A record of atmospheric 210Pb deposition in The Netherlands
Introduction
Nuclides of the 238U decay chain are widely used as tracers in atmospheric research, oceanography and marine geology. Budget studies in oceanography and determination of sediment accumulation rates are based on a constant supply of 210Pb to the sea. Pb-210 is produced in the sea water by in situ decay of 226Ra, via 222Rn and a few short-lived isotopes. Ocean waters have a 226Ra concentration of approx. 1.3 Bq m−3 (Cochran et al., 1990). Another main supplier of 210Pb to the continental shelf is atmospheric deposition (Bacon et al., 1994; Beks, 1997).
Atmospheric 210Pb (T1/2=22.3 years) is produced by in situ decay of gaseous 222Rn (T1/2=3.8 days). The source of atmospheric 222Rn is decay of 226Ra (T1/2=1602 years) in continental soils and in seawater. The world-wide average flux of 222Rn to the atmosphere is approx. 15 mBq m−2 s−1 from continental soils (Rama et al., 1961; Samuelsson et al., 1986), and 0.2 mBq m−2 s−1 from ocean waters (Samuelsson et al., 1986). Nazaroff (1992)found 22 mBq m−2 s−1 for radon exhalation from continental soils. The continents can thus be regarded as extended sources of atmospheric 222Rn. The mean aerosol residence time in the troposphere is approx. 5 days (Turekian et al., 1977; Lambert et al., 1982; Samuelsson et al., 1986), which is relatively short compared to the half-life of 210Pb. Due to the fast scavenging of 210Pb by precipitation there is a disequilibrium between 222Rn and 210Pb in the lower atmosphere (troposphere).
The world-wide downwards flux of 210Pb, in atoms m−2 s−1, should equal the upwards flux of 222Rn (Chamberlain, 1991). The activity is defined aswhere N is the number of atoms, so that the activity flux F isThe downwards 210Pb flux should therefore range between 8 mBq m−2 day−1 (oceanic level) and 600–900 mBq m−2 day−1 (continental level). As the continents cover approximately one-third of the earth's surface, the global average 210Pb deposition is 200–300 mBq m−2 day−1.
Because the continental and oceanic 222Rn exhalation rates differ by two orders of magnitude, the regional and local meteorology plays a significant role in the atmospheric deposition flux of 210Pb in a coastal area. Dörr et al. (1983) found a strong correlation between wind direction and 222Rn concentration in the air at a research platform in the German Bight, approx. 150 km NE of Groningen. In August 1980 northern winds corresponded to low 222Rn concentrations and southern winds brought continental air over the platform with high 222Rn concentrations. With predominant westerly winds only a small portion of the rain in the North Sea area will be of (European) continental origin. As the winds from North America take at least 4 days to cross the Atlantic Ocean, most 222Rn from the American continent has already decayed to 210Pb, and has been likely deposited over the ocean.
Variations in 222Rn concentration in the air are caused by a combination of temporal and geographic variations. Seasonal variations in 222Rn are mentioned by Lambert et al. (1990), who found in Antarctican air a summer concentration three times the winter concentration. Land coverage with snow or ice in the winter prevents 222Rn exhalation. Moisture content of continental soils also determines the 222Rn exhalation rate (Appleby and Oldfield, 1992). Geographic variations in The Netherlands are described by de Meijer (1992), who found mean annual radon concentrations in groundlevel air varying from <3 Bq m−3 to >6 Bq m−3 over 30-km distance.
Here we present temporal and spatial variations in the deposition of atmospheric 210Pb in The Netherlands over a 5-year period. This time-series was started to obtain an estimate for the atmospheric input in the 210Pb budget of the North Sea. The presented data sets may serve as a start for a more accurate estimation of the atmospheric deposition over the North Sea. Apart from longer data sets of the presented sites are some additional coastal sampling sites desirable in the UK, Denmark or Norway. The option of having some permanent sampling stations in the North Sea itself is as desirable as unrealistic.
Section snippets
Sample collection and analytical methods
Sampling was based on total deposition (dry+wet) of atmospheric 210Pb. Rain collectors were situated in the north-east (Groningen, 53°18′N 6°35′E) and north-west (Texel, 53°01′N 4°48′E) of The Netherlands (Fig. 1). The site in Groningen was 20 km from the sea to the north and 70 km to the west. The site at Texel is 10 km from the sea to the west and 100 m to the east. The most frequent wind direction is west in Groningen and south-west to west at Texel. The average wind speed is higher at Texel
Results and discussion
The time-series of total 210Pb deposition in Groningen and at Texel (Fig. 2A,B) show highly variable atmospheric fluxes. In Groningen the range was 20–2305 mBq m−2 day−1 with a median value of 138 mBq m−2 day−1 (n=193). At Texel the range was 1–3515 mBq m−2 day−1 with a median value of 146 mBq m−2 day−1 (n=305). As the sampling periods, especially in Groningen, vary in length (6–28 days) these measurements only give a first impression of the variation in deposition fluxes. Although the 210Pb
Conclusions
In the annual 210Pb deposition in The Netherlands we observed variations of a factor 2, both geographically and in time. The geographical position on the continent, with predominantly westerly oceanic winds, determines the mean long-term deposition flux. An almost random statistical behaviour of precipitation determines the fluctuation in the daily 210Pb flux (0–8865 mBq m−2 day−1, median value 10 mBq m−2 day−1). In particular the number of heavy rains or thunder storms determines the annual
Acknowledgements
The authors thank Rineke Gieles for taking care of the radiochemistry. This is NIOZ contribution 3268.
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