Deposition, abatement and environmental fate of pollutants in urban green ecosystems: Suggestions from long-term studies in Siena (Central Italy)

Long-term biomonitoring of atmospheric pollutants in Siena shows that organisms in urban ecosystems are an overlooked source of information on the deposition and environmental fate of chemicals that are not measured by automatic monitoring devices. Quercus ilex leaves and epiphytic lichens intercept airborne particulates and can be used as reliable quantitative biomonitors of metal deposition. Soils beneath holm oak trees have an enhanced sink capacity for metals and a noteworthy faunal diversity. Earthworms and land snails inhabiting those soils can help to evaluate metal bioavailability, their potential trophic transfer and health risks. Biological crusts, mosses and mollusks from vegetated urban walls appear even more reliable biomonitors of metal deposition. Although Siena is a town with its own architectural and climatic features and moderate atmospheric pollution mainly caused by vehicular traffic and domestic heating, our overview offers suggestions and guidance that can be adapted across different urban contexts, for enhancing the role of urban green ecosystems in the abatement and monitoring of atmospheric pollutants. Comparisons among the capabilities of different tree species to improve urban air quality indicate that in Mediterranean towns it would be difficult to select a better alternative to Q. ilex, although this species emits VOCs and contributes to O3 formation, a pollutant now of major concern. Due to the decreased atmospheric concentrations of SO2 and the scarce sensitivity of lichens to O3, in many towns lichen biodiversity seems no longer a valid indicator of air quality.