Partitioning of Formic and Acetic Acids to the Gas/Aerosol Interface

Atmospheric aerosol particles are abundant and ubiquitous in both indoor and outdoor environments. Organic acid species in aerosols have an understudied effect on the environment and human well-being due to their unique interfacial properties. While it is widely accepted that the chemical and physical evolution of aerosol particles in the atmosphere is closely connected to interface-related phenomena, interface-specific methods suitable for investigating these topics in situ have only recently become available. In this study, we elaborate on the recent development of vibrational sum frequency scattering (VSFS) spectroscopy as an interface-specific method for the in situ investigation of the surfaces of submicron aerosol particles suspended in gas. We demonstrate the ability of VSFS to characterize formic and acetic acids, alone and together, at the surfaces of aqueous aerosol particles. We evaluate molecular ordering at the gas/aerosol particle interface by examining polarization-resolved VSFS spectra. Lastly, we quantitatively analyze the interfacial affinity of simple organics for the air/ liquid interface, for submicron aerosol surfaces and bulk planar liquid surfaces. These results further support the use of VSFS as an analytical technique for probing the surfaces of small droplets and aerosols and provide insights into differences between the chemistry of planar and highly curved surfaces, with potential significance for atmospheric aerosol chemistry.