Pilot study on road traffic emissions (PAHs, heavy metals) measured by using mosses in a tunnel experiment in Vienna,...

Over the last few years there has been extensive research for new indicators providing information about deposition resulting from road traffic and tunnel experiments received special attention in emission research. Mosses have been used for the estimation of atmospheric heavy metal and PAH depositions for more than three decades, although they were used only a few times for estimating ambient air pollution caused by traffic. In the current study, the suitability of using a moss species for monitoring road traffic emissions inside a tunnel was evaluated. This was a first-time ever attempt to use plants (mosses) as bioindicators in a tunnel experiment. Specifically, two relevant questions were examined: 1) Do mosses accumulate toxic substances derived from road traffic emissions under the extremely adverse conditions which can be found in a tunnel, and 2) Which substances can mainly be attributed to road traffic emissions and therefore be taken as efficient and reliable indicators for motor vehicles?

For the first time a biomonitor (the moss species Hylocomium splendens (Hedwig) B.S.G.) was used in a road tunnel experiment to analyse emissions from road traffic. Moss samples were exposed for four weeks in wooden frames (size 10 cm x 10 cm), covered by a thin plastic net with a mesh size of 1 cm x 1 cm. 17 elements, mainly heavy metals, and the 16 EPA-PAHs together with coronene were analysed by ICP-AES, AAS and GC-MSD.

Enrichment factors, calculated by comparing post-experiment concentrations to those of a background site, were high for most PAHs, especially benzo(g,h,i)perylene (150.7), coronene (134.7), benzo(a)anthracene (125.0), indeno(1,2,3-c,d)pyrene (79.8), chrysene (78.1), pyrene (69.6) and benzo(b)-fluoranthene (67.4), and among the other elements for Sb (73.1), Mo (59.6), Cr (33.9), As (24.1), Cu (19.6), and Zn (17.1). All these substances can thus be taken as indicators for road traffic pollution. Concentrations were also significantly higher in the tunnel mosses for all investigated substances than along busy roads outside tunnels. Cluster analysis revealed groups of substances which could sensibly be attributed to various sources (abrasion processes, Diesel combustion) and enrichment in the various particle size classes.

The extreme high concentrations in the analysed moss samples from inside the tunnel were due to higher concentrations in the ambient tunnel air, and the fact that already deposited chemical substances are not lost by rain, as well as efficient uptake capacities even under the extremely adverse conditions in a tunnel. In accordance with previous studies our results suggest that PAHs are better indicators for emissions from the burning process than heavy metals.

As in open fields, mosses are suitable indicators for monitoring traffic emissions in tunnels. In addition to biomonitoring in open fields, in tunnel experiments mosses are even better indicators, because the confounding effects of other sources of pollution and the 'noise' in the accumulation process (e.g. washout through wet deposition) are minimised. The results of our study demonstrate the usefulness of mosses for surveying heavy metals and PAH emissions and deposition arising from road traffic sources, even under the extremely adverse conditions of the tunnel environment.

It can be considered that biomonitors like mosses are a suitable alternative to technical particle filters inside tunnels. They are easy to handle, low in costs and valuable information regarding traffic emissions can be obtained.

The results of this pilot-study proved the feasibility of the method, however, should be corroborated by further investigations based on a sample set that allows for generalization of the findings and might even include other moss species. A comparison of technical measurements with the biomonitoring method could lead to a more general acceptance of the results.