## 3 Results and Discussion
### Focus on 2019 and Link with Wildfires  
This led us to focus on the analyses of the 2019 case. The data are available for the following stations: Peterhof, Toravere, Helsinki, Helsinki Lighthouse, and Gustav Dalen. The analyses of the annual trends clearly show that the summer AOD values at these stations were significantly higher than in the rest of the year. Table 2 shows the summer AODs for these stations. The data show that the AOD levels in April, July, and August were significantly higher than those for the annual averages, which leads to the conclusion that during spring and summer in 2019, the aerosol episodes dominated the aerosol optical depth values for the entire year. Moreover, compared to the other summer trend AOD level (Figure 8), the 2019 summer is higher: it was always around 0.15 for each station (Helsinki Lighthouse, Irbe Lighthouse, Gustav Dalen Tower, and Helsinki). In fact, most of the time, the AOD level was under 0.15 for the other years (2007 to 2009, 2015 to 2018 and 2020) or was not the same at each station, which did not allow us to make conclusions (2006, 2010, 2013, 2014). In some cases, due to the small number of data, for example, from 1999 to 2005, we have only two values of the maximum AOD level; therefore, we cannot make any conclusions. However, another study was conducted for the year 2002 (see [34] for the entire study and results). In this study, the high AOD level (0.7 for August 2002) during 2002 was related to a very warm and long period (of seven months from February to September 2002) increasing the number of wildfires. \begin{table}
\begin{tabular}{l c c c c c} \hline
**AERONET Station** & **April** & **June** & **July** & **August** & **Annual Average** \\ \hline Peterhof (500 nm) & 0.266 & 0.112 & 0.159 & 0.150 & 0.128 \\ \hline Gustav Dalen (510 nm) & 0.200 & 0.137 & 0.125 & 0.172 & 0.137 \\ \hline Helsinki (500 nm) & 0.170 & 0.131 & 0.161 & 0.168 & 0.137 \\ \hline Helsinki Lighthouse (510 nm) & No data & 0.077 & 0.143 & 0.173 & 0.146 \\ \hline Toravere (500 nm) & 0.144 & 0.119 & 0.148 & 0.154 & 0.134 \\ \hline \end{tabular}
\end{table} 
Table 2: AOD values (level 2.0) at the Peterhof, Toravere, Helsinki, Helsinki Lighthouse, and Gustav Dalen stations for March and three summer months of 2019. In the following section, we will focus on the period of April to August 2019 to examine the origin of this unusual level of AOD during this period. To explain this higher average of AOD, we have made further analyses and provided information on extreme wildfire episodes across Europe during this period. The fire episodes in the selected countries are provided in Table 3. The selection was based on the vicinity of particular countries towards the Baltic and prevailing summer wind direction patterns (southerly), as well as on data availability (this is why there isn't mention of Belarus in the following). Apart from Ukraine, all the other countries had much worse fire situations in 2019 than for the average of 2009-2018. That is both in terms of the number of fires and burnt areas (except for Sweden and Ukraine). The fire situation in Europe as of 25 June 2019 is shown below in Figure 9.  
Figure 8: AOD level during summer through the years at each station. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Country**} & \multicolumn{3}{c}{**Number of Fires**} & \multicolumn{3}{c}{**Burnt Area (ha)**} \\ \cline{2-6}  & **2019** & **2009–2018** & **2019 as \% of** & **2019** & **2009–2018** & **2019 as \% of** \\  & & **Average** & **Average** & **Average** & **Average** \\ \hline Finland & 1458 & 1238 & 118 & 565 & 525 & 108 \\ \hline Germany & 1523 & 789 & 193 & 2711 & 514 & 528 \\ \hline Latvia & 1107 & 552 & 201 & 805 & 596 & 135 \\ \hline Lithuania & 279 & 171 & 163 & 200 & 107 & 187 \\ \hline Poland & 9635 & 7141 & 135 & 3572 & 3110 & 115 \\ \hline Romania & 425 & 274 & 155 & 2496 & 1678 & 149 \\ \hline Sweden & 5483 & 4391 & 125 & 1233 & 4730 & 26 \\ \hline Ukraine & 1261 & 1992 & 63 & 1065 & 3720 & 29 \\ \hline \hline \end{tabular}
\end{table}