Dear Colleagues,

Since CO₂ has been recognized as the most important anthropogenic greenhouse gas, owing to its significant impact on global warming and climate change, there are a substantial number of studies investigating the status of CO₂ in the atmosphere in the past and present and how it will change in the future. In support of the upcoming global stocktake in 2023, new methods are needed to determine the extent of human emissions’ impact on the global carbon cycle and climate change. Top-down carbon flux inversion by assimilating atmospheric carbon dioxide (CO₂) measurements is a useful tool for evaluating the global carbon budget and was used in the 2019 Refinement to the 2006 Intergovernmental Panel on Climate Change (IPCC) Guidelines for National Greenhouse Gas Inventories. Ground-based measurements provide highly accurate continuous data and have improved our understanding of global carbon flux. However, the sparseness and spatial inhomogeneity of the existing ground-based network limits our ability to infer consistent global- and regional-scale CO₂ sources and sinks, especially for tropical and high-latitude regions. To improve observation coverage, tailor-made satellites have been developed that provide atmospheric greenhouse gas (GHG) measurements with unprecedented precision.

China plays an important role in the global carbon budget as a major source of anthropogenic carbon due to its rapid economic development, although it simultaneously represents a region of increased carbon sequestration due to a number of reforestation projects. In China, a series of ambitious projects aiming to mitigate carbon emissions have been in effect for the last 15 years, including the first Chinese greenhouse gas monitoring satellite mission (TanSat), which is supported by the Ministry of Science and Technology of China, the Chinese Academy of Sciences, and the China Meteorological Administration. The TanSat mission was initiated in 2011 and successfully launched on 22 Dec 2016. TanSat started acquiring and archiving data operationally in March 2017.

TanSat carries two instruments on board: the Atmospheric Carbon Dioxide Grating Spectrometer (ACGS) and the Cloud and Aerosol Polarimetry Imager (CAPI). ACGS is a state‐of‐the‐art hyperspectral grating spectrometer allowing XCO₂ retrievals by measuring backscattered sunlight in three NIR/SWIR bands: the oxygen (O₂) A‐band (758–778 nm with ~0.04 nm spectral resolution), the weak CO₂ band (1,594–1,624 nm with ~0.125 nm spectral resolution), and the strong CO₂ band (2042–2082 nm with ~0.16 nm spectral resolution). CO₂ column information is largely drawn from the weak CO₂ band, which includes a series of strong but unsaturated CO₂ lines that respond to even small variations in atmospheric CO₂. The O₂ A-band hyperspectral measurement contains information on aerosol and cloud scattering both in total scattering amount and scattering vertical distribution. The strong CO₂ band provides information on aerosol and cloud scattering at longer wavelengths, in conjunction with information on CO₂ and water vapor. The CAPI is a multi‐band imager that provides radiance measurements in five bands (365‐408 nm, 660‐685 nm, 862‐877 nm, 1,360‐1,390 nm, 1,628‐1,654 nm) from UV to NIR. To obtain more information on aerosol size, which has a significant impact on the wavelength dependence of aerosol optical properties, CAPI includes two polarization channels (660‐685 nm and 1,628‐1,654 nm) to measure the Stokes parameters. TanSat flies in a sun‐synchronous low Earth orbit (LEO) with an equator crossing time of around 13:30 local time. The swath width of TanSat measurements is ~18 km across the satellite track and contains nine footprints each with a size of 2 km × 2 km in nadir.

It has been 6 years since TanSat launched and 11 years since the TanSat mission kick-off. This Special Issue will provide an overview of the latest progress and research on TanSat measurement, focusing especially on the satellite and instrument technics, retrieval algorithm, data application, and calibration/validation. TanSat is China’s first carbon satellite providing XCO₂ measurement to scientific research on the global carbon cycle. The next generation of the TanSat mission is current in the design phase, whose goal is to support global stocktake and China’s carbon peaking and carbon neutrality goals.

For this Special Issue, “China's First Dedicated Carbon Satellite Mission”, we are inviting contributions on new scientific results covering methods and applications as well as overview papers on topics such as:

• Measurement technics, incl. satellite and instrument performance;
• Retrieval algorithm and XCO₂ data product;
• Carbon flux inversion and data assimilation;
• TanSat data application, e.g., CO₂ emission/sink investigation;
• Val/Cal for TanSat measurement.

Papers that exploit TanSat data application in global and regional carbon monitoring research are especially encouraged. All types of papers are welcome, especially research articles.

Dr. Dongxu Yang
Prof. Dr. Yi Liu
Prof. Dr. Hartmut Boesch
Prof. Daren Lyu
Guest Editors

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• carbon dioxide
• satellite remote sensing
• carbon flux inversion
• data assimilation
• retrieval algorithm
• measurement, verification support (MVS)