On October 7-8, CPO’s Atmospheric Chemistry, Climate, and Carbon Cycle (AC4) Program is supporting a workshop called CO2-Urban Synthesis and Analysis in Boston, Massachusetts. Cities have emerged as leaders in U.S. efforts to reduce greenhouse gas emissions, but knowledge on how to quantitatively track emissions and assess mitigation efforts is lacking. This workshop is the last of three that helped launch a network bringing together leading scientists, practitioners, and policy makers to exchange information on how to better understand urban greenhouse gas emissions. Hosted by Boston University, the final workshop will focus on synthesizing science across data-rich U.S. cities, estimating and comparing carbon emissions, sharing the results with the user community, and outlining next steps for necessary research.