The project was carried out under the German Federal Ministry of Education and Research (BMBF) funding measure “Urban Climate in Transition.” The first phase began on 1 June 2016 and ran for three years, while the second phase continued the work of the joint project 3DO+M “Three‑dimensional observation and modelling of atmospheric processes in cities.” Subproject 6, referred to as Module B, was responsible for processing existing observations and designing new measurement campaigns to support the development, validation and application of the building‑resolving urban climate model PALM4U. Twelve Module B subprojects contributed to the evaluation and scientific use of PALM4U during Phase 2.

A newly equipped mobile measurement laboratory was deployed in eight intensive observation periods (IOPs) in Berlin and Stuttgart. The laboratory recorded temporally highly resolved data for nitrogen oxides (NO, NO₂), ozone (O₃), carbon monoxide (CO), carbon dioxide (CO₂), ammonia (NH₃), particle size distribution and number concentration. These measurements provided a detailed spatial and temporal picture of urban pollutant concentrations and were used both for model evaluation and for direct air‑quality applications. The laboratory’s high‑accuracy instrumentation enabled the detection of subtle variations in pollutant levels, such as differences between weekdays and weekends and temperature‑dependent emission behaviour.

In the Heslacher Tunnel, the project continued tunnel studies to investigate vehicle emission characteristics under controlled conditions. The data were used to quantify the heterogeneity of urban pollutant concentrations, revealing significant spatial variability that must be captured by high‑resolution models. Local ozone formation was examined through model studies that incorporated actual precursor measurements. The PALM4U chemistry module CBM4 was compared with an explicit zero‑dimensional model (MCM‑3.3.1). The comparison showed that CBM4 reproduces local ozone production reasonably well, while differences emerged in cumulative ozone production when a polluted air mass was transported from urban to rural areas. Photostationary equilibrium (PSE) simulations based on the simple PALM4U chemistry module were performed for two case studies: VALM01 in Berlin during winter 2017 and VALM04 in Stuttgart during summer 2018. These simulations highlighted the importance of background ozone and biogenic volatile organic compounds (VOCs) in determining urban ozone levels.

Urban‑surrounding studies addressed background ozone, the contribution of biogenically emitted VOCs, cold‑air flows, and urban heat‑island effects in the Stuttgart area. The shadow effect on the NO/NO₂ ratio in a street canyon was investigated using PALM4U simulations, demonstrating that shading can significantly alter the photochemical balance. A further application involved simulating the surroundings of Berlin’s Ernst‑Reuter‑Platz, linking PALM4U to the regional chemistry and transport model EURAD‑IM and visualising high‑resolution oxidant formation. These results illustrate the versatility of PALM4U for both local and regional air‑quality assessments.

The project’s findings were comprehensively published in a monograph in 2020, and the data and model evaluations continue to inform urban climate and air‑quality research. The collaboration among multiple German research institutes, supported by BMBF funding, has produced a robust, building‑level urban climate model that integrates detailed observations with advanced chemical and physical processes, providing a scientifically grounded tool for addressing current and future urban climate and pollution challenges.