Estimation of the solar-induced natural variability of the tritium concentration of precipitation in the Northern and Southern Hemisphere

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 significantly 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.