The JJJ Carbon Monitoring System---Support Carbon Neutral Goal with a High-Resolution Atmospheric CO2 Monitoring Network from Ground, Air and Space for Beijing-Tianjin-Hebei City Cluster

China has committed to peak its carbon emissions before 2030 and achieve carbon neutral before 2060, known as the Dual Carbon Goals (DCG). To support the DCG, monitoring and assessment of progresses in emissions reduction have become a central research focus. Here we describe the Beijing-Tianjin-Hebei (JJJ) Carbon Monitoring System, established over the last 5 years through a collaborative effort of 18 research institutions and universities. The monitoring capability includes ground-based high-precision reference stations, a dense network of medium accuracy low-cost sensors, carbon satellites, vertical profile samplers, and mobile platforms. Observations using the network have shown CO2 variations from diurnal, synoptic to seasonal-interannual timescales, as well as dramatic on-road changes during the COVID-19 pandemic. High-resolution modeling and data assimilation at km-scale reveal major deficiencies in global carbon emissions datasets at regional/city scale. The results have now been used in emissions assessment by national and local environmental protection agencies. Similar systems are now being developed in other Chinese cities.