Deposition of naturally occurring 7Be and 210Pb in Northern Finland.

Abstract

This study presents observations of naturally occurring 7Be and 210Pb in total deposition collected in three deposition sampling stations located in three towns in Northern Finland at Rovaniemi, Sodankylä and Ivalo. The deposition samples were measured in three-month intervals and the collected data covered the years from 1991 to 2017 in Rovaniemi and the years from 1999 to 2017 in Sodankylä and Ivalo. The 7Be deposition was observed to be dependent on the amount of precipitation where higher amount of precipitation meant higher 7Be deposition. The 7Be deposition showed a distinctive seasonal cycle where the highest 7Be deposition values were observed occur during July-September and the lowest values during January-March. From the annual total deposition 72-84% occurred during the months of April-September (2nd and 3rd quarters). The precipitation normalized enrichment factor (α) determined the excess or deficiency of 7Be relative to the amount of rain. The most excess of 7Be in the deposition was observed during April-June (2nd quarter) while the most 7Be depleted precipitation occurred during January-March (1st quarter). The large excess of 7Be in precipitation during April-June (2nd quarter) was an indication of injections of 7Be rich stratospheric air into troposphere. By using simultaneously collected 7Be aerosol concentration data at Rovaniemi and Ivalo, the 7Be deposition velocities were determined. The deposition velocities (VD) were also found to have a strong seasonal cycle where low velocities were observed during winter and higher ones during summer. The 7Be deposition time series were also compared with other time series that are known to affect ground level 7Be aerosol concentrations. These were precipitation, sunspot number (SSN), galactic cosmic ray flux (GCR), Arctic Oscillation (AO), North Atlantic Oscillation (NAO) and Scandinavian Pattern (SCAND) indices. The Pearson correlation and wavelet coherence (WTC) was used to analyze possible correlations between the time series. The analyses showed that the coherences were location dependent. Rovaniemi 7Be deposition data correlated strongly with rain and weakly with the SCAND index at the 6-8 year scale. The Sodankylä 7Be deposition data correlated strongly with the precipitation and SSN and GCR and weakly with AO and NAO indices. Ivalo 7Be deposition data correlated with strongly with 7Be aerosol concentration data, SSN, GCR and NAO index and weakly with the AO index. The WTC analysis revealed anti-phase coherences with the AO and NAO indices at 4-6 year scale. The 210Pb deposition data was sparse and only available from 2011 onwards. 210Pb deposition had a seasonal cycle where higher deposition values were observed during summer and lower ones deposition during winter. The annual 210Pb deposition was determined at Rovaniemi and Sodankylä during 2014. The values of 7Be/210Pb ratio was found to vary significantly seasonally, higher 7Be/210Pb ratios were measured during summer and lower ones during winter.