Résumé
<p>Numerous studies have already shown the possibility of tracing the sources, the<br>compositions, and the concentration of atmospheric pollutants deposited on plant<br>leaves. In environmental geochemistry, inter-element and isotope ratios from<br>chemical element assays have been used for these purposes. Alternatively,<br>environmental magnetism represents a quick and inexpensive asset that is<br>increasingly used as a relative indicator for concentrations of air pollutant on bio<br>accumulator surfaces such as plants. However, a fundamental issue is still pending:<br>Do plants in urban areas represent a sink for fine particles that is sufficiently effective<br>to improve air quality? This is a very topical issue because some studies have shown<br>that the foliage can trap fine particles by different dry deposition processes, while<br>other studies based on CFD models indicate that plant hedges in cities can hinder<br>the atmospheric dispersion of pollutants and therefore increase pollution at the level of<br>emission sources such as traffic. To date, no consensus was made because several<br>factors not necessary well known must be taken into account, such as, PM<br>concentration and size, prevailing wind, surface structures, epicuticular wax, to<br>mention just a few examples. A first step toward the understanding of the impact of<br>urban greens on air quality is the precise determination of the deposition velocity (Vd)<br>parameter. This latter is specific for each species and it is most of the time<br>underestimated in modeling-based studies by taking standard values.<br>In that perspective, we built a wind tunnel (6 m long, 86 cm wide and 86 cm high) to<br>perform analogical experiments on different endemic species. All parameters are<br>controlled, i.e, the wind speed, the nature and the injection time of pollutants (Gasoline<br>or Diesel exhausts, brakes or tires dust, etc…). We can provide the PM concentrations<br>upwind and downwind of natural reconstituted hedges by two dustmeters (LOACs -<br>MétéoModem). Beforehand, parameters such as the hedge resistance (%) or the leaf<br>area index (LAI) have been estimated for each studied specie to allow comparability<br>between plants removal potential. The interest would ultimately combine PM<br>concentration measured by size bins from the LOACs with magnetic measurements<br>(ARM, IRM100mT, IRM300mT and SIRM) of plant leaves. The idea is to check whether it<br>would be possible to precisely determine in situ the dust removal rate by urban greens<br>with environmental magnetism measurements. Up to now, we have carried out on<br>different endemic species such as Elaeagnus x ebbingei leaves and Mediterranean<br>pine needles, the results of which will be presented.</p>