Comment on egusphere-2025-1573

Abstract. The present study concerned the Arctic troposphere, providing an experimental database on nuclear aerosols that has improved considerably since 1999. The activity concentrations of 238Pu, 239+240Pu and 241Am were determined in the surface air of Hornsund, Spitsbergen, during 2007–2021. A multivariate approach was employed, incorporating meteorological data, 7Be, 137Cs, 210Pb records and isotopic ratios, to explain the transuranium dynamics of changes and provenance. 238Pu and 239+240Pu levels were comparable to recent observations from various locations, whereas 241Am exhibited particularly high values, with a maximum of 354 Bq/m3 detected in 2019. The highest activity concentrations of 6.61 nBq/m3 for 238Pu and 15.51 nBq/m3 for 239+240Pu identified in 2015 could be due to fly ash particles following wildfires in proximity to the Chernobyl zone. During the remaining period, 239+240Pu was correlated with seasonal processes, such as local resuspension and horizontal tropospheric transport of haze layers from remote areas. Similar mechanisms likely regulated a portion of 238Pu, but to a lesser extent. Unexpected single incidents of 237Np were encountered in 2013, 2014 and 2018. Furthermore, activity ratio evaluation demonstrated a frequent occurrence of 238Pu enrichment regarding known nuclear events. Unusual elevated levels of 241Am, 238Pu and 237Np were not associated with environmental processes; therefore, the possibility of recent anthropogenic emissions should be considered. Trajectory simulations performed in 2019 showed prominent transport pathways to the Hornsund from northern Eurasia.