## 3 Results and Discussion
### Transport of Aerosols by Wind and Link with Wildfires  
Another important point to consider in the analyses of the AOD in 2019 is the wind transport of aerosols. Meteorological events have an impact on aerosol concentration, AOD, and their circulation in the atmosphere, such as mesoscale ones like sea breeze (SB) or nocturnal low-level jet (LIJ), which significantly influence coastal air quality thanks to their ability to sufficiently mix different types of aerosols. Moreover, SB events are linked with an increase of PM\({}_{1}\) and PM\({}_{2.5}\) concentrations [38]. PM\({}_{1}\) represents particles with a diameter less than 1.0 \(\upmu\)m (fine particles), PM\({}_{2.5}\) for particles under 2.5 \(\upmu\)m, and PM\({}_{10}\) for particles under 10 \(\upmu\)m (both fine and coarse particles). In Figure 12, we focus on the particle transport by the wind related to the wildfires identified before: this figure represents the circulation of the wind and the quantities of particles at different sizes (PM\({}_{10}\), PM\({}_{2.5}\), PM\({}_{1}\)) with a specific value located at (59.50\({}^{\circ}\) N, 24.63\({}^{\circ}\) E), with the green circle in the area of the three stations: Helsinki, Peterhof, and Toravere. We chose specific dates (22 May 2019, 25 May 2019, 13 June 2019, 21 July 2019, and 23 August 2019) for which we have a high value of AOD in Figure 11d in the area of the three stations: Helsinki, Peterhof, and Toravere. For 22 May 2019 and 25 May 2019, the AOD is between 0.21 and 0.27 at each station; for 13 June 2019, it is around 0.4 for all the stations; and for 21 July 2019 and 23 August 2019, it is around 0.2. Figure 11: AODs at 500 nm (level 2.0) from the Helsinki, Peterhof, and Toravere between May and August 2019. (**a**) AODs at 500 nm (level 2.0) from the Helsinki station between May and August 2019. (**b**) AODs at 500 nm (level 2.0) from the Peterhof station between May and August 2019. (**c**) AODs at 500 nm (level 2.0) from the Toravere station between May and August 2019. (**d**) AODs at 500 nm (level 2.0) from each station between May and August 2019. On 22 May 2019, the wind was essentially coming from the southeast and the Black Sea area, passing by Ukraine, Belarus, Latvia, Lithuania, and Estonia. We have a similar quantity of PM\({}_{1}\) and PM\({}_{2.5}\) (40 \(\upmu\)g\(\cdot\)m\({}^{-3}\) and 48 \(\upmu\)g\(\cdot\)m\({}^{-3}\)) and a high value of PM\({}_{10}\) (69 \(\upmu\)g\(\cdot\)m\({}^{-3}\)). This high value of PM\({}_{10}\) can be explained by the excessive number of wildfires this year, according to Table 3: 201% of the average 2019 for Latvia and 163% for Lithuania. Thus, the wind transports the particles from these countries to the area of the three stations, making a high value of AOD. It is the same process for the two other days:  
On 25 May 2019, the wind came from Poland and Germany, where there is a high value for each PM\({}_{10}\), PM\({}_{2.5}\), and PM\({}_{1}\), and transported particles to the area we are studying. This can be illuminated by the number of wildfires (according to Table 3) in Poland (135%) and Germany (193%) during 2019. On 13 June 2019, the wind came from the Black Sea area and transported particles from there to our area of study, which explains the high level of AOD around 0.4 on this day (Figure 11d). On 21 July 2019, the wind came from France, Germany, and Russia, with a high level of particles in both of these areas resulting in a high level of PM\({}_{10}\), PM\({}_{2.5}\), and PM\({}_{1}\) for this day, between 60 and 100 \(\upmu\)g\(\cdot\)m\({}^{-3}\). Moreover, according to Figure 10, around 25 July 2019 was a period of a heatwave with temperatures higher than 40 \({}^{\circ}\)C in France and 35 \({}^{\circ}\)C in Germany, making wildfires easier to spread, corroborating the 193% of the average 2019 wildfires in Germany (Table 3). On 23 August 2019, there was a high level of particles around Europe, especially Italy and Germany, and around Russia and Kazakhstan.