NbS as an urban monitoring strategy in post-fire environmental analysis of PM components

In recent years, there has been a progressive increase in the number of fires in both natural and urban areas. These events lead to a significant increase in the concentration of particulate matter (PM) in the atmosphere and can have a major impact on the health of the exposed population. Assessing the exposure of the population to dust released by fires requires the installation of samplers at several locations around the area of interest and the study of the spatial distribution of the different PM components emitted. However, the installation of a large number of dust samplers is often not feasible in a timely manner and requires sampling with expensive equipment several days after the event at a few locations. On 9 July 2022, a major fire occurred in the Torre Spaccata area at a car depot, affecting an area with approximately 60,000 people exposed to particulate emissions from the fire. A Nature-based Solutions (NbS) approach was used to assess exposure to the released dust, which allowed a high number of samples to be obtained quickly and cheaply from numerous measurement points by using tree species present in the area. This approach, aimed at exploiting the invasiveness of the Ailanthus altissima species to assess its potential as a biomonitor of air pollution, allowed the sampling and chemical analysis of total suspended particulate matter (TSPM) deposited on the leaves of the plant at 24 sites homogeneously distributed over the area affected by the fire (approximately 15 km2). For each site, A. altissima leaves were sampled both in the days following the fire and one year later. Dust deposited on the leaves was extracted in deionised water and analysed for 40 elements by ICP-MS. The leaves were then dried and mineralised to determine the concentration of elements adsorbed by the leaf surfaces. The elemental concentrations obtained at the 24 measurement sites were interpolated by inverse distance weighted (IDW) and mapped in GIS to geo-reference their distribution in the study area [1]. Elemental tracers were identified both from emission sources present in the area, such as vehicular traffic (Fe, Mn) and soil resuspension (Be, Co and Zr), and from different PM components emitted by fire through biomass burning (Cs, Li and Rb), brakes, tyres (Sn, Zn) and car bodies (Al, Nb, Sn). The emission of these components was 10 times higher than that measured one year after the fire. Leaf blades were efficient at adsorbing mainly PM-soluble species. Finally, tracer concentration maps allowed the localisation of the areas with the greatest impact of the different PM components. In conclusion, this approach proved to be very efficient for a low-cost assessment of the spatial variability of tracers from several typical urban emission sources, such as traffic and biomass burning, as well as occasional sources such as fires.