## 1 Introduction  
Stromboli (Figure 1) is the northernmost of the Aeolian Islands, a volcanic archipelago located off the northern coast of Sicily (Southern Italy). Here, a persistent volcanic activity takes place from several vents located within a crater terrace at 750 m a.s.l. in the upper portion of the Sciara del Fuoco. Its eruptive style is characterized by 5-20 Strombolian explosions per hour [1] with a large amount of actively and passively gas emitted through several small eruptive vents located in the crater terrace, within which we can recognize three main features--namely, the northeast (NE), the central (C), and the southwestern (SW) vents. In addition to the summit degassing, a peripheral discharge of gas, due to dissolved volatiles from the coastal aquifer and soil degassing controlled by tectonic discontinuities, has to be considered. Strombolian activity is sustained by a delicate dynamic equilibrium between deep input and shallow volatiles magma degassing [2]. Fluids are continuously released by magma convection in a shallow magma reservoir, during which the ascending less-dense gas rich-magma substitutes the denser degassed magma that in turn sinks down [3; 4; 5]. The shallow magma reservoir releases volatiles composed of H\({}_{2}\)O, CO\({}_{2}\), SO\({}_{2}\), H\({}_{2}\)S, HF, and HCl (condensable gases), and some non-condensable gases (e.g., He, H\({}_{2}\), N\({}_{2}\), CO, CH\({}_{4}\)) during both eruptive activity and inter-eruptive periods [6; 7; 8]. Variations between passive degassing and Strombolian activity is recorded in the gas geochemistry, which suggest a shallower magma body within the upper conduits that feed the former degassing and CO\({}_{2}\)-rich gas bubbles coming from major depths (>4 km) [5]. The persistent activity of the volcano, characterized by passive degassing and mild Strombolian activity, is sometime interrupted by effusive eruptions, major and paroxysmal explosive events. In the last three decades, four effusive eruptions occurred in 1985, 2002-2003, 2007, and 2014, two of which characterized by paroxysmal events (5 April 2003 and 15 March 2007). The 2002-2003 eruption took place after a long period of absence of effusive activity after the previous one in 1985, allowing the superficial system to recharge in gases and to increase its eruptive/explosive potential. This effusive eruption, which began on 28 December 2002 and ended on 22 July 2003, was the longest in the last period, lasting 206 days and leaving the surface system of the island of Stromboli poor in gas. Instead, the next eruptions that occurred in 2007 and 2014 were much shorter (only 35 and 105 days, respectively) and were characterized more by a structural weakness of the volcanic building than by a real uprise of deep magma in large quantities. In fact, the amount of lava emitted during these last two eruptions was correlated with the emptying of the magmatic column from the top to the bottom and regulated by the height of the fracture that triggered the effusive eruption [9]. After the 2014 effusive eruption, a low eruptive activity period was recorded until May 2017, when more intense explosions started [10; 11]. Since then, nine major explosions and two overflows occurred between July 2017 and December 2018 [10]. In particular, four major explosions occurred in 2017 (26 July, 23 October, 1 November, and 1 December), and five occurred in 2018 (7 and 18 March, 24 and 26 April, 18 August). The two overflows were recorded on15 December 2017 and 6 December 2018, respectively. The 2017 major explosions occurred from the central and/or south-western crater vents, whereas in 2018 explosive activity was more distributed among the three main craters [10]. Both overflows occurred from the north-eastern crater vent [10]. Figure 1: Stromboli map with the location of the STR02 and STR01 stations (CO\({}_{2}\) fluxes measurements) on the summit and peripheral areas of the volcano in the Pizzo sopra La Fossa and Scari, respectively.