Thermal infrared dust optical depth and coarse-mode effective diameter over oceans retrieved from collocated MODIS and CALIOP observations

In this study, we developed a novel algorithm based on the collocated Moderate Resolution Imaging Spectroradiometer (MODIS) thermal infrared (TIR) observations and dust vertical profiles from the Cloud–Aerosol Lidar with Orthogonal Polarization (CALIOP) to simultaneously retrieve dust aerosol optical depth at 10 inline-formulaµm (DAODinline-formula_10 µm) and the coarse-mode dust effective diameter (inline-formulaD_eff) over global oceans. The accuracy of the inline-formulaD_eff retrieval is assessed by comparing the dust lognormal volume particle size distribution (PSD) corresponding to retrieved inline-formulaD_eff with the in situ-measured dust PSDs from the AERosol Properties – Dust (AER-D), Saharan Mineral Dust Experiment (SAMUM-2), and Saharan Aerosol Long-Range Transport and Aerosol–Cloud-Interaction Experiment (SALTRACE) field campaigns through case studies. The new DAODinline-formula_10 µm retrievals were evaluated first through comparisons with the collocated DAODinline-formula_10.6 µm retrieved from the combined Imaging Infrared Radiometer (IIR) and CALIOP observations from our previous study (Zheng et al., 2022). The pixel-to-pixel comparison of the two DAOD retrievals indicates a good agreement (inline-formulaR∼0.7) and a significant reduction in (inline-formula∼50 %) retrieval uncertainties largely thanks to the better constraint on dust size. In a climatological comparison, the seasonal and regional (inline-formula $M10inlinescrollmathml\mathrm{normal\; 2}{}^{\circ }\times \mathrm{normal\; 5}{}^{\circ }$ 34pt11ptsvg-formulamathimg8a76e0febdc6ccff4a2b5651953fe28a acp-23-8271-2023-ie00001.svg34pt11ptacp-23-8271-2023-ie00001.png ) mean DAODinline-formula_10 µm retrievals based on our combined MODIS and CALIOP method are in good agreement with the two independent Infrared Atmospheric Sounding Interferometer (IASI) products over three dust transport regions (i.e., North Atlantic (NA; inline-formulaR=0.9), Indian Ocean (IO; inline-formulaR=0.8) and North Pacific (NP; inline-formulaR=0.7)).

Using the new retrievals from 2013 to 2017, we performed a climatological analysis of coarse-mode dust inline-formulaD_eff over global oceans. We found that dust inline-formulaD_eff over IO and NP is up to 20 % smaller than that over NA. Over NA in summer, we found a inline-formula∼50 % reduction in the number of retrievals with inline-formulaD_eff>5 inline-formulaµm from 15 to 35inline-formula^∘ W and a stable trend of inline-formulaD_eff average at 4.4 inline-formulaµm from 35inline-formula^∘ W throughout the Caribbean Sea (90inline-formula^∘ W). Over NP in spring, only inline-formula∼5 % of retrieved pixels with inline-formulaD_eff>5 inline-formulaµm are found from 150 to 180inline-formula^∘ E, while the mean inline-formulaD_eff remains stable at 4.0 inline-formulaµm throughout eastern NP. To the best of our knowledge, this study is the first to retrieve both DAODpage8272 and coarse-mode dust particle size over global oceans for multiple years. This retrieval dataset provides insightful information for evaluating dust longwave radiative effects and coarse-mode dust particle size in models.