First time AOD measurements obtained during night-time using lunar POM radiometer at Tor Vergata site (Italy)

Sun/sky radiometers are one of the finest instruments to observe and understand the effect of aerosols in the atmosphere. By looking at the Sun using radiometers and using Beer’s law to measure the attenuation, we retrieve the aerosol properties during the daytime. This method has been used for many years to understand aerosol properties. AERONET and SKYNET are two of the most important international networks. On the contrary, data during night-time is still in the provisional phase for both AERONET and SKYNET. As part of the SKYNET network, we have successfully implemented an algorithm to retrieve the aerosol optical depth (AOD) using the moon as a source of light. This study aims to present the first SKYNET results obtained during night-time at Tor Vergata  (41.84° N, 12.65° E; 117 m. a.s.l.), located 20 km SE from Rome city. The site is equipped with lunar radiometers from both AERONET and SKYNET networks, which make it a well-suited location to observe the atmospheric column both during the day and at night. In this study, we will present the time series of AOD for daytime and night-time,  and the intercomparison with the AERONET observations for the common channels. We will also present, following the WMO directives, the differences between AERONET and SKYNET for the four common channels (500 nm, 675 nm, 870 nm, 1020 nm) in order to check for consistency of measurements. The importance of night-time AOD is filling the nocturnal data gaps, maintaining the continuity between two successive daytime data, as well as understanding the daily dynamics of aerosols. We still envisage the improvement in the retrieval algorithms and cloud screening methods. The improvement of results at Tor Vergata will help us to set up the algorithm at other sites in the SKYNET, with the ultimate objective of filling the data gaps specially in higher latitudes.

Keywords: Lunar radiometer, Aerosol optical depth, SKYNET

Acknowledgements: The current analysis has been done in the frame of the COST Action CA21119 HARMONIA, supported by COST (European Cooperation in Science and Technology). The Spanish Ministry of Economy and Competitiveness also fund the research through project PID2022-138730OB-I00. The participation of G. Kumar has been supported by the Santiago Grisolia program fellowship GRISOLIAP/2021/048. We thank AERONET, PHOTONS and SKYNET for their scientific and technical support.