Abstract: Most people on earth live in cities and they are responsible for the majority of greenhouse gas emissions. Cities are also where exposure to poor air quality is most frequent and most variable. Understanding and managing the path to net-zero greenhouse gas emissions, improved public health and lower public health inequities requires a view into the emissions and atmospheric chemistry of cities with the fine-grained detail that allows evaluation of specific processes and variations from one neighborhood to another. In this talk, I will describe the development of the Berkeley Environment, Air Quality and CO2 Network (BEACO2N http://beacon.berkeley.edu/about/), a dense network for mapping urban CO2, NOx, CO, O3 and aerosol. Each node of the network contains multiple optical and ChemFET sensors which accurately quantify the trace gases and are calibrated in the field using a variety of cross-referencing methods and comparisons with traceable standards. Integration of the BEACO2N maps with simple Gaussian plume models and sophisticated inversions employing high resolution weather models provide unique observational constraints on spatial and temporal patterns of CO2 and other emissions. Examples from the COVID shelter-in-place and testing models of fuel efficiency vs. vehicle speed in the SF Bay Area will be described along with prospects for further extension to other cities and other chemicals.