Molecular level Insights on the Photosensitized Chemistry of Nonanoic Acid in the Presence of 4-Benzoylbenzoic Acid at the Sea Surface Microlayer

Abstract. Atmospheric chemistry and aerosol-water interactions significantly impact Earth's climate by influencing the energy budget. Organic compounds concentrated at air-water interfaces, such as the sea-surface microlayer (SML), are key contributors to atmospheric aerosols and undergo complex photochemical reactions. This study combines sum-frequency generation (SFG) spectroscopy and mass spectrometry (MS) to investigate the photochemical interactions of nonanoic acid (NA) and 4-benzoylbenzoic acid (4-BBA) at the air-water interface under varying solar spectra, pH, and salinity conditions. SFG spectroscopy detected aromatic signals at the interface, unreported in prior studies using bulk techniques, highlighting the partitioning of non-surface-active compounds to the organic surface layer. The study demonstrates that 4-BBA acts both as a photosensitizer and a photoproduct precursor, with its photolysis being more active under shorter UV wavelengths. Reaction mechanisms were found to depend on solar spectrum, pH, and salinity, with salinity accelerating photoreaction rates by increasing surface concentrations of 4-BBA. These findings emphasize the need to account for environmental variables such as light intensity, geographic location, and atmospheric conditions when modeling photochemical processes. The results provide insights into surface-bulk photochemical coupling and their implications for aerosol formation across diverse natural water systems, from oceans to cloud droplets.