Investigating Vienna’s methane budget with local observations of turbulent fluxes and total column mole fractions

Like other cities across the globe, Vienna has announced an ambitious climate target of reaching net zero emissions of greenhouse gases (GHG) by 2040. Developing and implementing appropriate urban polices and measures to reach this goal requires robust understanding and quantification of the emissions of all GHGs, including Methane (CH₄). According to the Austrian provincial emission inventory (Bundesländer Luftschadstoff-Inventur) that sets the current baseline for Vienna’s planned emission reductions, CH₄ emissions contribute only ca. 1% of the city’s total GHG budget (87 of 8387 kt CO₂eq in 2021; 100-year global warming potential). However, urban sources of methane are associated with large uncertainties (e.g. leaks from gas distribution networks, post-meter fugitive emissions) with inventories often producing substantial underestimations of these emissions. The Vienna Urban Carbon Laboratory is currently investigating the CH₄ budget of Austria’s capital city with local atmospheric observations of turbulent fluxes and total column mole fractions. Since May 2022, CH₄ fluxes are being measured at the Arsenal radio tower using an eddy covariance system installed 144 m above the city.  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 CH₄ mole fractions. This conference contribution will discuss the temporal and spatial patterns in the CH₄ fluxes derived directly from eddy covariance observations so far, as well as the inverse estimates of summer CH₄ emissions from the EM27/SUN observations. The two independent, observation-based methods will provide robust, and policy-relevant indications on the potential accuracy (or potential bias) in Vienna’s CH₄ inventory.