Correlations between physicochemical properties of emitted diesel particulate matter and its reactivity

Plenty of publications have highlighted differences in individual physical or chemical properties of emitted diesel particulate matter (PM) and have correlated them to soot reactivity. An overview is given in the first part of the paper. Our study presented in the second part includes the determination of physical and of chemical properties of the emitted PM, which was generated at different boost and injection pressures on a modern light-duty diesel engine. The individual properties have been opposed to the reactivity of the PM. Afterwards, these correlations are discussed. The experiments show wide-ranging distinctions in soot reactivity (up to 162°C). The determined differences in soot structures as well as in primary particle sizes are small and do not correlate clearly to soot reactivity. Hence, there has to be a further property which influences soot reactivity to a high extent. PM composition (oxygen, ash content) is identified as a further property impacting soot reactivity. The various ash contents show good correlations to the soot oxidation behavior. The distinctions in ash contents can be explained by the different sampling durations because of large changes in emitted particulate mass, due to the high differences in particle size and number emissions. The source of the ash (Ca, Zn, Al) is attributed to inorganic elements from fuel, lubricating oil and engine wear. In addition, the oxygen content in soot is a further chemical property impacting soot reactivity.