A global land aerosol fine-mode fraction dataset (2001-2020) retrieved from MODIS using hybrid physical and deep learning approaches

The aerosol fine-mode fraction (FMF) is valuable for discriminating natural aerosols from anthro-pogenic ones. However, most current satellite-based FMF products are highly unreliable over land. Here, we developed a new satellite-based global land daily FMF dataset (Phy-DL FMF) by synergizing the advantages of physical and deep learning methods at a 1 degrees spatial resolution covering the period from 2001 to 2020. The Phy-DL FMF dataset is comparable to Aerosol Robotic Network (AERONET) measurements, based on the analysis of 361 089 data samples from 1170 AERONET sites around the world. Overall, Phy-DL FMF showed a root-meansquare error (RMSE) of 0.136 and correlation coefficient of 0.68, and the proportion of results that fell within the +/- 20% expected error (EE) envelopes was 79.15 %. Moreover, the out-of-site validation from the Surface Radiation Budget (SURFRAD) observations revealed that the RMSE of Phy-DL FMF is 0.144 (72.50% of the results fell within the +/- 20% EE). Phy-DL FMF showed superior performance over alternative deep learning or physical approaches (such as the spectral deconvolution algorithm presented in our previous studies), particularly for forests, grasslands, croplands, and urban and barren land types. As a long-term dataset, Phy-DL FMF is able to show an overall significant decreasing trend (at a 95% significance level) over global land areas. Based on the trend analysis of Phy-DL FMF for different countries, the upward trend in the FMFs was particularly strong over India and the western USA. Overall, this study provides a new FMF dataset for global land areas that can help improve our understanding of spatiotemporal fine-mode and coarse-mode aerosol changes. The datasets can be downloaded from https://doi.org/10.5281/zenodo.5105617 (Yan, 2021).