Statistical Assessment of the Influence of Saharan Dust on Direct Solar Radiation in Bulgaria

Abstract

Mineral dust can attenuate surface direct solar radiation, but its statistical signature is often difficult to isolate from the much stronger influence of cloud cover. This study provides a statistical assessment of the influence of Saharan dust on direct solar radiation in Bulgaria during the period 1 July 2020–30 June 2025. The analysis combines daily pyranometer observations from Sofia, Sandanski and Ahtopol with satellite-derived dust aerosol optical depth at 550 nm (DAOD550) and ERA5 total cloud cover. Direct solar radiation was normalized by daily extraterrestrial radiation on a horizontal surface in order to reduce the influence of solar geometry and day length. A sequence of five linear regression models was used to test the robustness of the estimated dust effect under progressively stricter controls for cloud cover, station-related differences and residual seasonality. In the principal full-sample model, DAOD550 retained a statistically significant negative coefficient (β = −0.0804, p = 0.002), while in the nearly clear-sky subset the effect became stronger (β = −0.1935, p < 0.001). The obtained results suggest that Saharan dust transport is associated with a measurable attenuation of direct solar radiation at the surface over Bulgaria, with the dust-related signal becoming more clearly detectable when cloud influence is reduced.