Seasonal Variability of Xe‐133 Global Atmospheric Background: Characterization and Implications for the International Monitoring System of the Comprehensive Nuclear‐Test‐Ban Treaty

Global simulations of the atmospheric dispersion of worldwide industrial Xe-133 releases have revealed a large spatial and day-to-day variability of the resulting so-called Xe-133 atmospheric background. Most stations of the International Monitoring System (IMS) of the Comprehensive nuclear-Test-Ban Treaty Organization actually detect Xe-133 regularly. Measured levels are explained by a varying combination of local and distant industrial sources and can interfere with discrimination of nuclear test signatures. Therefore, a better understanding of the Xe-133 atmospheric background is needed. In this study, a validated 2year simulation data set and a 4year measurement data set of Xe-133 activity concentrations have been analyzed in order to characterize possible seasonal variations of the Xe-133 atmospheric background due to atmospheric circulation, with a focus on (i) global distributions, (ii) occurrences of detections by the IMS network, and (iii) time series of monthly averages at IMS stations. Results show a larger spatial extent of the atmospheric background during winter months, which translates into a larger number of detections on the IMS network during winter months for both hemispheres. Some IMS stations present a significant seasonal variability in terms of levels, or both in terms of levels and origins. However, not all IMS stations are subject to seasonal variations, given their location with respect to sources and large-scale atmospheric circulation. In addition, a first set of predicted information about expected levels of atmospheric background at IMS stations not yet operational is provided, given the current knowledge of sources.