Ice nucleating particle flux density estimated with radon tracer methods during conditions dominated by thermally driven mountain winds

Abstract. Spatiotemporal variability of tropospheric ice nucleating particle (INP) concentrations, driven by a combination of surface sources and atmospheric transport mechanisms, distinctly modulates their effect on clouds. However, limited understanding of INP emission patterns currently impedes predictions of this variability, particularly for INPs active at little supercooling, e.g. temperatures ≥ -10 °C (INP_-10). Here, we apply two implementations of the radon tracer method (RTM) to quantify the INP_-10 flux density between source regions and a high-altitude site in central Europe during thermally driven planetary boundary layer (PBL) intrusion events. By using observations at a high-altitude site, the chosen implementations of the RTM are applied directly at an altitude where INPs can impact clouds, and at a location where atmospheric conditions regularly change between air masses characteristic of free tropospheric or “baseline” conditions and conditions distinctly influenced by air masses that have experienced recent contact with the terrestrial surface. Median emission flux densities estimated based on our implementation of the traditional RTM using two different radon exhalation maps were approximately 360 INP_-10 m^-2 h^-1 and 250 INP_-10 m^-2 h^-1, respectively. The median emission flux density modelled in a Bayesian framework was about 720 INP_-10 m^-2 h^-1. Initial estimates seem to indicate a limited role of thermally driven transport on fair-weather days in the annual INP_-10 export above the Alps. Relating the calculated flux densities to other kinds of observations, we demonstrate that top-down approaches investigating mean INP flux densities on local to regional scales under particular meteorological conditions can contribute to an improved understanding of INP dynamics at cloud height.