## 3 Results and Discussion
### Focus on the Particle Size: Fine and Coarse Aerosols  
Following this assessment, it was necessary to study the trend in aerosol particle size variations for all the analyzed stations (fine and coarse at wavelengths around 500 nm) (Figure 7).  
Globally, fine and coarse particles follow the same trends through the years, always with a higher fine mode distribution than coarse mode distribution at each station.  
For the Gotland station, between 1999 and 2003, we observed a clear increase in fine mode particle domination, which is consistent with the similarly observed increase in the aerosol number concentration during the same period. In the case of the three stations situated in Finland and Sweden (Helsinki, Gustav Dalen Tower, and Helsinki Lighthouse), it is evident that both the fine and coarse aerosol particle trends show a slight downward trajectory. The decline is very subtle, to the extent that one could even consider their evolution as relatively constant over time. However, there is a clear domination of fine  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Station** & **Years** & **AOD Difference** & **Increase/Decrease** & **Standard Deviation** & **Curve Coefficient** \\ \hline Gotland & 2001–2006 & 0.046 & 145.63\% (I) & 0.0246 & \(7.29\times 10^{-4}\) \\ \hline Gustav Dalen & 2005–2022 & 0.025 & 131.25\% (I) & 0.0135 & \(5.36\times 10^{-4}\) \\ \hline Helgoland & 2000–2014 & \(-\)0.035 & 85.48\% (D) & 0.0461 & \(-7.81\times 10^{-4}\) \\ \hline Helsinki & 2008–2021 & 0.001 & 101.29\% (I) & 0.0201 & \(7.29\times 10^{-4}\) \\ \hline Helsinki Lighthouse & & & & & \\ \hline Peterhof & 2014–2021 & \(-\)0.036 & 73.53\% (D) & 0.01387 & \(-2.73\times 10^{-3}\) \\ \hline Toravere & 2002\(-\)2021 & \(-\)0.106 & 55.65\% (D) & 0.03398 & \(-3.03\times 10^{-3}\) \\ \hline \hline \end{tabular}
\end{table}
Table 1: Multiannual AOD changes at the AERONET stations selected for this study.  
Figure 7: Fine and coarse mode particle distribution for each station over the years (level 2.0); wavelength around 500 nm.  
particles over coarse aerosol particles. Again, 2019 shows a \"jump\" in fine and coarse mode values.  
Finally, for the last three stations located in Estonia (Toravere), Germany (Helgoland), and Russia (Peterhof), the decline in fine aerosol particles is significantly more pronounced, while the decline in coarse particles remains relatively discrete. Moreover, the overall concentration of coarse particles is also comparatively lower than that of fine particles.  
It is interesting to note that the deviation observed in the aerosol concentration during the year 2019 is also clearly visible in the trend of fine aerosol particles. Indeed, in 2019, a significantly higher value of fine particles was recorded for each station.  
As a reminder from the introduction, fine and coarse mode particles are impacted by wildfires and burning biomass (BB), which is the burning of living and dead vegetation and includes human-initiated burning of vegetation for land clearing and land-use change, as well as natural lightning-induced fires [10]. Thus, studying the 2019 case more particularly to explain the variation of AOD compared to other years and external events' impact on aerosols is needed.