212Pb as tracer for PM deposition on urban vegetation

Abstract

212Pb concentration in outdoor air is closely correlated with fine suspended particulate matter in the atmosphere. Thanks to this association, this isotope can be used to trace the sinking processes of particulate matter due to the vegetation, also providing accurate estimations of the deposition velocity on foliar surfaces. This approach is particularly effective in areas with high thoron fluxes and, consequently, high 212Pb fluxes from soil.The contribution of vegetation to the improvement of air quality (AQImp) in the municipality area (MA) of Rome (Latium, Italy), almost entirely located on Th-enriched volcanic soils, was estimated by studying 212Pb deposition velocity on the grasses (0.9–2.5mm·s−1) and on the most common tree classes, namely conifers (1.5–15mm·s−1), evergreen (1–4mm·s−1) and deciduous (0.2–1.5mm·s−1).212Pb activity in outdoor air was determined by gamma spectrometry after air pumping with accumulation on cellulose filters and after collection on artificial electrostatically charged surfaces (ECS). The high 212Pb activity values obtained from this analysis (0.90±0.6Bqm−3 and 0.58±0.15Bqm−3, respectively near and far from the soil) are consistent both with the average regional thoron flux from volcanic soils (2.9·104Bqm−2·h−1) and with the thoron flux measured in the volcanic soils of the study area. Thoron and 212Pb fluxes were also measured both in laboratory and in the field under different soil moisture conditions. The total AQImp for the period from September 2014 to September 2015, calculated after the classification of the MA of Rome into six classes of vegetation, provided a value of 0.20 corresponding to 2.3 Tons per day of removed PM10. The role of grasslands in the PM10 removal, the contribution of the vegetation to the improvement of AQImp and the possibility of improving the sinking efficiency of green areas by increasing conifer trees coverage were also highlighted.