Morphological characteristics of diesel emission particles from image analysis of electron microscopy images

Morphological properties: shape, fractal dimension, primary particle and agglomerate size plus particle size distributions of Dp < 1 µm diesel agglomerates were studied. Emission source was Euro 2 type approved bus engine (ox cat or no cat) with varying engine and sampling specific formation conditions. Properties were analysed from 2D electron microscopy (EM) images. Traditional high vacuum SEM and TEM were applied. Physical state and morphology of ambient air released exhaust particles is made up of factors related to engine, load, fuel, aftertreament, dilution and sampling. Morphology of particles is important when describing particle geometry, optical shape and aerodynamic behaviour. Shape (surface area), in addition to aerodynamic size, control the effectiveness of particles within respiratory tract. Fractal dimension is a fundamental quantity for understanding agglomeration and growing mechanisms i.e. dynamic behaviour of diesel particles. A thermophoretic sampling system was developed to collect on an EM grid a representative, non-size segregated particle sample population from diluted exhaust. This result was compared with the more conventional pump sampling of diluted particle emission. Samples were collected on carbon coated TEM grids. Characteristics of particle populations were analysed from SEM images (20 - 50 000x magnification). Individual primary particles and agglomerates were analysed from TEM images, as a function of particle size. A semiautomatic digital image processing program1 was developed for analysing large particle numbers and for repeatability, as well as for characterisation of individual agglomerates. Given are size distribution quantities for populations and morphological magnitudes for single agglomerates. Calculated is particle equivalent area A, equivalent diameter Deq, max. length Lmax and max. orthogonal width Wmax as well as perimeter p distributions for Deq > 20 nm particle populations, and e.g. size distribution of primary particles, overlapping coefficient Cov and fractal dimension Df for individual agglomerates. Thermophoretic sampling with max 37oC dT worked well, even though increased dT would be favourable to shorten sampling times. Comparison to pump sampling with low enough face velocity and short enough sampling time yielded close to identical particle size distributions despite anticipated different trapping mechanisms. Sizes and morphological magnitudes of particle populations were quite satisfactorily analysed with the new image analysis program. The judgement is, however, in a natural way sensitive to preliminary human eye thresholding of the image. Sampling of wet and dry emission yielded only slightly different agglomerate size distributions from image processing. Deficiency of high vacuum EM imaging for native volatile particle constituents could not be heeled with trials of ESEM sampling and imaging technique. Minimum and maximum sizes of primary particles within all TEM images were Deq 7 and 55 nm, respectively. Primary particle mean radius in individual agglomerates ranged 8 - 20 nm. Automating fractal dimension Df calculation was found still very challenging due to limited capability of the program to identify and define primary particles correctly from contrasts in image grey scale or from perimeter curvature. Optional manual selection and sizing of primary particles was added in program. Fractal dimension variation from 2D images of particles was between 1.53 - 1.77.