## 4 Discussion
### Source of the High Amount of SO\({}_{2}\) Detected in 1991
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **Anomalous Day [2021]** & **Amount of the Anomaly 1** & **Visual Check of the Anomaly** \\ \hline \multirow{8}{*}{Aerosol} & 25 March & 103.88\% & Spatially coincide with the calderica of the \\  & 27 March & 19.38\% & Canarias volcano system \\  & 17 May & 19.38\% & Spatially coincide with the calderica of the \\  & 18 May & 325.25\% & Canarias volcano system \\  & 21 May & 38.09\% & Canarias volcano system \\  & 8 June & 346.98\% & \\  & 9 June & 834.43\% & \\  & 10 June & 180.31\% & It seems to be a sandstorm from the \\  & 11 June & 285.23\% & Sahara desert. \\  & 12 June & 4.29\% & \\  & 12 July & 164.26\% & It is not centred on the volcano, but an \\  & 13 July & 86.10\% & indirect effect cannot be totally excluded. \\  & 15 August & 105.87\% & It seems not related to the volcano \\  & 16 August & 76.08\% & \\ \hline \multirow{8}{*}{CO} & 24 March & 16.85\% & Emission close to the volcano \\  & 25 March & 84.75\% & It seems to be due to another source and \\  & 19 April & 16.61\% & later transported over the research area \\ \cline{1-1}  & & & Not clear as there is another phenomenon \\ \cline{1-1}  & 15 May & 3.59\% & but also emission potentially related to \\ \cline{1-1}  & 1 August & 7.30\% & The emission seems to come from \\ \cline{1-1}  & 2 August & 30.76\% & Not related to the volcano. \\ \cline{1-1}  & 9 August & 93.07\% & It could be related to the volcano. \\ \cline{1-1}  & 10 August & 13.09\% & It could be related to the volcano. \\ \cline{1-1}  & 19 August & 2.88\% & It does not seem to be related to the volcano \\ \cline{1-1}  & 20 August & 24.88\% & It does not seem to be related to the volcano \\ \cline{1-1}  & 26 August & 38.37\% & Spatially not localised above the volcano. \\ \cline{1-1}  & 4 September & 32.02\% & It could be related to other volcanoes in \\ \cline{1-1}  & 7 September & 143.52\% & \\ \cline{1-1}  & 8 September & 100.41\% & It could be related to the volcano even if it is \\ \cline{1-1}  & 9 September & 85.10\% & a very wide anomaly \\ \cline{1-1}  & 10 September & 29.40\% & \\ \cline{1-1}  & 13 September & 91.86\% & \\ \cline{1-1}  & 14 September & 174.76\% & \\ \cline{1-1}  & 15 September & 232.07\% & Some days are spatially well related to \\ \cline{1-1}  & 16 September & 50.25\% & the volcano. \\ \cline{1-1}  & 18 September & 33.58\% & \\ \cline{1-1}  & 19 September \({}^{2}\) & 15.27\% & “Co-eruption” emission. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Cont._It is outstanding to note that from about 40 days before the eruption, there are several recordings of seismic events and anomalies of CO and DMS in the atmosphere and even _TEC_ in the ionosphere. In this case, we suppose that the uplift of magma could have produced some degassing depicted by the atmospheric anomalies. Finally, the last uplift produced significant cracks in the upper crust, highlighted by the very high seismicity in the last days before the eruption, a clear pre-eruption process. Despite this, other explanations of atmospheric and ionospheric anomalies are still possible. For example, they could be due to atmospheric weather perturbations or external solar disturbances, even if the solar condition has been checked and reported quiet. Therefore, the bottom-up lithosphere-atmosphere-ionosphere propagation effect of the identified anomalies is one of the possible interpretations of the phenomena. Future studies need to demonstrate the interaction mechanism between the geo-layers, excluding other possible explanations and sources. Comparison with Other Works and Longer Term Preparation Effects of the Eruption of 19 September 2021  
[PERSON] et al. [89] identified an increase of seismicity and degassing some years before this eruption, i.e., in October 2017 and February 2018, revealing that the preparation for the eruption of September 2021 had a long preparation even some years before. For this purpose, we investigated the atmospheric time series from 1980 until the \"La Palma\" eruption in 2021.