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

The coarse and giant mineral dust particles have been documented to be ubiquitous throughout long-range transports by in-situ measurements, while the long-term ground-based remote sensing observations, such as Aerosol Robotic Network (AERONET), have limited evidence showing similar discoveries. Although it is challenging to interpret the discrepancies between the two fundamentally different observations, it is achievable to examine the remotely sensed dust coarse mode particle size distribution (DC-PSD) by studying the radiative closure from visible (VIS) and near-infrared (NIR) to thermal infrared (TIR) spectra, which is mainly sensitive to coarse and giant dust particles.

Therefore, in this study, we implement a radiative closure study from VIS-NIR to TIR for a dust case over Santa Cruz Tenerife (28.473N, 16.247W) on June 19, 2022, based on observations from AERONET, Micro-Pulse Lidar Network (MPLNET) and the spaceborne Atmospheric Infrared Sounder (AIRS). Including most of the uncertainty sources from necessary assumptions, our result shows that ~95% of the TIR brightness temperatures simulated based on AERONET-retrieved DC-PSD among the range of error is warmer than that from AIRS, meaning that using the AERONET-retrieved properties can hardly make a radiative closure with TIR observations. Afterward, we adjusted the AERONET-retrieved DC-PSD to be larger by referring to the in-situ measured Fennec-SAL PSD while keeping the total surface area unchanged. We found that the sensitivity of AERONET observations to this change in DC-PSD is mainly within 5%, while using the adjusted DC-PSD can very much improve the TIR radiative closure.

The result indicates that although the AERONET-retrieved DC-PSD is well-established in VIS-NIR channels, it is highly possible to miss a portion of coarse particles due to the limited sensitivity in VIS-NIR and lead to underestimated extinction in TIR. Combining VIS-NIR and TIR observations can improve the retrieval of DC-PSD, which is important in evaluating dust total mass concentration.