Mobile Ground-Based Measurements of CO2 Fluxes in Vienna

Urban carbon dioxide (CO₂) emissions contribute significantly to global anthropogenic greenhouse gas emissions. However, accurately monitoring emissions in individual cities remains a substantial challenge that could hinder effective local mitigation measures. In Vienna, a novel approach integrating stationary, tall-tower eddy covariance (EC) and CO₂ mole fraction measurements with mobile ground-based sampling of CO₂ concentrations was developed and tested. The mobile system consisted of a car equipped with a LI-COR 7200RS gas anaylzer and GPS device, recording location and CO₂ concentrations at 2 m height every second along a predefined route that traversed the flux footprint of the EC system (LICOR 7500DS and Gill Windmaster Pro). At the tower, continuous, parrellel measurements of turbulent fluxes and CO₂ mole fractions at 144 m above the surface were made by the said EC system and a Picarro isotope analyzer (G2131-i), respectively. The mobile measurements were averaged into 100 x 100 m grid cells and combined with the tall tower Picarro observations to derive vertical gradients in CO₂ molar concentations for each ha of the city intersected by the route. Surface fluxes were derived from these gradients using the aerodynamic resistance approach, based on Monin-Obukhov Similarity Theory (MOST) and the turbulence variables recorded by the EC system. These fluxes were compared to a downscaled CO₂ emissions inventory at both hectare-scale and zonal resolutions. Initiated in August 2024, the measurement campaigns, based on 2-hour midday surveys, have shown strong alignment between mobile measurements and inventories. This conference contribution will discuss the potential value of combining mobile and stationary measurements methods for investigating urban CO₂ fluxes. Currently, additional measurement campaigns are ongoing to assess the consistency of results across seasons. Furthermore, maps of LAI will be introduced to investigate whether vegetation explain temporal and spatial divergence between inventory and mobile measurement-derived fluxes.