Spatial distribution of traffic-related pollutants from tailpipe-to-road-to-ambient: the case of ultrafine-particles

The major objective of this work was to analyse the spatial variation of traffic air pollution, especially focussing on ultrafine particles (particles smaller than 100 nm, UFPs). Measuring any potential effect of any traffic air pollutant crucially requires, indeed, a comprehensive understanding of its spatiotemporal variations and distribution. To this respect, UFPs clearly emitted by internal combustion engines are extremely variable in both space and time, and this variability is believed to be an important issue to assess traffic air pollution fate and exposure. The process from emission at the tailpipe to concentration in ambient air was analysed using a two stage structure, namely, ‘tailpipe-to-road’ and ‘road-to-ambient’. The analysis was applied firstly for major traffic air pollutants, and then to UFPs. The methodology was developed by considering air pollutants’ concentration in terms of frequency distributions (FD) of the related space series. The dilution process (indicated by a LogNormal FD) was shown to be not the dominant process to fit the spatial variations of traffic air pollutants, which were conversely found to be well-fitted by the Gaussian FD. This revealed that when traffic emissions in an urban area can be considered as sources spatially diffused (no highways, etc.), the diffusion process in the atmosphere (represented by the Gaussian FD) dominates the distribution in space of such pollutants. This level of analysis has enhanced a deeper comprehension of the problem revealing governing factors and principal components of traffic air pollution variations in both space and time, and at both the stages. There appear at least two directions to proceed from the present study. First, the pursuing towards a stage 0 for the analysis of UFPs engine-to-tailpipe. Then, the assessment of UFP spatial distribution from engine-to-tailpipe-to-road-to-ambient. This could close the circle suggested by this Ph.D., with the potential to finally indicate to what extent UFPs measured at the exhausts of a single engine could reflect the outdoor concentrations in ambient air in a whole urban area.