Grain type and size of particulate matter from diesel vehicle exhausts analysed by transmission electron microscopy

The aim of this research was to apply a simple and quick method of size and shape characterization by TEM to diesel exhaust particles from large-capacity, high-performance trucks. Particulate matter (PM) samples were collected while the engines were idling. Investigation of this type of emission is essential because vehicles are idling at stop lights, in traffic, or during slow movement, goods loading and unloading. In these conditions, PM emission cumulates in a small area. It was found that PM from vehicle exhaust emissions can be divided into three groups: soot, irregular-shaped particles and circular particles. Irregular-shaped particles and soot aggregates were present in the exhausts of the three types of vehicle tested. Circular particles were identified only in the samples collected from exhaust emissions from the MAN vehicle, and were present in small amounts. The average surface area was in the range of 0.06 microm2 to 0.24 microm2. Mean perimeter fluctuated from 2.09 microm to 4.14 microm, and Feret diameter from 0.21 microm to 0.31 microm. Circularity was in the range of 0.12 to 0.30. Aspect ratio was around 0.30 to 0.45. Feret diameter seems to be a good parameter to define the mean size of particles, but does not take into account the influence of the shape. Therefore, this measurement seems to be useful just in the case of spherical or very rounded particles, not for all diesel PM. Thus, it is necessary to consider circularity or aspect ratio for DPM characterization.