Accumulation of airborne trace elements in mosses, lichens and synthetic materials exposed at urban monitoring stations: Towards a harmonisation of the moss-bag technique

Abstract

Mosses, lichens and cellulose filters were exposed for 17 weeks at four urban monitoring stations in Naples (S Italy) to assess the accumulation of airborne Al, As, Ba, Ca, Cd, Co, Cr, Cu, Fe, Hg, K, Mg, Mn, Mo, Ni, Pb, Ti, V, and Zn. In each site, the element accumulation was significantly higher in the moss Hypnum cupressiforme than in the lichen Pseudevernia furfuracea. Acid washed mosses accumulated the highest amount of trace elements, but the differences in element concentrations among the moss samples exposed after water washing and different devitalisation treatments (acid washing, oven drying and water boiling) and between the lichen samples exposed with and without the nylon bag were not statistically significant. The cellulose filters showed the lowest accumulation capability. The reciprocal ordination of sites and exposed materials showed an increasing contamination gradient (especially for Pb, Cu and Zn) from the background site to the trafficked city streets; this pattern was undetectable from PM₁₀ data recorded by the automatic monitoring devices operating in the four exposure sites. The element profile in exposed materials did not change substantially throughout the urban area and particles of polluted urban soils seem the main source of airborne metals in Naples. Through a comprehensive evaluation of the results from this and previous studies, a protocol is suggested for the moss-bag monitoring of trace element deposition in urban environments.

Introduction

Active monitoring of persistent air pollutants by moss and lichen transplants gives spatial and temporal information on sources and deposition patterns of atmospheric contaminants (e.g. Vingiani et al., 2004, Giordano et al., 2005, Aničić et al., 2009, Fernández et al., 2009, Giordano et al., 2010). In urban and industrial areas lacking indigenous cryptogams the data from exposed moss- and lichen-bags are an important complement to those recorded by automatic monitoring devices, because they allow characterising persistent contaminants associated to airborne particles, their possible sources and biological effects (Bargagli, 1998). The moss bag technique has been used for active biomonitoring for the past 40 years, but there is still no standardised protocol that enables application of the technique as a tool to monitor air quality (for a review see Ares et al., 2012).

Previous moss- and lichen-bag surveys in Italian urban environments (Tretiach et al., 2007, Adamo et al., 2007, Adamo et al., 2008a, Adamo et al., 2008b, Giordano et al., 2009, Tretiach et al., 2011) showed that the moss Hypnum cupressiforme and the lichen Pseudevernia furfuracea, exposed alive (after water washing) or devitalised with different pre-treatments (oven drying and acid washing) showed a statistically significant increase in concentrations of several trace elements according to the exposure time and environmental characteristics of the exposure sites. As a rule, in the same exposure conditions, the element accumulation in moss-bags was higher than in lichen-bags and the element uptake increased during rainy periods. In particular, moss devitalised through dry heat treatments (“oven drying”) appeared to preserve an extraordinary element accumulation capability, keeping low the variability of pre- and post-exposure elemental composition. The results were unaffected by variations due to moss metabolic activity and the moss-bag monitoring allowed the identification of the traffic as the main sources of metals and the establishment of spatial and temporal trends of element deposition (Tretiach et al., 2007, Adamo et al., 2008a, Giordano et al., 2009, Tretiach et al., 2011, Adamo et al., 2011).

To contribute to the settlement of standardised guidelines for the active monitoring of airborne trace elements in urban environments, the same moss and lichen species used in the above cited works (H. cupressiforme and P. furfuracea) were exposed during the summer for 17 weeks in Naples (S Italy) at four automatic monitoring stations, that provided meteorological and pollution data for the whole exposure period. The main objectives of this research were: (i) to assess the element accumulation in the moss exposed after a novel devitalising procedure (i.e. the inactivation, breakdown and partial dissolution of pectin chains through boiling of the material in distilled water); (ii) to compare the results of this moss pre-treatment with those of already tested procedures (water washing, acid washing and oven drying); (iii) to evaluate for the first time the influence of the nylon net on the element accumulation in lichens, by exposing the thalli either with and without the nylon bag; (iv) to compare the element accumulation in mosses and lichens with that in cellulose filters exposed in the same conditions (in Adamo et al. (2007) comparison was made with a quartz fibre filter, Whatman QMA 1851047, and a cation exchange filter, Pall ICE 450).

Through a comprehensive evaluation of the results from this and previous exposure experiments, the paper aims at outlining a protocol for the active monitoring of trace elements in urban areas.