This project developed a method for quantifying the abundance in urban air of larger organic compounds emitted from diesel engine exhaust.  These compounds exist in the gas phase and are thought to be significant contributors to secondary organic aerosol formation (SOA) in urban areas, resulting in PM2.5 exceedances of national air quality standards in summer time.  Diesel engine emissions are becoming a more important source of primary and secondary air pollutants in US urban areas as a result of reductions in emissions and improving fuel efficiency of spark ignition vehicles.  Measuring the abundance of the organic gases emitted from diesel engines, in particular compounds with 12 to 25 carbon atoms (C12 to C25), is analytically challenging due to their low volatility and wide range of isomeric forms.  This project investigated using proton transfer reaction mass spectrometry (PTR-MS) to measure the abundance of organic compounds associated with diesel emissions in lab and field studies, and investigated their atmospheric processing rates in a large flow tube photoreactor that was constructed for this project.  The project involved developing a thermal desorption sampler to accumulate the diesel organic compounds and then desorb them into the PTR-MS instrument.  The instrument was operated at a low electric field strength (80 Td) to reduce the occurrence of dissociation reactions.  We demonstrated that this approach allowed for measuring the total abundance of long chain alkanes (> C12) in laboratory studies of diesel fuel and diesel exhaust.  Field sampling in a winter time environment (Yakima, WA) demonstrated that the technical approach for measuring tracers of diesel engine emissions worked.   Measurement of diesel engine emissions from a diesel- electric generator demonstrated that diesel engines emit a broad of range of monoaromatic compounds, bicyclic aromatic compounds, and cycloalkanes.   The project performed an extensive investigation of the sensitivity and product ion distribution from a wide range of hydrocarbons to better understand the general utility of PTR-MS measurements in identifying vehicle exhaust organics in urban air.  We determined that the PTR-MS is sensitive to long chain alkanes and cycloalkanes but these compounds typically yield M-H+ product ions rather than M+H product ions.

 The study also constructed a 3 foot diameter Teflon film flow tube photoreactor that allowed for the continuous monitoring of exhaust photochemical processing, simulating urban atmopsheric chemical processing.  Exhaust from a diesel-electric generator and diesel and gasoline vehicles was sampled using an ejector diluter which diluted the exhaust up to 100 fold before flowing into the photoreactor.  Measurements were made of NOx, CO, CO2, O3, organic compounds, PM size distribution, and speciated PM using an aerosol mass spectrometer.  Photooxidation of diesel exhaust rapidly produced ozone and organic PM2.5.  However the mass spectrum of the PTR-MS was complicated by the generation of many photoproducts from organic compounds that interfered with the measurement of most aromatics and naphthalene’s.  Photochemical studies of diesel exhaust will require mass spectrometers with mass resolution of at least 4000 to separate the occurrence of photoproducts from these precursors.  The work from this study demonstrated that in principle the PTR-MS analytical technique can be adapted to measure the large organic compounds emitted in diesel engine exhaust and thus will be a useful monitoring tool for diesel engine organic compound emissions in urban environments.

Last Modified: 04/29/2016
Modified by: B. Thomas Jobson