Mid-infrared complex refractive indices for oleic acid and optical properties of model oleic acid/water aerosols

Abstract

Frequency-dependent, complex refractive indices for oleic acid in the mid-infrared have been retrieved from aerosol extinction spectroscopy from 750 to 5000 cm⁻¹. The major absorption bands can be attributed to the relevant vibrational modes of the acid, including those that originate with the hydrogen bond induced dimerization through the carboxylic group. The newly reported indices for oleic acid are used to investigate the optical properties of model core–shell aerosols formed by coating a water droplet with a layer of oleic acid. The ability of a non-linear least-squares program to retrieve the structural parameters for core–shell particles is also explored. The initial results indicate that these parameters can be determined for such aerosols as long as the particles give rise to a sufficient amount of scattering in their extinction spectra. Low-scattering spectra, however, can yield suspicious results. Some limits on the application of the retrieval procedure to particles observed in the field are also discussed.