Global atmospheric monitoring of radionuclides and noble gases: Insights into the interhemispheric transport of anthropogenic emissions

The radioactivity measurements registered in the aftermath of the Fukushima accident by the International Monitoring System (IMS) network provide evidence of North-to-South air mass transport. On a smaller scale, similar evidence is also provided by the episodic releases of both radioactive particles and noble gas from civil nuclear facilities. These anthropogenic releases are collected by the stations of the IMS radionuclide (RN) network during normal routine operations. Atmospheric transport modelling (ATM) calculations enable identification of possible source areas of these radionuclide detections. It is demonstrated that the IMS stations located in the band between the Equator and the Tropic of Capricorn can occasionally be influenced by anthropogenic radioactive emissions from facilities located in the Northern Hemisphere. North-to-South air mass transport is however a rather rare - but observed - phenomenon. It is more frequently observed between January and April. ATM backward simulations show that the actual pathways of air masses depend on the prevailing wind direction and vary from station to station. In most cases air masses need less than 14 days (which is the maximum number of days for ATM operational backward simulations at the CTBTO) to be transported from the emission source and go across the Equator. However, for more isolated IMS stations, strongly influenced by trade winds like RN51 (Kavieng, Papua New Guinea) or RN26 (Nadi, Fiji), it can be demonstrated that this operational criterion is often not adapted as transport of air masses from release to collection points can be much longer. It is therefore recommended that the ATM backward simulations performed at the CTBTO are extended to at least 21 days in routine operations.