## 2 Description of the Study Area and the Methods
### Data and Measurements
The AOD data were not always collected and available at the same periods; however, the selected stations provide good geographical and multiannual coverage of the region. The following AERONET stations were selected for the AOD analyses (Figure 2). The Gustav Dalen Tower station is managed by the Swedish Maritime Administration. It is located in the northern Baltic Proper (58.594\({}^{\circ}\) N and 17.467\({}^{\circ}\) E), approximately 10 nautical miles off the Swedish coast, at an elevation of 25 m a.s.l. The data were collected between 2005 and 2023. The station used a CIMEL sun photometer with the following measurement wavelengths: 412 nm, 443 nm, 490 nm, 532 nm, 551 nm, 667 nm, 870 nm, 1020 nm. The Helsinki station is located at 60.20373\({}^{\circ}\) N and 24.96065\({}^{\circ}\) E, at an elevation of 52.8 m a.s.l. The CIMEL sun photometer is installed on top of the Finnish Meteorological Institute's main building. It is an urban-type station and is characterized by its location at high latitude and in the vicinity of the sea. The data were collected at the station between 2008 and 2023. The station was equipped with a CIMEL sun photometer (wavelengths: 340 nm, 380 nm, 440 nm, 500 nm, 675 nm, 870 nm, 1020 nm, 1640 nm). The Helsinki Lighthouse station is managed by the Finnish Maritime Administration. It is located in the Gulf of Finland (59.949\({}^{\circ}\) N and 24.926\({}^{\circ}\) E), approximately 15 nautical  
Figure 2: Location of AERONET stations around the Baltic Sea. miles southeast of the harbor of Helsinki. The data were collected at the station between 2006 and 2019. The station was equipped with a CIMEL device (wavelength: 412 nm, 443 nm, 490 nm, 532 nm, 551 nm, 667 nm, 870 nm, 1020 nm). The Gotland AERONET station (57\({}^{\circ}\)55\({}^{\prime}\) N, 18\({}^{\circ}\)57\({}^{\prime}\) E) lies in the northern part of the island of Gotland, 50 m inshorc, at an elevation of 10 m a.s.l. Owing to the location of the island in the central Baltic Sea, this station is often referred to as a representative of Baltic Sea aerosol conditions. The data were collected between 1999 and 2021. The station was equipped with a CIMEL device (wavelengths: 340 nm, 380 nm, 440 nm, 500 nm, 675 nm, 870 nm, 1020 nm). The Toravere station is located at 58.25500\({}^{\circ}\) N and 26.46000\({}^{\circ}\) E, at an elevation of 70 m a.s.l. This site is located about 20 km southwest of Tartu, and the data were collected between 2001 and 2023. The station was equipped with a CIMEL device (wavelengths: 340 nm, 380 nm, 440 nm, 500 nm, 675 nm, 870 nm). The Helgoland station is located at 54.17786\({}^{\circ}\) N and 7.88736\({}^{\circ}\) E on Helgoland Island, at an elevation of 60 m a.s.l and some 35 miles from the mainland. The data were collected between 1999 and 2015 at the following wavelengths: 340 nm, 380 nm, 440 nm, 500 nm, 675 nm, 870 nm, 1020 nm. The Peterhof station is located at 59.88100\({}^{\circ}\) N and 29.82600\({}^{\circ}\) E, at an elevation of 58 m a.s.l. The data were collected between 2013 and 2012 at the following wavelengths: 340 nm, 380 nm, 440 nm, 500 nm, 675 nm, 870 nm, 1020 nm.