Tritium has been long recognized as a useful tracer for the study of atmospheric transport, ocean circulation, and the global water cycle. In addition, the application of tritium measurements in various fields has grown signif- icantly in the last few decades. Since 1963, the atmospheric test-ban treaty, bomb tritium concentrations in precipitation have significantly declined. Therefore, in the last two decades, global tritium concentration of precipitation (including anthropogenic and natural sources) has almost reached a steady-state level. The aim of this study is to estimate the temporal variation of the natural tritium concentration of precipitation during the past decades. To do this, we use a backward predicting time-series model that exploits the correlation between precipitation tritium concentration and the secondary neutron flux in the atmosphere. The measured tritium time series of 21 Northern and two Southern Hemispheric stations are used, while neutron monitor (NM) data, which are widely compared to the production rate of cosmogenic isotopes in the atmosphere, is used as an external variable for the model. Backward predicting SARIMAX statistical models are fit on the period 2001–2018 and provide estimates of the natural precipitation tritium levels for the bomb peak period 1960–2000. Evaluation of backward estimations on the 1990–2000 test period yields RMSE measures between 0.5 and 4.6 TU for four of the 23 investigated stations, pointing out locations where the neutron flux is a good predictor of the precipitation tritium concentration.

The relationship between the atmospheric concentration of cosmogenic isotopes, the change of solar activity and hence secondary neutron flux has already been proven. The temporal atmospheric variation of the most studied cosmogenic isotopes shows a significant anti-correlation with solar cycles. However, since artificial tritium input to the atmosphere due to nuclear-weapon tests masked the expected variations of tritium production rate by three orders of magnitude, the natural variation of tritium in meteoric precipitation has not previously been detected. For the first time, we provide clear evidence of the positive correlation between the tritium concentration of meteoric precipitation and neutron flux modulated by solar magnetic activity. We found trends in tritium time series for numerous locations worldwide which are similar to the variation of secondary neutron flux and sun spot numbers. This variability appears to have similar periodicities to that of solar cycle. Frequency analysis, cross correlation analysis, continuous and cross wavelet analysis provide mathematical evidence that the correlation between solar cycle and meteoric tritium does exist. Our results demonstrate that the response of tritium variation in precipitation to the solar cycle can be used to help us understand its role in the water cycle.

Carbon-14 (half-life 5730 years) is produced by the reaction of secondary thermal neutrons derived from the primary cosmic-ray flux on nitrogen9. This 14C is incorporated into the terrestrial carbon cycle within 1–2 years as is most convincingly demonstrated through the14C signal of anthropogenic nuclear testing10. Miyake et al.2,3,4 were the first to study the annual signal of 14C in tree rings to reveal annual excursions, which were much larger than those observable in the decadally resolved international radiocarbon calibration curve (IntCal20). Specifically, they observed spikes in 14C activity at 774–775 CE and 993–994 CE2,4. The 774 CE event was first confirmed in a bristlecone pine record4, and indeed, this work encouraged many subsequent studies looking for both these events and searches for other events. Büntgen et al.5 summarizes 14C series of trees for 774–775 CE from 34 locations and another set of trees for 993–994 CE from 10 locations around the globe. Another rapid event at 660 BCE has been reproduced in different records and is therefore widely accepted. Separately, other rapid changes that may show more complex solar dynamo phenomena or combinations of solar and galactic events at 5480 BCE and 813 BCE have been observed.

The methane emissions from the Hungarian Pannonian Basin are not well qualified, due to a lack of measurements of CH4 mole fraction and δ13CCH4 in the air. This study reports methane measurements in air samples from Hungary, placing them in the context of regional and global background data, to investigate the inputs to the methane burden in Central Europe. CH4 mole fraction and δ13CCH4 from the Hungarian tall tower station, Hegyhátsál, and additional data from Mace Head (Ireland) and Zeppelin (Svalbard) are used with back trajectory modeling to identify central European source areas and their seasonal variation between the summer vegetation and winter heating periods. Methane measurements in air masses sampled in the European interior, have significantly higher maxima and seasonal amplitudes than at the Mace Head and Zeppelin European background sites. The mean CH4 mole fraction value is about 80 ppb higher than the comparable marine background, and values above 2,000 ppb were frequently observed between February 2013 and December 2015. The mean δ13CCH4 value −47.5 ± 0.3‰ (2σ) was comparable to values at all three monitoring sites, but specific pollution events were detected at Hegyhátsál. Concentration weighted trajectory modeling, meteorological parameters, stable carbon isotopic composition (δ13CCH4), and Miller-Tans analysis show that the main factors influencing CH4 at the Hegyhátsál, apart from diurnal and seasonal changes in the planetary boundary layer, are emissions from residential heating and industrial CH4 emissions during the winter.