CALIPSO-based aerosol extinction profile estimation from MODIS and MERRA-2 data using a hybrid model of Transformer and CNN.

Acquiring aerosol vertical distribution information is crucial to accurately quantify the aerosol radiation effect on climate and understand the environmental pollution mechanism of the atmosphere. Passive remote sensing has shown its capability to gain large-scale, high spatiotemporal resolution aerosol vertical information such as aerosol layer height (ALH). However, it is still challenging to extract detailed aerosol vertical distribution information, e.g., aerosol extinction profile (AEP), from passive observations. To fill this gap, this study proposed a hybrid model of Transformer and convolutional neural network (CNN) to estimate the AEP from passive multispectral remote sensing (MODIS) measurements with the aid of three-dimensional reanalysis data (MERRA-2). Specifically, the model is learned to estimate the AEP, which is called AproNet, by using the active space-borne lidar (CALIPSO) data as supervised information. Besides, we design a shape invariant loss (SIL) to better capture the shape characteristics of the AEP and incorporate an auxiliary scene awareness loss (SAL) to enhance the model's generalization capacity and physical reliability outside the CALIPSO orbit. The extensive quantitative experiments show that the AEPs estimated by the proposed model agree well with the CALIPSO measurements with an overall performance of IOA=0.821, R=0.800, MAE= 0.014, and RMSE= 0.041, respectively. Qualitative comparisons also demonstrate the model's reliability in estimating the aerosol three-dimensional spatial distribution. Independent year test and comparisons with ground-based lidar measurements further indicate the robustness of the proposed model despite some degradation in performance. However, the incompleteness and uncertainty of the CALIOP products limited the performance of the proposed model to some extent. In the future, the model needs to be further physically constrained and strengthened with more data sources to improve reliability. In general, this study paves the way for acquiring aerosol extinction profiles with high spatiotemporal resolution over a large geographical space.