Towards a harmonization of the “bag technique”: trace metal uptake by variously treated biological materials

Biomonitoring of air pollutants by moss and lichen transplants has been widely used over the past years, especially to monitor trace elements, since it gives useful information on their deposition, particularly in those urban and industrial areas lacking of indigenous moss and lichen species. Moreover, this technique provides information on contaminants associated to airborne particles which in European urban areas are not routinely checked, hindering the identification of pollutant sources crucial to undertake effective policies to knock down air pollutants. Nevertheless, the current lack of standardization at international level, and the scarce knowledge on the factors influencing element uptake make this technique still open to criticism. In this experiment moss and lichen were exposed for 17 weeks at four urban air quality monitoring stations of Naples city. Trace element uptake was evaluated in moss material before and after different devitalisation treatments (acid washing, oven drying and the novel water boiling) and in lichen exposed with and without the nylon bag. A major uptake was overall recorded in moss materials. Uptake differences among moss materials were not significant (P = 0.378), as well as the difference between lichen exposed with and without nylon bag (P=0.532). The ordination of sites/materials in relation to element uptake indicates a contamination gradient of the investigated sites, but element profiles did not differentiate substantially in the different areas of the city suggesting that atmospheric deposition in Naples urban area is qualitatively homogeneous only differing quantitatively in diverse monitoring sites. The overall high and significant correlations observed among and between geogenic and antropogenic elements suggest that polluted urban soil could represent a massive source for most elements. The implementation of devitalized moss bag could represent a ready-to-use robust tool for routine monitoring of air pollutants.