## 2 Description of the Study Area and the Methods
### Data and Measurements  
In terms of aerosol studies, the region is both difficult and important to study. It is a very cloudy sea and deeply impacted by aerosols produced in various anthropogenic activities, both locally and from distant sources. Due to the cloudiness, satellite aerosol studies are limited, and sun photometry seems like a proper tool for Baltic aerosols' measurements in that case. For our study, we chose AERONET data, as there are a number of NASA AERONET sunphotometer stations located around the Baltic Sea and they provide aerosol optical properties with good quality [31]. The AERONET program (AErosol RObotic NETwork), initiated by NASA and LOA-PHOTONS (CNRS), comprises a coalition of ground-based remote sensing aerosol networks. This initiative maintains a comprehensive and easily accessible database of aerosols' optical, microphysical, and radiative properties. They are essential for aerosol research, satellite retrieval validation, and collaboration with other databases. Standardization of sun photometers, including calibration, data processing, and distribution, ensures measurement unity and coherence. Using CIMEL Electronique CE318 multiband sun photometers, the program conducts direct measurements of solar and sky radiation across multiple spectral bands (between 340 to 1020 nm) with the latest CE318-T model capable of nocturnal measurements of lunar spectral radiation. Based on the Beer-Bouguer law, optical depth calculations account for factors such as Rayleigh scattering, ozone absorption, and gaseous pollutants to extract the aerosol optical depth (AOD). AERONET observations, focusing on spectral AOD, inversion products, and precipitation water, and leverage evolving data-processing algorithms with the latest version 3.0 enhancing data analysis capabilities, offer us three data quality levels: Level 1.0 (unc screened), Level 1.5 (cloud-screened), and Level 2.0 (cloud-screened and quality-assured). All the derivative products from the AOD depend on these levels and are submitted to additional quality checks. Each AOD measurement consists of a triplet observation per wavelength, taken every 30 s over one minute. This variability provides insight into data quality. During large air mass periods, measurements are made at 0.25 air mass intervals, while at smaller air masses, they are taken every 15 min. Cloud variations, usually greater than those of aerosols, cause observable triplet variations useful for cloud detection. The 15 min interval also allows for an extended assessment for cloud contamination. Moreover, the daily average takes into account all the points during the day (more than three required), and the monthly average is based on the daily average [31; 32]. To ensure the increasing quality of data in the future, some research and developments are in process. Notably, between the PHOTONS network, the original French component of the AERONET network and the CIMEL company created a specialized sun photometer for measuring linear polarization at wavelengths between 340 and 1640 nm. Several of these \"Dual Polar\" instruments are already in the testing phase at sites such as Lille (France) and Beijing (China). Another example is CIMEL developing a portable single wavelength (532 nm) lidar system, CAMLTM. The CAMLTM CE 370-2 has the ability to profile atmospheric cloud and aerosol structures and to retrieve aerosol optical and dynamic properties. [33]. These studies and developments are essential to improve AOD measurements through diverse weather conditions. In order to search for the multiannual trends and extreme aerosol optical depth (AOD) cases, we selected seven AERONET stations, which are located around the Baltic Sea and provide long-term data, and chose level 2.0 and \(\lambda\) = 500 nm (AOD_500/550). The AOD is a measure of the extinction effect of atmospheric aerosols and is typically used as a parameter to assess the degree of air pollution.