Using tall tower flux measurements for GHG emissions monitoring in cities: Emerging results and perspectives from the Vienna Urban Carbon Laboratory

The substantial urban contribution to global anthropogenic greenhouse gas (GHG) budgets underlines the importance of improved GHG emissions monitoring in cities. Reducing urban emissions of carbon dioxide (CO₂) and methane (CH₄) will be critical to mitigating climate change; yet, GHG budgets of individual cities as quantified by emission inventories can be very uncertain. This is due to a lack of appropriate activity data and emission factors for compiling city-scale inventories or uncertainties in spatial downscaling of regional/national emissions.

The Vienna Urban Carbon Laboratory is currently investigating how monitoring of CO₂ and CH₄ emissions in Austria’s capital city can be supported by a range of atmospheric measurement methods, including a tall-tower application of eddy covariance flux measurements. Cities can represent non-ideal conditions for eddy covariance due to the aerodynamically rough surface conditions and spatial heterogeneity in GHG sources (and sinks). Nonetheless, if biases/errors caused by these factors are acceptable, the method provides a potentially significant advantage in that net urban emissions can be directly inferred from the measured vertical turbulent fluxes. Since December 2017, CO₂ fluxes at 144 m above the surface have been measured using an eddy covariance system deployed at the A1 Arsenal radio tower in Vienna’s city centre. The original rationale for the tall tower approach was to partially mitigate the aforementioned challenges of urban eddy covariance (e.g. to get above the deep urban roughness layer and measure in the surface layer) and to increase the area of the city sampled by the flux footprint. In May 2022, the observations at the tower were expanded to measure CH₄ fluxes, as well as atmospheric mixing ratios of CO₂ and its stable carbon isotope composition. Furthermore, between May and July 2022, a parallel measurement campaign with four ground-based, sun-viewing FTIR spectrometers (EM27/SUN) was conducted to measure horizontal gradients in total column CO₂ and CH₄ concentrations.

This conference contribution will present an analysis of the tall-tower eddy covariance measurements of CO₂ and CH₄ fluxes and discuss potential applications within the scope of operational emissions monitoring. In addition to discussing the encouraging agreement between eddy covariance measurements and local CO₂ emission inventories for the years 2018 to 2020, the initial eddy flux-inventory comparison for CH₄ will be presented. Moreover, planned analyses (and initial results, where available) on several relevant fronts will be briefly discussed:  comparison of the eddy fluxes with inverse modelled CO₂ and CH₄ fluxes using differential column concentration measurements; comparison of partitioned CO₂ fluxes with source-sector emission estimates derived from local inventories and measurements of stable carbon isotope composition of atmopsheric CO₂. Finally, trends in CO₂ fluxes between 2018 and 2022 will be presented to highlight the potential early indicator function and immediate societal benefits  that urban eddy covariance can provide.