Assessment of Dust Size Retrievals Based on AERONET: A Case Study of Radiative Closure From Visible-Near-Infrared to Thermal Infrared

Dust particles suspended in the atmosphere span a wide size range (0.001-100 mu m). Notably, super-coarse dust particles (diameter >10 mu m) have been observed to be more abundant than what climate models suggest. Theoretically, these super-coarse particles present little radiative signatures in visible to near-infrared (VIS-NIR) but significantly affect the thermal infrared (TIR) radiation. This study addresses the question of whether remote sensing techniques operating in the VIS-NIR can capture these dust particles. We use side-by-side observations associated with a dust plume in both VIS-NIR and TIR to assess whether the dust properties, including the size distribution, inferred by VIS-NIR observations can generate well-matched radiative signatures with TIR observations. We found that the simulated outgoing radiation at the top of the atmosphere in TIR using the VIS-NIR-inferred dust properties is greater than the observations because of not enough dust extinction, potentially led by the absence of super-coarse dust. By introducing an extra super-coarse mode in the size distribution, we found a better match with the TIR observation, while the dust optical properties retrieved in VIS-NIR can be mostly conserved. Our result demonstrates the importance of combining VIS-NIR and TIR observations to retrieve the dust size distribution.