Environmental radionuclides as tracers for transport processes in snow.

Radionuclides are useful tracers for the determination of transport processes from the atmosphere to the hydrosphere by snowfall. Particularly in high altitudinal regions radionuclides released to the environment are efficiently removed from the atmosphere by snow. Once deposited, they will be concentrated in the snow cover due to evaporation and sublimation as long as ambient temperature stays low and no melt water runoff occurs. Short-term releases to surface waters after snowmelt initiation in spring may lead to high concentrations having an impact on water quality. To trace radionuclide transport in the aquatic environments related to snow on Mt. Zugspitze, in addition to their concentrations, comprehensive knowledge about the type of precipitation and the size of the aerosols, they are bound at, the snow cover development, the influence of meteorological conditions on snow alteration and melt water runoff are necessary. A brief description of methods for the investigation of the transport of ubiquitously distributed environmental radionuclides from their atmospheric deposition over their behaviour in the snow cover at Zugspitzplatt to the recovery in the surface water at Partnach spring is given. Applications performed in research projects of the authors’ work group since 2011 on Mt. Zugspitze are introduced and highlight the advantages of the location for studying water and radionuclide budgets related to snow. The results help to trace the pathways of radioactive particles from the atmosphere to aquatic environments. Consequently, in cases of extensive radionuclide releases to snow covered environments, peak discharges and the scope of action for countermeasures can be predicted to mitigate the impact on water quality and human radiation exposure

Washout of particle-bound radionuclides for characterised rain and snow events on Mt. Zugspitze, Germany.

Washout of radionuclides by precipitation is regarded to be an efficient deposition process depending on various parameters. Precipitation in the form of snow is suspected to be most efficient in scavenging of aerosol particles. Accidental releases of radionuclides, as seen in the recent nuclear accident of Fukushima-Daiichi in March 2011, have had serious impacts on the terrestrial environment, especially in the mountainous regions where major precipitation has fallen as snow. Supposing that wet deposition depends strongly on the type and  haracteristics of the precipitation event we present field experiments on the quantification of wet deposition of radionuclides by snow and rain events. The experiments took place in the free atmosphere at the Environmental Research Station Schneefernerhaus on Mt. Zugspitze, Germany (2,650 m a.s.l.). Snowfall event characterisation by  eteorological and snow microphysicalb parameters was carried out using a 2D video disdrometer combined with a precipitation gauge. Washout of aerosol particles in the size range from 10-2 μm to 10 μm was quantified operating an electrical low pressure cascade impactor ELPI+. Trace analysis was performed by ultra-low level gamma spectrometry (HPGe detectors) of cosmogenic 7Be, of terrigenic 210Pb (radon decay product) and of 137Cs from resuspended fallout of the Chernobyl accident in precipitation and bulk aerosol samples. Dependencies of washout efficiencies of the radionuclides on precipitation rates and snow parameters will be presented. The results shall help to improve radionuclide transport models used in modern decision support systems for emergency management (e.g. ARTM, RODOS), which do not include snow parameters so far. Proposal is made how to incorporate such parameters in future

Influence of melt-freeze-cycles on the radionuclide transport in homogeneous laboratory snowpack.

Radionuclides released to the environment and deposited with or onto snow can be stored over long time periods if ambient temperature stays low, particularly in glaciated areas or high alpine sites. The radionuclides will be accumulated in the snowpack during the winter unless meltwater runoff at the snow base occurs. They will be released to surface waters within short time during snowmelt in spring. In two experiments under controlled melting conditions of snow in the laboratory, radionuclide migration and runoff during melt-freeze-cycles was examined. The distribution of Cs-134 and Sr-85 tracers in homogeneous snow columns and their fractionation and potential preferential elution in the first meltwater portions were determined. Transport was associated with the percolation of meltwater at ambient temperatures above 0 °C after the snowpack became ‘ripe’. Mean migration velocities in the pack were examined for both nuclides to about 0.5 cm h-1 after one diurnal melt-freeze-cycle at ambient temperatures of -2 °C to 4 °C. Meltwater fluxes were calculated with a median of 1.68 cm h-1. Highly contaminated portions of meltwater with concentration factors between 5-10 against initial bulk concentrations in the snowpack were released as ionic pulse with the first meltwater. Neither for caesium nor strontium preferential elution was observed. After recurrent simulated day-night-cycles (-2 °C to 4 °C), 80 % of both radionuclides were released with the first 20 % of snowmelt within 4 d. 50 % of Cs-134 and Sr-85 were already set free after 24 h. Snowmelt contained highest specific activities when the melt rate was lowest during the freeze-cycles due to concentration processes in remaining liquids, enhanced by the melt-freeze-cycling. This implies for natural snowpack after significant radionuclide releases, that long-time accumulation of radionuclides in the snow during frost periods, followed by an onset of steady meltwater runoff at low melt rates will cause the most pronounced removal of the contaminants from the snow cover. This scenario represents the worst case of impact on water quality and radiation exposure in aquatic environments

Deposition von Umweltradionukliden mit Schnee auf drei Alpengipfeln.

