2. Biomonitoring of trace elements atmospheric deposition using mosses
Mosses (bryophytes) have been recognized as the most effective type of organisms for biomonitoring of air quality based on their numerous anatomical and physiological features, widespread occurrence and tendency to accumulate and retain pollutants.
Terrestrial mosses have several advantages as biomonitors of trace element atmospheric deposition when compared to higher plants: (1) they lack developed root system; (2) low variability of morphology through the growing season; (3) they lack leaves cuticula; (4) high surface-to-volume ratio; and (5) high cation exchange capacity (CEC) of cell walls.
These include indigenous naturally-occurring forms, moss transplants, moss bags and peat profiles (Sphagnum moss species).
2a. Passive moss biomonitoring
Mosses have often been used in passive biomonitoring where native species are used in the area under study involving only two major steps: collection and analyses of moss samples. Mosses may be found growing on forest or open floors, rocks, tree barks or walls, sampled at the site from interest and analyzed to obtaine results which reflect the level of pollution of the site.
Two moss genera, Brachythecium sp. and Eurhynchium sp. were sampled from two parks in the city of Belgrade, analyzed by instrumental neutron activation analyses (INAA) and this data were used for estimation of the trace elements atmospheric deposition.
In the laboratory, the samples were carefully cleaned from all dead material and attached litter, then only green and green-brown moss upper parts from the two–three last years were analyzed. The trace elements determination were performed by instrumental neutron activation analyses (INAA) and this data were used for estimation of the trace elements atmospheric deposition.
2b. Active moss biomonitoring
In urban areas where native mosses are often scarce or absent, active biomonitoring using the so-called “moss bags technique” has been developed. The moss bags technique initially involves the collection of a moss species from its natural, relatively unpolluted habitat. The moss was than cleaned from extraneous materials (litter, soil..), packed into nylon net-bags and suspended in area of the interest. Sphagnum moss species are by far the most frequently used to prepare moss bags.
Active biomonitoring with moss bags (Sphagnum girgensohnii Russow) were used to examine atmospheric deposition in the urban area of Belgrade. The samples was analysed by instrumental neutron activation analyses (INAA) and atomic absorption spectrometry (AAS). Significant accumulation of the most elements occurred in the exposed moss bags when compared with the initial moss element content. Also, the role of water supply on the moss accumulation ability was investigated and experiment with dry and irrigated (wet) moss bags was carried out. The suitability of the above mentioned moss species as an alternative method for air quality studies in urban areas was tested and compared to atmospheric deposition estimated from the bulk sampling devices, i.e. instrumental monitoring technique.
The exposure period is especially critical in moss biomonitoring surveys. If exposure time is too long, saturation of exchange sites on the moss membrane may occur, and preferential displacement or exchange of elements as well. This can be avoided by using the minimum exposure periods necessary for detecting trends. The accumulative properties of the Sphagnum girgensohnii moss bags with time were investigated in one of our study. The concentrations of 49 elements were determined by inductively-coupled plasma mass spectrometry (ICP-MS). S. girgensohnii moss bags, both dry and wet, showed a linear trend of accumulation during 1–5 months of exposure for a majority of the studied elements. Very similar trends of element accumulation for dry and wet moss bags indicates that both methods of exposure may be used as appropriate tools for biomonitoring of trace elements in air. However, in cases of relatively low air pollution load wet moss bags could be a better choice by showing higher accumulation capacity, presumably due to prolonged vitality of the moss. In this case exposure periods of up to 5 months may be preferable in order to improve the sensitivity of the method.
The method of active moss biomonitoring with Sphagnum girgensohnii could be used as screening monitoring of atmospheric trace elements pollution in urban areas.
2c. Active moss biomonitoring of trace element distribution in canyon streets, tunnels and public
In urban environments and especially in those areas where population and traffic density are relatively high, human exposure to hazardous substances is expected to be significantly increased. This is often the case near busy traffic streets in city centre, tunnels, etc. where urban topography and microclimate may cause poor air conditions giving rise to contamination hotspots. High pollution levels have been observed in street canyons which are a term frequently used for urban streets flanked by buildings on both sides with poor pollutants dispersion. Within these streets, pedestrians, cyclists, drivers and residents are likely to be exposed to pollutant concentrations exceeding current air quality standards.
The total number of air quality monitoring stations or sampling locations within a city is usually limited. However, active biomonitoring of pollutants using moss bags could be a practical and cost effective approach (dense net of monitoring sites).
Active moss biomonitoring survey of trace element air pollution in the Belgrade canyon streets was performed with the main aim to evaluate possibilities of using Sphagnum girgensohnii moss bags for investigation of the vertical distribution patterns of selected trace elements in an urban environment. The selected streets are situated in heavily traffic area of the down town with additional contribution of public garages in overall pollution in this area.
In several public garages within the selected street canyons the moss bags monitoring of trace elements was carried out in parallel with instrumental measurements of particulate matter (PM10). The measuring points were at the entrance ramps which are close to billing counter where people work. The comparison between instrumental and biomonitoring technique will be done, as well as estimation of health risk for people employed in public garages. The moss bags were also exposed in the tunnel ("Terazijski tunel"), in front of the tunnel entrance and out of tunnel with the aim to estimate a spatial gradient of trace element concentrations in the air.
The results confirmed that the use of Sphagnum girgensohnii moss bags is a simple, sensitive and inexpensive way to monitor the small-scale inner-city spatial distribution of ambient trace elements content. This study shall provide deeper insights into pollution patterns of urban street canyons and shall deliver an easy applicable, extremely cost efficient tool for urban monitoring.
The results obtained on small scale surveys will form a qualitatively new database useful for mathematical modelling that predict levels and identify sources of atmospheric pollution on a large scale based on a small scale monitoring in the study areas. For modelling Operation Street Pollution Model (WinOSPM) will be involved. By applying conventional input data (dimensions of the canyons, traffic density, meteorological conditions) and data obtained by this active moss biomonitoring study a validation of these models can be given.
