Third international challenge to model the medium- to long-range transport of radioxenon to four Comprehensive Nuclear-Test-Ban Treaty monitoring stations.

In 2015 and 2016, atmospheric transport modeling challenges were conducted in the context of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) verification, however, with a more limited scope with respect to emission inventories, simulation period and number of relevant samples (i.e., those above the Minimum Detectable Concentration (MDC)) involved. Therefore, a more comprehensive atmospheric transport modeling challenge was organized in 2019. Stack release data of Xe-133 were provided by the Institut National des Radioéléments/IRE (Belgium) and the Canadian Nuclear Laboratories/CNL (Canada) and accounted for in the simulations over a three (mandatory) or six (optional) months period. Best estimate emissions of additional facilities (radiopharmaceutical production and nuclear research facilities, commercial reactors or relevant research reactors) of the Northern Hemisphere were included as well. Model results were compared with observed atmospheric activity concentrations at four International Monitoring System (IMS) stations located in Europe and North America with overall considerable influence of IRE and/or CNL emissions for evaluation of the participants' runs. Participants were prompted to work with controlled and harmonized model set-ups to make runs more comparable, but also to increase diversity. It was found that using the stack emissions of IRE and CNL with daily resolution does not lead to better results than disaggregating annual emissions of these two facilities taken from the literature if an overall score for all stations covering all valid observed samples is considered. A moderate benefit of roughly 10% is visible in statistical scores for samples influenced by IRE and/or CNL to at least 50% and there can be considerable benefit for individual samples. Effects of transport errors, not properly characterized remaining emitters and long IMS sampling times (12-24 h) undoubtedly are in contrast to and reduce the benefit of high-quality IRE and CNL stack data. Complementary best estimates for remaining emitters push the scores up by 18% compared to just considering IRE and CNL emissions alone. Despite the efforts undertaken the full multi-model ensemble built is highly redundant. An ensemble based on a few arbitrary runs is sufficient to model the Xe-133 background at the stations investigated. The effective ensemble size is below five. An optimized ensemble at each station has on average slightly higher skill compared to the full ensemble. However, the improvement (maximum of 20% and minimum of 3% in RMSE) in skill is likely being too small for being exploited for an independent period.

Influence of mineral dust on changes of 7Be concentrations in air as measured by CTBTO global monitoring system.

Atmospheric Transport Modelling (ATM) results were combined with 7Be observations collected during the 2009-2015 period by the three radionuclide stations from the International Monitoring System (IMS), located in Mauritania (18.1 N, 15.9 W), Kuwait (29.3 N, 47.9 E) and Panama (9.0 N, 79.5 W), to study the influence of Saharan dust on changes in 7Be surface concentrations. It is demonstrated that for long-range transport (>3000 km), the overall impact of Sahara can be reproduced using a single point source located in the Bodélé depression (17.0 N, 18.0 E). To monitor the arrival time of dust plumes at the IMS stations, a series of 14-day forward simulations with daily releases from the Bodélé, during dusty episodes between 2009 and 2015, were generated. In total 1020 simulations with the output at the surface level (0-150 m) and 420 simulations with the output at 9 vertical layers ranging from the surface up to 10 km, were analysed. In the simulations, the analysed meteorological input data provided by the European Centre for Medium-Range Weather Forecasts (ECMWF) were used. It is demonstrated that an influx of dust at high levels (3-10 km) tends to locally increase surface 7Be concentrations in area under the influence of subsiding dust plume. It is also shown that an influx of dust at lower altitudes (up to 1 km) will have the opposite effect on surface concentrations. In case dust is present in the whole column of atmosphere, its final impact depends on the ratio between its amount in the upper layers (3-10 km) and lower layers (0-1 km). In consequence an increase up to 30% or a decrease up 20% in daily 7Be surface values may be observed during such an episode. On a monthly scale a few episodes related to an increase of 7Be values or its decrease may follow each other. It was estimated that on average the presence of dust leads to the increase of 7Be mean monthly surface values. The largest increase was noted at the station MRP43, of about 4.1 ±â€¯1.3%; and the smallest at the stations KWP40, of about 2.0 ±â€¯1.6% and PAP50, of about 2.0 ±â€¯1.0%, respectively.

Long-range transport of Xe-133 emissions under convective and non-convective conditions.

To investigate the transport of xenon emissions, the Provisional Technical Secretariat (PTS) operates an Atmospheric Transport Modelling (ATM) system based on the Lagrangian Particle Dispersion Model FLEXPART. The air mass trajectory ideally provides a "link" between a radionuclide release and a detection confirmed by radionuclide measurements. This paper investigates the long-range transport of Xe-133 emissions under convective and non-convective conditions, with special emphasis on evaluating the changes in the simulated activity concentration values due to the inclusion of the convective transport in the ATM simulations. For that purpose a series of 14 day forward simulations, with and without convective transport, released daily in the period from 1 January 2011 to 30 June 2013, were analysed. The release point was at the ANSTO facility in Australia. The simulated activity concentrations for the period January 2011 to February 2012 were calculated using the daily emission values provided by the ANSTO facility; outside the aforementioned period, the median daily emission value was used. In the simulations the analysed meteorological input data provided by the European Centre for Medium-Range Weather Forecasts (ECMWF) were used with the spatial resolution of 0.5°. It was found that the long-range transport of Xe-133 emissions under convective conditions, where convection was included in the ATM simulation, led to a small decrease in the activity concentration, as compared to transport without convection. In special cases related to deep convection, the opposite effect was observed. Availability of both daily emission values and measured Xe-133 activity concentration values was an opportunity to validate the simulations. Based on the paired t-test, a 95% confidence interval for the true mean difference between simulations without convective transport and measurements was constructed. It was estimated that the overall uncertainty lies between 0.08 and 0.25 mBq/m3. The uncertainty for the simulations with the convective transport included is slighted shifted to the lower values and is in the range between 0.06 and 0.20 mBq/m3.

Influence of precipitation on (7)Be concentrations in air as measured by CTBTO global monitoring system.

Data collected by the International Monitoring System (IMS) during 2009-2012 were used to study influence of precipitation and relative humidity on changes in (7)Be concentrations in atmosphere. The significant decrease in (7)Be concentrations, corresponding to measurements collected by stations located within Intertropical Convergence Zone (ITCZ) is demonstrated. This effect can be attributed to the process of enhanced wet deposition within the ITCZ. To quantify this effect data collected by IMS stations within ITCZ were thoroughly analyzed. It was found that the atmospheric content of (7)Be strongly decreases under the rain conditions. The rain mediated depletion of (7)Be to half of its before rain value, needs about 62 h in case of light precipitation, while in the case of moderate precipitation about 38 h is needed. In addition the evaluated impact of humidity showed that increase in relative humidity by 20%, for example from 70% ± 5% to 90% ± 5% causes almost a double decrease in beryllium concentration in surface air.