Diesel engine particulate emissions: A comparison of mobility and microscopy size measurements

Abstract

Sub-micron particulates emitted from diesel engines have serious negative effects on human health and environment. Accurate characterizations of such particulate matter based on independent diagnostic techniques are necessary to monitor, regulate, and model atmospheric pollution levels. The relevant sizes, in addition to mass, are of special interest as they determine the transport, toxic, optical, chemical, and deposition properties of particulates. While there is abundant information in the aerosol literature on the apparent size distributions of diesel particulates measured with mobility sizers, it is unclear how well such commercial instruments can characterize aggregated particles due to spherical interpretation and potential alterations of physical properties within extensive sampling and dilution lines. In this study, these experimental concerns were assessed by comparing equivalent size measurements from a differential mobility analyzer to the actual physical dimensions and morphologies obtained from direct TEM observations of particulates sampled from the exhaust of a diesel engine with rapid thermophoretic deposition technique. Because mobility analyzers yield an overall equivalent particulate size, the measured mobility sizes were much larger than the microscopy spherule diameters, which dictate the available surface area for their interactions with the surrounding atmosphere. Mobility size distributions were also narrower with smaller sizes compared to the TEM aggregate measurements. The average mobility diameters were found to be approximately half the average microscopy aggregate gyration diameters for the present diesel engine operating conditions. Some of the differences in fractal dimension of diesel soot aggregates were also discussed in view of the present experimental results.