The impact of different atmospheric phenomena to cosmogenic 22Na/7Be ratio

Abstract

The isotopic ratio of two cosmogenic isotopes, 22Na and 7Be, has been recognized as a possible tracer for atmospheric dynamics. We studied how different atmospheric phenomena affect the 22Na/7Be ratio that are known to affect the ground level 7Be concentrations. The 22Na/7Be ratio was studied in the following context: estimation of aerosol residence time, in the intrusions of stratospheric air after Sudden Stratospheric Warming (SSW) events, seasonal variations due to different atmospheric mixing conditions and finally the interannual variations caused by the large-scale atmospheric circulation such as North Atlantic Oscillation (NAO), Arctic Oscillation (AO), Scandinavian Pattern (SCAND) and Quasi-Biennial Oscillation (QBO). This study utilized airborne radioactivity monitoring data from three Finnish stations located in Rovaniemi (66.51 °N, 25.73 °E), Kajaani (64.08 °N, 27.71 °E) and Kotka (60.48 °N, 26.92 °E). The theoretical calculations were performed to see how the 22Na/7Be ratio changes as a function of altitude during when produced by the galactic cosmic rays. The calculations showed that the 22Na/7Be ratio is higher in the stratosphere and lower in the troposphere. At mid-stratosphere level the ratio was 0.001, at tropopause level approximately 0.0007 and at ground level approximately 0.00045. Due to the different half-lives of 22Na and 7Be the mean atmospheric residence time can be estimefrom the radioactive decay law by assuming an initial activity concentration ratio of 0.0007 and using the measured ground level 22Na/7Be ratios. This resulted in the median atmospheric residence time as 91–95 days. The atmospheric impact on 22Na/7Be ratio was studied at different time scales starting from short-term impact from SSW, the known seasonal cycle and variations at interannual timescale. SSW events caused rapid increases in the ground level 7Be and 22Na concentrations while the impact on 22Na/7Be ratio was relatively small. A clear seasonal cycle in the 22Na/7Be ratio was observed where the 22Na/7Be ratio was at low value during November–February, increased during March–May, stayed at stable high value during June–July and steadily decreased during August–October. The long-term average for 22Na/7Be ratio was established to be 1.1 × 10−4 by using the Kotka data from 1993 to 2020. During 1995–2009, the 22Na/7Be ratio was below the long-term average, and during 2010–2020, generally above. The interannual variations in the 22Na/7Be ratio were studied by using wavelet transform analysis of the Kotka data. The analysis showed 4-6-year and 11-year periodicities. The wavelet coherence analyses with AO, NAO, SCAND, QBO and Southern Oscillation (SO) indices showed that the shorter periodicity was caused by the NAO/AO effects the atmospheric circulation in Scandinavia and hence the transport of the isotopes from upper atmosphere to ground level. The observed coherence with the Southern Oscillation Index (SOI) starting from 2012 onwards appeared at an approximately 3–4year scale while the AO and NAO had a coherence at an approximately 6-year scale. The observed 11-year periodicity was caused by the Solar cycle which modulates the 7Be and 22Na production in the upper atmosphere.