Molecular Characterization of Light Absorbing Compounds in Secondary Organic Aerosols Generated from Photo-oxidation of Toluene

Light absorbing constituents of organic aerosol, or "brown carbon (BrC)", have been recently recognized as an important contributor to climate forcing. However, little is known about fundamental relationship between the chemical composition of BrC and its light absorption properties. Furthermore, both composition and optical properties of BrC are highly variable for SOA generated from different precursors. It has been suggested that among the myriad constituents of organic aerosol only few strong chromophores may determine the overall light absorption properties, making the identification of the chromophores particularly challenging. Toluene is one of the major anthropogenic VOCs in urban air. Chamber studies have shown that secondary organic aerosols (SOA) generated from photo-oxidation of toluene under high NOx conditions exhibit BrC properties in contrast to those formed under low NOx conditions. In this study, liquid chromatography coupled with UV/Vis spectroscopy and high resolution mass spectrometry was used to identify the light absorbing organic molecules in toluene SOA. It is demonstrated that nitroaromatic compounds with 2 or 3 nitro-groups are major BrC chromophores of toluene SOA in visible light range. Their structures are predicted by highly accurate measurement of elemental compositions and fragmentation spectra obtained from MS/MS analysis.