The use of atmospheric radionuclides as tracers of atmospheric processes: what can we learn about aerosol transport?

Radionuclides have been traditionally employed in environmental studies as tracers of environmental processes, providing timescales of processes and trace substances involved in terrestrial transport, exchange and mixing processes (Froehlich and Masarik, 2010). In the field of atmospheric sciences, the research on cosmogenic and terrigenous radionuclides has been crucial for a better understanding of several basic processes, such as interhemispheric transport, stratosphere-troposphere exchange (STE) and timescales of atmospheric dynamics. Moreover, it also provided important insights into transport and/or deposition processes of atmospheric pollutants. In particular, Radon-222 and its progenies in the atmosphere have been widely utilized as powerful tracers to quantify atmospheric processes that include: i) source tracking and transport (within and between troposphere and stratosphere) time scales of air masses, including the stability and vertical motion of air masses; ii) removal rate constants and residence times of aerosols; iii) deposition velocities and washout ratios of aerosols; iv) sources of continental dust in an air mass; v) flux to and exchange between environmental systems of other gaseous species (e.g., CH4, Hg0); and vi) processes of attachment of metal ions to atmospheric aerosols. In addition, radon is increasingly being used as a tool for quantifying stability influences on urban pollutant concentrations. This work presents a brief compendium of research results achieved exploiting the use of terrigenous radionuclides in different sites located in the Mediterranean basin. Specifically, we will demonstrate how the use of long-term time series of the crustal 210Pb radionuclide produced by 222Rn decay collected at two different sites of different altitudes in the Mediterranean basin was effectively used together with trajectory statistics to study the role of the Mediterranean sea as a major aerosols reservoir layers (Brattich et al., 2016). As a second point, we will show how the study of trends in radon-222 progeny can help to elucidate the causes of downward PM10 trends observed in the last decades in Europe, offering alternative explanations in terms of a combination of meteorological factors juxtaposed over decreasing anthropogenic emissions (Brattich et al., 2020). We will finally demonstrate how 222Rn can be used provide as a uniquely efficient tool for atmospheric circulation in complex topography conditions. In fact, the use of smart use of wind speed and anemological parameters may lead to source localizations and source characterization, such as height of emission above the surface on the basis of radon whose behavior is spatially and temporally well recognizable.