Specific organic compounds have been used as tracers of biological and anthropogenic source inputs to the marine atmosphere1–8. This tracer approach uses the unique molecular signatures or fingerprints for various lipid compound classes of marine and terrestrial plants to identify the sources of the organic substances in aerosols9. However, many of these substances are not stable and undergo transformation reactions. For example, unsaturated fatty acids, which are major constituents of marine and terrestrial plants, have not been frequently detected in remote marine aerosols3,6,7. The mechanism for the transformation of unsaturated fatty acids in the atmosphere has not been elucidated. In this paper, we report the discovery of a homologous series of ω-oxocarboxylic acids (C4–C14; C9 being maximum) in marine aerosols. We propose that these compounds are oxidation products of unsaturated fatty acids in the marine environment. The discovery of ω-oxoacids, along with additional aerosol, sea water, and rain data for mono- and di-carboxylic acids, has led us to postulate a photo-induced oxidative reaction scheme for unsaturated fatty acids in the marine atmosphere and surface seawater.