## 3 Results
### Atmospheric Investigation
#### 3.2.2 Atmospheric Composition Investigation (SO\({}_{2}\), DMS, Aerosol and CO)  
Figure 10 represents the time series of SO\({}_{2}\) from 23 March until 19 September (included). The red dashed line represents its value in 2021, i.e., the year of the eruption, while the colour bands indicate 1.0, 1.5, and 2.0 standard deviations of typical historical values (blue line). It is possible to note that only two days (circled by red oval) exceed the two standard deviations: 15 and 16 August 2021, which are a little more than one month before the eruption and are better investigated in the following. Figure 11 shows the maps of distributions of the two most anomalous days (i.e., 15 and 16 August 2021) in the SO\({}_{2}\) time series. From the map, it is possible to depict that the maximum concentration of SO\({}_{2}\) not only coincides with La Palma island, but also with the same pixel where Cumbre Vieja vent is located (i.e., the Southern-West side of the island La Palma), even if the resolution of the atmospheric data is too low to confirm that the emission was from this volcano vent. On the other side, on 16 August 2021, a small presence of a western plume of SO\({}_{2}\) was likely emitted by the volcano. Despite this, the Atlantic Ocean between Canarias islands and the North African coast experienced a wide cloud of SO\({}_{2}\) that could have partially influenced the time series. Contrarily, the maximum  
Figure 9: Geomagnetic \(Z\) (Centre) component of geomagnetic field measured by GUI and TAM ground observatories. The signal has been filtered into 5. A linear fit is reported in E as a dashed line. The linear fit equations are also provided inside panel E where \(``t\"\) is the time in days. SO\({}_{2}\) value of 15 August 2021 was about double that of 16 August 2021. This could mean that the Cumbre Vieja volcano's emission was much more intense, supporting its possible volcanic source. The time series of DMS is illustrated in Figure 12, and it shows a large number of anomalous days grouped in three periods circled with red ovals: (1) from 17 to 23 May 2021, (2) from 8 to 12 June 2021, and (3) from 15 to 19 August 2021. On all other days, the curve is inside two standard deviations, so we consider it a normal oscillation of the value for this area. It is a pity to note that the distribution maps of some of these anomalous days in Figure 13 seem to exclude that DMS was emitted straight from the volcano. In fact, all the maps show higher concentrations in the sea, suggesting more of a direct emission from the oceanic sea water as proposed, for example, by [PERSON] and [PERSON] [79]. Despite this, we cannot exclude some indirect processes, like an increase in the temperature of the ocean floor due to the uplift of magma that could increase the algae in the seawater that are associated with high concentrations of dimethyl sulphide [34]. Figure 11: Distribution maps of the SO\({}_{2}\) most anomalous days around La Palma volcano (represented by a white star). The values are extracted at 23 UT. Figure 10: SO\({}_{2}\) time series in the six months before La Palma volcano eruption of values estimated at 23 UT. The years 1982 and 1991 have been automatically excluded from the analysis due to the presence of outliers. Aerosol Optical Thickness (AOT) time series investigation in the six months before \"La Palma\" volcano eruption is provided in Figure 14. Several days show a significant anomalous positive increase of aerosol above La Palma volcano, but it is important to check the parameter more deeply, as it could be affected by other factors like sandstorms from the Sahara desert [80]. For this purpose, we have produced the map of all anomalous days, and in Figure 15, we reported some of them. In particular, Figure 15C (9 June 2021) and Figure 15F (15 August 2021) show a very high aerosol concentration from the Southeast, i.e., close to the Sahara Desert, and they are likely due to sandstorms.