Beryllium-7 and 210Pb atmospheric deposition measured in moss and dependence on cumulative precipitation
Graphical abstract
Correlation between cumulative activity of 7Be and 210Pb measured in moss samples normalized by the cumulative precipitation.
Introduction
Natural airborne radionuclides 7Be and 210Pb are frequently used as tracers in atmospheric studies (Lal and Peters, 1962, Vecchi and Valli, 1997, Baskaran, 2011). They have very different origins. Beryllium - 7 is primarily created in stratosphere, and upper troposphere by way of interactions of high-energy particles of cosmic radiation with nuclei of the most abundant elements in the air, nitrogen and oxygen (Lal et al. 1958). After formed, 7Be attaches to aerosol due to high chemical reactivity. Thus, 7Be follows all atmospheric paths of transport and deposition of aerosol (Papastefanou and Ioannidou, 1995, Aldahan et al., 2001, Kaste et al., 2002). Soil containing Uranium, is the dominant source of 210Pb. As a member of 238U decay chain, 222Rn as a noble gas, escapes all chemical bounds and enters the atmosphere. When 222Rn decays, daughter elements are produced that have a high affinity for aerosol are produced. 210Pb, the only one long living daughter product of 222Rn with half life of 22 years, becomes involved in atmospheric transport as well as deposition processes (Baskaran and Naidu, 1995, Koch et al., 1996, Preiss et al., 1996, Dibb, 2007). Measurement of 7Be and 210Pb activity concentration (in Bq m− 3) in atmosphere is usually done by air sampling (Kulan et al., 2006, Cho et al., 2007, Papastefanou, 2009, Carvalho et al., 2013, Yang et al., 2013). Air is collected by a calibrated vacuum pump through a filter to trap aerosol. After chosen collection time the activity concentration from filter is measured by the standard gamma spectrometry, following either some chemical treatment or in bulk. Aerosols removed from atmosphere by dry or wet deposition can be collected by deposition samplers (Wallbrink and Murray, 1994, Hirose et al., 2004, Du et al., 2008, Renfro et al., 2013). After some period of time, collected content in depositional vessel is measured by gamma spectroscopy device to determine the activity the radionuclide of interest in fallout (in Bq m− 2). Temporal variations of 7Be and 210Pb activity in air, as well as in precipitation, have been a subject of numerous studies (cf. Feely et al., 1989, Azahra et al., 2003, Ioannidou et al., 2005, Yamamoto et al., 2006, Doering and Akber, 2008, Kikuchi et al., 2009, Mann et al., 2011). Seasonal variations of 7Be and 210Pb in air and fallout were observed, including 11-year periodicity of 7Be concentration in the atmosphere as a consequence of solar modulation of arriving cosmic radiation (Cannizzaro et al., 2004, Papastefanou and Ioannidou, 2004, Kulan et al., 2006). In some cases correlation between measured airborne radionuclide and some atmospheric parameters (precipitation) has also been found (Caillet et al., 2001, Lozano et al., 2012, Tositti et al., 2014).
Currently, terrestrial mosses are widely used in environmental monitoring (Rühling and Tyler, 1973, Buse et al., 2003). Measurement of element concentrations in moss has been shown to be very suitable for studying the atmospheric deposition of heavy metals as well as other trace elements, including radionuclides (Godoy et al., 1998, Delfanti et al., 1999, Dowdall et al., 2005). Widespread occurrence, simple sampling procedure, and gamma spectroscopy measurement (without any chemical treatment of moss samples) make mosses a very useful medium in investigation and monitoring of airborne radionuclide deposition. In order to use 7Be or 210Pb effectively as a tracer (Krmar and Matavuly, 2007, Krmar et al., 2009), to study different atmospheric processes, their depositional characteristics should be known and a comprehensive understanding of their variability in the atmosphere, as well as dependence on a number of atmospheric parameters. In previously and later published papers (Ishikawa et al., 1995, Guebuem et al., 2000, Caillet et al., 2001, Ioannidou and Papastefanou, 2006, Zhu and Olsen, 2009) a moderate direct correlation was observed between precipitation (which washes-out the atmosphere and deposit activity) and total measured depositional fluxes of airborne radionuclides. On the contrary, concentrations of airborne radionuclide measured in near ground air usually show a week inverse correlation in respect to the precipitation amount (Likuku 2006). It can be expected because precipitation washes-out atmosphere and after rain or snow, lower concentrations of airborne activity are measured until sedimentation from higher layers of atmosphere brings new amount of radionuclides.
The main goal of this paper is to explore some techniques which can provide more insight into possible relationships between measured activities of airborne radionuclides and amount and the duration of precipitation. Krmar et al. (2009) pointed out that 7Be and 210Pb activities measured in mosses are not correlated with precipitation amount and the duration of precipitation, despite the fact that content of radionuclides in moss tissues depends on depositional fluxes. This can be explained since mosses cumulate activity, i.e., an amount of some airborne radionuclide in moss sample is cumulative measure of radionuclide deposition and decay and/or physical removing. Thus a different approach in analysis of relations between precipitation and moss activity has to be considered. Our objective for this paper is to investigate (i) the use data of cumulative precipitation and the cumulative duration of precipitation; and (ii) explore relationships between those datasets and measured values of the 7Be and 210Pb activities that provide a dataset of “cumulative activities” of airborne radionuclides in mosses.
Section snippets
Materials and methods
Samples of naturally growing mosses were collected from single location (~ 300 m2) from the horizontal eaves of the Department of Biology in Novi Sad, Serbia. One sample per week was taken during 14 months. The green part of the moss that contains 3–4 annual growth segments was used for analyses. Samples were cleaned from all mechanical impurities (leaves and other plant material), and dried until constant mass was reached. Dry mosses (from 22 to 48 g) were packed in cylindrical plastic containers
Cumulative activity of 7Be and 210Pb in mosses as a function of time
The results of measurements of activity concentrations of 7Be and 210Pb in selected moss samples are presented in Fig. 1, modified from (Krmar et al. 2009). From Fig. 1b it is seen that values of 210Pb activity concentration do not show prominent seasonal variation. In contrast to that Fig. 1a depicts that a prominent increase of activity concentration of 7Be appears at the beginning of the summer. Published results of air concentrations or deposition measurements of 7Be display on a strong
Conclusions
In this paper we focused on how to achieve better understanding of precipitation and duration of events that potentially have influence on variability of the 7Be and 210Pb depositional processes, and consequently, on modes of accumulation of airborne radionuclides in mosses, using measured activity concentrations of 7Be and 210Pb in moss samples and precipitation data. This is important to understand since use of mosses as a medium for gathering airborne radionuclides opens up a lot of
Acknowledgments
This paper was realized as a part of the project “Studying climate change and its influence on the environment: impacts, adaptation and mitigation” (III43007) financed by the Ministry of Education and Science of the Republic of Serbia within the framework of integrated and interdisciplinary research for the period 2011–2014.
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