Investigating the contribution of biogenic sinks and sources to Vienna’s CO2 budget using bottom-up modelling and tall-tower flux measurements

The importance of accurate monitoring of carbon dioxide (CO₂) emissions from cities is underlined by the substantial urban contribution to global fossil fuel combustion. Typically, cities quantify emissions of CO₂ using inventories and also use these models to design appropriate local mitigation policies and measures. However, inventories of individual cities can be uncertain (lack of appropriate activity data and emission factors, uncertainties in spatial downscaling) and furthermore often do not include estimates for the sector Land Use, Land-use Change and Forestry (LULUCF). While the relative, total contribution of LULUCF to a city’s annual CO₂ balance may be assumed small, such an assumption should be verified. Furthermore, biogenic fluxes of photosynthesis and respiration may indeed be significant at higher temporal resolutions and omitting these fluxes can limit the conclusions drawn from comparisons of city CO₂ inventories with estimates based on atmospheric CO₂ observations.

The Vienna Urban Carbon Laboratory is currently investigating how monitoring of CO₂ emissions in Austria’s capital city can be supported by a range of atmospheric measurement methods, including a tall-tower, urban application of eddy covariance. Despite the focus on atmospheric observations, the project is also investigating the contribution of biogenic fluxes to Vienna’s net CO₂ budget. A LULUCF model of annual carbon stock changes has been developed following the IPCC guidelines using inter alia local forest inventory data and spatially-explicit data on land use and urban tree crown cover. Parrellel to this, work is underway to implement spatially- and temporally resolved simulations of vegetation CO₂ fluxes using semi-empirical models of photosynthesis and respiration. Ultimately, integrating these results (together with bottom-up estimates of human respiration) will provide a more meaningful comparison between the local CO₂ inventory with the fluxes derived from the eddy covariance measurements.