Seasonal variation of night-time accumulated Rn-222 in central Italy

Abstract

Observations of Radon-222 taken in the central Italy site of L’Aquila since 2004 have been analysed with simultaneously collected meteorological data and other atmospheric trace gas concentrations to better understand the physical drivers of the seasonal variability of Radon-222 in the atmospheric boundary layer and to assess the role of different mechanisms controlling the tracer abundance immediately above the Earth surface. Dynamical removal due to turbulent convective motions that are highly dependent on atmospheric meteorological conditions is found to be the dominant controlling process. The observed radon concentrations are negatively correlated to horizontal wind speed, which is an indirect measure of atmospheric vertical motions (through mass continuity), and to mixing ratios of surface ozone. In a site where the impact of local photochemical production is limited, surface ozone is another atmospheric trace species whose abundance is largely controlled by horizontal advection and vertical mixing, the latter with a main source region located in the free troposphere instead of at the surface. Monthly averaged day–night changes of radon concentration are found to be clearly anti-correlated to surface temperature (T) (correlation coefficient R = −0.96) because the latter acts as a driver of boundary layer turbulence and is in turn correlated to the magnitude of the radon soil flux. The rate of increase of the radon concentration during meteorologically stable nights can be used to infer to the magnitude of the monthly averaged radon soil flux (Φ) and its statistical uncertainty. A positive/negative correlation of Φ is found with T and soil moisture retention (W), respectively (R T = 0.93 and R W = −0.90).