Retrieval of Aerosol Optical Properties from Spectral Solar Irradiance Using Radiative Transfer Modelling in Mountain and Coastal Environments

Ground-based spectral observations of solar radiation provide a powerful constraint on aerosol–radiation interactions and support the validation of satellite aerosol products. This study presents a radiative transfer modelling and inversion framework for the retrieval of aerosol optical depth (AOD) and single scattering albedo (SSA) from spectrally resolved direct normal irradiance (DNI) and global horizontal irradiance (GHI) measurements in the 300–1100 nm wavelength range.

The methodology exploits the complementary sensitivity of DNI and GHI to aerosol extinction and absorption across the measured spectral region. Forward simulations are performed using a radiative transfer model, while an optimal estimation inversion scheme is applied to retrieve aerosol optical properties by minimizing the spectral residuals between modelled and measured irradiances.

The approach is applied at two contrasting environments in Greece: a high-altitude continental background site at Kalavryta and a coastal site at Epanomi. These locations represent distinct aerosol regimes influenced by long-range transport, boundary-layer dynamics, and marine contributions. Site-specific atmospheric profiles, surface albedo, and solar geometry are explicitly accounted for in the simulations.

Independent validation is performed using co-located Sun photometer measurements providing reference AOD and SSA products. For cloud-free conditions, retrieved AOD at 500 nm shows a mean bias below 0.05 and a root-mean-square error of 0.04–0.06, depending on site and aerosol load. SSA retrievals exhibit mean deviations below 0.05 in the visible range, with increased sensitivity under moderate to high aerosol loading. The coastal site demonstrates enhanced absorption variability linked to mixed marine-continental aerosol, while the mountain site is dominated by aged continental and transported aerosol.

The results demonstrate that combined spectral DNI and GHI measurements can robustly constrain aerosol optical properties with high temporal resolution, offering a complementary ground-based observational capability for aerosol monitoring, radiative studies, and satellite validation activities.