Dendroanalytical monitoring of atmospheric pollution with LA-ICP-MS: a case study from Villadossola (VCO, Italy).

Tree rings yearly formed in wood are a wide and complex source of data, potentially offering hints on the impact of atmospheric pollution. Dendroanalysis, the method of analysing tree-rings for trace metal pollution, is based on the assumption that element concentrations in wood represent the availability of those elements in the environment in which the tree has grown. Tree ring counting combines the chronological information to the chemical data, providing a potentially powerful tool for precise dating of atmospheric pollution events. While root uptake is generally recognised, trace element assimilation through leaves is still questioned, especially in the case of superior plants. Therefore changes in xylem chemistry would reflect both changes in exposure to metal deposition and changes in soil chemistry. Unlike deciduous plant leaves, conifers needles have a life span of several years, rendering these plants more sensitive to atmospheric pollution. In addition, previous studies already acknowledged pine needles as environmental indicators (Dongarrà et al., 2003). To test the applicability of dendroanalysis for retrospective environmental monitoring, the site of Villadossola has been selected, where a serious atmospheric pollution event, due to uncontrolled emissions of black smokes from a smelter, was documented in 1989-1990. Air quality data produced in the 1990's indicate that the main emitted pollutants were Cd, Zn, Pb, Mn, Ni e Cu (ARPA Piemonte, 1990). In addition, in 1995, an investigation on the heavy metals content in soils surrounding Villadossola allowed to estimate the deposition, evaluate the bioavailability and calculate the migration rate within the soil (Facchinelli et al., 2003). Wood cores from 8 conifer trees, ranging in age from 23 to 77 years were sampled with a Pressler borer at about 300 m from the emission source. Tree ring analyses are performed at the CNR-IGG, Pavia, using a Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). This technique offers the potential for quick and non-destructive analysis, with minimal sample preparation and high spatial resolution. The selected analytical spot dimensions (80 microns) allow repeated measurements to be performed on the same ring. Preliminary results are obtained on a core extracted from a 66 years old Pine tree (Pinus nigra) (Folin C., 2004). The analysed elements are: Li, Be, Mg, Ca, Sc, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Se, Rb, Y, Mo, Cd, Sn, Sb, Cs, La, Ir, Pt, Au, Hg, Pb, Th, U. As no internal standard is presently available, all signals are normalised to Mg, an essential macronutrient whose concentration has been determined by ICP- AES after acid dissolution. Repeated analysis of the same ring showed a good analytical reproducibility for all elements. Temporal trends indicate that several heavy metals (Cu, Pb, Ni, Mn, Zn, Cd) are significantly above background levels. Pb, Cu and Ni show peaks exactly corresponding to the years of documented smoke emission, testifying for a strong foliar uptake for these elements. Cd, Zn and Mn also show significant enrichments, although not perfectly coinciding. These discrepancies are likely due to translocation mechanisms within the wood. Statistical data treatments (factor and cluster analyses) also confirm the association of these elements. Results clearly show the importance of foliar adsorption as a metal uptake mechanism and indicate that LA-ICP-MS can be considered, in our case, a valid analytical technique for precisely dating circumstanced atmospheric pollution events. The comparison with results obtained on other conifer wood samples, currently ongoing, will allow evaluating the response of these living organisms to environmental pressure and verifying their reliability for retrospective environmental monitoring.