Abstract
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.
Original language | English |
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Publication status | Published - 2006 |
MoE publication type | Not Eligible |
Event | 10th ETH Conference on Combustion Generated Nanoparticles - Zürich, Switzerland Duration: 21 Aug 2006 → 23 Aug 2006 |
Conference
Conference | 10th ETH Conference on Combustion Generated Nanoparticles |
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Country/Territory | Switzerland |
City | Zürich |
Period | 21/08/06 → 23/08/06 |