How to parametrize the absorptivity of organic aerosols in atmospheric radiative models remains uncertain. Here we report that the lambda = 400 nm absorbers in model aerosol mixtures elute as weakly polar species in reversed-phase chromatography. Typical among them, the m/z = 269 (C12H13O7, O/C = 0.58) isomers detected by mass spectrometry possess C=O groups linked by C=C bridges. More polar species, such as the m/z = 289 (C11H13O9, O/C = 0.82) polyacids, are instead colorless. On this basis we argue that visible absorptivity, which develops from extended conjugation among chromophores, may not increase monotonically with oxidation level. (C) 2014 Elsevier B.V. All rights reserved.