Particulate matter dispersion in an urban-industrial environment and deposition on Quercus ilex leaves: laboratory and modelling analysis

A number of studies have focused on urban trees to understand their mitigation capacity of air pollution. Present PhD project aims to describe Particulate Matter (PM) accumulation on tree leaves in a specific urban environment as case study, both in terms of quantity and quality. Furthermore, experimental data obtained from laboratory analysis are crossed with an air pollution model designed for urban environments, in order to relate PM deposition to main polluting sources. PM deposition on Quercus ilex leaves was quantitatively analyzed in four districts of the City of Terni (Italy) for three periods of the year. Fine (between 0.2 and 2.5 µm) and Large (between 2.5 and 10 µm) PM fractions were analyzed. Mean PM deposition value on Quercus ilex leaves was 20.6 µg cm-2. Variations in PM deposition correlated with distance to main roads and downwind position relatively to industrial area. Epicuticular waxes were measured and related to accumulated PM. For Fine PM deposited in waxes we observed a higher value (40% of total Fine PM) than Large PM (4% of total Large PM). Model chosen to perform the air quality simulation on the study area was the TAPM (The Air Pollution Model) and ran for 1 year period on study area (Jan 2012 – 2013). This model allowed describing PM dispersion pattern within the city, according to the main polluting sources position. TAMP implementation evidenced areas potentially more polluted in the case study city and the local effect of air pollution sources. Qualitative analysis is obtained through the application of a monitoring and characterization protocol for PM deposited on Q.ilex leaves. Four trees were selected as representative of urban environments based on their proximity to a steel factory and a street. PM from quantitative analysis was characterized using scanning electron microscopy and energy dispersive X-ray spectroscopy, inferring the associations between particle sizes, chemical composition, and sampling location. Modeling of particle size distributions showed a tri-modal fingerprint, with the three modes centered at 0.6 (factory related), 1.2 (urban background), and 2.6 μm (traffic related). Chemical detection identified 23 elements abundant in the PM samples. Principal component analysis recognized iron and copper as source-specific PM markers, attributed mainly to industrial and heavy traffic pollution respectively. A similar technique implemented for qualitative analysis is being used in the “European Sampling Campaign” a huge PM characterization project, in collaboration with Antwerp University within COST FP1204. Platanus sp. leaves were collected in 20 different cities and analyzed. SEM/EDX was supported by SIRM (Saturation Isothermal Remnant Magnetization) analysis. First results are reached, nevertheless the work is still in progress: the largest part of analyzed particles is fine PM; not direct association between site and PM dimension or density on leaves is found. PM quality is different between upper or lower leaf side. Iron is confirmed as “technology” marker. There is a relation between Fe concentration and SIRM results. Results from this study allow to increase our understanding about air pollution interactions with urban vegetation and could be hopefully taken into account when guidelines for local urban green management are realized. Upscale of qualitative results on leaf area basis provided a useful indicator for strategic evaluation of harmful PM pollutants using tree leaves.