Seasonal and Diurnal Variations in XCO2 Characteristics in China as Observed by OCO-2/3 Satellites: Effects of Land Cover and Local Meteorology

Monitoring the dynamics of atmospheric CO2 is crucial for enhancing comprehension of the carbon cycle. Using column-averaged dry-air mole fraction of CO2 (XCO2) data collected by the Orbiting Carbon Observatory (OCO)-2 and OCO-3 satellites during 2020-2021, this study explored seasonal and diurnal variations in XCO2 characteristics in typical land cover biomes in China, and investigated their relationships with meteorological drivers. Results showed that XCO2 products retrieved by OCO-2 and OCO-3 have good agreement with Total Carbon Column Observing Network measurements, with average deviations of 0.8 and 1.2 ppm, respectively. The satellite observations revealed XCO2 hotpots located mainly in central and eastern China, and areas of low XCO2 values in western China, with a seasonal curve that was highest (lowest) in spring (summer). The largest seasonal cycle amplitude (similar to 9 ppm) of XCO2 was observed in forest areas, highlighting its key role in carbon exchange. Additionally, XCO2 was found to have a near-sinusoidal diurnal pattern, characterized by rapid decrease in the early morning as photosynthesis resumed after sunrise, as indicated by the sun-induced chlorophyll fluorescence (SIF), a peak at around midday, and subsequent decrease as SIF increased after mid-afternoon. Urban regions had the highest diurnal cycle amplitude (similar to 6 ppm) among biomes. Statistical analyses revealed seasonal shift and nonlinear variation in the relationships between XCO2 and meteorological variables, suggesting that CO2 uptake is influenced by favorable humidity conditions. These relationships also provide insight into the sensitivity and adaptability of XCO2 to meteorological factors in diverse ecosystems such as savanna and