Determining the Optical Properties of Secondary Organic Aerosols using UV/Vis Spectroscopy
Atmospheric light-absorbing carbon (LAC) particles, which include black carbon (BC) and brown carbon (BrC) particles play an important role in global climate change. Currently, optical properties of BC are well defined, with studies showing absorption of light by BC at wavelengths above ~500 nm. At lower wavelengths of the visible light spectrum (300-500 nm), light absorption by BrC may be substantial. However, the extent of absorption is dependent on accurate knowledge of the optical properties of BrC, which are currently not well established. Optical properties of secondary organic aerosols (SOA), a variant of BrC, have been determined, but only for a limited subset of source types and atmospheric conditions. Furthermore, the studies have not quantified optical properties such as mass-specific cross sections (MAC) and complex refractive indices (k). As a result, the extent of their influence in the atmosphere remains poorly understood. In this study, fulvic acid was used as a model to validate the experimental procedure for the analysis of several different samples of varying concentrations and compositions of SOA generated by the oxidation of volatile organic compounds (VOC), specifically 1,2,4-trimethylbenzene. SOA were dissolved in water and analyzed using UV/Vis spectroscopy. Absorption values determined using the UV/Vis spectra were then incorporated into the calculation for MAC and k.