ABSTRACT
Particulate and vapor phase emissions in the diluted exhaust of a light-duty diesel engine designed for Euro 5 application have been sampled. The engine was operated in three modes, and samples were collected from the exhaust without aftertreatment but also with aftertreatment by an exhaust oxidation catalyst and particle filter. The samples were analyzed by two-dimensional gas chromatography with time-of-flight mass spectral detection. The results show overall removal efficiencies for the organic compound mass by the combination of oxidation catalyst and particle filter of 50, 56, and 74% for the high-speed/high-load, low-speed/low-load, and high-speed/low-load conditions respectively. The results are clearly indicative of substantial repartitioning of the particulate and vapor components within the abatement devices and show an apparently reduced efficiency for the removal of high-molecular-weight alkanes under high-speed/high-load conditions relative to lower-molecular-weight compounds, although this may be due to alkane formation by thermocracking of other species. A notable feature is the presence of oxygenated compounds in the emissions, which are not present in the fuel. These are increased under high-speed/high-load conditions, and the results suggest the formation in the aftertreatment devices as well as in the combustion process.
Subject(s)
Dust , Vehicle Emissions , Catalysis , Gases , Oxidation-ReductionABSTRACT
Diesel engine emissions are by far the largest source of nanoparticles in many urban atmospheres, in which they dominate the particle number count, and may present a significant threat to public health. This paper reviews knowledge of the composition and atmospheric properties of diesel exhaust particles, and exemplifies research in this field through a description of the FASTER project (Fundamental Studies of the Sources, Properties and Environmental Behaviour of Exhaust Nanoparticles from Road Vehicles) which studied the size distribution-and, in unprecedented detail, the chemical composition-of nanoparticles sampled from diesel engine exhaust. This information has been systematized and used to inform the development of computational modules that simulate the behaviour of the largely semi-volatile content of the nucleation mode particles, including consequent effects on the particle size distribution, under typical atmospheric conditions. Large-eddy model studies have informed a simpler characterization of flow around the urban built environment, and include aerosol processes. This modelling and engine-laboratory work have been complemented by laboratory measurements of vapour pressures, and the execution of two field measurement campaigns in London. The result is a more robust description of the dynamical behaviour on the sub-kilometre scale of diesel exhaust nanoparticles and their importance as an urban air pollutant.
ABSTRACT
A number of major research questions remain concerning the sources and properties of road traffic generated particulate matter. A full understanding of the composition of primary vehicle exhaust aerosol and its contribution to secondary organic aerosol (SOA) formation still remains elusive, and many uncertainties exist relating to the semi-volatile component of the particles. Semi-Volatile Organic Compounds (SVOCs) are compounds which partition directly between the gas and aerosol phases under ambient conditions. The SVOCs in engine exhaust are typically hydrocarbons in the C15-C35 range, and are largely uncharacterised because they are unresolved by traditional gas chromatography, forming a large hump in the chromatogram referred to as Unresolved Complex Mixture (UCM). In this study, thermal desorption coupled to comprehensive Two Dimensional Gas-Chromatography Time-of-Flight Mass-Spectrometry (TD-GC × GC-ToF-MS) was exploited to characterise and quantify the composition of SVOCs from the exhaust emission. Samples were collected from the exhaust of a diesel engine, sampling before and after a diesel oxidation catalyst (DOC), while testing at steady state conditions. Engine exhaust was diluted with air and collected using both filter and impaction (nano-MOUDI), to resolve total mass and size resolved mass respectively. Adsorption tubes were utilised to collect SVOCs in the gas phase and they were then analysed using thermal desorption, while particle size distribution was evaluated by sampling with a DMS500. The SVOCs were observed to contain predominantly n-alkanes, branched alkanes, alkyl-cycloalkanes, alkyl-benzenes, PAHs and various cyclic aromatics. Particle phase compounds identified were similar to those observed in engine lubricants, while vapour phase constituents were similar to those measured in fuels. Preliminary results are presented illustrating differences in the particle size distribution and SVOCs composition when collecting samples with and without a DOC. The results indicate that the DOC tested is of very limited efficiency, under the studied engine operating conditions, for removal of SVOCs, especially at the upper end of the molecular weight range.
