Diurnal variability of aerosol optical depth is important for various applications, including satellite aerosol data validation, radiative forcing computations, studies of aerosol interaction with humidity and clouds, and also public health. Aerosol optical depth measurements acquired through the ground-based Aerosol Robotic Network are analyzed. Analysis of the diurnal cycle over major urban/industrial areas within the network showed a prevailing pattern of the optical depth increase by 10--40% during the day at most sites. Diurnal variability range is <10% over various sites where dust aerosol is a major contributor to optical depth. Sites in southern Africa influenced by distant sources of biomass burning aerosol show no diurnal cycle, while the presence of local sources causes a diurnal trend with a maximum aerosol loading observed in the afternoon hours. Over oceans, because of the very low optical depth, even 20% departure from the daily average is practically within the measurement uncertainty.