This study presents the development and pilot implementation of a conceptual, practice-oriented Urban Digital Twin (UDT) used for environmentally sensitive mobility management within the pilot region of Leipzig (Germany). The UDT uses modular traffic simulations (SUMO-simulation of urban mobility), real-time sensor data and floating car data to create dynamic, near-real-time models of intermodal urban mobility systems and to evaluate mobility measures. Through a collaborative, co-production process with municipal stakeholders, the study focuses on scenario-based simulations to assess the potential impact of traffic scenarios under upcoming air pollution limits. The Leipzig case serves as a test environment for validating the technical feasibility of the conceptual UDT framework, applying a co-production process and discussing transferability of the approach. The results show the potential of UDTs to support responsive, data-driven decision-making using the developed workflow manager. In doing so, this study provides a transferable methodological basis for cities that aim at supporting urban sustainability transformations using digital twin technology.