  Saturated fatty acids are abundant organic compounds in oceans and sea
sprays. Their photochemical reactions induced by solar radiation have recently
been discovered as an abiotic source of volatile organic compounds, which serve
as precursors of secondary organic aerosols. However, photoabsorption of
wavelengths longer than 250 nm in liquid saturated fatty acids remains
unexplained, despite being first reported in 1931. Here we demonstrate that the
previously reported absorption of wavelengths longer than 250 nm by liquid
nonanoic acid [CH3(CH2)7COOH)] originates from traces of impurities (0.1% at
most) intrinsically contained in nonanoic acid reagents. Absorption cross
sections of nonanoic acid newly obtained here indicate that the upper limit of
its photolysis rate is three-to-five orders of magnitude smaller than those for
atmospherically relevant carbonyl compounds.
