Significance Recent efforts to reduce greenhouse gas emissions have focused on cities due to intensive emissions, viable policy levers, and interested stakeholders. Atmospheric observations can be used to independently evaluate emissions, but suitable networks are sparse. We present a unique decadal record of atmospheric CO ₂ from five sites with contrasting urban characteristics that show divergent trends in CO ₂ emissions across a city. Comparison with population growth reveals a nonlinear relationship that may reflect how urban form affects CO ₂ emissions. Four state-of-the-art global-scale emission inventories capture the nonlinear relationship with population density but not the divergent long-term trends across the city. This demonstrates that CO ₂ monitoring networks can provide insight into urban carbon cycle processes and provide policy-relevant information to urban stakeholders.