## 4 Discussion
In addition, the carrier effects due to CO\({}_{2}\) fluxes may further contribute to thoron emission [52]. In brief, hydrothermal alteration of the caldera occurs on the surface soils and, here, the thoron, normally outgassed from rock, is influenced, producing increasing emissions, also sustained by CO\({}_{2}\)[61]. Alteration of temperature and pressure within the Campi Flegrei hydrothermal system also reflects the variation in gradients of temperature and pressure recorded at the nearby Rn stations, which show a high correlation factor (>0.7) with Rn activity concentration. The same comparisons in Figures 4 and 5 were made in [1] for the \({}^{222}\)Rn signals, at both sites and over the same period. Almost the same correlations were found among the different independent signals, and quite similar conclusions were also drawn. Thus, the surface emission of \({}^{220}\)Rn behaves like \({}^{222}\)Rn emission, both influenced by geo-indicators of the evolution of the current volcanically induced unrest of the Campi Flegrei caldera. The different intensities of trend curves and residuals, lower in MSA than in OLB, occurring equally for \({}^{222}\)Rn and \({}^{220}\)Rn, are likely due to the different distances of the two sites from the main active degassing area, i.e., Pisciarelli fumarole, the center of the current unrest of the Campi Flegrei caldera, where the hydrothermal activity is concentrated [35] (Figure 1). Therefore, the intensity difference of the surface \({}^{220}\)Rn signals can be well explained by variation in the flux of the hydrothermal-volcanic CO\({}_{2}\) carrier gas, lower in MSA with respect to the higher effect in OLB, which is located inside the central unrest area. This result suggests the extension of the area affected by the current Campi Flegrei crisis, larger than the area of seismicity and of intense hydrothermal activity [1]. Figure 6 shows the activity concentration values of \({}^{222}\)Rn versus those of \({}^{220}\)Rn, at the OLB (Figure 6a) and MSA (Figure 6b) sites during the entire monitoring period of 2011-2017. The two graphs testify a linear correlation between the gas isotopes (R\({}^{2}\) is 0.65 for OLB and 0.54 for MSA), which most likely means that they have been carried out by the venting of CO\({}_{2}\) gas, favored by crustal stress/strain changes [53; 62]. Two enlargement sides are also evident in Figure 6, indicating that different gas sources are needed to explain the Rn data. A lower part, that exhibits radon-thoron values of almost the same order of magnitude, can denote Rn variations more sensitive to the shallower sources at lower activity concentration level [62]. In contrast, a higher part, that shows low thoron values corresponding to significant and high radon values, naturally indicates \({}^{222}\)Rn coming from deep sources compared to \({}^{220}\)Rn only from shallow sources (ground surface) [62]. This fact intuitively suggests that a certain amount of \({}^{220}\)Rn comes from natural outgassing at the surface, and it is not carried by CO\({}_{2}\) gas together with the \({}^{222}\)Rn, probably coinciding with periods of less, quiet and moderate hydrothermal and seismic activity. The scattered graph of the data from MSA in Figure 6b, compared to the clustered one from OLB in Figure 6a, underlines two important aspects. The first one is the substantial difference in the two signals at MSA and OLB, differently influenced by the local meteorological parameters, due to the location of the Rn stations; outside at MSA was the most influenced, with respect to the OLB site inside a gallery [27; 37]. The second aspect is the distance of both sites with respect to the central area of the current unrest of the Campi Flegrei caldera, which is closer to OLB than to MSA, which markedly influences \({}^{222-220}\)Rn emissions [1].