What can gross alpha/beta activities tell about Po and Pb in the atmosphere?

Po and Pb represent the most abundant part of atmospheric aerosol long-lived natural radioactivity. Moreover, Pb-Po monitoring in the atmosphere can be of interest for tracking extreme natural events that can enhance the level of alpha/beta radioactivity in air. In this paper, we question the possibility to use routine gross alpha/beta measurements in order to monitor Po, Pb and Po/Pb ratio in the atmosphere. Based on joint gross alpha/beta measurement and Pb-Po specific determination on 16 atmosphere samples, we show that (i) gross beta activity systematically overestimates Pb activity due to the presence of interfering beta emitters and (ii) gross alpha activity mostly reflects Po activity even if an underestimation is always observed due to alpha particle attenuation in the sample. In order to determine Po/Pb ratio, we discuss the advantages of using gross alpha activity time variation compared to specific Po measurements. Finally, the gross alpha/beta ratio appear to be a good proxy of the Po/Pb ratio when large-scale variations are to be monitored. We report a first reference level for the activity level of airborne Po in France of 13 ± 6 μBq/m.