Quantification of diesel fuel semi-olatile organic compounds by PTR-MS

Description

The formation of secondary organic aerosol (SOA) in urban areas is not well understood, with models underestimating organic mass loadings by up to a factor of 10. It is thought that there is a large pool of organic carbon that goes unmeasured in urban areas due to its semi-volatile nature and the inherent difficulty in measuring low mixing ratios of such species by gas chromatographic techniques. It has been proposed (reference) that components of organic particulate matter emitted from diesel exhaust might partition back to the gas phase as the exhaust plume becomes diluted, creating a pool of reactive semi-volatile compounds that act as precursors for SOA. For the upcoming CARES field experiment in Sacramento, CA, we will deploy a new sampling method to measure long-chain alkanes and other constituents of diesel exhaust that may contribute to SOA. The approach will be to use a proton transfer reaction mass spectrometer (PTR-MS) to measure organics coupled to a custom-built thermal desorption system that pre-concentrates SVOC from the air. Laboratory experiments show that the PTR-MS has an equivalent sensitivity to n-alkanes larger than C12, indicating that direct protonation of these species by the H3O+ reagent ion. For n-alkanes with fewer than seven carbon atoms, there is no apparent reaction with H3O+. The response to n-alkanes > C12 was measured to be 1/5 of that of toluene. The alkanes were shown to fragment to a common set of ions, allowing for the total n-alkane and iso-alkane abundance to be determined from the measurement of six ion fragments. Analysis of diesel #2 fuel by PTR-MS shows the fuel is dominated by alkanes, as expected, but with significant abundance of alkylbenzenes (up to mass of 232 amu) that may be important urban air SOA precursors. The poster will describe the lab experiments to quantify PTR-MS response to n-alkanes and other diesel exhaust components and describe initial results of the thermal desorption sampling of SVOC by PTR-MS.