China　has　committed　to　peak　its　carbon　emissions　before　2030　and　achieve　carbon　neutrality　before　2060,　known　as　the　dual　carbon　goals　(DCGs).　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　6years　through　a　collaborative　effort　of　16　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　time　scales,　as　well　as　dramatic　on-road　changes　during　the　COVID-19　lockdown.　High-resolution　modeling　and　data　assimilation　at　kilometer　scale　reveal　major　deficiencies　in　global　carbon　emission　datasets　at　regional/city　scale.　The　inversion　found　the　total　fossil　fuel　CO2　emissions　of　the　City　of　Beijing　to　be　117　megatonne　(Mt)　but　much　lower　than　the　national-databased　inventories　(152　MtCO2).　The　results　have　been　used　in　emission　assessment　by　national　and　local　environmental　protection　agencies.　Similar　systems　are　now　being　developed　in　other　Chinese　cities.　SIGNIFICANCE　STATEMENT:　China＇s　carbon　neutral　goal　by　2060　requires　our　ability　to　monitor　and　assess　progresses　in　its　path　to　decarbonization　with　unprecedented　resolution　at　city　and　regional　scale　where　most　of　the　CO2　emissions　occur.　Since　2017,　a　collaborative　effort　from　16　research　institutions　has　built　a　carbon　monitoring　system　for　the　Beijing-Tianjin-Hebei　(JJJ)　city　cluster.　Data　from　a　network　of　ground-,　air-,　and　space-based　observations　showed　CO2　variations　at　diurnal-,　synoptic-　to　seasonal-interannual　time　scales,　as　well　as　dramatic　on-road　changes　during　the　COVID-19　lockdown.　High-resolution　modeling　and　data　assimilation　at　kilometer　scale　reveal　major　deficiencies　in　global　carbon　emission　datasets.　The　results　are　being　used　in　emission　assessment　by　national　and　local　environmental　protection　agencies,　adopted　at　other　Chinese　cities.