![]() ![]() A similar annual flux was observed in 2019 however, the substantially lower flux in 2020 likely reflected emissions reductions associated with the COVID-19 pandemic. The 2018 flux of 10.89 kt CO 2 km −2 was in close agreement with the annual CO 2 emissions estimate of 10.19 kt CO 2 km −2 derived from an official emissions inventory (excluding large point source emissions). Indeed, the calculated annual fluxes suggested that positive and negative 30-min biases were largely offset when averaging over longer time intervals. While the influence of CO 2 advection could not be quantified, the pronounced morning flux peaks indicated that CO 2 released to the atmosphere during these partially decoupled, night-time periods accumulated to some extent and was subsequently mixed vertically with the morning increase in boundary layer depth. Furthermore, variations in weekend/public holiday CO 2 fluxes from night-time to morning periods suggested a partial night-time decoupling between turbulent exchange at 144 m and the net fluxes at the surface. Weekend/public holiday diurnal trends in CO 2 fluxes indicated an influence of vegetation fluxes on net CO 2 flux variations between midday and late afternoon in spring and summer. Weekday-weekend patterns as well as weekday diurnal variations in CO 2 generally followed respective trends in local traffic counts however, flux correlations with hourly traffic counts on weekends and public holidays were much weaker. For flows from the south-east, when flux footprints covered areas of built-up and industrial land yet were dominated by green urban spaces and arable land, the seasonality was much less marked and average monthly CO 2 fluxes were generally lower, particularly in winter. When analysing separate trends for the two dominating wind directions, this seasonality was most pronounced for the flows from the north-west when flux footprints overlapped with Vienna's most populated and urbanised districts and fluxes. Average monthly CO 2 fluxes followed seasonal trends in temperature-based proxies of heating demand, with the highest fluxes observed in the winter months. Overall CO 2 flux data availability after quality control filtering (49%) was comparable to that observed by other studies of urban and suburban eddy covariance measurements. ![]() This study reports on three years (2018–2020) of turbulent CO 2 fluxes measured at 144 m above the city of Vienna, Austria using an eddy covariance system installed on the A1 Arsenal radio tower. ![]()
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