Mapping contiguous XCO₂ by machine learning and analyzing the spatio-temporal variation in China from 2003 to 2019

As China is the world's largest CO₂ emitter, it is important to understand the spatio-temporal variation of atmospheric CO₂ to reduce carbon emissions. Satellite remote sensing for carbon monitoring has been widely used and studied because of its long-term and large-scale characteristics. However, the satellite data results are very sparse with significant gaps due to narrow swath and other factors on CO₂ retrieval. The simple interpolation methods ignore the influential factors of CO₂ and loss the spatial resolution, which leads to the inability to quantify the spatio-temporal variation well. This study developed a machine learning method that considers carbon emissions, vegetation, and meteorology. Using the column-averaged dry-air mole fraction of CO₂ (XCO₂) data of SCIAMACHY, GOSAT, and OCO-2, we derived monthly-scale contiguous XCO₂ data across China from 2003 to 2019 with 0.25° resolution. The results showed a good agreement with the satellite measurements, with the bias and standard deviation of 0.11 and 1.38 ppmv for the validation dataset, respectively. Moreover, the results were consistent with the model simulation and in-situ sites, indicating the ability to reflect long-term spatio-temporal variation with a finer texture. We analyzed the spatial distribution, seasonal variation, and long-term trends of XCO₂ in China, revealing that the machine learning method has comparable performance to model simulations. The results showed that XCO₂ is dominated by anthropogenic emissions spatially and has a clear seasonal cycle, with a larger amplitude the further north. The long-term trend shows the XCO₂ increased by an average rate of 2.17 ppmv per year from 2003 to 2019 in China, which is consistent with the global. The method and data can further study the carbon cycle and climate change.