The characterisation of diesel exhaust particles – composition, size distribution and partitioning

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.

Technical note: Use of an atmospheric simulation chamber to investigate the effect of different engine conditions on unregulated VOC-IVOC diesel exhaust emissions

“…The compound class-dependant removal efficiencies for the investigated DOC were 39 ± 12 % and 83 ± 3 % for the aliphatics (branched and straight-chain) and singlering aromatics, respectively. A typical DOC is expected to remove 50 to 70 % of the total HC emissions (Johnson, 2001;Alam et al, 2016). For the investigated compounds, the total DOC HC removal efficiency is at the lower limit of this expectation.…”

“…The removal of nalkanes in the DOC have been found to decrease with increasing carbon chain length, a result of the greater number of adjacent sites in the DOC required to achieve absorption (Yao, 1980;Russell and Epling, 2011), supporting the results shown here. However, recently Alam et al (2016) investigated the HC removal efficiency of a similar specification DOC (i.e. mixed platinum and rhodium) for C 12 to C 33 nalkanes, among other species.…”

Urban atmospheric chemistry: a very special case for study

“…20 Such particles are liable to shrink by evaporation as they advect into cleaner air within the urban environment and are largely absent from particle size distributions measured at surrounding rural sites. Measurements in Barcelona have provided evidence for the contribution of particles formed by regional nucleation processes 21,22 and there appear to be contributions both from advection of particles formed outside of the city as well as a separate population of particles which form within the Barcelona atmosphere.…”

The Effect of Varying Engine Conditions on Unregulated VOC Diesel Exhaust Emissions

“…The DOC removed 46 ± 10 % of the ∑SpVOC emissions, with 83 ± 30 3 % for the single-ring aromatics and 39 ± 12 % for the aliphatics (branched and straight-chain). A typical DOC is expected to remove 50 to 70% of the total HC emissions (Johnson, 2001;Alam et al, 2016). For the investigated compounds, the total DOC removal efficiency is at the lower limit of this expectation.…”

“…Furthermore, it was found that the exhaust gas composition varied depending primarily on combustion efficiency and DOC HC removal efficienc y, both 30 which are strongly dependent on working temperature (Korin et al, 1999;Roberts et al, 2014;Majewski and Khair, 2006;Russell and Epling, 2011). Diesel exhaust emissions contain thousands of compounds ranging from ~ C5 to C22, with contributions of up to C33 from lubricant oil (Alam et al, 2016;Gentner et al, 2017). Only a proportion of these emissions were speciated in this study.…”