Molecular characterization of organic aerosols using high-resolution mass spectrometry

Description

Characterization of composition and chemical transformations of OA is both a major challenge and the area of greatest uncertainty in aerosol research. Particularly, little is known about the fundamental relationship between the chemical composition and physicochemical properties of OA. The effect of atmospheric aging on these properties is also poorly understood. Applications of high-resolution electrospray ionization mass spectrometry (HR-ESI/MS) for structural characterization of OA constituents has been demonstrated recently, and currently this is a rapidly growing area of research in aerosol chemistry. We have conducted a number of field and laboratory studies that utilized HR-ESI/MS for comprehensive characterization of OA. We used HR-ESI/MS for characterization of the chemical composition of biomass burning aerosols (BBA). Our results indicated that BBA contained a variety of distinct, biomass-specific, characteristic peaks in ESI/MS spectra that can be used as unique markers for different types of biofuels. In addition to a large number of oxygen-containing polar organic compounds, we identified a significant number of N-heterocyclic compounds that have not been previously described in the literature. Because of the presence of non-bonding electrons and double bonds, N-heterocylcic compounds often contain chromophore moieties responsible for light absorption in the UV/Vis region. Our current projects focus on the application of high-resolution desorption electrospray ionization mass spectrometry (DESI-MS) for detailed chemical characterization and studies of chemical aging of OA collected on substrates. DESI-MS offers unique advantages both for detailed characterization of chemically labile components in OA that cannot be detected using more traditional ESI-MS approach, and for studying chemical aging of OA. DESI-MS combined with MS/MS experiments were used to examine chemical aging of SOA in the presence of gaseous ammonia. Exposure of SOA to ammonia resulted in measurable changes in the light absorption properties of the sample. We demonstrated that ammonia-mediated chemical aging results in formation of highly conjugated nitrogen-containing species that are responsible for light-absorbing properties of the aged SOA. That study presented an important step towards understanding the formation of light-absorbing OA (brown carbon) in the atmosphere.