Retrieval of nighttime AOD values with all-sky cameras

Measuring aerosol properties such as aerosol optical depth (AOD) at nighttime is crucial for understanding the overall impact of aerosols on climate, especially during polar night. Nocturnal AOD measurements can be obtained using Moon photometers; however, the Moon is not present throughout the entire night, resulting in a lack of AOD observations during more than half of the nighttime period. Star photometers, which use different stars as targets, can fill this gap. Nevertheless, only a few star photometers are currently operating worldwide, as they are expensive instruments and present several challenges for unattended operation.

Within this framework, this study explores the use of all-sky cameras, capable of capturing images of the entire sky vault, to extract the incoming irradiance of several visible stars and use this information to retrieve nighttime AOD both under Moon presence and during Moon-free periods. A new methodology to extract star irradiances from all-sky camera images is proposed, together with a Langley-plot technique to estimate AOD from these irradiances (Román et al., 2025). The resulting AOD values were calculated at several locations and compared with Moon photometer measurements. The comparison shows a high correlation between both datasets at all sites. On average, the proposed method overestimates Moon photometer AOD values by approximately 0.02, with a precision of about 0.03–0.04.

These results indicate that all-sky cameras could provide a viable solution to fill the nighttime AOD observation gap on a global scale, owing to their low cost and fully automatic operation.