Abatement of intermediate- and semi-volatile aerosol emissions within road transport

Intermediate/semi-volatile organic compounds (I/SVOCs) emitted from transport are currently unregulated and are believed to contribute significantly to secondary aerosol formation. A key knowledge gap remains regarding the characterisation and enhancement of their abatement via catalytic exhaust aftertreatment systems, particularly in gasoline-fuelled road vehicles where data are scarce. To elucidate potential I/SVOC abatement mechanisms, this study comprehensively characterised emissions from a modern light-duty gasoline internal combustion engine (ICE) upstream and downstream of a three-way catalyst (TWC) and a gasoline particulate filter (GPF).

Undiluted gaseous emissions, including selected volatile organic compounds (VOCs), were directly measured in real time using an MKS multi-gas Fourier transform infrared spectrometer (FTIR). The exhaust was diluted using a two-stage Dekati® eDiluter™ Pro system, maintaining a representative and consistent dilution ratio and temperature. Subsequently, particle number size distribution over the range 1.2 nm – 440 nm was achieved with a combination of two mobility particle size spectrometers (MPSSs) operated in parallel as well as an advanced particle size magnifier coupled with a condensation particle counter (PSM-CPC). A purpose-built adsorption tube and filter sampler (AFS) was employed to simultaneously capture gas- and particle-phase I/SVOC emissions, which were then analysed via comprehensive two-dimensional gas chromatography time-of-flight mass spectrometry (GC×GC-ToF-MS).

Experiments conducted under “low load” engine conditions revealed a total hydrocarbon (THC) removal efficiency of > 96% for the TWC. Results demonstrated some particle removal over the TWC, primarily in the nucleation mode (featuring a considerable I/SVOC fraction); however, total particle numbers were decreased by > 99.7% over the GPF, with removal efficiency not dropping below 99.2% for any individual size bin.  Analysis of engine-out I/SVOCs indicated a high prevalence of alkylbenzenes, polycyclic aromatic compounds (PAHs), and alkyl-PAHs, with most PAHs and oxygen-containing compounds found in the particle phase with monoaromatics found in the gas phase. This study offers insights into the dynamics of I/SVOCs within the exhaust aftertreatment system, aiding the identification of their sources and the development of targeted mitigation strategies.

Acknowledgement:

This research was funded by the European Union’s Horizon Europe research and innovation programme within the AEROSOLS project under grant agreement number 101096912 and the UK Research and Innovation (UKRI) under the UK government’s Horizon Europe funding guarantee [grant numbers 10092043 and 10100997].