Sensitivity studies of nighttime top-of-atmosphere radiances from artificial light sources using a 3-D radiative transfer model for nighttime aerosol retrievals

By accounting for surface-based light source emissions andtop-of-atmosphere (TOA) downward lunar fluxes, we adapted the sphericalharmonics discrete ordinate method (SHDOM) 3-dimensional (3-D) radiativetransfer model (RTM) to simulate nighttime 3-D TOA radiances as observedfrom the Visible Infrared Imaging Radiometer Suite (VIIRS) Day/Night Band(DNB) on board the Suomi-NPP satellite platform. Used previously for daytime3-D applications, these new SHDOM enhancements allow for the study of theimpacts of various observing conditions and aerosol properties on simulatedVIIRS-DNB TOA radiances. Observations over Dakar, Senegal, selected for itsbright city lights and a large range of aerosol optical depth (AOD), wereinvestigated for potential applications and opportunities for using observed radiances containing VIIRS-DNB "bright pixels" from artificial light sources to conduct aerosol retrievals. We found that using the standard deviation (SD) of such bright pixels provided a more stable quantity for nighttime AOD retrievals than direct retrievals from TOA radiances. Further, both themean TOA radiance and SD of TOA radiances over artificial sources aresignificantly impacted by satellite viewing angles. Light domes, theenhanced radiances adjacent to artificial light sources, are strongfunctions of aerosol properties and especially aerosol vertical distribution, which may be further utilized for retrieving aerosol layerheight in future studies. Through inter-comparison with both day- andnighttime Aerosol Robotic Network (AERONET) data, the feasibility ofretrieving nighttime AODs using 3-D RTM SHDOM over artificial light sourceswas demonstrated. Our study shows strong potential for using artificiallight sources for nighttime AOD retrievals, while also highlighting largeruncertainties in quantifying surface light source emissions. This studyunderscores the need for surface light emission source characterizations asa key boundary condition, which is a complex task that requires enhancedinput data and further research. We demonstrate how quality-controllednighttime light data from the NASA's Black Marble product suite could serveas a primary input into estimations of surface light source emissions fornighttime aerosol retrievals.