Relationships between morphology and optical properties of vehicle-emitted soot

Abstract

Soot emissions from vehicles have effects not only on public health but also on the Earth’s radiative balance. This pollutant is the main absorber of visible solar radiation and is considered to be the second factor contributing to global warming after CO2. Large and compact soot agglomerates have a higher single-scattering albedo than small and irregular ones. Among morphological and optical studies, most of them focus on the effect of the particle size, but not so many on the irregularity of agglomerates. In this work, the methodological approach is based on the combination of original experimental techniques (in-situ measurement of morphological and optical properties of soot particles emitted from a vehicle on a rolling bench) with theoretical developments (using fractal analysis and optical modelling). A typical conventional European road vehicle was used as a soot generator. Individual soot agglomerates were sampled and visualized with transmission electron microscopy (TEM), and their images were subjected to digital treatment and their morphological parameters were used as input for modelling. In addition, a double dilutor-Differential Mobility Analyzer (DMA)-Condensation Particle Counter (CPC)-Cavity Attenuated Phase Shift spectrometer (CAPS) assembly was used, which allows for classification of particles based on their mobility diameter and to simultaneously measure the optical extinction and scattering coefficients of the agglomerates. It was observed that, in general, the diameter of gyration obtained from TEM images underestimate the electrical mobility diameter, especially for large agglomerates. It was also observed that the diameter of primary particles and the power-law prefactor increase with the agglomerate size, while opposite trend was found for the fractal dimension and for the coordination number. Both the mass-specific absorption and scattering cross-sections (MAC and MSC) of agglomerates were observed to decrease with increasing agglomerate size, as consequence of decreasing fractal dimension.