Temporal variability of beryllium-7 fallout in southwest UK
Introduction
Beryllium-7 (7Be) (t1/2 53.3 days) is a cosmogenic radionuclide produced in the upper atmosphere by cosmic ray spallation of nitrogen and oxygen. Upon fallout, its affinity with sediment (Kaste et al., 2002) has led to its widespread use as a sediment tracer in terrestrial and fluvial-marine environments (Taylor et al., 2013, Walling, 2012). With continued development of 7Be application as a tracing tool it is necessary to improve our knowledge regarding the temporal variability of fallout (Walling et al., 2009). A number of factors affect fallout temporal variability by influencing 7Be availability in surface air. For example, rates of 7Be production are dependent upon cosmic ray activity and can, therefore, be influenced by solar cycles with 7Be concentrations in surface air demonstrating a negative correlation with sunspot number (Ioannidou et al., 2005). On a seasonal basis, concentrations of 7Be in surface air are influenced by mixing of stratospheric air with the upper troposphere (stratosphere-troposphere exchange (STE)). Higher rates of production in the stratosphere and longer residence time of 7Be-bearing aerosols creates a concentration gradient relative to the troposphere (Doering and Akber, 2008a). This concentration gradient can be reduced during folding of the tropopause, which encourages mixing of the troposphere with stratospheric air and this commonly occurs during spring at mid-latitudes (Feely et al., 1989). Convective circulation during warmer months can also influence seasonal variation in 7Be concentrations in surface air by driving the downward transport of 7Be-enriched air from the upper troposphere (Doering and Akber, 2008a, Feely et al., 1989, Ioannidou et al., 2005). Because 7Be-bearing aerosols are readily scavenged by precipitation (Ioannidou and Papastefanou, 2006), seasonal patterns in rainfall can also affect surface air concentrations by removing available 7Be (washout) (Doering and Akber, 2008a, Feely et al., 1989). Wet deposition is the dominant pathway of 7Be flux to the Earth's surface with dry deposition accounting for <10% (Ioannidou and Papastefanou, 2006, Wallbrink and Murray, 1994). 7Be deposition (Bq m−2) is, therefore, well correlated with rainfall (mm) (Ayub et al., 2009, Caillet et al., 2001, Doering and Akber, 2008b, Gonzalez-Gomez et al., 2006, Mabit et al., 2014) although some variation in depositional flux can be attributed to changes in 7Be activity in rainwater (Bq L−1). This is influenced by atmospheric processes such as washout and circulation of enriched air (Baskaran, 1995, Caillet et al., 2001, Gonzalez-Gomez et al., 2006). There is also some evidence to suggest that 7Be activity in rainwater can be influenced by rainfall intensity with more effective scavenging of aerosols by fine rainfall droplets (Ioannidou and Papastefanou, 2006).
The availability of 7Be in surface air and its subsequent fallout is, thus, influenced by a complex combination of factors, which are likely to contribute to temporal variability of 7Be activity in rainwater and deposition. In the UK, data regarding the temporal variability of 7Be fallout are scarce and, to the best of the authors' knowledge, no published, peer-reviewed data exist for southwest England. This contribution provides a record of monthly 7Be fallout for Plymouth, southwest England (average annual rainfall 1007 mm (1981–2010) (Met Office)) spanning a two-year period (2009–2011). Potential controls upon temporal variability of 7Be rainfall activity and deposition are discussed and, in addition, data are compared to secondary available records obtained for other areas of the UK during the same study period.
Section snippets
Method
A total of 30 rainfall samples were collected from a flat roof area located ∼50 m a.m.s.l (∼15 m above ground level) on the Plymouth University campus (50° 22′ 55″ N, 04° 08′ 34″ W), over a 25 month period from February 2009 to March 2011. A tipping-bucket rain gauge with 0.2 mm resolution and 10 min logging intervals was located adjacent to the sample site. Total (wet and dry) fallout was sampled in plastic containers (3 L) using 0.05 m2 area funnels. Prior to deployment, 10 mL HCl (2.5 M) was
Rainfall activity concentrations and temporal variability
Data for monthly 7Be activity concentrations in rainfall and mean monthly rainfall are shown in Table 1. The range of 7Be activity concentrations over the study period in Plymouth was 1.56 (±0.13) to 2.67 (±0.20) Bq L−1 which agrees well with the values reported for other areas of the UK (Short et al., 2007). Fig. 2 shows a general trend of higher activity during the spring/summer months and lower activity during the autumn/winter periods. The mean activity concentration for February to
Discussion
Higher 7Be activity in rainfall during spring/summer months in Plymouth corresponds to findings of raised 7Be concentrations in surface air during spring and summer periods owing to decreased stability of air masses encouraging convective circulation within the troposphere (Daish et al., 2005, Doering and Akber, 2008a, Doering and Akber, 2008b, Ioannidou et al., 2005). This circulation can transport 7Be-enriched air from the upper troposphere to surface layers where it is readily scavenged by
Conclusions
7Be activity concentration in rainfall sampled in Plymouth was generally higher during the spring/summer months and this was potentially influenced by stratosphere to troposphere exchange (STE) and convective circulation within the troposphere. Comparison with wider UK sites demonstrated significant spatial variability in 7Be rainwater activity although it is difficult to identify controlling factors with limited local meteorological data. Data for Chilton and Aberporth showed much higher
Acknowledgements
This paper represents a contribution to the International Atomic Energy Agency Coordinated Research Project D1.20.11 under Research Agreement IAEA contract UK/15538 and has been finalised within the framework of the International Atomic Energy Agency Coordinated Research Project D1.50.17 under Research Agreement IAEA contract 20524/R0.
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