Direct Observation and Quantitative Measurement of OH Radical Desorption During the Ultraviolet Photolysis of Liquid Nonanoic Acid

Ultraviolet (UV) photolysis of fatty acid surfactants which cover the surfaces of atmospheric liquid aerosols and are found in the oceans such as nonanoic acid (NA) has recently been suggested as a source of hydroxyl (OH) radicals in the troposphere. We used laser-induced fluorescence to directly observe OH radicals desorbed from the surface of neat liquid NA as a primary photoproduct following 213 nm irradiation. The upper limit of photoreaction cross section for the OH radical desorption was estimated to be 9.0(4.1) $\times$ 10$^{-22}$ cm$^2$, which is only 1.2$\pm$0.8% of the photoreaction cross section established for the photolysis of gas-phase acetic acid monomers. Vibrational sum-frequency generation spectroscopy for liquid NA revealed the hydrogen-bonded, cyclic, dimer structure of the NA molecules at the liquid surface. This dimerization can inhibit the formation of OH radicals and lead the present low photochemical reactivity of liquid NA.