Intercomparison of nighttime aerosol optical depth retrievals from both reflectance-based and city light-based methods using VIIRS DNB data

Abstract. Using observations from the Visible Infrared Imaging Radiometer Suite (VIIRS) Day/Night Band (DNB), two different nighttime aerosol optical depth (AOD) retrieval methods were evaluated and inter-compared. The first approach is a lunar reflectance-based retrieval method, using reflected moonlight in a manner similar to daytime retrievals. The second approach utilizes changes in light patterns over regions with artificial light sources due to the upward diffusion of light by aerosol particles. Both retrieval methods were implemented over Dakar, Senegal for 2017 and 2018. Retrievals from both approaches were evaluated against ground-based solar and lunar AErosol RObotic NETwork (AERONET) data, as well as daytime AOD retrievals from the Moderate Resolution Imaging Spectroradiometer (MODIS). Additionally, impacts of using the Miller and Turner lunar model for estimating Top-Of-Atmosphere (TOA) lunar spectral flux for AOD retrievals were also studied. Findings suggest that while both retrieval methods show skill in retrieving nighttime AOD by qualitatively identifying over-ocean aerosol plume locations and quantitatively comparing with solar and lunar retrieved AERONET data, cloud contamination and variations in lunar properties are factors that need to be carefully quantified in future studies for accurate nighttime aerosol retrievals using VIIRS DNB data. This study suggests that there are sampling issues from both approaches, but the combined use of both retrieval methods can increase the sampling rate for nighttime aerosol retrievals by more than 50 %.