A combined dataset of path-averaged and in-situ measurements of greenhouse gases to inform on the sensitivities to localized source patterns and transport effects in the urban atmosphere.

Urban areas are a major and growing contributor to anthropogenic greenhouse gas (GHG) emissions and are thus an important target for emission reduction efforts. However, measurement-based information for planning, implementing, and monitoring such reduction efforts on city scales is rarely available to policymakers and stakeholders. Such monitoring systems typically rely on three key components: measurements of GHG concentrations (or turbulent fluxes), modeling of the atmospheric transport and prior information on the spatial and/or temporal structure of the emissions. The high spatial and temporal heterogeneity of urban areas and their emissions is especially challenging for atmospheric transport models and gridded inventories, which are currently pushed to resolutions of kilometers and below in an effort to accurately represent these effects. However, GHG concentration measurements are often performed by in-situ systems and are thus not necessarily representative for the kilometer scales on which measurements, transport modeling and prior information are typically compiled. This becomes increasingly important with the ever-improving quality of measurements, models, and inventories themselves.

We present a dataset of urban path averaged concentration measurements of CO₂ and CH₄ and their comparison to co-located in-situ measurements. The path averaged measurements are taken along a 1.55 km long path over the city of Heidelberg, Germany. The observatory utilizes FTIR spectroscopy and is now in continuous operation since February 2023. Analysis of the path averaged and co-located in-situ measurements reveals differences of up to 20 ppm in CO₂ for specific wind directions, which are most likely a result of a local atmospheric transport phenomenon. Further, the two measurements show differences in CH₄, which are likely a result of different sensitivities to local emissions. Overall, the data indicate a clear but different sensitivity of either measurement approach to localized source patterns. Thus, the dataset enables the assessment of the representativeness of the different measurement approaches and of the performance of atmospheric transport models and emission inventories in the urban environments.