## 4 Discussion
The variation in CO\({}_{2}\) fluxes can explain the wide range of soil \({}^{220}\)Rn activity concentration, from surface ground layers (\(<\)1 m) to migrated out into the atmosphere. Figure 4: (**a**) Thoron residuals (anomalies) at OLB and MSA sites compared with the fumarolic tremor (RSAM) registered at Pisciarelli fumarole. (**b**) Thoron trends at OLB and MSA sites compared with the maximum vertical displacement at Campi Flegrei, the background seismicity (i.e., the cumulative number of days with earthquakes since 2000) and the hydrothermal CO\({}_{2}\) flux trend from Pisciarelli vent. The accuracy of the measured data, i.e., the error bars, is an approximately constant 4%. This explanation of the concatenation of interrelated geophysical and geochemical events moved by pressure gradients, however, cannot fully explain the outgassing range of \({}^{220}\)Rn from the surface ground, considering the very short half-life of the \({}^{220}\)Rn, which makes escape from its point source within the rock and filtrate through the ground into the atmosphere very difficult. Therefore, to shed light on such anomalies and trends observed in the present work, two recent papers [55,56] come to our aid, which both experimentally proved the increase in Rn (both isotopes) emanating power induced by thermal gradients due to magma dynamics in active volcanic areas [57]. Such a result is particularly true in volcanic areas where intense hydrothermal alteration is present, such as at the Campi Flegrei caldera [23,58]. Progressive phenomena of increasing temperature can produce surface \({}^{220}\)Rn emissions even almost four orders of magnitude higher than those measured during rock deformation, micro-fracturing and failure [56]. Thermal gradients can contribute significantly to produce, superficially, \({}^{220}\)Rn emissions spatially heterogeneous and non-stationary in time, resulting in a transient state dictated by temperature gradients and the carrier effects of gases [55]. Thus, the increase in thoron emanation coefficient may lead to higher and anomalous concentrations of thoron in surface soil gas [55,59]. In our case, this phenomenon caused by thermally induced reactions on the thoron signal is demonstrated in Figure 5a, where the increasing pattern of the temperature of the Campi Flegrei hydrothermal system [20,35,50], which strongly characterizes the fumarolic activity (RSAM) of the caldera [48], well matches with the increasing trend of the thoron at OLB and MSA sites. In [55], it is stated that positive \({}^{220}\)Rn anomalies in surface soils of active volcanic areas are addressed to repeated cycles of stress due to increasing thermal pressures, leading to the opening or reactivation of fractures and to changes in rocks as a function of  
Figure 5: Comparison of temperature (**a**) and pressure (**b**) of the Campi Flegrei hydrothermal system, estimated at Pisciarelli fumarole, with the fumarolic tremor (RSAM) registered at Pisciarelli fumarole and with thoron trends at OLB and MSA sites. temperature. Actually, this is what we assumed to occur in the Campi Flegrei hydrothermal system (Figure 4a). Taken together, all the different independent signals that we have discussed are likely affected by the significant rise in temperature and pressure of the hydrothermal system at the Campi Flegrei caldera, which began two decades ago and is still ongoing [18; 33] (Figure 5). In these scenarios, the temperature increase may induce thermal devolatilization phenomena on surface thoron emission and the fluid pressure may increase, favoring ground deformation, earthquakes and the flux of the hydrothermal gas [1]. Thus, these hydrothermal alterations, induced by increases in the temperature and pressure, frequently modeled in terms of enhanced heat flow with related stream discharge and stress-induced micro-fracturing with an increase in porosity and permeability, act as perturbation mechanisms that contribute to produce anomalies in surface thoron outgassing, spatially heterogeneous and non-stationary in time [55; 60].