Im Rahmen des Projektes „Virtuelles Alpenobservatorium“ (Verbund der Konsortialpartner der Umweltforschungsstation Schneefernerhaus in Kooperation mit anderen europäischen Höhenforschungsstationen) wurde am 2 . Juli 2015 am Sonnblick ein Schneeprofil beprobt, um die Verteilung der Umweltradionuklide Be - 7 , Pb - 210 und Cs - 137 darin zu bestimmen . Im Teilprojekt „Auswirkung des Klimawandels auf den alpinen Wasserhaushalt und die Umweltradioaktivität“ werden mit Hilfe dieser Tracernuklide die Transportpfade und - prozesse radioaktiver Partikel von der Atmosphäre über die Schneedecke ins Schmelz - und Oberflächenwasser untersucht . Die Aufnahme der Gesamtschneeprofile auf Zugspitze (D, 2 . 420 m ü . NN), Jungfraujoch (CH, 3 . 400 m ü . NN) und Sonnblick (A, 3 . 110 m ü . NN) dient dazu, den Ist - Zustand der deponierten Umweltradioaktivität an unterschiedlichen Lokalitäten zu ermitteln, um klimabedingte Änderungen des Wasser - und Radionuklidhaushaltes für die Zukunft daraus abzuleiten

Snow event classification with a 2D video disdrometer - a decision tree approach.

Snowfall classification according to crystal type or degree of riming of the snowflakes is import for many atmospheric processes, e.g. wet deposition of aerosol particles. 2D video disdrometers (2DVD) have recently proved their capability to measure microphysical parameters of snowfall. The present work has the aim of classifying snowfall according to microphysical properties of single hydrometeors (e.g. shape and fall velocity) measured by means of a 2DVD. The constraints for the shape and velocity parameters which are used in a decision tree for classification of the 2DVD measurements, are derived from detailed on-site observations, combining automatic 2DVD classification with visual inspection. The developed decision tree algorithm subdivides the detected events into three classes of dominating crystal type (single crystals, complex crystals and pellets) and three classes of dominating degree of riming (weak, moderate and strong). The classification results for the crystal type were validated with an independent data set proving the unambiguousness of the classification. In addition, for three long-term events, good agreement of the classification results with independently measured maximum dimension of snowflakes, snowflake bulk density and surrounding temperature was found. The developed classification algorithm is applicable for wind speeds below 5.0 m s -1 and has the advantage of being easily implemented by other users

On the consistency of 2-D video disdrometers in measuring microphysical parameters of solid precipitation

Detailed characterization and classification of precipitation is an important task in atmospheric research. Line scanning 2-D video disdrometer devices are well established for rain observations. The two orthogonal views taken of each hydrometeor passing the sensitive area of the instrument qualify these devices especially for detailed characterization of nonsymmetric solid hydrometeors. However, in case of solid precipitation, problems related to the matching algorithm have to be considered and the user must be aware of the limited spatial resolution when size and shape descriptors are analyzed. Clarifying the potential of 2-D video disdrometers in deriving size, velocity and shape parameters from single recorded pictures is the aim of this work. The need of implementing a matching algorithm suitable for mixed- and solid-phase precipitation is highlighted as an essential step in data evaluation. For this purpose simple reproducible experiments with solid steel spheres and irregularly shaped Styrofoam particles are conducted. Self-consistency of shape parameter measurements is tested in 38 cases of real snowfall. As a result, it was found that reliable size and shape characterization with a relative standard deviation of less than 5 % is only possible for particles larger than 1 mm. For particles between 0.5 and 1.0 mm the relative standard deviation can grow up to 22 % for the volume, 17 % for size parameters and 14 % for shape descriptors. Testing the adapted matching algorithm with a reproducible experiment with Styrofoam particles, a mismatch probability of less than 3 % was found. For shape parameter measurements in case of real solid-phase precipitation, the 2-DVD shows self-consistent behavior