Light extinction by atmospheric particles is strongly dependent on the size, chemical composition, and water content of the aerosol. Since light extinction by particles directly impacts climate and visibility, measurements of the extinction at various relative humidities (RHs) are needed. In this work, the optical growth factors, f RHext (80%RH, Dry) have been measured using cavity ring-down aerosol extinction spectroscopy at 532 nm for particles of varying organic/sulfate compositions. Specifically, slightly soluble, multifunctional aromatic compounds resulting from biomass burning have been investigated. In general, the organic compounds studied exhibit much smaller optical growth than inorganic compounds such as ammonium sulfate. Also, a linear relationship between mass fraction organic and optical growth has been observed for most organic compounds studied, in agreement with previous studies of more water-soluble organics. The role of particle density for mixtures that do not follow a linear relationship is also explored.