  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.
