Biomagnetic Monitoring vs. CFD Modeling: A Real Case Study of Near-Source Depositions of Traffic-Related Particulate Matter along a Motorway

Abstract

A test site located along a 12-lane motorway east of Montpellier, France, is used to evaluate the potential of biomagnetic monitoring on traffic-related particulate matter (PM) to parametrize a computational fluid dynamics (CFD) simulation of the local airflow. Two configurations were established on the site with three vegetated flat-top earth berms of a basic design, and a fourth one was located windward to the traffic roofed with a 4-m-high precast concrete wall. As a first step, PM deposition simultaneously on plant leaves, on low-cost passive artificial filters, and on soils was estimated from proxies supplied by magnetic and X-ray fluorescence measurements on both sides of the motorway. These latter revealed that traffic-related pollutants are present on soils samples highlighted with a clear fingerprint of combustion residues, and wears of breaks, vehicles, and highway equipment. Maximum PM accumulations were detected in the lee of the berm–wall combination, while no significant deposition was observed on both sides of the flat-top earth berms. These results are in line with measurements from PM µ-sensors operated by the regional state-approved air quality agency. Finally, we compared the experimental measurements with the outcomes of a computational fluid dynamics (CFD) modeling based on the Reynolds-Averaged Navier–Stokes (RANS) equations that consider the traffic-induced momentum and turbulence. The CFD modeling matches the experimental results by predicting a recirculated flow in the near wake of the berm–wall combination that enhances the PM concentration, whereas the flat-top berm geometry does not alter the pollutants’ transport and indeed contributes to their atmospheric dispersion.

Benefits and Limitations of Environmental Magnetism for Completing Citizen Science on Air Quality: A Case Study in a Street Canyon

“…For that, it seems inescapable to emancipate from the other potential PM sources and thus the regional background level. The solution is to use the relative concentrations of Fe‐bearing nanoparticles expressed as a relative change (RC) of the IRM intensity (Letaïef et al., 2020): $$$\mathrm{R}\mathrm{C}=\frac{\mathrm{I}\mathrm{R}{\mathrm{M}}_{\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{f}\mathrm{f}\mathrm{i}\mathrm{c}}-\mathrm{I}\mathrm{R}{\mathrm{M}}_{\mathrm{r}\mathrm{e}\mathrm{f}\mathrm{e}\mathrm{r}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{e}}}{\mathrm{I}\mathrm{R}{\mathrm{M}}_{\mathrm{r}\mathrm{e}\mathrm{f}\mathrm{e}\mathrm{r}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{e}}}$$$ where IRM reference is the background IRM intensities measured from samples collected at a monitoring site not associated with the local traffic source. The location of the reference deposimeters and their positioning is essential in our approach.…”

“…The accumulation was observed visually by microscopy and detected magnetically, while the metal enrichments were not obvious. Finally, a recent study compared the IRM 1000mT relative changes values with a numerical modeling for PM 2.5 dispersions along a motorway (Letaïef et al., 2020). The strengths of this model based on computational fluid dynamics (CFD) with the Reynolds‐Averaged Navier–Stokes (RANS) equations is to consider the traffic‐induced momentum and turbulence.…”

“…It consists of studying the magnetic properties of such surfaces after they have been exposed to a polluted air. Many studies have already shown the correlation between magnetic properties of Fe‐bearings nanoparticles with both traffic‐related PM concentrations (Hofman et al., 2017; Letaïef et al., 2020; Matzka & Maher, 1999; Rea‐Downing et al., 2020) and trace metals concentrations such as Pb, Co, Ti, Zn, Sr (Hofman et al., 2014, 2020; Lu et al., 2016). It is also important to mention that Fe‐bearing nanoparticles are ubiquitous, especially in urban PM (Thorpe & Harrison, 2008), and easy to detect because they show a ferromagnetic ( sensu lato ) signal (Gonet & Maher, 2019).…”

“…Furthermore, IRM intensity may be subject to instabilities due to the magnetic viscous effect of variable relaxation times (Néel, 1949) if sufficiently small magnetic nanoparticles are present as often observed in magnetic monitoring survey (Letaïef et al., 2020). As it is routinely done in our laboratory, samples were placed in a zero‐field following the IRM acquisition until the magnetization was stable enough.…”

“…To achieve these objectives, BREATHE was implemented in three test cases that were sources of conflict between citizens and policy makers: along a motorway (Letaïef et al., 2020), around a waste energy valorization center, and in an urban street canyon. The present paper will focus on the results and feedback from a part of BREATHE completed in 2021: the study carried out with and for the citizens on the traffic impact in a street canyon located in an ancient neighborhood of the city of Montpellier, France.…”