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

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 (2^nd and 3^rd 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 (2^nd quarter) while the most ⁷Be depleted precipitation occurred during January-March (1^st quarter). The large excess of ⁷Be in precipitation during April-June (2^nd 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 (V_D) 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.