes, such as chemical reactions and phase transformations, which determine the ability of sea-spray particles to scatter or absorb solar radiation and promote cloud formation. Sea-spray aerosol particles have major yet poorly understood influence on the state of the atmosphere. Although non-linear vibrational spectroscopy is a reliable technique for probing the nature of aerosol interfaces, resolving the spectral features into specific structural and dynamical properties of the interface poses substantial difficulties. Here, computer simulations are used to disentangle strictly surface-sensitive contributions from bulk-dependent effects at a model sea-spray aerosol, which allows for a detailed, molecular-level characterization of the interfacial properties.