Short-term variability of atmospheric aerosol optical properties over Ioannina using the CALITOO Sun Photometer

Introduction
Atmospheric aerosols play an important role in the Earth’s climate system through their interaction with solar radiation and their role in cloud formation, while also being associated with adverse health effects. Their complex and variable properties and their strong temporal and spatial variability stress the necessity for intensifying monitoring efforts worldwide. This study presents the results of a short-term campaign conducted in Ioannina, NW Greece, aiming at characterizing atmospheric aerosol optical properties.

Methodology
The campaign was held between May 26 and June 5 2026, using a CALITOO sun photometer, obtaining hourly Aerosol Optical Depth (AOD) measurements under clear-sky conditions at three wavelengths (465 nm, 540 nm, and 619 nm). These measurements enabled the determination of the Ångström Exponent (AE) and the investigation of AOD and AE diurnal and day-to-day variability, thus providing valuable insights into particle size characteristics.

Results
Noticeable day-to-day variations in aerosol optical properties were recorded. AOD was higher (daily mean values up to 0.27 at 465 nm) during the first days of the campaign, followed by lower and more stable values after May 29 (not exceeding 0.2 at 465 nm). AE values generally exceeded 1, indicating the predominance of fine aerosol particles, while few AE values lower than 1 were associated with elevated AOD, pointing to links between increased aerosol loadings and coarse mode particles and secondary aerosol formation. Meteorological conditions, particularly precipitation events, greatly influenced aerosol optical properties through washout/rainout.

Conclusions
Overall, the results demonstrate the capability of the CALITOO sun photometer to monitor short-term aerosol variability and to provide information on aerosol loadings and size distributions. Furthermore, the importance of coarse mode and secondary particles in the Ioannina basin was further corroborated, highlighting the complexity of the aerosol population in the area and the importance of continuous monitoring in the area.